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DISCUSSIONS OF THE FARADAY SOCIETY No. 27, 1959 ENERGY TRANSFER WITH SPECIAL REFERENCE TO BIOLOGICAL SYSTEMS THE FARADAY SOCIETY Agents f o r the Society’s Publications : The Aberdeen University Press Ltd. 6 Upper Kirkgate, Aberdeen Scotland@ The Faraday Society and Contributors, 1960 PUBLISHED . . . 1960 PRINTED IN GREAT BRITAIN AT THE UNIVERSITY PRESS ABERDEENA GENERAL DISCUSSION ON ENERGY TRANSFER WITH SPECIAL REFERENCE TO BIOLOGICAL SYSTEMS 14th-16th April, 1959 A GENERAL DISCUSSION on Energy Transfer with Special Reference to Biological Systems was held at Nottingham University on the 14th, 15th and 16th April 1959. The President, Dr. E. W. R. Steacie, O.B.E., D.Sc., LL.D., F.R.S., was in the Chair and over 200 members and visitors were present. Among the distinguished overseas members and guests welcomed by the President were the following : Dr.S. J. Adelstein (Harvard), Mr. J. Amesz (Leiden), Prof. W. Berends (University of Delft), Dr. R. Braams (Utrecht), Prof. Milton Burton (University of Notre Dame), Prof. Britton Chance (Philadelphia), Dr. J. J. Chang (Bethesda), Prof. S. Claesson (University of Uppsala), Prof. M. Dole (North- western University), Dr. L. N. M. Duysens (Leiden), Dr. H. Feilchenfeld (Hebrew University), Prof. Th. Forster (Stuttgart), Dr. M. Furst (Washington Square College), Dr. H. R. Gersmann (Amsterdam), Dr. Z. R. Grabowski (University of Warsaw), Dr. W. A. Hagins (Bethesda), Dr. B. Hargitay (Bruxelles), Prof. R. Havemann (Humboldt University), Dr. J. Higgins (Free University of Brussels), Prof. and Mrs.F. H. Johnson (Princeton University), Dr. J. de Jonge (Eindhoven), Prof. H. Kallmann (Washington Square College), Dr. Tsoo E. King (Oregon State College), Dr. J. Kratohvil (University of Zagreb), Prof. R. L. Livingston (University of Minnesota), Dr. W. Looney M.I.T.), Prof. R. Lumry (University of Minnesota), Dr. K. Minnaert (Amster- dam). Prof. W. J. Moore (Indiana University), Dr. A. Muller (Karlsruhe), Dr. J, O h (Zurich), Dr. L. Paoloni (Rome), Dr. H. Pietsch (Humboldt University), Prof. E. Rabinowitch (University of Illinois), Dr. M. P. Reddy (Saclay), Prof. R. A. Robinson (University of Malaya), Dr. B. Rosenberg (New York University), Mr. K. Rosengren (University of Lund), Prof. M. Schoen (Munich), Dr. D. Schulte-Frohlinde (Heidelberg), Prof.H. Shull (Indiana University), Mr. J. W. Steketee (Eindhoven), Prof. Stig. Sunner (University of Lund), Dr. G. Szasz (Zurich), Dr. A. Szent-Gyorgyi (Woods Hole, Mass.), Mr. P. Taylor (Philadelphia), Prof. A. Terenin (Leningrad), Mr. D. Timm (University of Lund), Dr. A. R. van Dyken (Washington), Dr. I. Wadso (University of Lund), Dr. B. E. Wahler (Berlin), Dr. A. Weller (Stuttgart), Dr. and Mrs. G. M. Wyman (Frankfurt).A GENERAL DISCUSSION ON ENERGY TRANSFER WITH SPECIAL REFERENCE TO BIOLOGICAL SYSTEMS 14th-16th April, 1959 A GENERAL DISCUSSION on Energy Transfer with Special Reference to Biological Systems was held at Nottingham University on the 14th, 15th and 16th April 1959. The President, Dr. E. W. R. Steacie, O.B.E., D.Sc., LL.D., F.R.S., was in the Chair and over 200 members and visitors were present.Among the distinguished overseas members and guests welcomed by the President were the following : Dr. S. J. Adelstein (Harvard), Mr. J. Amesz (Leiden), Prof. W. Berends (University of Delft), Dr. R. Braams (Utrecht), Prof. Milton Burton (University of Notre Dame), Prof. Britton Chance (Philadelphia), Dr. J. J. Chang (Bethesda), Prof. S. Claesson (University of Uppsala), Prof. M. Dole (North- western University), Dr. L. N. M. Duysens (Leiden), Dr. H. Feilchenfeld (Hebrew University), Prof. Th. Forster (Stuttgart), Dr. M. Furst (Washington Square College), Dr. H. R. Gersmann (Amsterdam), Dr. Z. R. Grabowski (University of Warsaw), Dr. W. A. Hagins (Bethesda), Dr. B. Hargitay (Bruxelles), Prof. R. Havemann (Humboldt University), Dr.J. Higgins (Free University of Brussels), Prof. and Mrs. F. H. Johnson (Princeton University), Dr. J. de Jonge (Eindhoven), Prof. H. Kallmann (Washington Square College), Dr. Tsoo E. King (Oregon State College), Dr. J. Kratohvil (University of Zagreb), Prof. R. L. Livingston (University of Minnesota), Dr. W. Looney M.I.T.), Prof. R. Lumry (University of Minnesota), Dr. K. Minnaert (Amster- dam). Prof. W. J. Moore (Indiana University), Dr. A. Muller (Karlsruhe), Dr. J, O h (Zurich), Dr. L. Paoloni (Rome), Dr. H. Pietsch (Humboldt University), Prof. E. Rabinowitch (University of Illinois), Dr. M. P. Reddy (Saclay), Prof. R. A. Robinson (University of Malaya), Dr. B. Rosenberg (New York University), Mr. K. Rosengren (University of Lund), Prof.M. Schoen (Munich), Dr. D. Schulte-Frohlinde (Heidelberg), Prof. H. Shull (Indiana University), Mr. J. W. Steketee (Eindhoven), Prof. Stig. Sunner (University of Lund), Dr. G. Szasz (Zurich), Dr. A. Szent-Gyorgyi (Woods Hole, Mass.), Mr. P. Taylor (Philadelphia), Prof. A. Terenin (Leningrad), Mr. D. Timm (University of Lund), Dr. A. R. van Dyken (Washington), Dr. I. Wadso (University of Lund), Dr. B. E. Wahler (Berlin), Dr. A. Weller (Stuttgart), Dr. and Mrs. G. M. Wyman (Frankfurt).CONTENTS PAGE ~ O T H SPIERS MEMORIAL LECTURE- Transfer Mechanisms of Electronic Excitation. By T. Forster . . 7 I. MODES OF ENERGY TRANSFER FROM EXCITED AND UNSTABLE IONIZED STATES- Intramolecular and Intermolecular Energy Conversion Involving Change of Multiplicity. By G. Porter and M.R. Wright Outer and Inner Mechanism of Reactions of Excited Molecules. By A. Weller . Energy Transfer in Aromatic Vapours ; The Benzene-Sensitized Fluor- escence of Anthracene Vapour at 2652 A. By B. Stevens Viscosity and Temperature Effects in Fluorescence. By E. J. Bowen . Light and High Energy Induced Energy Transfer in Liquid and Rigid Organic Scintillators. By F. H. Brown, M. Furst and H. Kallmann Energy Transfer in Fluorescent Plastic Solutions. By J. B. Birks and K. N. Kuchela. . On Luminescence Decay Times and their Relation to Mechanisms of Energy Transfer in Radiation Chemistry of Liquids. By M. Burton and H. Dreeskamp . Energy Transfer in Polyethylene and Polyethylene + Polybutadiene Mixtures During Gamma Irradiation. By M. Dole and T. F.Williams . . 18 28 34 40 43 57 64 74 Energy Transfer in Systems of Connected Organic Molecules. By A. Terenin, E. Putzeiko and I. Akimov . . 83 GENERAL DIsCuSSIoN.-Prof. H. C. Longuet-Higgins, Dr. D. H. Whiffen, Dr. A. Weller, Prof. G. Porter, Prof. R. Livingston, Dr. M. R. Wright, Mr. F. Wilkinson, Prof. A. Terenin, Dr. J. P. Simons, Prof. J. Weiss, Dr. R. A. Ford, Prof. H. Kallmann, Prof. D. D. Eley, Dr. B. Rosenberg, Mr. G. Jackson, Dr. G. Weber, Dr. B. Stevens, Mr. P. J. McCartin and Mr. E. Hutton, Dr. J. B. Birks, Dr. F.ZH. Brown, Dr. M. Furst, Prof. M. Burton, Prof. R. Lumry . 94 11. ENERGY MIGRATION IN ORGANIZED BIOLOGICAL SYSTEMS- Introductory Paper. By A. Szent-Gyorgyi . . 111 The Semiconductivity of Organic Substances. Part 3. Haemoglobin and Some Amino Acids.By M. H. Cardew and D. D. Eley . 115 Charge Transfer Processes in Biological Systems. By R. Mason. . 129 Electronic Energy Transfer in Haem Proteins. By G. Weber and F. J. W. Teale . . 134 56 CONTENTS PAGE Modified Reactivity of Haemoglobin Following Light Absorption. The Role of the Triplet State in Reactions Sensitized by Chlorophyll. Fluorescence Yield Against Velocity Relationships in the Hill Reaction of Chloroplast Fragments. By R. Lumry, B. Mayne and J. D. Primary Photochemical and Photophysical Processes in Photosynthesis. By E. Rabinowitch . . 161 Spectrophotometric Studies on Pyridine Nucleotide in Photosynthetic Cells and Cellular Material. By J. Amesz and L. N. M. Duysens . Radiationless Migration of Electronic Excitation in Retinal Rods.Reaction Rate Control of Light Emission in Bioluminescent Systems. Stabilization of " Steady States " of Cytochromes at Liquid Nitrogen Structure-Function Interrelationships in Mitochondria1 Electron Trans- Models for Some Energy-Transporting Substances of Biological Interest. The Transfer Potential of Enzyme Substrate Compounds. By H. By Q. Gibson . . 142 By R. Livingston and A. C. Pugh . . 144 Spikes . . 149 173 By W. A. Hagins and W. H. Jennings . * 180 By F. H. Johnson, H. Eyring and J. J. Chang . . 191 Temperatures. By B. Chance and E. L. Spencer, Jr. . . 200 port and Oxidative Phosphorylation. By D. E. Green . , 206 By M. J. Cowan, J. M. F. Drake and R. J. P. Williams Gutfreund . . 226 . . 217 GENERAL DxscussIoN.-Dr. J. H. Turnbull, Prof. J. Weiss, Prof. J. A. V. Butler, Dr. A. Szent-Gyorgyi, Dr. Britton Chance, Prof. E. C . Baughan, Dr. L. Paoloni, Prof. D. Eley, Dr. R. Mason, Prof. H. C . Longuet-Higgins, Mr. Patrick Taylor, Prof. H. Kallmann, Dr. J. B. Birks, Prof. G. Porter, Dr. F. A. Bovey, Dr. A. Weller, Prof. A. Terenin, Prof. R. Lumry, Dr. F. W. J. Teale, Prof. E. Rabinowitch, Dr. B. Rosenberg, Dr. L. N. M. Duysens, Dr. J. Amesz, Dr. G. Weber, Dr. E. R. Redfearn, Prof. D. E. Green, Prof. T. E. King, Dr. N. Uri, Dr. R. J. P. Williams, Dr. I. Wadso, Dr. S. J. Adelstein, Dr. D. J. R. Laurence, Dr. D. T. Elmore, Dr. H. Gutfreund . . 232

ISSN:0366-9033    

DOI:10.1039/DF9592700001

出版商:RSC    

年代:1959

数据来源: RSC

摘要:

10TH SPIERS MEMORIAL LECTURE TRANSFER MECHANISMS OF ELECTRONIC EXCITATION BY TH. FORSTER Laboratory of Physical Chemistry, Technische Hochschule, Stuttgart Received 14th April, 1959 Let me say at the beginning that I feel extremely honoured by the invitation to deliver the 10th Spiers Memorial Lecture. Though I never had the pleasure of meeting Mr. Spiers, whose death occurred before I began my own studies, I am well aware of his contributions to the development of your Society. There is no doubt that he has determined essentially the character of these Discussions by which the Faraday Society has become famous throughout the world. When I was informed that your Society planned to devote this Discussion to the subject of Energy Transfer, I felt fascinated. Twelve years ago, when I began to occupy myself with this subject, it was a very neglected one.In the meantime, many people have become interested in it, and its applications range from radiation physics to biology. While the Discussion isconcerned with energy transfer of any possible mechan- ism, I should like to restrict this Introductory Lecture to a more specific mechanism of energy transfer. This is the transfer of electronic excitation energy between otherwise well-separated atomic or molecular electronic systems. The most simple case is that of two distinct atoms in the empty space where electronic excitation of one atom may result in excitation of the other one. Similar cases are of atoms or molecules in solution or in crystalline environment, provided this environment does not allow electronic transfer from one to the other.Furthermore, we should exclude the trivial case of an excitation transfer that consists in the emission of one quantum of light by the first atom or molecule followed by re-absorption by the second one. This mechanism can be under- stood easily by the familiar laws of optics and, is, therefore, of not much interest, even if it may contribute to transfer in special cases. It is only the non-radiative transfer of excitation occurring during the short lifetimes of excited electronic systems which we shall consider here. Although this mechanism is a very specific one, it seems to be of general occur- rence. It is responsible for the phenomenon of sensitized fluorescence -of atoms and molecules which has been observed in the vapour phase as well as in solution and in the crystalline state.It leads to the so-called concentration depolarizution of fluorescence and sometimes contributes to concentration quenching. Moreover, it plays an essential part in the properties of organic scintillators and of certain types of inorganic crystalline phosphors. Finally, it contributes to reactions observed in radiation chemistry and in the photochemistry of biological systems, and it is considered today even in connection with other biological processes. The first observation of energy transfer was made by Cario and Franck (1922) in their classical experiments on sensitized fluorescence of atoms in the vapour phase. A mixture of mercury and thallium vapour, when irradiated with the light of the mercury resonance line, shows the emission spectra of both atoms.Since thallium atoms do not absorb the exciting light, they can get excited only indirectly by an excitation transfer from mercury atoms. A transfer by re- absorption is impossible here. Therefore, this transfer must be a non-radiative one with a mercury atom as the donor or sensitizer and the thallium atom as the 78 TENTH SPIERS MEMORIAL LECTURE acceptor. Unfortunately, in this case it cannot be decided whether the transfer occurs between distant atoms or during a normal collision or even in a labile molecule formed as an intermediate. This decision, however, was possible in similar cases, as in the mercury-sensitized fluorescence of sodium and in the mutual sensitization of the fluorescence of different mercury isotopes.In these cases, the transfer occurs over distances very much larger than those in normal collisional separations. A more complete discussion of these and similar cases has recently been given by Livingston.2 Similar observations of sensitized fluorescence have been made with molecular vapours. The experiments of Terenin and Karyakin 3 with naphthalene as sensitizer and acridine as acceptor may be mentioned as an example. Another case will be reported later by Dr. Stevens.4 More numerous are the investigations on sensitized fluorescence in solution, some examples of which are presented in table 1. Only such cases are considered here, where both sensitizer and acceptor are at low concentrations in an inert solvent. Without exception, the transfer takes place from a sensitizer absorbing at lower wavelengths to an acceptor absorbing at higher ones, because a transfer in the opposite direction would be impossible for energetic reasons.As we shall see later, a moderate red shift is favourable to this kind of transfer. TABLE 1 .-SENSITIZED FLUORESCENCE IN SOLUTIONS sensitizer acceptor remarks ref. phenosafranine tetrabromresorufin qualitatively 5 trypaflavine rhodamine-B only sensitizer 6, 7 benzo flavine rhodamine-B sensitizer life-time 8 chlorophyll-b chlorophyll-a both components 9 1 -chloroanthracene perylene quantitatively 10, 11, 12 quenching quantitatively and many other systems measured The first observations of sensitized fluorescence in solution, though of a more qualitative nature, were made very early by J.Perrin and Choucroun.5 In our own first experiments with trypaflavine and rhodamine,69 7 only the quenching in sensitizer fluorescence resulting from excitation transfer could be followed quanti- tatively. Nevertheless, transfer over a separation of 70 8, was established and the non-trivial kind of this transfer recognized by quenching experiments which indicated a decrease in lifetime of the excited sensitizer. Similar results were obtained by Galanin and Levshin 8 for a large number of similar systems, where the decrease in sensitizer lifetime was measured directly. The first measurements where the intensities of both fluorescence components could be followed quanti- tatively were those of Watson and Livingston 9 with both chlorophylls.Of special importance are the experiments of Bowen, Brocklehurst and Livingston 10-12 with 1 -chloroanthracene and perylene where any possible trivial mechanism was excluded with special care. Some of their results are reported in fig. 1. With a constant ratio of both components, sensitizer and acceptor absorb constant fractions of the exciting light. The increase in perylene fluorescence with increasing concentration must, therefore, result from energy transfer by chloroanthracene. At the same concentrations, a decrease in chloroanthracene fluorescence due to this transfer is observed. Furthermore, the total quantum yield of fluorescence increases with concentration. This results from the fact that chloroanthracene by itself has low fluorescence efficiency due to internal quenching.This internal quenching is diminished when the lifetime of the excited chloroanthracene molecule is decreased by excitation transfer to perylene. A trivial re-absorption process would not shorten the lifetime of the sensitizer and, therefore, not increase the total fluorescence yield.T. FORSTER 9 Further experiments have shown that in this case the transfer occurs not over collisional distances but over the mean intermolecular distances of sensitizer and acceptor, corresponding to a concentration of 10-3 to 10-2M. This is demon- strated by the fact that sensitization occurs with similar half-value concentrations in solution of very different viscosities and even in organic glasses at low tem- perature. The possibility of the formation of a complex between sensitizer and acceptor molecules was excluded by the additivity of the absorption spectra and the different dependence on concentration to be expected in this case.It must be concluded, therefore, that excitation transfer of a non-trivial nature occurs over the mean distances between statistically distributed molecules which are about 40A in this case. X 3 o--------”----o I 12 14x I FIG. 1 .-Fluorescence of mixed solutions in benzene of 1 -chloroanthracene and perylene in 5 : 1 molar ratio (taken from Bowen and Brocklehurst 10). Table 2 summarizes some qualitative features of this kind of long-range transfer and of some more or less trivial mechanisms. The non-trivial transfer differs from re-absorption transfer by its independence of the volume of the solution, by the decrease in sensitizer fluorescence lifetime, and by the invariability of the TABLE 2.-cHARACTERISTIC PROPERTIES OF TRANSFER MECHANISMS non-trivial transfer reabsorption complexing encounter dependence on none increase none none volume dependence on none none none decrease viscosity sensitizer decreased unchanged unchanged decreased lifetime sensitizer fluor- unchanged changed unchanged unchanged escence spectrum absorption unchanged unchanged changed unchanged spectra sensitizer fluorescence spectrum.It differs from short-distance collisional transfer by its independence of solvent viscosity and from transfer within a molecular complex by the constancy of absorption spectra and the decrease in sensitizer fluorescence lifetime.In most cases, some of these different properties allow a decision between trivial and non-trivial transfer mechanisms. Further dis- criminations may be made by quantitative studies of these properties.10 TENTH SPIERS MEMORIAL LECTURE Let us now consider the mechanism of this long-range excitation transfer in more detail. It may be astonishing that such a transfer is possible at all during the short lifetimes of excited molecules which usually are of the order of 10-8 sec. One should consider, however, that this is actually a long time for electronic systems, where orbital motions occur during 10-15 sec. Therefore, the comparatively weak interaction between distant atoms or molecules may be sufficient for ex- citation transfer, provided some kind of resonance condition is fulfilled.This was recognized already by J. Perrin 13~14 who formulated a first theory of excitation transfer between molecules in solution based on the principles of classical physics. This theory, as well as its later quantum-mechanical refinement by F. Perrin 15 predicted transfer distances of more than 100 8, but was insufficient quantitatively. The simpler case of energy transfer between atoms has been treated by Kallmann and London 16 who arrived at similar transfer distances. FIG. 2.-Simplified energy level diagram of sensitizer (S) and - . . ... - - - - acceptor (A). radiative transitions non-radiative transitions transfer transitions. I coupled transitions We shall here consider the molecular case only, where the superposition of electronic and nuclear vibrational levels is essential.Fig. 2 represents the sim- plified energy level diagram of both molecules. During the absorption process, the sensitizer is excited to one higher vibrational level of its first electronic ex- citation state. From there it is converted to lower vibrational levels of the same electronic state by obtaining thermal equilibrium with the surrounding medium. In solution or in other condensed systems, this thermal relaxation takes place during 10-13-10-12 sec. It may be assumed for simplicity that the temperature is low enough for the excited molecule to remain in its lowest vibrational level for the rest of its lifetime of about 10-8sec duration. After this time-interval it returns to the ground state by spontaneous radiative or non-radiative processes.Let us now suppose that the energy difference for one of these possible de- activating processes in the sensitizer molecule corresponds exactly to that for a possible absorption transition in a nearby acceptor molecule. Then, with sufficient energetic coupling between these molecules, both processes may occur simul- taneously, resulting in a transfer of excitation from sensitizer to acceptor. With the broad spectra of polyatomic molecules in solution, there is always sufficient coincidence between sensitizer and acceptor transitions, if the absorption spectrum of the acceptor overlaps the fluorescence spectrum of the sensitizer. Therefore, a moderate red shift of the acceptor spectra towards those of the sensitizer is favourable to this kind of transfer.With regard to this condition, this kind of energy transfer is often called " resonance transfer " or " transfer by inductive resonance ". This peculiar kind of resonance condition results from thermal relaxation together with the Franck-Condon principle which act here essentially in the same manner as in producing the common Stokes shift.T. F ~ R S T E R 11 It may be somewhat confusing that this condition is similar to that for re- absorption of the sensitizer fluorescence by the acceptor. Nevertheless, the mechanism of the non-trivial process is an entirely different one, leading to transfer before the emission of sensitizer fluorescence takes place. Also it needs some amount of mutual coupling between the electronic systems of both molecules and can, therefore, take place only over limited distances.This coupling is strongest if the corresponding optical transitions in both molecules are allowed for electric dipole radiation. Then these transitions are coupled not only to the radiation field but also to each other. Naturally, the interaction energy is of a dipole-dipole nature, depending on an inverse propor- tionality to the third power of the molecular distance. The probability of energy transfer is then proportional to the square of this interaction energy and decreases, therefore, with the sixth power of the distance.17 A quantitative treatment leads to the following expression for the rate constant of the transfer process : Here v is the wave number, E(V) the molar decadic extinction coefficient, f ( v ) the spectral distribution of fluorescence (measured in quanta and normalized to unity on a wave number scale), N Avogadro's number and T; the intrinsic or radiative lifetime of the excited sensitizer.n is the refractive index of the solvent, R the mutual distance between both molecules and K an orientation factor. More specifically, this is where $SA is the angle between the transition moment vectors of both molecules while 4s and #JA are the angles between these respective vectors and the direction S --f A. K = cos 4SA 3 cos 4s . cos#A The average value for a random directional distribution is 19 ~2 = 4. Eqn. (1) may be rewritten more conveniently as Here 7s is the actual mean lifetime of the excited sensitizer. It is connected to and to the quantum yield v i of the sensitizer fluorescence (without transfer) by 0 0 r s = T~ .rs . (3) Obviously Ro is the critical transfer distance for which excitation transfer and spontaneous deactivation of the sensitizer are of equal probability. From eqn. (1) to (3) one gets This is valid for any thermal equilibrium distribution over the vibrational levels of both molecules, provided the spectra are taken at the corresponding tem- perature. The transfer probability is independent of the exciting wavelength even if higher electronic states of the sensitizer are involved. As is to be expected, Ro increases with the quantum yield of the sensitizer and with the overlap of the spectra. In typical cases, Ro-values from 50 to 100 A have been calculated. These formulae become invalid when energy transfer occurs before thermal equilibrium is established.This would be expected for gases under low pressure where thermal relaxation is slow, but also in liquid or solid medium when, due to strong interaction, the transfer is very rapid. In these cases, the transfer may take place from the vibrational level obtained by excitation directly and depend, therefore, on the exciting wavelength. This stands in some analogy to the phenomenon of resonance fluorescence of molecular vapours where the emission spectra show such a dependence.12 TENTH SPIERS MEMORIAL LECTURE Under extremely strong interaction, the transfer may even be faster than molecular vibrations. In this case the absorption spectra are no longer additive and it would be diacult to consider the excitation even temporarily localized at one molecule or the other.This is realized in some cases of molecular aggregation, preferably of alike molecules,20-22 and will not be considered further. Let us return again to energy transfer under conditions of thermal equilibrium in the vibrational levels. Even in this case, eqn. (1) to (4) are not generally valid because they refer to dipole-dipole interaction only. Therefore, eqn. (1) is rather the first term of an expansion in powers of R-1. Other terms must be considered when the transfer occurs over small distances of when dipole-dipole interaction is weak because of forbidden optical transitions in sensitizer or acceptor. Such forbiddeness may result from molecular symmetry or from spin inter- combination (e.g.for transitions between singlet ground states and triplet excited states). In both cases, it must be considered first that transitions of this kind are never strictly forbidden. Symmetry-forbidden transitions become partially allowed in combination with certain molecular vibrations, intercombination transitions by mixing of states with different multiplicities, especially in the presence of atoms with higher nuclear charges. Such transitions occur, therefore, in absorption as well as in emission. Naturally, the extinction is low and the emission delayed, being a typical phosphorescence in the case of intercombination transitions. Eqn. (1) to (4) should represent, at least approximately, even then the inverse sixth power term of our expansion.If the optical transition is forbidden in the sensitizer but allowed in the acceptor, eqn. (4) still predicts large transfer distances in as far as the fluorescence (or phos- phorescence) yield 7s is high. This results from the fact that the slower transfer rate calculated from eqn. (1) is compensated by a longer lifetime of the excited sensitizer, in as far as deactivation does not occur mainly by non-radiative pro- cesses. According to these considerations, long-range excitation transfer should be possible under suitable conditions even from the triplet state of a sensitizer to the singlet state of an acceptor. On the other hand, a forbidden transition on the acceptor side results in low EA(V) so that eqn. (4) predicts only short transfer distance. Actually, this may be somewhat larger due to higher-order terms.For a symmetry-forbidden transition where dipole-dipole interaction is small, the transfer might be determined by dipole-quadrupole interaction and show an inverse 8th power dependence. As Dexter 23 has demonstrated, the transfer occurs over distances larger than those of molecular contact even in that case. Finally, at small distances, exchange terms in the interaction operator must be considered.23 These are essential if the transition is intercombination forbidden in the acceptor so that neither dipole-dipole nor higher multipole interaction leads to strong coupling. Electron-exchange interaction allows transfer only under conservation of the total multiplicity of the system, e.g. between tr;plet states of both sensitizer and acceptor.It needs some overlap of the electronic clouds of both molecules and occurs, therefore, at shorter distances only. Since resonance transfer does not need strong interaction, the transfer distances should be some- what larger than contact separations. So even then transfer may occur between molecules otherwise considered as independent of each other. The occurrence of energy transfer between triplet states has been demonstrated by Terenin and Ermolaev 24-27 with solid solutions of benzophenone or benz- aldehyde as sensitizers and substituted naphthalenes as acceptors. The transfer distances are about 14A which is much larger than calculated from eqn. (4) for dipole-dipole interaction in these cases. As already stated by the authors, an exchange mechanism as described before should be considered here.It is quite natural that excitation transfer may not only occur between different molecules but also between separate electronic systems of the same molecule. Weissman,28 Sevchenko and co-workers,29~ 30 and also Crosby and Kasha 31 haveT . FORSTER 13 observed that in aromatic rare-earth chelate complexes, the rare-earth luminescence is sensitized by absorption in the aromatic component. Weber 32 and Teale 33 have reported similar observations with molecules containing two independent aromatic systems. Phosphorescence experiments demonstrating intramolecular excitation transfer between triplet levels have been performed by Ermolaev and Terenin.27 Returning to excitation transfer between different molecules we must adapt our treatment of the elementary process between two molecules to the conditions in solution where molecules are distributed at random. Let us suppose that energy transfer occurs only from sensitizer to acceptor molecules and that the inverse sixth power dependence of pure dipole-dipole interaction holds.We shall assume further that Brownian translational movement of all molecules is slow enough so that each individual transfer process may be considered at constant distance. On the other hand, Brownian molecular rotation will be considered to be much faster than transfer, so that the average value of the orientation factor ~2 = 5 may be used. These conditions are realized approximately in solutions of moderate viscosity.In this case a straightforward calculation 78 349 35 leads to the following expression for the quantum yield TA of the acceptor fluorescence : with I - !&>) , vrr c 2 co x = - - (+(x) : error function). 3000 co = ~ 4rrNRJ ' Here vZaX is the maximum quantum yield of the acceptor fluorescence obtained either by direct excitation or by complete transfer. The acceptor concentration is expressed by the dimensionless quantity x. The reference concentration co which may be called a critical transfer concentration corresponds to an average of one acceptor molecule in a sphere of the radius Ro. The expression (5) is represented in fig. 3. Another expression which considers transfer to the nearest acceptor molecule only has been used by Dexter.23 In solutions of higher viscosity as well as in cases of a very rapid transfer, the assumption of rapid Brownian rotation is not valid. In these cases the transfer must be calculated for all individual orientations and averaged afterwards.An approximate calculation based on a largely simplified model 36 leads to somewhat different function, represented also in fig. 3. For the inverse 8th power law of dipole-quadrupole coupling, Dexter 23 has calculated the dependence on con- centration, assuming transfer to the nearest molecule only. Experimental data have often been represented by simpler formulae. With the assumption of a sharp transfer distance with instantaneous transfer for shorter and no transfer for longer distances, the simple exponential formula results. By a formal kinetic treatment of excitation transfer concurring with the spontaneous deactivation of the sensitizer one arrives at an equation of the Stern- Volmer type : These different functions are compared to each other in fig.4 with the parameters a and /? so adjusted as to give a half-value concentration equal to that of eqn. (5). These values are a = 1.42 and /3 = 2.05. Identical slopes at lower concentrations would be obtained with a = = 1.57 while the " theoretical " value of a would be 1.00. It should be mentioned here that, notwithstanding the inverse sixth14 TENTH SPIERS MEMORIAL LECTURE I 10 CICO FIG. 3.-Relative intensity of sensitizer fluorescence against acceptor concentration for dipole-dipole coupling. left curve : right curve : molecules with fixed orientations.fast rotating molecules ; 1 : 2 : 3 : X 0.1 I 10 100 cico FIG. 4.-Relative intensity of sensitizer fluorescence against acceptor concentration. 1. calculated for dipole-dipole coupling and fast rotating molecules, 2. approximated by exponential formula eqn. (7), 3. approximated by Stern-Volmer formula, eqn. (8).T. F ~ R S T E R 15 power dependence on distance, eqn. (6) as well as the other formula predict a linear increase of sensitization at low concentrations. An expression for the decrease in sensitizer lifetime with increasing acceptor concentration has been derived by Galanin 34 for dipole-dipole coupling. In correspondence with experimental data,6* 8 it predicts that the decrease in lifetime is markedly slower than that of the intensity of sensitizer fluorescence.As was stated before, the conditions for excitation transfer are optimal if the electronic level of the acceptor is somewhat lower than that of the sensitizer. Therefore, the conditions are not optimal for transfer between alike molecules. They are most unfavourable at very low temperatures where only the 0,O transition is common to absorption and fluorescence. But they are more favourable at higher temperatures where some vibrational levels are thermally excited and, correspond- ingly, absorption and fluorescence spectra of the same molecule overlap to some extent. Fig. 5 tries to illustrate these different conditions at low and higher temperatures. Apart from less optimal overlap, the conditions for excitation transfer are the same for alike molecules, and eqn.(1)-(4) may be applied for dipole-dipole coupling. I 1 I I I D I T=O I I I I l l I I I l l I I I I I T>O I I I I I I I I l l I I I l l I I I l l I I t l l I I I 1 I I I J FIG. 5.-Simplified energy level diagram for transfer between alike molecules (notations as in fig. 2). Naturally, excitation transfer between alike molecules can occur in repeated steps. So the excitation may migrate from the absorbing molecule over a con- siderable number of other ones before deactivation occurs by fluorescence or some other process. Though this kind of transfer cannot be recognized from fluor- escence spectra, it may be observed by the decrease of fluorescence polarization with increasing concentration. This so-called " concentration depolarization " which was discovered by Gaviola and Pringsheim 37 (1924) results from an ex- citation transfer to molecules with different orientations from the absorbing one.With typical dyes, it has been observed at concentrations of about 10-3 M where the mean distances are 70 A. The possibility of excitation migration complicates all experiments on sensitized fluorescence under higher sensitizer concentrations. Extreme cases are those, where the sensitizer itself is used as the solvent in which the acceptor is present at minor concentration. One of the best known systems is that of crystalline an- thracene containing traces of naphthacene as an impurity. In such crystals the green fluorescence of naphthacene is observed 389 39 even at molar ratios lower than 10-5, where the distance from an average anthracene molecule to the nearest naphthacene molecule is about lOOA.This seems somewhat large for a single- step transfer of excitation. The other possibility is that of migration transfer where the excitation travels from one anthracene molecule to another in some kind of Brownian movement until it reaches the neighbourhood of one of the few naphthacene molecules present.16 TENTH SPIERS MEMORIAL LECTURE A recent observation by Lyons and White 40 is in favour of this second mechan- ism. These authors found that with a molar ratio of 10-5 no fluorescence sen- sitization occurs at 4°K. Such a suppression of excitation transfer would be expected for migration transfer between alike molecules which, as was stated before, should essentially depend on thermal vibrations (cp.fig. 5). A similar case has been reported by Kanda and Sponer 41 for benzaldehyde in solid toluene where sensitization of phoshorescence is suppressed at low temperature. Other arguments favouring migration-transfer in such crystalline systems have been given by Schmillen 42 from life-time measurements. On the other hand, Bar and Weinreb 45 have shown that a liquid solution of anthracene in toluene (ca. 2 x 10-3 M) still shows fluorescence sensitization when most of the toluene is replaced by an inert solvent. This is in favour of a single- step transfer. It seems natural that in such systems the prevalence of one or the other mechanism would largely depend on the acceptor concentration and on the properties of both molecules.Similar considerations may apply to the observa- tions of sensitized fluorescence in natural pigments by Duysens 44 and 0thers.45-47 We have confined our interest here mainly to the phenomena of sensitized fluorescence and phosphorescence where excitation is created by absorption of a quantum of light and finally results in emission of another quantum. But our considerations are also valid for other mechanisms of primary excitation or of final deactivation. Another possibility of excitation is that by high-energy radi- ation with x-, fl-, or y-rays. In these cases, excitation occurs either directly or indirectly via primary ionizations. The excitation, after having been converted to the lowest excited state in the usual way, will undergo similar transfer processes under suitable conditions.The usefulness of organic scintillators depends essen- tially on this possibility as investigations of Kallman and Furst,'@ Swank and Buck,499 50 Hartwick,Sl Knau 52 and others have demonstrated. Prof. Kallman 48 and Dr. Birks 53 will discuss this problem in more detail. Excitation produced by light or by high-energy radiation often results in chemical processes instead of luminescence. This leads to photochemical or radiochemical reactions. In these cases, too, production and final disappearance of excitation may be separated by one or more steps of excitation transfer. In the usual photochemistry of gases and solutions, the conditions for this are not favourable because the absorbing substances are commonly used in low con- centrations.However, other conditions prevail in radiochemical reactions for which only absorbing solvents exist. As Prof. Burton 54 and Prof. Dole 55 will later deal with this topic, I shall not further discuss it here. In biological systems the conditions for excitation transfer are very favourable because nature often prefers high local concentrations of absorbing matter. Prof. Rabinowitch 56 and Prof. Lumry 57 will tell us how far this possibility seems actually to be used in photosynthesis. As Prof. Szent-Gyorgyi 58 has emphasized, nature may use this possibility even in ordinary biochemical reactions. In order to arrive at a better understanding of excitation transfer and of its role in molecular energy exchange, further knowledge of the many competing processes like internal conversion and deactivation is necessary.These problems will be discussed by Dr. Bowen,59 Prof. Porter60 and Prof. Livingston61 and Dr. Weller 62 in their respective contributions. Furthermore, we should not forget that excitation transfer is by no means the only possible mechanism of energy transfer. Electron transfer as well as proton transfer must also be taken into consideration, and it is very fortunate that Prof. Terenin's 63 paper is devoted to these aspects of our general subject. 1 Cario and Franck, Z. Physik, 1923, 17, 202. 2 Livingston, J. Physic Chern., 1957, 61, 860. 3 Terenin and Karyakin, Isvest. Akad. Nauk. S.S.S.R., ser. Fiz., 1951, 15, 550. 4 Stevens. this Discussion.T. FORSTER 17 5 Perrin J. and Choucroun, C.R. hebd.Se'ances Acad. Sci., 1929, 189, 1213. 6 Forster, 2. Elektrochem., 1949, 53, 93. 7 Forster, 2. Naturforsch., 1949, 4 4 321. 8 Galanin and Levshin, J. exp. th. Fiz., 1951, 21, 121. 9 Watson and Livingston, J . Chem. Physics, 1950, 18, 802. 10 Bowen and Brocklehurst, Trans. Faruduy SOC., 1953, 49, 1131. 11 Bowen and Livingston, J. Amer. Chem. SOC., 1954,76, 6300. 12 Bowen and Brocklehurst, Trans. Favaduy SOC., 1955, 51, 774. 13 Perrin, J., 2me conseil de Chimie SoZvuy (Gauthier and Villar, Paris, 1925), p. 322. 14 Perrin, J., C.R. hebd. Skances Acad. Sci., 1927, 184, 1097. 15 Perrin, F., Ann. Chim. Physique, 1932, 17, 283. 16 Kallmann and London, 2. physik Chem. B, 1928,2, 207. 17 Forster, Naturwiss., 1946, 33, 166. 18 Forster, Ann. Physik, 1948, 2, 55. 19 Forster, Fluoreszenz orgunischer Verbindungen (Vandenhoeck and Ruprecht, 20 ref.(19), p. 254. 21 Lavorel, J. Physic. Chem., 1957, 16, 1600. 22 Forster and Kasper, Z. physik Chem., 1954, 1, 275. 23 Dexter, J. Chem. Physics, 1953, 21, 836. 24 Terenin and Ermolaev, Dokludy Akad. Nuuk, 1952, 85, 547. 25 Ermolaev, Doklady Akad. Nauk, 1955, 120, 925. 26 Terenin and Ermolaev, Trans. Faruday SOC., 1956, 52, 1042. 27 Ermolaev and Terenin, 8me rkunion annuelle de la Socie'te' de Chimie Physique (Paris, 28 Weissman, J. Chem. Physics, 1942, 10, 214. 29 Sevchenko and Morachevskii, Isvest. Akad. Nuuk S.S.S.R. ser. Fiz., 1921, 15, 628. 30 Sevchenko and Trofimov, J. exp. th. Fiz., 1951, 21, 220. 31 Crosby and Kasha, Spectrochim. Acta, 1958, 10, 377. 32 Weber, Nature, 1957, 180, 1409. 33 Weber and Teale, Trans. Faruduy SOC., 1958, 54, 640. 34 Galanin, J. exp. th. Fiz., 1955, 28, 485. 35 Antonoe-Romanoescu and Galanin, Akad. Nuuk S.S.S.R., 1957, 3, 389. 36 In this model the spherically symmetrical distributions of sensitizer and acceptor molecules have been approximated by tetrahedral distributions of different mutual orientations. Gottingen, 1951), p. 85. 1958). 37 Gaviola and Pringsheim, 2. Physik, 1924, 24, 24. 38 Bowen, J. Chem. Physics, 1945, 13, 306. 39 Bowen, Mikiewicz and Smith, Proc. Physic. SOC. A, 1949, 62, 26. 40 Lyons and White, J. Chem. Physics, 1958, 29, 223. 41 Kanda and Sponer, J. Chem. Physics, 1958, 28, 798. 42 Schmillen in Halbleiter und Phosphore, Berichte des internationulen Colloquiums in 43 Bar and Weinreb, J. Chem. Physics, 1958, 29, 1412. 44 Duysens, Nature, 1951, 168, 548. 45 French and Young, J. Gen. Physiol., 1952, 35, 387. 46 Bannister, Arch. Biochem. Biophys., 1954, 49, 222. 47 Brody, J. Chim. Physique, 1958, 55, 942. 48 Brown, Furst and Kallman, this Discussion, and further references quoted there. 49 Buck and Swank, Physic. Rev., 1952, 87, 191. 50 Swank and Buck, Physic. Rev., 1953, 91, 927. 51 Hartwick, J. Chem. Physics, 1957, 26, 323, 1463. 52 Knau, Z. Naturforsch., 1957, 12a, 881. 53 Birks, this Discussion. 55 Dole and Williams, this Discussion. 56 Rabinowitch and Brody, this Discussion. 57 Lumry, Mayne and Spikes, this Discussion. 5 8 Szent-Gyorgyi, J. Chim. Physique, 1958,55, 916. 59 Bowen, this Discussion. 60 Porter and Wright, this Discussion. 62 Weller, this Discussion. Gurn?isch, 1956 (Vieweg and Sohn Braunschweig, 1958), p. 445. 54 Burton, this Discussion. 61 Livingston, this Discussion. 63 Terenin, this Discussion.

ISSN:0366-9033    

DOI:10.1039/DF9592700007

出版商:RSC    

年代:1959

数据来源: RSC

摘要:

I. MODES OF ENERGY TRANSFER FROM EXCITED AND UNSTABLE IONIZED STATES INTRAMOLECULAR AND INTERMOLECULAR ENERGY CON- VERSION INVOLVING CHANGE OF MULTIPLlCITY BY GEORGE PORTER AND M. R. WRIGHT Dept. of Chemistry, The University, Sheffield 10 Received 16th January, 1959 Radiationless transitions between states of different multiplicity are considered with special reference to conversion between triplet and singlet energy levels. Rates of this conversion have been measured for naphthalene and anthracene and rate constants are given for the fist-order intramolecular process, the second-order process involving two triplet molecules and also of quenching by other species, particularly paramagnetic ions. It is established that the first-order rate is strongly viscosity dependent and this is attrib- uted to a structural difference between the two states. Conversion from the triplet state is induced by paramagnetic molecules and ions but the quenching rate constant shows no correlation with magnetic susceptibility. A general theory of " paramagnetic quenching " is proposed in which the function of the quenching molecule is one of overall spin conservation.All processes, including energy transfer, which involve a change in total electron spin momentum have a low probability compared with the corresponding processes in which spin momentum is conserved. Nevertheless it is now quite clear that conversion between electronic states of different multiplicity is often of primary importance in changes which involve excited molecules. The reason for these apparently conflicting statements lies in the high probability of radiation- less conversion between electronic states which may, in the absence of spin and other restrictions, have a rate constant exceeding 1011 sec-1.If a change of multip- licity is involved the rate may be reduced by a factor of 104 but is still high enough to compete effectively with other modes of deactivation of the excited state. Since the ground state of most molecules is a singlet, the multiplet of most interest is the triplet and, in particular, the triplet state of lowest energy. This has the following properties. (i) It is the lowest excited electronic state of the molecule. (ii) It has a lifetime, even in fluid solvents, which is typically of the order of 10-4 sec, i.e. several orders of magnitude greater than that of the excited singlet states.(iii) Its chemical behaviour is usually characteristic of a biradical. The triplet state is now frequently postulated as an intermediate in energy transfer processes and in chemical and biochemical change but only in a very few cases has its role been established. Most work on the triplet state has been concerned with the radiative process of phosphorescence in rigid media and only recently have data become available concerning its properties in the more common fluid solvents. By means of the flash photolysis technique it is possible to observe the triplet state directly by means of its absorption spectrum and to follow its concentration as a function of time after irradiation. In this way, fairly extensive data have now been 18a b c d e FIG.1 .-Oscillographic records of the decay of triplet naphthalene in (a) n-hexane, (6) water, (c) ethylene glycol, ( d ) paraffin r ] = 33 cp, ( e ) paraffin 3 = 167 cp. Time units : msec. [To face page 19.G . PORTER AND M. R. WRIGHT 19 accumulated on the triplet states of a variety of molecules. Here, two fundamental processes involving change of multiplicity will be discussed. The first is the intra- molecular process of radiationless conversion between triplet and singlet states and the second is the deactivation of the triplet state by other molecules, par- ticularly those which themselves have multiplicities higher than singlet. The experimental findings on each of these processes are somewhat unusual, and have not received a satisfactory explanation.RESULTS The data to be presented were obtained by a combination of flash photolysis and spectrophotometric recording. The apparatus has already been briefly described 1 and further experimental details will be given elsewhere. In this paper we are concerned mainly with first-order decay constants of the triplet state in various solvents and the constants of quenching by paramagnetic substances. r time, psec FIG. 2.-First-order plots of the decay of triplet naphthalene in water and in the presence of various ions. Concentration of Zn2+ and Ga3+ was 5 x 10-4 M and of all other ions 2.5 x 1 0 - 4 ~ . 0 No ion ; A [Cu(CN)z]- ; Ga3+ ; 0 Zn2+ ; x Ce3f; A Nd3+ ; Gd3+. The type of record obtained, as well as the two principal effects with which we are concerned, are illustrated by fig.1 and fig. 2. The former shows oscillographic records of the absorption by the triplet state of naphthalene in solvents of varying viscosity, as a function of time. Fig. 2 shows first-order decay plots derived from this type of trace which illustrate the effect of ionic solutes on the lifetime of triplet naphthalene in water. The first-order decay curves are not exactly linear and it is found that there is a small contribution from second-order (triplet-triplet) quenching which becomes quite consider- able at high triplet concentrations. The method of analysis of these curves which is used to derive first- and second-order decay constants has been described.1 As a result of the second-order contribution the first-order rate constants in earlier work were consistently higher than those reported here.In the flash photographic work of Porter and Windsor,2.3 rather high triplet concentrations were measured and owing to the lower accuracy of the method, separation of first- and second-order processes was not possible. Our rate constants are also slightly lower than those of Livingston and Tanner4 who used the present method but found no second-order contribution.20 C H A N G E OF MULTIPLICITY The occurrence of a second-order process immediately suggested a possible explanation of the viscosity dependence of triplet decay and it was therefore necessary to study the effect of viscosity on first- and second-order rates separately. The second-order rate was viscosity dependent as expected but the data in tables 1 and 2 show that the rate of first-order decay is also largely controlled by solvent viscosity.There are evidently some constitutional effects as well, which is not surprising since macroscopic viscosity is only approximately related to diffusion coefficients and to effects on a molecular scale. At the higher viscosities, when the encounter rate is truly diffusion controlled, the rate constant in related solvents, e.g. paraffins 1 and 2, is approximately inversely proportional to viscosity. The limiting values in rigid solvents refer to the radiationless process and are derived from phosphorescence lifetimes. TABLE 1 .-FIRST-ORDER (kl) AND SECOND-ORDER (k2) DECAY CONSTANTS OF TRIPLET NAPHTHALENE IN VARIOUS SOLVENTS AT 2 0 ° C .SECOND-ORDER CONSTANTS ARE BASED ON THE LIMITING VALUE E = 10,oOo AND C IS A CONSTANT WHICH IS LESS THAN UNITY solvent viscosity (cp) k1(sec- 1) ck2 (1. mole-1 sec-1) n-hexane 0.3 1.2 x 104 2.1 x 109 water 1-1 7.5 x 103 4.1 x 109 ethylene glycol 21.1 9.7 x 102 2-2 x 108 paraffin 1 33.0 1.5 x 103 3.9 x 108 paraffin 2 167 3.1 x 102 8.0 x 107 rigid glass very high < 1 < 106 TABLE 2.-FIRST-ORDER (kl) AND SECOND-ORDER (k2) DECAY CONSTANTS OF TRIPLET ANTHRACENE IN VARIOUS SOLVENTS AT 20°C k2 (1. mole-1 sec-1) solvent viscosity (cp) kl (sec-1) n-hexane 0.3 1.3 x 103 1.6 x 1010 tetrahydrofurane 0.5 2.3 x 103 1.0 x 1010 ethylene glycol 21.1 2.8 X 102 8.8 x 108 paraffin 1 33.0 5.6 X 102 1.2 x 109 rigid glass very high < 10 < 106 paraffin 2 167 1.6 X 102 2.5 x 10s The rate constants of quenching of triplet naphthalene by various ions, determined in the same way, are given in table 3.Of the ions investigated, diamagnetic ions have no effect (or at high concentrations possibly a small negative effect) whilst paramagnetic ions all quench the triplet state, though the efficiency varies over a wide range and is apparently quite unrelated to the magnetic susceptibility, provided the ion is paramagnetic. TABLE 3.-RATE CONSTANTS OF QUENCHING OF TRIPLET NAPHTHALENE BY IONS IN WATER AND ETHYLENE GLYCOL ko(1. mole-1 sec-1 x 10-7) ion K+ Zn2+ Ga3 + CU(CN)~- cu2+ Ni2 + co2+ Cr3 + Fe2+ Fe3 + Mn2+ Nd3 + Gd3 + in water 0.00 & 001 0 0 f 0.2 0 0 f 0.1 0 0 f 0.1 7-5 f 0.7 2.3 rt 0.4 5.0 &- 0 6 6.9 f 0.6 2.9 i 0.4 2.8 rt 0.4 - - - in ethylene glycol 0 f 0.1 0 f 0.2 0 rt 0.2 7.3 0.7 2.4 i 0.3 4-4 rt 0.4 3.8 f 0.3 1.6 f 0.2 0.04 k 0.005 0.007 f 0.002 - - - no.of unpaired electrons 0 0 0 0 1 2 3 3 4 5 5 3 7 paramagnetic susceptibility (Bohr magnetons) diamagnetic diamagnetic diamagnetic diamagnetic 1-93 3-21 5.01 3-82 5-30 5.85 5.8 1 3.60 8.01G . PORTER A N D M . R . WRIGHT 21 Preliminary data indicate that the same is true of anthracene triplet quenching, but that the quenching rate constants of a particular ion depend both on the triplet molecule and on the solvent. The only other paramagnetic molecules, for which quenching constants are available, are given in table 4. The rate constants of quenching by 0 2 and NO are taken from the results of Porter and Windsor.3 The effects are certainly general to other molecules, as well as anthracene, but rate constants have not yet been accurately determined.TABLE 4.-RATE CONSTANTS OF QUENCHING OF TRIPLET ANTHRACENE BY PARAMAGNETIC MOLECULES IN HEXANE SOLUTION quenching molecule no. of unpaired electrons k (1. rno1e-l sec-l) Q 0 2 2 4 x 109 NO 1 4 x 109 triplet anthracene 2 1.6 x 1010 DISCUSSION INTRAMOLECULAR RADIATIONLESS CONVERSION BETWEEN TRIPLET AND SINGLET STATES It is not always appreciated that the appearance of phosphorescence in a wide variety of molecules and its absence in fluid solvents is largely unexplained. In molecules such as the aromatic hydrocarbons, radiationless crossing from the upper singlet state S1 to the triplet TI occurs with a rate constant which is typically of the order of 108 sec-1 in rigid media, whilst the apparently similar conversion from TI to the ground state SO is not observed in rigid media and must have a rate less than 10-1 sec-1 in benzene and other molecules with similar radiative triplet lifetimes.If the rates of the two radiationless conversions were of com- parable magnitude, no phosphorescence would be observed, and this is the reason for the absence of phosphorescence in ordinary solutions and gases. Studies of the kinetics of triplet state decay in different solvents have shown that a number of bimolecular processes may occur, e.g. quenching by oxygen or a second triplet, but that, in the absence of such quenchers, the triplet state has a natural lifetime which is generally much less than the radiative life and which must be attributed to the process of radiationless conversion to the ground state.2 The second-order decay would of course be diffusion controlled in view of its high rate and a viscosity dependence is to be expected and is, in fact, found.The results in table 1 and 2 establish that the first-order decay, which is the predominant process at low intensities, is also a function of solvent viscosity. I t is clearly unprofitable to consider the triplet decay as a process which is rate-determined by energy transfer to the solvent, particularly since the rate attains its maximum value in the gas phase.5 It has also been established that the first- order process being considered occurs without the intervention of any molecules other than those of the solvent.We must therefore conclude that the rate con- stants measured are those of the intramolecular radiationless conversion process from the triplet state T1 to the ground state So. Energy transfer to solvent occurs after crossing and is not rate-determining. The outstanding problem is why the radiationless crossing TI-& should be viscosity dependent to such an extent that it is totally inhibited in rigid media. The radiationless transition probability between two states i and j is propor- tional to the square of the matrix element Wg of the perturbation function W which in turn is given by where # j and $j are the eigenfunctions of the two states i and j . The eigenfunctions can be separated into a product of electronic-rotational and vibrational functions which are independent to a first approximation and the electronic function can22 CHANGE OF MULTIPLICITY again be separated into a product of spin and co-ordinate functions.The spin functions of two electronic states of different multiplicity are orthogonal so that, in so far as the separation of the functions is a good approximation, the transition probability is zero, but it becomes finite in the presence of spin-orbit interaction. The electronic eigenfunctions will be modified only slightly by the solvent and the dependence of transition probability on viscosity cannot be explained in any general way by a difference in the electronic terms of the matrix element. The position with regard to rotation is less clear. The selection rule for perturbations in isolated diatomic molecules is that both states must have the same total angular momentum, i.e.AJ= 0. In solution, free rotation is inhibited but the con- servation of angular momentum is still at least as probable as in the isolated molecule. Further theoretical work on this point would be helpful but explana- lions of the viscosity effect in terms of inhibited rotation are made very unlikely by the experimental fact that predissociations and internal conversions are ob- served, even in rigid media, with high probability, e.g. crossings from state S1 to TI. A general selection rule based on conservation of total angular momentum would apply to all such transitions and it is probable that rotation can be neglected in discussing the transition probability as it can in discussing the Franck-Condon principle for radiative transitions.6 The part of the matrix element depending on the vibrational eigenfunction is W; = J flWV$'dr where Wv is the part of the interaction energy depending on nuclear co-ordinates and $7 and $7 are the vibrational eigenfunctions.The transition probability therefore depends on the overlap of vibrational eigenfunctions and the Franck- Condon principle is valid just as in the more familiar case of radiative transitions in diatomic molecules. A high transition probability will be found only when, classically, the system can pass from one state to the other without a large alter- ation of position or momentum. In a polyatomic molecule and in a viscous medium this restriction may be very important if the equilibrium configuration of the molecule in the two states is different.Recent work on simple molecules has shown that such is often the case.7 The structures of molecules in their triplet states, and particularly aromatic ones in which we are most interested, are not known and evidence obtained from the weak absorption spectra or diffuse emission spectra in rigid media is at present uncertain. We believe, however, that the low radiationless transition probability from the triplet to the ground state and its viscosity dependence in a wide variety of molecules must be interpreted as showing that the triplet state has an equilibrium nuclear configuration which is considerably different from that of the ground state and that the configurational change which is necessary to attain a position with low reverse-crossing probability is inhibited by clamping of the distorted structure by the viscous or rigid solvent.There are theoretical reasons for believing that the structure of the triplet state even of a molecule as simple as benzene, may differ significantly from that of the ground state. Because of the Pauli principle the probability distributions of two electrons in different orbitals with respect to each other are different in singlet and triplet states. For two electrons in a circle, Dickens and Linnett 8 have shown that $s#s* = (3.r>2(1 + cos [(m - 4 ( 4 2 - 41)11, #T#T* = (inP(1 - cos [(m - nX+2 - 41>I>, where 4s and $T are the wave functions of the singlet and triplet systems, m and n axe the quantum numbers of the two orbitals, and $2 - $1 is the angular separation of the electrons.The products $$* are proportional to the probability that the electrons have the angular separation $2 - $1. It is seen that, for the singletG . PORTER AND M. R . WRIGHT 23 state, the probability is a maximum when the electrons are coincident whilst in the triplet state the electrons have zero probability of being in the same place. Electron repulsion will change the situation quantitatively but the difference remains as is shown by the large splitting of singlet and triplet levels in aromatic molecules. In benzene the instantaneous electron distributions in the three states of interest may be schematically represented as follows : singlet So singlet S1 triplet TI The most stable nuclear configuration of a single Dewar form of TI would probably be folded about the vertical axis, a structure suggested on quite different grounds by Lewis and Kasha.9 Experimental evidence, which may indicate a different equilibrium configur- ation in the triplet and singlet ground states comes from the results of Craig, Hollas and King,lo following work of Evans.11 It is found that the radiative life- time of the triplet state of benzene calculated from the integrated absorption coefficients is greater than 700 sec, earlier work being incorrect owing to the presence of oxygen. On the other hand, the measured lifetime from phosphor- escence decay is 7 sec.These findings would be in accordance with our con- clusion that transition from the triplet state occurs to a ground state molecule with a configuration very different from the equilibrium one.If the structural difference between triplet and ground singlet states of benzene is accepted there is little difficulty in extending the arguments to most other aromatic molecules. It is interesting to note that structural isomeric differences between these states were originally proposed as an explanation of the phos- phorescent state 12 but were later discarded in favour of the triplet state theory. In our view both spin and structural restrictions are necessary to the appearance of phosphorescence of long duration. INTERMOLECULAR PROCESSES OF TRIPLET STATE DEACTIVATION All energy transfer processes from excited states must involve other molecules but we have seen that, when only an inert solvent is present, energy transfer is not itself the rate-determining process.In the presence of certain molecules, however, the rate of deactivation of the triplet state is greatly enhanced. There are undoubtedly several mechanisms of quenching by which this may occur, some of which involve electron transfer, hydrogen atom transfer or other chemical reactions. Since most chemical reactions result in the formation of an addition compound or of two doublet radicals so that spin conservation is always possible, we shall not be concerned with such processes here. A quenching molecule may induce a change of multiplicity in a second molecule without chemical change in at least three ways : (i) It may induce perturbations, and particularly increase spin-orbit coupling, so that the spin selection rule is partially broken down.(ii) A transfer of electronic energy may take place, spin momentum being conserved by excitation of the quencher to a state of different multiplicity. (iii) Conservation of total spin momentum may be made possible during the encounter without, necessarily, any multiplicity change of, or energy transfer to, the quenching molecule.24 CHANGE OF MULTIPLICITY We shall be concerned mainly with the third of these processes which gives a new interpretation of so-called " paramagnetic quenching ". Since the quench- ing molecule is unchanged it will be appropriate to describe the process as " cata- lyzed spin conservation ". Perturbations resulting in a breakdown of the spin selection rule may be brought about, in principle, by heavy atoms or by a magnetic field including the magnetic field of a neighbouring molecule.No effect which can be attributed to heavy atom catalysis of radiationless conversion between triplet and singlet states has been found. Table 3 shows that diamagnetic ions such as Zn2+ and Ga3+ have no measurable quenching effect on the triplet state of naphthalene in water. Livingston and Tanner4 found no marked effect of carbon disulphide or of substituted benzenes, e.g. bromobenzene, on the triplet state lifetime of anthracene. Furthermore approximate measurements of triplet yields in solution indicate that heavy atoms in the solvent have no measurable effect on the Sl-Tl crossing either. On the other hand, paramagnetic molecules have a considerable effect on triplet state lifetimes and this has been attributed by some workers to magnetic perturbations, and breakdown of the spin selection rule.The data given in table 3 makes this interpretation very unlikely. Not only is there a wide variation in efficiency but in a related group, such as the ions of the first transition series, there is no correlation whatever with magnetic susceptibility except that all para- magnetic ions quench to some extent and diamagnetic ions have no effect. Quenching by magnetic perturbation would be expected to be a general and rather unspecific effect of all paramagnetic molecules which was closely related to mag- netic susceptibility as it is in the process of nuclear spin change in the para-ortho hydrogen conversion induced by these same ions.ENERGY TRANSFER WITH SPIN CONSERVATION In a collisional process it is only necessary that the total spin momentum of the whole system be conserved rather than that of each separate partner. One process by which this may be achieved is by " spin transfer " from the excited molecule A to the quencher Q. This is automatically accompanied by energy transfer since a change of electronic state of both partners is involved. If the quenching molecule Q is a singlet this is the only process (apart from chemical reaction) by which a change in multiplicity of A can be induced whilst conserving spin momentum. In the case of a triplet state of A the process becomes A* (triplet) + Q (singlet) -+ A (singlet) + Q* (triplet).It is therefore a necessary condition for this type of quenching that the quencher Q shall have a triplet level lower than that of A. Such cases have been found by Terenin and Ermolaev 13 for transfer from the triplet state of benzophenone and a number of similar molecules to naphthalene and its derivatives. The rate con- stants are very low in rigid solvents and are not yet known in ordinary solutions. CATALYZED SPIN CONSERVATION All paramagnetic molecules which we have investigated quench the triplet state to some extent and we have given reasons why this is not to be attributed to the magnetic field alone. In many of the cases given in table 3, chemical re- action, particularly electron transfer, might reasonably be expected but it is highly improbable that this is the genera! explanation of the quenching action of para- magnetic molecules.There is no correlation with the oxidation-reduction poten- tials of the quenchers and, since both oxidizing and reducing ions are effective, it would be necessary to postulate formation of both the negative and positive radical ions of the hydrocarbons. Most of the paramagnetic molecules studied have low-lying energy levels and quenching might therefore be interpreted simply as an electronic-energy transferG . PORTER AND M . R. WRIGHT 25 process. This may indeed contribute to the quenching mechanism when the energy levels are favourably situated. But there are many difficulties in accepting this as the general mechanism of " paramagnetic quenching ". It is improbable that nitric oxide has a quartet level lower than the triplet level of anthracene and no correlation with the excited levels of the paramagnetic molecules and their quenching efficiency can be found.An alternative explanation of the quenching effect of paramagnetic molecules on the triplet state will now be given. The collisional process A (triplet) + Q (singlet) -+ A (singlet) + Q (singlet) is forbidden by spin conservation rules.14 On the other hand the process is allowed if the multiplicity of Q is higher than singlet. More generally, in the process A* (S = X) + Q(S = y ) -+ A(S = x - 1) + Q(S = y ) , where S is the spin quantum number and x and y are integral or half integral numbers with x > 1, the change is allowed by spin conservation rules for the overall system provided y > 0.We immediately obtain a common property of paramagnetic molecules which distinguishes them from singlet molecules. The otherwise forbidden spin change of A becomes allowed in the presence of a paramagnetic Q without necessarily involving energy transfer or other change in Q. Molecules of solvent must of course be present to remove the excess energy of A after the transition, just as occurs in the intramolecular conversion. This quenching mechanism is quite distinct from magnetic perturbation effects and indeed its rate will be shown to be independent of magnetic susceptibility provided the quencher is not a singlet. It is useful to consider the process in two steps : (i) the formation of the collision complex AQ, and (ii) the dissociation of AQ to A and Q.The quenching rate constant will depend on the following factors : (i) spin-spin coupling between A and Q in the complex AQ, (ii) the lifetime of AQ, (iii) a spin statistical factor. Consider first the spin statistical factor. If x < y , the possible spin quantum numbers of the complex AQ are y + x , y + x - - 1 , . . . y - x , and the total statistical weight g is given by Of these possible states the only ones which can give the required products with spin x - 1 and y are y + x - I , . . . y - x + l , the statistical weight of these states being g q = 2 ( y + x - l ) + 1 + . . . 2 ( y - x + 1 ) + 1 . The probability that AQ will have a spin momentum which correlates with the required products is g,/g, the general expressions for which are and26 CHANGE OF MULTIPLICITY Values of gq/g for values of x and y of interest are given in table 5.X TABLE 5.-sTATISTICAL FACTORS FOR THE PROCESS Y If we confine ourselves to the process given, where the multiplicity of Q is unchanged and that of A must decrease, the only possibilities for the dissociation of AQ are to the products or to the original reactants unless x > 2. Two possi- bilities may now be distinguished. CASE 1 The complex AQ is stable with respect to dissociation to the original reactants. In this case the probability of dissociation to products with altered spin is unity and the statistical probability of the overall process is given directly by gq/g. CASE 2 The energy of AQ with respect to original reactants is small compared with kT.In this case the probability of formation of products with changed spin will be given by probability that AQ will dissociate to A(S = x - 1) = statistical weight of A(S = x - 1) sum of statistical weights of all allowed states of A - -5 gt and values of gp/gt are given in table 5. The overall probability of conversion of A in this case is given by the product gqgp/ggt in the final column. In the last two rows, referring to quintet quenching, the factor gp/gt = 3 does not include the small probability of singlet formation. The interesting conclusion emerges from the figures of table 5 that, on purely statistical grounds, all paramagnetic molecules have an equal probability of in- ducing the transition between a triplet and a singlet state. Differences between quenching rate constants will therefore arise as a result of the other two factors mentioned above and, since these are not related in any direct way to the magnetic susceptibility, the experimental finding that molecules with unpaired electrons are quenchers but that their efficiency is not related to the multiplicity, becomes comprehensible.The large difference in quenching rates between different molecules is now to be considered in terms of spin-spin interaction and the lifetime of the collision complex. These two factors are closely related since both depend on the overlap of the orbitals of the unpaired electrons in A and Q. If there is no interaction between the spin moments, each molecule must individually conserve spin mo- mentum and no change will occur.If interaction is small quenching will occur with a probability less than that calculated on purely statistical grounds.G. PORTER AND M. R. WRIGHT 27 The rates in tables 3 and 4 fall broadly into three groups : GROUP 1 GROUP 2 GROUP 3 On passing from group 1 to group 3 the orbitals of the unpaired electrons be- come increasingly deep-seated as we go from p to d to f electrons. The d electrons of the transition metal ions overlap readily with orbitals of other molecules but they form complexes in aqueous solution which shields the unpaired electrons and reduces overlap with electrons of the triplet. The f electrons of the rare earths are known from many different lines of evidence. e.g. magnetic suscepti- bility theory15 to have relatively little interaction with the solvent or other environment.The three examples of group 1 on the other hand are typical radicals or bi- radicals and the collision complex formed with a triplet state probably has a stability of at least several kcal. Spin-spin interaction will therefore be strong and the complex may have a considerable life. In view of the strong interaction, case 1 is almost certainly applicable here and the statistical factor is therefore g,/g. The product of the reciprocal of this factor and the quenching rate constant is in close agreement with the calculated diffusion-controlled encounter rate. It is to be expected that the radiationless transition probability is increased in the presence of an efficient paramagnetic quencher only by an amount correspond- ing to the difference between a spin forbidden and a spin allowed transition, i.e. by a factor of about 104. Now the lifetime of triplet anthracene in n-hexane in the absence of quenchers is 10-3 sec so that its lifetime when the spin restriction is removed should be about 10-7 sec. The average lifetime of the collision com- plex between triplet anthracene and oxygen, nitric oxide or a second triplet should therefore also be about lO-7sec which is much longer than the duration of an encounter not involving chemical interaction and is in accordance with kinetic studies of anthracene photosensitized oxidation.16 k m 1010 1. mole-1 sec-1 k w 5 x 107 1. mole-1 sec-1 k m 2 x 105 1. mole-1 sec-1 0 2 , NO, aromatic triplet metal ions of first transition series ions of lanthanide rare earths. We are grateful to the Royal Society for the loan of a monochromator and to the Geophysics research directorate, Air Force Cambridge research centre of A.R.D.C., USAF through its European Office for support of part of this work. 1 Porter and Wright, M. R., J. Chim. Physique, 1958, 55, 705. 2 Porter and Windsor, Faraday SOC. Discussions, 1954, 17, 178. 3 Porter and Windsor, Proc. Roy. SOC. A , 1958, 245, 238. 4 Livingston and Tanner, Trans. Faraday SOC., 1958, 54, 765. 5 Porter and Wright, F. J., Trans. Faraday SOC., 1955, 51, 1205. 6 Herzberg, Spectra of diatoinic molecules (Van Nostrand, 1950). 7 Ramsay, Ann. N. Y. Acad. Sci., 1957, 67, 485. 8 Dickens and Linnett, Quart. Rev., 1957, 11, 291. 9 Lewis and Kasha, J . Amer. Chem. SOC., 1944, 66, 2100. 10 Craig, Hollas and King, J. Chem. Physics, 1958, 29, 976. 11 Evans, J . Chem. SOC., 1957, 1351, 3885. 12 Lewis, Lipkin and Magel, J. Amer. Chenz. SOC., 1941, 63, 3005. 13 Terenin and Ermolaev, Trans. Faraday SOC., 1956, 52, 1042. 14 Wigner, Gottinger Nachrichten, 1927, 375. 15 van Vleck, Theory of Electric and Magnetic Susceptibilities (Oxford, 1932). 16 Livingston, J. Chim. Physique, 1958, 55, 887.

ISSN:0366-9033    

DOI:10.1039/DF9592700018

出版商:RSC    

年代:1959

数据来源: RSC

摘要:

OUTER AND INNER MECHANISM OF REACTIONS OF EXClTED MOLECULES BY ALBERT WELLER Labor f. physikal. Chemie, Wiederholdstr. 15, Stuttgart, Germany Received 16th February, 1959 The distinction between outer and inner mechanism (introduced by Forster 1) is out- lined in connection with the quenching of fluorescence. The same distinction is applied to acid-base reactions of excited molecules, where the inner mechanism is known in prin- ciple. The rate constants of these reactions exhibit a rough parallelism with the cor- responding equilibrium constants. Upper limiting values of the reaction rate constants are calculated using the equilibrium constants, the diffusion coefficients of the reactants, and steric factors (closely related to the statistical factors of the respective equilibria).Good agreement with the experimental rate constants proves that the " inner " proton transfer equilibrium is established in a time which is much shorter than the mean lifetime of the excited molecules. Exceptions occur with proton transfer reactions between nitrogen atoms. The influence of the environment on the inner mechanism is discussed. Reactions of excited molecules (A*) which lead either to quenching or to transformation 2-4 of fluorescence can be studied by fluorescence measurements, when the reaction rates are comparable with the reciprocal mean lifetime 701 of the excited molecules. For bimolecular reactions in solution the two reactants are required to form an encounter complex. Transfer of excitation energy to quenchers which absorb at longer wavelengths than the fluorescent molecules may occur over distances considerably larger than the encounter distance.5 This kind of reaction will not be treated in this paper.If, however, due to forbidden transitions, weak mutual coupling exists between the partners, the distance required for energy transfer will eventually become equal to the encounter distance. The different ways possible for the formation of the encounter complex (A*-Q) which have led to the distinction between diffusional (or dynamic) and static quenching may be represented in the following manner : diffusional static kl A* + Q +A*.Q k-1 f K" I A + Q + A.Q. This scheme includes only the outer mechanism which describes the intermolecular conditions of quenching. The additional reaction, k2 A*.Q + unstable intermediate or photoproduct (11) representing the proper quenching process, completes the reaction scheme for fluorescence quenching.The rate constant k 2 depends on the inner mechanism and determines to a certain extent the quenching effect. The influence of this rate constant k 2 on the overall reaction and its implications will be discussed below in connection with the acid-base reactions of excited molecules in water. Before this, however, the conditions for diffusional and static quenching will be considered. 28A . WELLER 29 The following expression for the concentration dependence of the relative quantum yieldflfo can be derived 6 from the complete reaction scheme (I) and (11) : where P is the probability of the occurrence of the inner mechanism.For P --f 1, the well-known expression : (3) f = 1 fo (1 + K"cQ)(l klCQTO) is obtained. The relaxation time of the association equilibrium in the ground state Teq depends on the sum of the equilibrium concentrations of the reactants which may be approximated by the total concentration cQ, because usuallycA < cQ : then The possible difference between the dissociation rate constants in the ground and excited state-due to a different amount of interaction in either state-is accounted for by the superscript O (indicating the ground state). On the other hand, the plausible assumption is made that the association rate constants in the ground and excited state are equal. The relative amounts of diffusional and static quench- ing will depend on the ratio of the two 7s.Static quenching occurs when req > TO. This condition implies that P -+ k2/[k2 + (l/ro)]. On the other hand, when Teq -S r o diffusional quenching occurs and P --f k2/(k2 + k-1). In either case, k2 must be at least comparable to the reciprocal mean lifetime for measurable quenching to occur. Since Teq decreases with decreasing viscosity, quenching will be essentially diffusional in solvents of low viscosity like water, except in such cases where k-i becomes small, because of strong electronic interaction between A and Q in the encounter complex. When the theory of diffusion-controlled reactions is applied to the diffusional type outer mechanism, it must be considered that non-stationary processes may be involved, due to the short time available for the reaction.In addition, the effect of electrostatic forces of ionic charges and the influence exerted on the overall reaction by the rate constant k2 must be taken into account. In the expression finally derived, ( 5 ) the exponential factor corrects for the non-stationary part of the reaction, and _ - f - exp 1- K~Q~flfro)'l fo 1 + kCQTO ' is the stationary overall reaction rate constant which for y = 1 becomes identical with the rate constant derived by Debye 8 for diffusion-controlled ionic reactions. These reactions depend on the relative diffusion coefficient, DA + DQ, the ratio, and the encounter distance a; E = dielectric constant. The number N' of par- ticles per millhole corrects for the dimensions (1. mole-1 sec-1) used for second- order rate constants.The factor y in eqn. (6) is the probability that the reaction ( k 2 ) will occur when the reactants have reached the distance a. Therefore y30 REACTIONS OF EXCITED MOLECULES furnishes information with regard to the inner mechanism. On the other hand, y2 appears in the exponential factor, so that with decreasing y the non-stationary part of the reaction decreases much more rapidly than the stationary part. It is in this respect that similar expressions which have been derived by Umberger and La Mer 9 and by Sveshnikov 10 differ from eqn. (5). In those equations the effect of the inon-stationary reaction decreases linearly with y. With diffusion-controlled reactions of more complex molecules, there may be involved a steric factor CJ which accounts for the possibility that no reaction will occur unless the reactants come into contact with their mutually reactive sites.In this case one can write Y = CJP, (8) where p is now the probability that the proper reaction (k2) takes place when a sterically favourable encounter of the reactants occurs. Since little is known about the inner mechanism of quenching, it seems worthwhile to apply the above formulae and considerations to other reactions of excited molecules, where the inner mechanism is known in principle. Such reactions may then serve as a model for quenching reactions. With acid-base reactions of the general type * kl * k2 * k3 * AHzA + B Z B + (AH-B)"Af"B + (A-HB)zA+ZB + A(ZA-1) + HB(ZB+l), (111) k-1 k-2 k-3 the excited acid, splitting off a proton, goes over into the excited conjugate base (and vice versa) ; the fluorescence of the species thus formed is an additional source of information. Expressions analogous to eqn.(6), derived 11 for the relative quantum yields flfo and f 'If '0 (where the dashed quantities refer to the species formed by the reaction), have been used to separate the stationary and non-stationary part of the reactions, so that values of the stationary overall rate constants can be no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 TABLE ~.-ACID-BASE REACTIONS IN WATER AT 25" AH+ B H3O+ +OH- H3O+ + CH3COO- AcrH+ + OH- * H30+ + R b R ~ H + C~H~COO- RbH + C3H7COO- RbH + CH3COO- NH: + A& H3P04 + Rb- R ~ H + HCOO- RbH f H2P04- HCOOH + Rb- Ac:H++ NH3 CH3COOH + $0- K 5.0 x 1015 5.7 x 104 2.2 x 103 650 115 1 02 88 49 26 8.7 0,204 0.115 0-039 0.01 1 kexpt.1. mole-' sec-1 13 f 2 x 1010 4 5 & 0.5 X 1010 1.85 & 0.15 X 1010 4.8 f 0.3 X 1010 2-86 * 0.15 X l o 9 2.76 & 0.15 X 109 2.90 IJ, 0.15 X 109 2-91 A 0.2 x 109 0.57 f 0.03 X 109 2.40 j, 0.15 x lOQ 6.0 f 0-4 X 108 2.8 & 0.2 X 108 0.22 i 0.02 x 108 0.33 f 0.02 x 108 14.6 8.58 6.02 10.1 2.08 2.00 2.23 1.97 2.65 2.58 2-09 2.44 3.05 2.08 12 13 4 15 15 15 15 16 4 15 16 15 4 15 (20°C) AcrH+ = acridinium kation ; RO- = 8-naphtholate ion ; ROH = 8-naphthol. Acr = acridine;A. WELLER 31 obtained if the mean lifetimes TO and TO’ respectively are known.* A number of these acid-base reactions thus evaluated are given in table 1. They are arranged in order of decreasing equilibrium constant K and it is seen that the overall rate constant k decreases roughly in the same order. All values are extrapolated to zero ionic strength.Two reactions, (1) and (2), measured by Eigen and co- workers 12913 are included in table 1. The relative diffusion coefficients have been calculated from ionic conductances and, for uncharged molecules, taken from the literature or evaluated by means of a nomogram given by Othmer and Thakar.14 Values of y, quoted in table 2, have been calculated from the rate constants k with the aid of eqn. ( 6 ) using a = 7.5 A. This later assumption seems justified for two reasons. (i) 7.5 A corresponds to two layers of water molecules between the proton-exchanging groups. (ii) With reaction (l), a < 7.5 leads to y Z 1. So, if a common value of a is chosen, it must not be smaller than 7.5 A.no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Y 1.0 0.59 0.35 0.53 0.24 0-24 0-23 0.24 0.037 0.17 0.05 1 0.022 0.001 5 0-0028 TABLE 2 kcalc. 1. mole-1 sec-1 13 x 1010 3.8 x 1010 1.8 x 1010 4.5 x 1010 2.9 x 109 2.8 x 109 3.1 x 109 3.2 x 109 4.4 x 109 2.9 x 109 6.8 x 108 3.5 x 108 1.7 x 108 0.34 X 108 With regard to the probability p , two limiting cases may be imagined con- sidering reaction scheme (111) : (i) The equilibrium, determined by the rate constants k2 and k-2, has a re- laxation time, (k2 + k-2)-1, much shorter than the mean lifetime of the excited molecules. As a result, equilibrium is established at once when an encounter of the reactants occurs. In this case p + p f = 1, (9) wherep’ refers to the back reaction of (111). then (ii) k2 and k-2 are much smaller than any other rate constant of scheme 111, p + p l - = a .(10) This case is realized with the slow reaction of pseudo-acids and will therefore Using eqn, (9), upper limits of p and pl can be calculated : not be considered here. where p = cm + a; p’ = 1/(1 + C), (1 1) p (T‘ ( D , + D,)6’ (eb - 1) p’ (T (D, + 0,)s (ed’ - 1) C = - = - K In this equation, which follows simply from eqn. (6) and (S), (T’/(T is the statistical factor which removes contributions to K due to symmetry. As an example, the equilibria associated with the reactions (7), (8) and (9) may be considered, where cr’lcr amounts to +, 3, *, respectively. Single values of (T may be obtained * Mean lifetime measurements have been carried out at the University of Giessen, Germany, by Dr.A. Schmillen, whose valuable co-operation is thankfully acknowledged.32 REACTIONS OF EXCITED MOLECULES approximately from the y-values of the first four reactions for which p is very near unity, due to the high value of the equilibrium constant ; thus y = U. Since no steric requirements are involved with H3O+ and OH- as reactants, u = 3 can be assigned to the acetate ion and to the naphtholate ion, and u = 3 to the acridine molecule. If u = 1 is assumed for the ammonium ion, u = + for ammonia follows from the statistical factor U'/U for the equilibrium. By similar arguments the other U-values of table 2 have been obtained. The molar rate constants which can now be calculated by means of are listed in table 2. The agreement with the experimental values is remarkably good, except with the reactions (9) and (13), in which the proton is transferred between nitrogen atoms.Quite analogous results are obtained with acid- base reactions of 3-acetylaminopyrene-5 : 8 : 10-trisulphonate (DH3-), in water 17 (table 3). For reactions (l), (6) and (7) the agreement is good, con- trary to reactions (2)-(5), in which, again, the proton is transferred between nitrogen atoms. no. 1 2 3 4 5 6 7 AH+B * DH3- + OH- Dh3- + (CH&NH Dk3- + CH3NH2 Dk3- + (CH3)3N P a DH3- + NH3 CH3COOH + 64- Dk3- + CH3COO- TABLE 3 K 1.3 x 107 8 x 104 2 x 104 800 210 140 0.0072 kexpt. 1. mole-1 sec-1 14.8 i 0.9 X 108 4-5 f 0.3 x 108 4.6 I0.3 x 108 2.2 f 0.15 x 108 2.34 f 0-15 X 108 11 1 5 x 108 0.077 & 0.008 X 108 kcalc. 1. mole-1 sec-1 14.5 X 108 6.5 X 108 7.8 X 108 5.8 X 108 10.4 x 108 12 x 108 0.087 x 108 These results suggest that the " inner " proton transfer equilibrium is estab- lished in a time much less than the mean lifetime of the excited molecules (so that eqn.(9) is valid), except in the proton transfer between nitrogen atoms. These remarkable exceptions are certainly connected with the absence of abnormal conductivity in liquid ammonia and amines. Finally, a few remarks are necessary about the influence of the environment on the inner mechanism. For acid-base reactions in water at room temperature no such influence seems to be involved. This is probably due to the Grotthuss- type migration of the proton through the hydration spheres and to the short dielectric relaxation time of water.It is, however, very likely that changes of the solvent configuration which are connected with the transfer of charges (proton or electron), become rate-determining, when the dielectric relaxation time of the solvent is longer than the mean lifetime of the excited molecules. In fact, it hasA . WELLER 33 been observed 18 with excited aromatic amines Ar*NH2 in different akohols, that the rate constant k2 of the reaction : k2 Ar*NH3+ * HOR + Ar*NH2 H2OR-t k-2 depends on the dielectric relaxation time of the solvent. The excited ammonium compounds are extremely strong acids with pK-values between - 2 and - 6, therefore k2 > k-2. Acidic alcoholic solutions of the ammonium compound exhibit at room temperature the fluorescence of the amine. This indicates that reaction (IV) goes practically to completion.When, however, the temperature is lowered, the fluorescence of the ammonium compound appears and the fluor- escence of the amine decreases. In different alcohols this fluorescence trans- formation occurs at different temperatures, depending on the temperature de- pendence of the dielectric relaxation time of the respective alcohol. This effect can be explained by the assumption that the configurational changes of the solvent, which are connected with the proton transfer become rate-determining at these temperatures. 1 Forster, Fluoreszenz org. Verb. (Vandenhoeck and Ruprecht, Gottingen, 1951), 2 Forster, 2. Elektrochem., 1950,54,42 and 531. 3 Weller, 2. Elektrochem., 1952, 56, 662 ; 1956, 60, 1044. 4 Weller, 2. Elektrochem., 1957, 61, 956. 5 Forster, 2. Naturforsch., 1949, 4a, 321. 6 Weller, unpublished results. 7 Weller, 2. physik. Chem., 1957, 13, 335. 8 Debye, Trans. Electrochem. Soc., 1942, 82, 265. 9 Umberger and La Mer, J. Amer. Chem. Soc., 1945, 67, 1099. 10 Sveshnikov, Acta physicochim., 1935, 3, 257. 11 Weller, 2. physik. Chem., 1958, 15 (Bonhoeffer-Gedenkband), 438. 12 Eigen and de Maeyer, 2. Elektrochem., 1955,59,986. 13 Eigen and Schoen, 2. Elektrochem., 1955,59,483. 14 Othmer and Thakar, Ind. Eng. Chem., 1953, 45, 589. 15 Weller, 2. physik. Chem., 1958, 17, 224. 16 Gurr, Diplomarbeit (Stuttgart, 1959). 17 Weller, 2. physik. Chem., 1958, 18, 163. 18 Urban and Weller, unpublished results. p. 199.

ISSN:0366-9033    

DOI:10.1039/DF9592700028

出版商:RSC    

年代:1959

数据来源: RSC

摘要:

ENERGY TRANSFER IN AROMATIC VAPOURS; THE BENZENESENSITIZED FLUORESCENCE OF ANTHRACENE VAPOUR AT 2652 A BY B. STEVENS Dept. of Chemistry, The University, Sheffield 10 Received 2nd February, 1959 The intensity of anthracene vapour fluorescence excited by the 2652 8, mercury line at 170°C is found to increase with the pressure of added benzene vapour. The lifetime of the excited anthracene molecule under these conditions is found from oxygen quenching measurements to be equal to the value found for the same molecule excited by the 3660 8, line, showing that no energy-dependent first-order deactivation of anthracene molecules takes place. Fluorescence enhancement in this case cannot therefore be due to collisional stabilization of the excited anthracene molecules, but must be due to energy transfer to anthracene from excited benzene molecules produced at this wavelength.The lifetime of the excited benzene molecule, determined from oxygen quenching measurements, together with the anthracene fluorescence intensity dependence on benzene pressure, enables a value of 7.6 8, to be calculated for the transfer distance. The transfer of electronic excitation energy plays an important role in photo- synthesis,l in the phenomenon of radiation protection 2 and in the concentration quenching of dye solutions.3 In these systems the efficiency of transfer, which may involve several hundreds of molecules, is often dependent on the nature of the medium which may control the relative dipole orientation of the transferring species or even participate in the transfer process itself.The part played by the medium will therefore be better understood if transfer processes can be studied in its absence, i.e. in the gas phase. Although it is not always possible to do this for molecules encountered in biological systems, since the temperatures required to produce appreciable vapour pressures would undoubtedly cause pyrolysis, it should be possible to investigate transfer in more thermally stable systems with similar electronic properties. The transfer of excitation energy between atoms and simpler molecules, as in the quenching of the mercury resonance line, often takes place with a greater efficiency than that anticipated on the basis of simple collision theory, especially if the molecule is unsaturated.4 Qualitative observations of transfer between more complex molecules include the naphthalene-sensitized fluorescence of acridine, acridonimine, certain phthalimide derivatives and indigo blue, and of aluminium 8-quinolinolate and Mg-phthalocyanine where direct excitation fails.5 Anti- Stokes sensitization of aniline vapour by indigo blue at 3900A and of benzene by aniline at 2800A have also been reported.6 To this author's knowledge, however, the only quantitative measurements of transfer between aromatic mole- cules in the gas phase are those of Dubois7 on the benzene-,!3-naphthylamine system at 2537 ,& which is complicated by the simultaneous collisional stabilization of excited 8-naphthylamine molecules by the sensitizing gas.This paper presents some recently accumulated data on the benzene-sensitized fluorescence of anthracene vapour excited by the Hg 2652A line at 170°C.EXPERIMENTAL MATERIALS Anthracene (m.p. 217") was purified as described elsewhere ; 8 Hopkins and Williams analar benzene was sublimed 3 times in vacuum to remove dissolved gases ; 0 2 was taken from a bulb supplied by the British Oxygen Co. sealed on to the vacuum line. 34B . STEVENS 35 APPARATUS Fig. 1 shows a plan of the optical circuit. The fluorescent vapour was contained in a rectangular quartz cell C measuring 2 x 2 x 5 cm with an absorption path of 1.8 cm, connected through a Hoke packless valve to a standard vacuum line equipped with storage bulbs, McLeod gauge and manometer for measuring gas pressures ; a lower limb of the cell containing solid anthracene rested in an aluminium block which could be electrically heated to control the anthracene pressure.The cell and the metal valve were contained in an asbestos hot-box fitted with a fan, a thermometer and 4 heating coils H. To avoid -J I FIG. 1.-Plan of optical system. PI, P2, P3-Mazda 27M3 photomulti- H-heating coils ; Q-quartz plates ; J-Corning 9863 filters ; C-quartz cell ; G-glass plate ; S-monochromator slit. pliers ; complications arising from Hg-photosensitized reactions it was decided to use the Hg 2652 A line isolated from a 125 W H.P. Hg vapour lamp by means of a Hilger-Watts quartz monochromator D222 with an exit slit width of 0.10 mm, The ratio of fluorescence intensityf, intercepted by a Mazda 27M3 photomultiplier P2, to the intensity R of a refer- ence beam reflected from a quartz plate at 45" to the incident beam was measured as previously described.9 In the sensitization runs, a glass plate was inserted in the fluores- cence beam to remove any radiation emitted by the benzene.A third photomultiplier P3 earthed through a Tinsley spot microammeter was used to measure intensities of trans- mitted light from which fluorescence was removed by a Corning 9863 filter. RESULTS Stern-Volmer plots obtained from the 0 2 quenching of anthracene and of benzene vapour fluorescence at 2652A and 170" are shown in fig. 2. With benzene, the liquid in the lower limb was cooled to - 196" during the addition of 0 2 and allowed to warm up after the valve was closed ; since the liquid temperature, and hence the benzene pressure, could not be carefully controlled in this way, absorption measurements at each 0 2 pressure were made simultaneously and the relative quantum yields fir, determined, thus the ordi- nate of fig.2 for benzene is foZ,/fl;, where IB and 1; are absorbed intensities in the presence and absence of 0 2 respectively. Assuming a collisional quenching efficiency of unity, and collision radii of 4.0, 3.0 and 1-7A for anthracene, benzene and 0 2 , the lifetimes 7A and '78 of excited anthracene and benzene molecules obtained from the appropriate quenching constants KQ are : anthracene (p = 0.76 mm) ; K, = 950 l./mole ; 7A = 2.6 x 10-9 sec ; benzene (p = C. 55 mm) ; KQ = 6000 l./mole; T~ = 2.3 x 10-8 sec,36 ENERGY TRANSFER IN AROMATIC VAPOURS Fig. 3 shows the variation of anthracene vapour fluorescence intensity with pressure of added benzene, the results being expressed as the ratio of intensity f, in the presence of benzene to the intensityfo in the absence of benzene.The temperature of solid anthra- cene was kept at 148.0" throughout, controlling the vapour pressure 8 at 0-76 mm. 3. I 1.c fa f 2.5 2.C I .! [02] mole/l. x lo4 (benzene quenching) I 2 I 3 I 4 I 9' / 9' k , , , , [02] mole/l. x 104 (anthracene quenchhg) 0, KQ = 6OOO I./mole 5 10 I5 2 0 25 x, KQ = 950 l./mole FIG. 2.-Stern-Volmer plots of 0 2 quenching data for anthracene and benzene at 2652 A and 170". 0-benzene ; x -anthracene. [benzene] molell. x 104 FIG. 3.Variation of relative increase j J f 0 of anthracene vapour fluorescence intensity with concentration of added benzene at 2652 8, and 170".B .STEVENS 37 Absorption data for anthracene and benzene at 2652A and 170" are shown in fig. 4. A pressure-broadening effect is observable at low anthracene pressures, the extinction coefficient approaching a constant value over the pressure range used in this work. Similar effects have been noted with 8-naphthylamine 10 and with anthracene 11 at 3660 A. benzene pressure, mm Hg 10 2 0 3 0 4 0 5 0 6 0 1 \ \ %\? -0.08 I I I 1 I I I I . 0.1 0 . 2 0.3 0.4 0 5 0.6 0 . 7 0.8 0.9 I anthracene pressure, mm Hg 0, qo=l8.6 1. mole-1 cm-1 x , rlo=523 1. mole-1 an-1 FIG. 4.-Variation of loglo (transmitted intensity/incident intensity) with vapour pressure of anthracene and of benzene at 2652 8, and 170". 0-benzene ; x -anthracene.From the slopes of the curves the following values are obtained for the decadic extinction coefficients taking 1.8 mm as the cell depth : anthracene : €2652 = 523 1. mole-' cm-1 at 170", benzene : €2652 = 18-6 1. mole-1 cm-1 at 170". DLSCUSSLON The lifetime of the anthracene molecule excited at 2652A under conditions such that self-quenching is negligible, is in excellent agreement with the value of 2.5 x lO-9sec obtained from previous quenching data12 at 3660A using the same collision diameters. The independence of T on wavelength shows that an energy-dependent first-order deactivation is not operative, hence the fluorescence enhancement produced by benzene cannot be ascribed to collisional stabilization of the excited molecule as for aniline,l3 p-naphthylamine 10 and perylene.14 It also indicates that internal conversion from the second excited singlet state of anthracene excited by 2652A, to the lowest excited singlet responsible for fluor- escence emission, is much faster than the process of emission itself so that only the first excited electronic state need be considered.The fluorescence enhancement is readily accounted for in terms of a benzene sensitized anthracene fluorescence. Since benzene does not exhibit self-quenching at the pressures used,7 the following scheme is the most probable :38 ENERGY TRANSFER I N AROMATIC VAPOURS 1 . 2. 3. 4. 5. 6. 7. A + /IV -+ A* A* -+ A 4- hv’ A * + D B* 4 B + hv” B * + D B * + A - + B + A * . B + h v + B* A and B refer to anthracene and benzene molecules respectively, the asterisk denotes an excited electronic state, and D is the product of a radiationless transition.Under photostationary conditions processes 1-7 lead to eqn. (1) for the measured quantity : where k, = rate constant of process iz, I, = intensity of light absorbed by anthracene in the presence of benzene IB = intensity of light absorbed by benzene in the presence of anthracene Z i = intensity of light absorbed by anthracene in the absence of benzene l o = incident intensity, d = cell depth. = 10[1 - eXp (- E,[A]d), Under the experimental conditions the optical density ranges from 0.02 in the absence of benzene to 0.07 in the presence of benzene at the highest concentration (1.72 x 10-3 mole/l.) in which case the exponentials can be expanded and higher terms than the first neglected to give I* - 1 0 4 4 1 4 I B - IoEBIBld, 1; - Io€,[A]d.Eqn. (1) now becomes i.e., fS/’o varies linearly with benzene concentration as is experimentally observed. The slope of the curve (fig. 3) provides the value whence with [A] = 2.77 x 10-5 mole/l. and extinction coefficients already given equal to the ratio of decadic k7/(ks + k6) = k7Tg = KS = 10 ‘5 X 103 I./mOk. Here K,, the sensitizing constant, is equal to the quenching constant for the quenching of benzene by anthracene, and TB is the lifetime of the excited benzene molecule in the absence of anthracene, which is obtained above from 0 2 quenching.B . STEVENS 39 If MA and MB are the respective molecular weights of anthracene and benzene and rAB is the quenching diameter for benzene quenching, or the sensitizing diameter for anthracene sensitization, then Ks = T~(N/lOoo)r&[8 nRT(kf~ $- MB)/kfAMB]* l./moky which with gives which is close to the sum (7.0 A) of the collision radii assumed for these molecules. It is concluded, therefore, that transfer in this system occurs with unit efficiency in collisions of the second kind.The energies of the 'Lb state of benzene and 1Bb state of anthracene are some 38,000 cm-1 and 39,000 cm-1 above the respective ground states,lS on which grounds efficient transfer would be expected ; presum- ably long range transfer is inefficient owing to the lack of suitable orientation at these distances which, however, is achieved in a collision between these planar molecules. TB = 2.3 X 10-8 sec, rAB = 7.6 A, APPENDIX The radiative lifetime ~ ; 2 of the benzene molecule may be estimated from the integrated absorption coefficient which is related to the oscillator strengthf.For this transition in the region 2200-2700 A Almsay and Laemme116 obtain the value f = 0.002 for the vapour at 170"C, whence from the relationship T: = 5.7 x 10-7 sec, v = 3-81 x 104 cm-1. with This is consistent with a quantum yield of benzene fluorescence of The author is grateful to Mr. E. Hutton for calculating this value of T& and to the Royal Society for a grant towards purchase of equipment. 1 e.g. see Rabinowitch, J. Physic. Chem., 1957, 61, 870. 2 e.g. see M. Lefort, Ann. Rev. Physic. Chem., 1958, 9, 123. 3 Levschin and Baranova, Izvest. Akad. Nauk, S.S.S.R. ser. fiz., 1956, 20, 424. 4 Norrish and Smith, Proc. Roy. SOC. A , 1940, 176, 295. 5 Terenin and Karyakin, Izvest. Akad. Nauk. S.S.S.R. ser.fiz., 1951,15, 550 ; Doklady 6 Prileshajewa, Acta physicochim., 1934, 1, 785. Prileshajewa and Klimova, Acta 7 Dubois, J. Physic. Chcm., 1959, 63, 638. 8 Stevens, J. Chem. Soc., 1953, 2973. 9 Bowen and Metcalf, Proc. Roy. SOC. A, 1951, 206,437. 10 Neporent, Zhur. Fiz. Khim., 1950, 24, 1219. 11 McCartin, unpublished data. 12 Stevens, Trans. Faraday Soc., 1955, 51, 610. 13 Neporent, Zhur. Fit. Khim., 1939, 13, 965. 14 Bowen and Veljkovic, Proc. Roy. SOC. A , 1956, 236, 1. 15 In the notation of Platt, J. Chem. Physic., 1949, 17, 484. 16 Almsay and Laemmel, Helv. chim. Acta, 1951, 34,462. Lavorel, J . Physic. Chem., 1957, 61, 864. Akad. Nauk. S.S.S.R., 1954, 96, 269. physicochim., 1937, 7, 163.

ISSN:0366-9033    

DOI:10.1039/DF9592700034

出版商:RSC    

年代:1959

数据来源: RSC

摘要:

VISCOSITY AND TEMPERATURE EFFECTS IN FLUORESCENCE BY E. J. BOWEN Physical Chemistry Laboratory, Oxford University Received 30th September, 1958 Fluorescence measurements have been the means of examining the nature of the transfer of electronic energy from one molecuie to another and from one part of a molecule to another part. In this paper the possibility is explored of using fluorescence measurements to investigate the change over from one electronic state of a molecule to another. Fluorescence is generally enhanced by lowering the temperature. The measurement of this effect is not easy because attention must be paid to changes of concentration, refractive indices, extinction coefficients, and band shapes, while oxygen quenching, concentration quenching and effects such as dimerization must be eliminated.Mr. J. Sahu has recently measured the quantum yields of fluorescence of solutions of substituted anthracenes in a number of organic solvents from - 70" to 70°C with due care about the above factors.1 The effect of temperature on the yield F may be interpreted by the scheme: A f hv -+ A*, (1) A* + A + hv', (2) A* --f A. (3) If process 2 is independent of temperature and process 3 needs activation energy E it follows that 2 ( l / F ) - 1 = k exp (- E/RT). All the 9-substituted anthracene derivatives studied showed approximate agreement with this equation, which implies that F tends to unity at T = 0°K. On the other hand, the yields for side-substituted anthracene derivatives appeared to tend to values less than unity at T = O"K, which could be explained by supposing that process 3 takes two courses, a temperature-dependent and a temperature-independent one.The above equation then becomes ( l / F ) - K = k exp (- E/RT), where K is a constant greater than unity. Unfortunately one is not able here to evaluate K, k, and E separately with any accuracy, as this would require more precise values of F than it has been possible to obtain. Where there are only two constants, as for the 9-substituted derivatives, this difficulty does not arise. If this mode of treatment of the data is correct the heat of activation E may be interpreted in terms of a point (or more probably an ill-defined region) where the potential-energy surfaces of the excited singlet and the ground states come together, and the temperature-dependent effect is associated with a direct energy degradation.The temperature-independent effect may then be related with the change over from the excited singlet state to the triplet without the need for appreciable activation. There may therefore be a means here of separating two types of energy degradation. For the 9-substituted anthracenes, where the number of constants is advantage- ously reduced to two, the k and E values obtained showed for organic solvents of several types a much clearer dependence on the anthracene than on the solvent. In viscous solvents, however, both constants appeared to be markedly higher for each solute. 40E . J . BOWEN 41 This was further investigated by using as solvents a series of paraffinic mixtures of different viscosities. Table 1 gives for these solvents the quantity T/q (absolute temperature/viscosity in poise) x 10-4.temp. "C - 70 - 50 - 30 - 10 10 30 50 70 A TABLE 1 B solvent C D 1.03 1-8 2.88 1.07 0.28 1 4.3 1 -64 0.484 0.171 6.02 2-40 0.807 0.350 8.23 3-31 1 *20 0.626 10.68 4-42 1.75 1 *09 5.68 2.42 1.80 In interpreting the results, the problem is to separate the effect of temperature on the solvent from that on the solute in respect of process 3 above. It was found that the empirical equation : (l/F) - 1 = k(T/q)1/4 exp (- E/RT) gave a good fit for all the results, with very little variation of k or E with solvent. This is shown in the tables 2, 3 and 4 below giving experimental and calculated values of the fluorescence yields F for three compounds.Constants for three other anthracenes are given in table 5. TABLE 2.-9-METHYL ANTHRACENE solvent A B C k= 6.7, k= 6-12, k = 6.05, E= 2350, E=2350. E=2350, temp. O C expt. calc. expt. calc. expt. calc. - 70 0.82 0.81 - 50 0.68 0.68 - 30 0.56 0.58 0.67 0.67 0.75 0.74 - 10 0.46 0.46 0.56 0.56 0-64 063 10 0.38 0.37 0.46 0.46 0.53 0.53 30 0.30 0.30 0.37 0.37 0.43 0.43 50 0.24 0.24 0.29 0.30 0.36 0-35 70 0.24 0.25 0.29 0.29 TABLE 3.-9-METHOXY ANTHRACENE solvent A B C k=516, k = 506. k= 545, E= 5000, E= 5000, E=5000, temp. "C expt. calc. expt. calc. expt. calc. - 70 0.97 0.98 - 50 0.91 0.92 - 30 0.79 0.80 0.89 0.85 - 10 0.63 0.64 0.72 0.70 0.76 0.75 10 0.46 0.46 053 0.52 0.58 0.58 30 0.31 0.31 036 0.37 0.40 0-40 50 0.20 0.20 0.23 0.24 0.26 0.27 70 0.15 016 0.16 0.18 D k= 5.26' E=2350.expt. calc. 0.73 0.73 0.62 0.62 0.51 0.51 0.42 0.42 0.34 0-34 D k= 553. E= 5000. expt. calc. 0.81 0.79 0.62 0.62 0.44 0.44 0.29 0.29 0.18 0.19 These tables show that irrespective of solvent a single value of E can be found for each solute, and that the introduction of the term (T/q)1/4 makes the remaining constant k vary rather little with change of solvent.42 VISCOSITY AND TEMPERATURE EFFECTS IN FLUORESCENCE temp. OC - 50 - 30 - 10 10 30 50 70 solute 9-ethyl 9-phen yl p-chloro A k= 18.1, E= 3500, expt. calc. 0-94 092 0.85 0.85 0.75 0.74 0.63 0.63 0.51 0.52 0.40 0.42 TABLE 4.-9 : 10-DICHLORO ANTHRACENE solvent B C k=20*3, k= 19.5, E= 3500, E= 3500, expt. calc. expt. calc. 0-80 0.79 0.83 0.82 0.68 0.68 0.73 0.73 0.56 0.56 0.60 0-60 0.46 0.45 0-50 0.50 0.35 0.35 0-40 0.40 TABLE 5 solvent A B C k E k E k E 8.42 2500 6.92 2500 7-07 2500 1.15 1800 1-17 1800 1-36 1800 52.5 2900 49.7 2900 44.0 2900 D k = 18-4.E= 3500. expt. calc. 0.85 0.86 0-76 0.77 0.64 0.64 0.53 0.52 0.43 0.42 D k E 6-48 2500 1.28 1800 A possible interpretation of these results may be found along the following (a) A* 3 A + hv', (b) A* 3 A', (4 A' --f A*, (4 A' + A 4- hv', (4 (f 1 Here A* represents the excited molecule in its lowest vibrational level, becoming A' on receiving activation energy E to reach a change-over point on to the ground- state potential-energy surface. It is assumed that both A* and A' can radiate with a rate constant kh that the rate of process c is given by kl exp (- E/RT), d by k2, and f by k3 (TI$. This last assumption implies that diffusion-type movements, less frequent in viscous solvents, permit process f to take place. On this hypothesis (1IF)- 1 = (ki/kf) eXP ( - E/RT) (T/q)/{T/T -k k2fk3 -k (kl/k3) eXP ( - E/RT) -k kf/k3). If kl and kf are relatively small the expression reduces to lines. A + hv -+ A*, A' + solv. -+ A. (kllkf) exp ( - E/RT) (T/q)/(T/q -k k2/k3). The empirical function (T/7)1/4 is fairly closely reproducible by the function k(T/$/(T/q + C) over the range of values here used with k 12-20 and C 0.2-1.4. If the thread of these arguments is sound it indicates that for a singlet-excited state molecule to degrade directly to the singlet ground state, thermal activation energy is necessary but not sufficient ; in addition, the requisite freedom to reach certain amplitudes of vibrational movement against the solvent viscosity is also needed. 1 Bowen and Sahu, J. Physic Chem., 1959,63,4. 2 Terenin, Acfu physicochim., 1948,18,210.

ISSN:0366-9033    

DOI:10.1039/DF9592700040

出版商:RSC    

年代:1959

数据来源: RSC

摘要:

LIGHT AND HIGH ENERGY INDUCED ENERGY TRANSFER IN LIQUID AND RIGID ORGANIC SCINTILLATORS* BY FELIX H. BROWN,? MILTON FURST,$ AND HARTMUT KALLMANN~ Received 5th February, 1959 In contrast to excitation by high energy where the primary energy is absorbed mainly in the bulk material, under light excitation the component which absorbs the incoming radiation may be varied in many solutions by using appropriate wavelengths. Thus the solute and solvent may each be separately excited. By comparing fluorescence under both conditions an absolute measurement of energy transfer occurring from solvent to solute can be made. The results of such determinations show that the probability of energy transfer from effective solvents approaches unity if the solute concentration is sufficient. This occurs particularly when the lowest excitation level of the solvent is energized, but there are indications that the quantum efficiency is near unity when higher levels are excited.Three modes of energy transfer are discussed. Experiments using rigid media aid in discriminating among these since material diffusion transfer does not occur. Definite differences are found in rigid and non-rigid media. Quenching materials are found to be less effective in rigid media. The use of an intermediate " solvent " produces fluor- escence enhancement in rigid as well as non-rigid media. Polymethylmethacrylate and polystyrene show somewhat different energy-transfer properties. It appears that migration transfer plays an important role, at least in rigid media. Transfer of excitation energy from bulk material to solutes has been investigated for many years. This investigation has recently gained impetus from scintillation work ; 1 in this case the bulk material is excited by high-energy radiation in which not only the lowest excited state but also higher states and ionization are produced. These experiments showed that considerable amounts (- 10 %) of the total energy absorbed in the bulk material is emitted by the solute.2 There were also indications that most of the transferred excitation energy comes from the lowest excited state of the solvent molecule.These indications were confirmed by light-excitation experiments which showed that the contributions to transfer made by ionization and higher excitation-energy states are small.In these experiments, it is the solvent which is excited by light ; a portion of the energy absorbed by the solvent is emitted as fluorescent radiation by the solute. Ionization and higher energy states can be completely excluded by choice of suitable wavelengths. When this is done energy transfer is found to proceed to the same extent as under high energy. The fluorescence described is molecular fluorescence and not technical fluorescence which is sometimes de- scribed in the literature.3 This signifies that absorption-reemission effects are negligible in these experiments. From these and other results it can be concluded that the following three modes are most probably responsible for the total energy transfer process from the solvent to the solute. (i) An excited solvent molecule in its movement (diffusion) through the solvent comes close enough to a solute molecule for excitation energy to go over to the * Work supported by U.S. Army Signal Corps Engineering Laboratories, Fort Mon- mouth, New Jersey.j- Dept. of Physics, New York University, New York 3, New York. 2 Dept. of Physics, New York University, New York 3, New York, and Dept. of Physics, Hunter College, Bronx, New York. 4344 ENERGY TRANSFER I N ORGANIC SCINTILLATORS solute molecule during the period of time they are near each other. This mode is called material diffusion transfer. (ii) The excitation energy (rather than the molecule) migrates through the solvent by jumping from one solvent molecule to a neighbouring one. Eventually the excitation energy comes close enough to a solute molecule, and the energy goes over to the solute.This type of energy transfer is designated migration transfer. (iii) There may be neither diffusion nor migration of excitation energy in the solvent, but instead the energy of the solvent molecule may be transferred directly to the solute in a single step (jump) over relatively large distances. This mode is designated single-step transfer.4 All three processes may occur in the solution with one being perhaps of major importance in a given solution. Another type of energy transfer, namely that by absorption of light and re- emission has been proposed.5 In the cases we have investigated there is practically no energy transfer by fluorescent radiation ; only the three processes mentioned above occur.LIGHT EXCITATION OF LIQUID SYSTEMS The use of light excitation in measuring energy transfer has limitations not present when high energy is used. In dilute solutions under high energy excita- tion, the primary energy is absorbed mainly in the solvent because its mass is so much greater than that of the solute. When light excitation is used the absorption spectra of solvent and solute must be suitably matched for this to occur. For occurrence of considerable energy transfer the solute concentration must be of the order of 0.01 M ; this is known from high energy measurements.6 Con- sequently at such concentrations the ratio of absorption of solute to solvent must be about 0.02 for practically all the energy to be absorbed by the solvent. Another limitation is that a number of solvents, worthy of study from the viewpoint of energy transfer, cannot be as easily excited by light because their absorptions lie too far in the ultra-violet. A notable example is p-dioxane, interesting because it has good energy transfer properties under high energy despite its lack of a con- jugated double-bond system, generally found in solvents which transfer well.After considerable experimentation, it was found that an adaptation of the Beckman DU spectrophotometer makes possible reliable measurements of energy transfer under light excitation. The basic experiment was a comparison of the fluorescence of the solute directly excited by the incoming radiation with its fluorescence when excited by energy transfer from the solvent.The direct light was essentially completely absorbed in the soIution in most cases. Possible stray light effects were checked by measuring energy transfer in the same solutions by reflection as well as transmission. On the whole the results were the same. These reflectance measurements were also made in order to decide the possible influences of geometry effects. In the transmission measurements the light was incident normal to the solution. The photomultiplier was along the line of incidence. A filter which did not pass the direct incident light was placed in front of the photomultiplier, which filter passed the fluorescence radiation ; in any event the direct light is almost entirely absorbed in the solution. In the re- flection arrangement, the photomultiplier and the incident light were on the same side of the solution, and the light did not go through any vessel.The overall results of these measurements were quite similar for " effective " and " poor " solvents.7 Both were essentially independent of exciting wavelength, and the relative intensities were the same at different concentrations. Since it is more convenient to use the transmission arrangement, in part because of greater in- tensity, most of the light excitation measurements were made in this way. Quartz cells with a 1 cm path length were used and the light source was a hydrogen lamp. The fluorescence of a given solute concentration was measuredF . H . B R O W N , M . FURST A N D H . KALLMANN 45 in a solvent not absorbing the incoming radiation and in another solvent which absorbs the incoming radiation. The determination of solute fluorescence in the non-absorbing solvent serves as a measure of the intensity of the incident light.(In some cases the fluorescence efficiency of the solute is not the same in both solvents as determined by comparing the fluorescent light output of the Same solute in both solvents using exciting wavelengths not absorbed by either solvent.) If the ratio of the light outputs of both solutions at a given wavelength is the same as when no light is absorbed by the solvent then there is 100 % energy transfer ; there is then no decrease of fluorescent light when the light is first ab- sorbed in the solvent and thereafter transferred to the solute. In fig. 1, 2 and 3, the ratio of the fluorescent light outputs is given for all wavelengths used ; those at which neither solvent absorbs are always to the right.exciting wavelength (A) FIG. 1.-Fluorescence of PBD in anisole and in pdioxane under u.-v. excitation. As an example, at X = 2500 A, in cyclohexane + fluoranthene (1 g/l.), cyclo- hexane does not absorb the incoming radiation whereas fluoranthene does. On the other hand, in anisole + fluoranthene (1 g/l.) the anisole absorbs more than 99 % of the incoming radiation. Even at higher fluoranthene concentrations little of the incident light is absorbed by the solute. In both solvents the fluor- escence emitted is that characteristic of fluoranthene. In cyclohexane there is no energy transfer from the solvent when the exciting wavelength is 2500A.In the other solvent (anisole), fluorescence occurs via energy transfer at excitation wavelengths less than 2800A since these wavelengths are absorbed only by the solvent. The ratio of the solute light output under energy transfer conditions to that when direct excitation occurs is a measure of energy transfer. Measurements show that the energy transfer in some solutions is very efficient and approaches 100 %. At large solute concentration some fraction of the incident light is also absorbed by the solute (e.g. lOg/l. of fluoranthene in fig. 2 at 2500A). In order to determine the exact amount of energy transfer, a cor- rection must be made for this direct absorption. The correction was not made in these figures since it is not large except in the wavelength region near the absorption edge of the solvent.Light-induced transfer was measured at different exciting wavelengths. This was done in order to determine the effect of higher solvent excitation levels. Pre- liminary work seemed to indicate that higher levels were less efficient than lower ones.4 These early measurements were not so accurate as those herein reported especially near the surfaces of the solution. The present, more reliable nieasure- ments indicate, however, that energy transfer is essentially independent of exciting46 ENERGY TRANSFER IN ORGANIC SCINTILLATORS wavelength. While wavelengths obtainable from the hydrogen lamp do not allow great variations in the electronic excitation levels, considerable differences in vibrational levels certainly occur. In anisole an electronic excitation level above the first excited level is also reached.Fig. 1-3 describe energy-transfer measurements under light excitation of wave- lengths down to 2300A. Fig. 1 and 2 utilize a solvent (anisole) known from measurements under high energy excitation to be effective for energy transfer. exciting wavelength (A) FIG. 2.-Fluorescence of fluoranthene in anisole and in cyclohexane under u.-v. excitation. I I I I 1 I I I I I I 7 4 0 0 2500 2600 2 7 0 0 2 8 0 0 2 9 0 0 3 0 0 0 3 1 0 0 3200 3300 3400 exciting wavelength (A) FIG. 3.-Fluorescence of fluoranthene in anethole and in cyclohexane under u.-v. excitation. Fig. 1 depicts results with 2-phenyl-5-(4-biphenylyl) oxadiazole [PBD]. This solute is the most efficient known; it emits in the near ultra-violet.Fluoranthene, shown in fig. 2, is a solute of medium efficiency, fluorescing in a wavelength region extending into the visible. The curve of fig. 2 at 10 g/l. of fluoranthene is almost horizontal and the same for wavelengths absorbed by the solute or by the solvent. Practically 100 % transfer occurs for most of the wavelengths below 2800& since the solvent absorbs nearly all of the incoming radiation. Fig. 3 describes experiments in a solvent (anethole) known from high-energy measurements to be " moderate " in its transfer properties. It is seen that under light excitationF . H . BROWN, M. FURST A N D H . KALLMANN 47 the energy transfer from the “moderate” solvent is also smaller than from the “ effective ” one. Even with 20 g/l.solute the fluorescence is considerably smaller when the energy is absorbed by the solvent than when it is absorbed by the solute. It is to be noted that anethole quenches the solute to a sizeable extent SO that the ratio at the right side is not unity. Some energy is absorbed by the solute at the highest concentration, so that the energy transfer is smaller than indicated by fig. 3. Similar results were obtained using 1-methylnaphthalene and p-xylene as solvents. Up to the present, few compounds have been tried partially because of the limitations discussed above. I I I I I ..A 5 10 15 I W PBD conc. (g/1.) FIG. 4.-Fluorescence of PBD in p-xylene under u.-v. and gamma-ray excitations. A direct comparison between energy transfer produced by light excitation and by high energy excitation has also been made.The exciting light source was alternated with a high energy exciting source, and the rest of the measuring ap- paratus was left unchanged. The results are shown in fig. 4 for y-radiation, for 2400 A, 2700 A and 3100 A exciting wavelengths. Energy transfer occurs in all cases except that of 3 100 A excitation because this light is not absorbed by the solvent. Excluding the last, the three curves coincide almost throughout ; only in the middle range are there small deviations, probably within experimental accuracy. The curves indicate that energy transfer produced under high energy is identical to that produced under light excitation, and that in both cases it occurs from the lowest excited state of the solvent.These experiments were undertaken to investigate whether the previously ob- served difference between gamma ray and light induced fluorescence was real or due to different arrangements. In the present measurements, geometry was the same in all cases, and it was ascertained that the results were independent of48 ENERGY TRANSFER I N ORGANIC SCINTILLATORS geometry. Thus we conclude that there is very little difference in energy transfer produced by gamma ray and light excitation and that the previously found differ- ence was due to geometric effects. Energy transfer occurs via the lowest excited state of the solvent; it is almost 100 % when light excites the solvent at high enough solute concentrations. This means that for 100 quanta absorbed in the solvent, 100 are transferred to the solute.Energy transfer produced under high energy and light excitations of various wavelengths are practically identical. This excludes considerable contribution of higher states and of ionization to energy transfer. The results also seem to show that there is 100 % transition from the second electronic level to the first, since under light excitation with 23008, the second electronic level of anisole is excited and no difference in transfer is observed (see fig. 1 and 2). The next problem is which of the three processes described in the introduction is most important for energy transfer. Experiments in rigid media were per- formed in order to find out something more about these processes. They are described in the following sections.The following conclusions can be drawn from these experiments. INVESTIGATIONS IN RIGID MEDIA It is possible to discriminate between the three processes as follows : process one, transfer by material diffusion, is excluded in rigid media ; thus only processes two and three can occur. Processes enumerated for energy transfer are also re- sponsible for quenching, in which the excited state of the solvent or solute is non-radiatively transferred to the ground state by interaction with some other molecule. The excited solvent molecule in a rigid medium can be quenched according to process two. The excitation energy migrates from solvent molecule to solvent molecule and eventually reaches the neighbourhood of a quencher where it undergoes a radiationless transition to the ground state.When migration of energy does not occur, as is the case for an excited solute, then energy quenching can occur only via process three. Since quenching is often not connected with a resonance process, it occurs when the excited solute molecule happens to be in the vicinity of a quenching molecule. Energy migra- tion from solute to solute molecule does not take place because of the large distance between these molecules, and transfer via radiation is excluded because little or none of its own radiation is absorbed by the solute. (Absorption effects can be determined by varying the sample thickness.) Thus for solute quenching in rigid media, only process three is significant, and only those excited so'ute molecules are quenched which are near a quenching molecule.The formula for solute quenching, occurring according to process three is (1) N is the number of molecules quenched per second ; c is the solute concentration, P gives the number of solute molecules excited per second, and VO(= $ 7 4 ) is the so-called quenching volume (assuming as a first approximation that when the distance between excited solute molecule and quencher is greater than YO, no quenching occurs, and when it is less than ro quenching occurs). Light emission by the unquenched molecules is then proportional to P exp (- voc). Energy transfer according to process three can be described by a formula like (1) in which c becomes the concentration of the substance to which energy is transferred, and vo the transfer volume, which is generally larger than the quenching volume which can be assumed to be of the order of the molecular radius cubed.This implies that quenching occurs only when a quenching molecule is within about 5 x 10-8 cm from the centre of the excited molecule. The eth part of the molecules are quenched at a quencher concentration c = l/q, generally of the order of 1 mole/l. In liquid media, quenching occurs at concentrations of about 0-1 M or even smaller but in these media quenching also occurs via process one N = P[1 - exp (- voc)].F . H . BROWN, M. FURST A N D H . KALLMANN 49 and from this process a larger quenching probability results than from process three. The value of v depends upon the lifetime of the excited molecule in all cases-only slightly in a non-resonance process and much more in a resonance process.Experimentally it is found in accordance with the above that solute quenching in rigid media is much smaller than in liquids (see table 1). TABLE SOL SOLUTE (0,045 M PPO) QUENCHING BY ~,CY.,X,S(’,X’,CC’-HEXACHLORO-~-XYLENE IN XYLENE AND POLYSTYRENE remaining solute fluorescence as percent of unquenched solute fluorescence xylene, yo polystyrene, yo quencher concentration 0.016 M 48 96 0.032 M 36 94 0-064 M 29 89 There may be a considerable difference between quenching of the solute and of the excited solvent molecule in a rigid medium since in the latter case quenching also can occur by energy migration (process two). Experiments indeed indicate that the solvent is more quenched than the solute. The next step is the investigation of energy transfer itself.In rigid media it is only necessary to discriminate between processes two and three. Energy transfer according to process two in liquids is given 9 by eqn. (2) (which also would hold for process one with a different value of Q) in which R + c in the denominator describes the concentration quenching of the solute. This term is unnecessary in rigid media because of the absence of concentration quenching which requires process one (see below). Pc P’c <Q+c)(R+c) --+ ’ I = I = P*[l - exp (- vc)]. (3) Eqn. (3) gives the energy transfer according to process three (same form as eqn. (1) discussed above). In this case, Y is the distance over which energy can be transferred during the lifetime of the excited solvent molecule. This distance can be quite large so that process three may contribute considerably to energy transfer even in rigid media.The value of r for transfer is greater than that of YO for solute quenching which latter is of molecular dimensions. Thus, according to formula (3), energy transfer may occur at relatively small concentrations, whereas solute quenching according to formula (l), occurs only at larger concentrations. The experiments were per- formed with polystyrene (PS) and polymethylmethacrylate (PMMA) as solvents. The results obtained from these polymers are separately discussed. Energy jumps through distance Y in one step. POLYSTYRENE (Ps) Quenching experiments yielded the following. Concentration quenching in polystyrene does not occur at any measured concentration.It also does not occur when large amounts of an intermediate “ solvent ” such as naphthalene are added. This bears out the contention that concentration quenching is due to material diffusion (process one). The absence of concentration quenching is evident in fig. 5, which depicts high energy induced fluorescence of 2 : 5-diphenyl- oxazole (PPO) in polystyre -e and in polystyrene plus 0.2 M naphthalene as inter- mediate solvent. There is no decrease of fluorescence at higher concentrations ; such a decrease is observed in liquids. Fluorescence under light excitation of the solute has also been measured, and the absence of concentration quenching confirmed.* * Such quenching occurs with triplet states because of their longer lifetimes.50 ENERGY TRANSFER I N ORGANIC SCINTILLATORS The next step was to introduce quenchers into polystyrene systems in order to measure their effect on the fluorescence under gamma rays and under direct solute excitation by light.These experiments show clearly that solute quenching by a PPO conc. (mole/]. monomer) (0-2 M)/PPO. FIG. 5.-Gamma-ray induced fluorescence of 0 PS/PPO and PS/naphthalene quencher conc. FIG. 6.-Gamma-ray induced fluorescence of PPO scintillators quenched by or., ay a, a‘, a’, a‘-hexachloro-p-xylene. 1. PS/naphthalene (0.2 M)/PPO (0.045 M). 2. PS/PPO (0.045 M). 3. XylenelPPO (0.023 MI. given compound is smaller in polystyrene than in liquid solvents. This is con- sistent with the absence of processes one and two for solutes in polystyrene solu- tions. Diphenylmercury and m,cc,a,m~,a’,cc’-hexachloro-p-xylene were used as quenchers.(Fig. 6 depicts results with the latter.) Both substances when tested in liquid media were found to be strong quenchers for the solute, 2 : 5-diphenyl-F. H . BROWN, M . FURST AND H . KALLMANN 51 oxazole (PPO). In polystyrene, however, a,a,a,a’,a’,a‘-hexachloro-p-xylene pro- duced almost no solute quenching and diphenylmercury quenched this solute by only about 20 oh at the relatively high concentrations employed. This indicates that the quenching radius of diphenylmercury in polystyrene is of the order of 10-7 cm according to formula (1). Solvent quenching is discussed later since it is related to the mechanism of energy transfer. Two basic types of energy-transfer experiments were performed.In one, polystyrene containing various solutes was excited by gamma rays, and the fluorescence determined as function of solute concentration ; in the other, similar solutions were measured with various amounts of naphthalene present as inter- mediate solvent (fig. 5). In these latter experiments the excitation energy of polystyrene is assumed to be transferred to naphthalene and then from naphthalene to the solute, similar to the sequence found in analogous liquid solutions. This assumption is borne out by experimental results (see below). Experiments without naphthalene show that polystyrene transfers energy to the solute quite well but larger solute concentrations than in “ effective ” liquid solvents are required. The total amount of energy transferred is a little smaller than in liquids.This was determined by comparing the light efficiency of the same solute in xylene and polystyrene under light excitation and high energy excitation. It may be recalled that the concentration at which energy is transferred is essentially determined by the speed of energy transfer and the lifetime of the excited solvent molecule. Since it is known from decay time measurements that this Metime is even longer in polystyrene than in xylene,lo one concludes that the necessity for larger solute concentration in polystyrene is due to an impairment of the energy transfer process itself. The simplest assumption would be that material diffusion, process one, does not occur in polystyrene, but is responsible for part of the energy transfer in xylene, and that energy migration, process two, in polystyrene is not so effective as processes one and two together in xylene.This assumption is not sufficient to explain all the results since it is found that the shape of the fluorescence against concentration curves observed in polystyrene containing effective solutes, with the exception of some special cases discussed below, is different from those of a liquid solution.11 They cannot be completely explained by either process two or three. It may be that a combination of both processes is responsible for the behaviour. Addition of naphthalene to polystyrene solutions has two effects. Smaller solute concentrations are required for maximum energy transfer. (0.2 M naphthalene is sufficient to bring about maximum effect.) Secondly, the shape of the concentration curve is altered.The experimental results in polystyrene solutions containing added naphthalene can be better represented by formula (3) (process three). The maximum light emission is the same with and without naphthalene. Smaller solute concentrations required for maximum fluorescence may be due to a longer lifetime of excitation energy in naphthalene compared to polystyrene. It is known that naphthalene and its derivatives in liquid solu- tions have somewhat longer lifetimes than does polystyrene.10 Lifetime differ- ences may, of course, not be the only factor. There is, however, another noteworthy point. It is observed, as mentioned above, that the change in solute concentration required for maximum fluorescence caused by addition of naphthalene is terminated at relatively small naphthalene concentrations (0.2 M).More naphthalene does not change the concentration curves further. At this naphthalene concentration all possible excitation energy is transferred from polystyrene to naphthalene. This is borne out by the ob- servation that, at this same naphthalene concentration, maximum fluorescence from naphthalene occurs under high-energy excitation in the absence of other compounds.11 If it is assumed that energy transfer from naphthalene to naph- thalene occurs, it can only occur via migration transfer (process two). Then the52 ENERGY TRANSFER I N ORGANIC SCINTILLATORS following difficulty arises. It would be expected that migration from naphthalene to naphthalene be bettered by continued increases in naphthalene concentration ; this would decrease the solute concentration required for a given amount of energy transfer.The absence of this decrease is a prominent feature of all solutes investigated in polystyrene plus naphthalene. The process of excitation energy migration from naphthalene to naphthalene when the molecules are not so close to each other as they are in solid or liquid naphthalene is not a very probable one although it is a resonance process. The reason is that the transition from the excited state (no vibrational excitation) to the ground state usually includes excitation of vibrational levels of the latter state. A transition to a vibrational level of the ground state does not release enough energy to excite another naphthalene molecule.Since the transition to the ground state without excitation of vibrational levels includes only a small portion of the total transition probability, the probability for a migration process entirely based on dipole transitions in naphthalene molecules is not very large. This probability of migration is further reduced because the total dipole transition moment for the lowest excited state of naphthalene is extremely small. A rough evaluation based on these considerations shows that energy should not migrate from naphthalene to naphthalene in a rigid medium over distances of 2 x 10-7 cm (equivalent to 0.2 M) during the lifetime of the excited naphthalene molecule. This leads to the conclusion that transfer from naphthalene to solute does not occur by energy migration (naphthalene -+ naphthalene + solute) but rather by a single-step transfer (process three) ; energy is directly transferred from naph- thalene to solute over relatively large distances.The probability of this process is much larger than that between molecules of naphthalene for two reasons. First, the respective dipole transition probability of the accepting solute molecule is larger than that of naphthalene, and the transfer to a solute is better than to another naphthalene molecule since the transfer probability is a product of two transition probabilities. Secondly, the difference in energy levels between naphthalene and solute is important. Because of this difference not only are zero-zero transitions in naphthalene capable of producing transfer to the solute, but also transitions to higher vibrational levels of the naphthalene ground state.Thus it is understandable that energy transfer from naphthalene to solute in rigid media does not occur via migration transfer but via single step transfer. These ideas follow Foerster’s.4 One would be inclined to assume a similar process for transfer of energy in polystyrene without naphthalene. But as already stated this is not borne out by the fluorescence against concentration curves observed in polystyrene. The only explanation we can offer at present is that processes two and three both occur. This will be investigated in more detail, especially by measuring lifetimes. We hope to report on this in the near future. In liquid systems the transfer parameter Q displays an amazingly small de- pendence upon the solute and its transition probability to the ground state as determined from absorption spectra.This may be due to the predominance of processes one and two, in which the ultimate transfer may occur between molecules very near each other. In polystyrene + naphthalene systems, however, in which it is assumed that process three predominates in the final transfer to the solute, it would be expected that v is more strongly dependent upon the solute used. More information about these processes is derived from experiments on the quenching of solvent molecules. Such quenching can be studied by observing the quenching of high energy induced fluorescence. It can be concluded that most of the fluorescence decrease observed is due to solvent quenching since it was determined that the solute is only relatively slightly quenched.When di- phenylmercury or ct,qx,a’,ct’,ct’-hexachloro-p-xylene is added to various poly- styrene systems, a considerable decrease in fluorescence is observed. This quenching is so strong that it is doubtful whether process three alone can accountF . H. BROWN, M. FURST AND H . KALLMANN 53 for it ; the quenching cross-section of these compounds for polystyrene would have to be unusually large. This strong quenching of polystyrene is therefore assumed to be due in part to migration of excitation energy in polystyrene. Here, indeed, migration of excitation energy can occur, since polystyrene molecules are close to each other. Further, transfer of excitation from one polystyrene molecule to another is probably due not only to dipole transitions but also to overlap of wave functions which may be appreciable for neighbouring atoms and molecules.Thus it seems that the strong quenching seen in fig. 6 comes about because excitation energy migrates in polystyrene until it comes close enough to a quencher molecule. The quenching behaviour is different when naphthalene is added. Large concentrations of naphthalene eliminate a sizeable portion of the fluorescence decrease ; naphthalene is considerably less quenched than polystyrene. This is at least partially attributed to the lack of energy migration between naphthalene. The excitation energy has no chance to come near the quencher molecule during its lifetime if the energy initially is localized far from the quencher.The number of naphthalene molecules close to a quencher molecule is not very large at the concentrations employed. Therefore this lack of quenching in naphthalene is a corroboration of the absence of energy migration in naphthalene. The results so far discussed are consistent with our previous ideas of energy transfer.12 There are some results, however, which seem incapable of being explained with these ideas alone. Some solutes have been found to have different energy-transfer behaviour. One such case is fluoranthene in polystyrene without and with added naphthalene. In fig. 7 at small concentrations a rather steep rise of high energy induced fluorescence, and then an almost continuous rise of this fluorescence over a large concentration range is observed.The difference between fluoranthene and other solutes is that energy transfer to fluoranthene increases constantly and does not reach a limiting value at the large concentrations used. If naphthalene is added, the fluorescence against concentration curve is parallel to the curve without naphthalene but the intensities are greater ; this means the same amount of energy transfer is accomplished at smaller solute concentrations. Similar behaviour is shown by chrysene. A simple interpretation would be that energy transfer from polystyrene (no naphthalene) to these solutes is poor, requiring larger solute concentrations than do other solutes. The same must then be true when naphthalene is present since the fluorescence curves are parallel.This is difficult to understand since with considerable amounts of naphthalene added, energy transfer from naphthalene to the solute would not be influenced any more by polystyrene and since in liquid solutions energy transfer to fluoranthene and chrysene is normal. One cannot simply interpret these fluorescence curves as due to a quenching, assuming, for instance, that some of the solute molecules are in positions where they are more easily quenched than in other positions and that this results in a decreased fluorescence. This assumption is untenable since under light excitation the fluorescent behaviour is quite normal; that is, a steep rise of fluorescence in- tensity in the range where absorption becomes completed and then a constant fluorescent light output.Therefore it is not the fluorescence efficiency of the solute which is different, but rather the energy-transfer process. It is noteworthy that the maximum light output of these polystyrene solutions under high energy is higher than that of respective liquid solutions. This higher light output is in accord with the higher fluorescence efficiency of these solutes in polystyrene observed when directly excited by light.11 The only explanation which we can present up to now is that the transfer of energy from the solvent to these solutes over large distances is small because of small transition moment. Calculations based on absorption strengths do not bear this out. Fluoranthene does have a small transition moment (shown by its long time-constant in liquid solutions,10~ despite the fact that fluoranthene is quenched), but this small transition probability is not sufficient to explain the54 ENERGY TRANSFER IN ORGANIC SCINTILLATORS difference between the fluorescence against concentration curves of, for example, fluoranthene and 2 : 5-diphenyloxazole.Furthermore, it would be expected that, in polystyrene, energy transfer occurs partially via migration of energy (process two), but then the smaller transition moment of fluoranthene would be much less important since excited polystyrene molecules can always be close enough to the fluoranthene molecule. In this respect it is also important to consider the curve when naphthalene is present which shows the same behaviour. This indicates that the same impeding effects occur for energy transfer from naphthalene to fluoranthene as from polystyrene to fluoranthene.At this stage it is difficult to pin down what impairs the energy transfer to these compounds in polystyrene. I I I 1 I I 1 I I ' OlOl ' 0 . 0 3 0.05 0 . 0 7 0.09 fluoranthene conc. (moles/l. monomer) FIG. 'I.-Gamma-ray induced fluorescence of PS/fluoranthene and PS/naphthalene + fluoranthene. 1. PS/naphthalene (0.2 M)/fluoranthene. 2. PS/fluoranthene. The results obtained from rigid polystyrene can be summarized as follows. Solute quenching is relatively small and presumably occurs only by process three. Quenching of polystyrene is greater ; it seems to occur by processes two and three. Quenching according to process two does not seem to occur for naphthalene when present as intermediate " solvent ".Even at relatively large naphthalene concentrations, it seems to be small ; so that naphthalene quenching goes on only by process three. Because process one is excluded, solute concentra- tion quenching (and shifts of emission spectra by formation of fluorescent dimers, as is found with pyrene in liquid solutions 13) does not occur. Some solutes in polystyrene, e.g. fluoranthene and chrysene, have a higher fluorescence efficiency when excited by light than they do in liquid solvents. This effect is similar to the viscosity effect which is reported for fluorescence light output of solutes in viscous media,l4 but the effect is not general. Polystyrene energy transfer is different from that from liquid solvents.Not only are larger concentrations required, but the shape of the fluorescence curve is quite different. This is tentatively interpreted as due to a combination of pro- cesses two and three in polystyrene solutions. Energy transfer via an intermediate " solvent " occurs in polystyrene as well as in liquids.F . H . BROWN, M . FURST A N D H . KALLMANN 55 It is conjectured that the transfer from naphthalene to the solute occurs by process three because this transfer reaches its maximum at such low naphthalene concentrations that migration between naphthalene molecules is improbable. POLYMETHYLMETHACRYLATE (PMMA) Experiments similar to those on polystyrene solutions were performed on polymethylmethacrylate (PMMA). Fig. 8 shows the high-energy induced fluor- escence of 2 : 5-diphenyloxazole in PMMA without and with naphthalene as intermediate “solvent”. The overall shapes of the curves are similar to cor- responding ones in polystyrene, but marked differences are present.Because of its structure, it was expected that PMMA be a “ poor ” solvent (with respect to PPO conc. (moles/l. monomer) FIG. 8.-Gamma-ray induced fluorescence of PMMA/PPO and PMMA/naphthalene/PPO. 1. PMMA/naphthalene (0-8 M)/PPO. 2. PMMA/PPO. energy transfer). This would be seen by a very gradual rise with solute con- centration of high energy induced fluorescence. Addition of naphthalene as inter- mediate ‘‘ solvent ” would be expected to make the rise considerably steeper, and relatively large naphthalene concentrations would be necessary to extract all the available energy from PMMA.These ideas were arrived at because PMMA does not contain a conjugated double-bond system (in most cases important to effective energy transfer) and because PMMA by itself exhibits even smaller fluor- escence under high-energy excitation than effective solvents. This latter may be considered as an indication that considerable quenching occurs, making the life- time of the excited state relatively short. The figure shows, on the contrary, that PMMA without naphthalene reaches its maximum fluorescence at relatively small solute concentrations, the same order as those encountered in polystyrene. This would make PMMA a “ moderate ” solvent for energy transfer. The maximum light output obtained from PMMA solutions under these conditions is only about one-third as great as that obtained from polystyrene solutions of the same solutes. The addition of various amounts of naphthalene not only shifts the rise of the fluorescence concentration curve to smaller concentrations as expected, but also raises the maximum light output to about twice that without naphthalene. Larger amounts of naphthalene are needed to extract all available energy from PMMA than are needed in polystyrene; maximum fluorescence is obtained with ap- proximately 1 M naphthalene.A possible interpretation of these results is that two different excited-energy states exist in PMMA with few transitions occurring between them; one having56 ENERGY TRANSFER IN ORGANIC SCINTILLATORS moderate transfer properties, due perhaps to a lifetime of medium duration, the other having poor transfer properties and probably a short lifetime.The excitation energy is extracted from the better transferring state by relatively small concen- trations of solute. Once this energy is extracted, the fluorescence appears to reach a saturation. The second state transfers so poorly that at the available solute concentrations very little of its energy is extracted. This second energy com- ponent, however, can be extracted by large amounts of naphthalene. The need for large naphthalene concentrations is in accord with the assumption that energy in the other excited state is less transferable. When large amounts of naphthalene are present, the PMMA + naphthalene + solute system emits only about 10 to 30 % less energy than the corresponding polystyrene system. The validity of this model for energy transfer from PMMA will be investigated by fluorescence measurements under light excitation of solvent and solute and by concomitant measurements of time constants. 1 Kallmann, Physic. Rev., 1950, 78, 621. Reynolds, Harrison and Salvani, Physic. 2 Broser, Kallmann and Martius, 2. Naturforsch., 1949, 4a, 204. Furst, Kallmann 3 Birks, Physic. Rev., 1954, 94, 1567. 4 Forster, Fluoreszenz Organischer Verbindungen (Vandenhoeck und Ruprecht, 5 Kallmann and Furst, Physic. Rev., 1951, 81, 853. Birks, Scintillation Counters 6 Furst and Kallmann, Physic. Rev., 1952, 85, 816. Furst, Kallmann and Brown, 7 Furst and Kallmann, Physic. Rev., 1955, 97, 583. Furst and Kallmann, J . Clzem. 8 Brown, Furst and Kallmann, J. Chim. Physique, 1958,55, 688. 9 Furst and Kallmann, Physic. Rev., 1952, 85, 816. Rev., 1950, 78,488. Ageno, Chiozotto and Querzoli, Acc. dei Lincei, 1949, 6, 626. and Kramer, Physic. Rev., 1953, 89, 416. Gottingen, 1951). (McGraw-Hill, New York, 1953). J. Chem. Physics, 1957, 26, 1321. Physics, 1955, 23, 607. 1O(a) Kallmann and Brucker, Physic. Rev., 1957, 108, 1122. 11 Brown, Furst and Kallmann, J. Int. Atomic Energy Agency, in press. 12 Kallmann and Furst, in Liquid Scintillation Counting Conference (Pergamon Press, New York, 1958), p. 3. Brown, Furst and Kallmann, J. Chim. Physique, 1958, 55, 688. Kallmann, Furst, and Brown, in Semiconductors and Phosphors (Interscience Publishers, New York, 1958), p. 269. (b) Swank and Buck, Rev. Sci. Instr., 1955, 26, 15. 13 Forster and Kasper, 2. Elektrochem., 1955, 59, 976. 14 Kallmann, Furst and Brown, in Semiconductors and Phosphors (Interscience Pub- lishers, New York, 1958), p. 269.

ISSN:0366-9033    

DOI:10.1039/DF9592700043

出版商:RSC    

年代:1959

数据来源: RSC

摘要:

ENERGY TRANSFER IN FLUORESCENT PLASTIC SOLUTlONS BY J. B. BIRKS AND K. N. KUCHELA The Physical Laboratories, The University, Manchester Received 24th March, 1959 Solvent-solute energy transfer in tetraphenyl-1 : 3-butadiene + polystyrene solutions has been studied by observing the fluorescence excitation spectra from 220-350 mp. At low con- centrations the transfer is purely radiative, but at c > 10-4 M non-radiative transfer occurs. The transfer is independent of excitation wavelength from 270-240 mp, but decreases at shorter wavelengths, probably due to the reduced excitation depth. There is some evidence for a change in the polystyrene emission spectrum when excited by 220 mp radiation. The results are compared with data on similar solutions, excited by ionizing radiations.Studies 1-4 of the fluorescence of polystyrene solutions of organic fluors, excited by ionizing radiations, have shown that efficient solvent-solute energy transfer occurs in these systems. If ionizing radiation is used for the stimulation of the fluorescence, the energy transfer is only one of several alternative or suc- cessive processes by which the initial ionization and excitation energy of the solvent molecules is dissipated, degraded, or ultimately converted into the observed solute fluorescence. If ultra-violet radiation is used for excitation, the energy transfer can be studied directly by observing the fluorescence excitation spectrum. In the present experiments this method, which has been previously used for liquid solutions,L 6 has been applied to a typical plastic solution system, that of 1 : 1’ : 4 : 4’-tetraphenyl-1 : 3-butadiene (T.P.B.) in polystyrene.The behaviour of this system for P-particle and a-particle excitation has been investigated by Swank and Buck.3 EXPERIMENTAL AND RESULTS The solutions, ranging in concentration c from 10-6 M to 10-2 M, were supplied by Messrs. Nash and Thompson, Ltd., and were prepared in the standard manner3 by the thermal polymerization of solutions of T.P.B. in styrene monomer. Cylindrical specimens, 2.2 cm diam. by 1 cm long, were machined from each solid solution. The ultra-violet absorption spectra of the solvent and solute (fig. 1) were measured with a Unicam SP 500 spectrophotometer. For polystyrene the extinction coefficient K was derived (i) from measurements on solutions in chloroform, extrapolated to 100 % concentration, and (ii) from measurements on thin polystyrene films.For T.P.B. the molar extinction coefficient E was derived from measurements (i) on solutions in poly- styrene at wavelengths down to 310 mp, and (ii) on solutions in cyclohexane. The rela- tive values of K and E determine small correction factors in the subsequent analysis. They can be estimated adequately for this purpose, despite the differences in the absorption in the liquid and solid phase. The fluorescence excitation spectra of the solution specimens were measured by a method similar to that of Birks and Cameron.6 The specimen was placed on a Chance OY 10 filter, adjacent to the photo-cathode of an E.M.I. 6097 B photomultiplier mounted vertically.The upper face of the specimen was illuminated with monochromatic radiation from a Unicam SP 500 spectrophotometer with a hydrogen lamp source, the horizontal exit beam being deflected vertically downwards by a polished aluminium mirror. The filter is transparent to the T.P.B. fluorescence, but it is opaque to the incident radiation and to most of the polystyrene fluorescence emission. The photomultiplier output 5758 ENERGY TRANSFER I N FLUORESCENT PLASTIC SOLUTIONS current was corrected for the small dark current component. Except at low concentra- tions, where there is a small contribution due to polystyrene fluorescence, the current is proportional to the intensity of the T.P.B. fluorescence. 4 10 3 10 \ \ \ \ A (mt4 FIG. 1.-Absorption spectra of (1) polystyrene (a) in chloroform, (b) solid, and (2) T.P.B.(a) in polystyrene, (6) in cyclohexane. 9-1 Z A (m) FIG. 2.-Fluorescence excitation spectra. Relative fluorescence quantum intensity I against wavelength X for specimens of concentration c. The relative quantum intensity of the incident radiation at different wavelengths was determined by replacing the specimen by a standard calibrating solution of 1 cm thick- ness, contained in a cylindrical fused silica vessel of similar dimensions to the specimen. 2 x 10-2 M solutions of 1-dimethylaminonaphthalene 7-sodium sulphonate in water, andJ. B . BIRKS AND K . N. KUCHELA 59 1-dimethylaminonaphthalene 5-sulphonic acid in N sodium hydroxide solution, which have constant fluorescence quantum efficiencies 7 at wavelengths from 210-400 mp, were used for calibration.Both solutions gave consistent and reproducible results. The I I I I 2 7 0 O l 2 2 0 2 3 0 240 2 5 0 2 6 0 (w) FIG. 3.-Energy transfer coefficient f against wavelength A, for specimens of concentration c. S C FIG. 4.-(a) Energy transfer coefficient f against concentration c (A = 240-270 mp). (6) Scintillation pulse height S against c (Swank and Buck 3). fluorescence intensity I of each specimen, normalized to a constant flux of incident quanta, was measured from 350 mp to 220 mp wavelength. Typical fluorescence excitation spectra are plotted in fig. 2. The energy transfer coefficient f, defined as the fraction of quanta initially absorbed by the solvent which are transferred to the solute, has been evaluated from the excitation60 ENERGY TRANSFER I N FLUORESCENT PLASTIC SOLUTIONS spectra.6 At X > 300m,u where the polystyrene is almost transparent, the T.P.B.fluor- escence is excited directly. Its intensity attains a limiting value l o in the higher con- centration specimens, where the incident radiation is completely absorbed by T.P.B. At lower concentrations Ill0 is approximately equal to the fraction of incident radiation absorbed by T.P.B. in the specimen, as calculated from E and K (fig. 1). At X < 270mp, practically all the incident radiation is absorbed by polystyrene in the lower concentration specimens, and assuming the fluorescence quantum efficiency of T.P.B. to be independent of wavelength, f = I/Zo. At higher concentrations a correction must be applied for the component of the T.P.B.fluorescence directly excited by the incident radiation,6 so that The values off as a function of excitation wavelength X for specimens of different c are plotted in fig. 3. f i s found to be independent of X for X = 240-270 mp, i.e. for excitation within the first absorption band of polystyrene. The variation off with c in this spectral region is plotted in fig. 4(a). FIG. 5.-Excitation at X = 270 m p : (a) direct ; I against concentration c ; (b) indirect (by polystyrene emission) ; I' against concentration c. The radiative component of the energy transfer, due to absorption of polystyrene fluorescence by T.P.B., was studied by a method similar to that of Swank and Buck.3 A thin disc of pure polystyrene, 0.05 cm thick, was placed on top of the specimen with a thin intermediate film of glycerine to reduce internal reflection at the interface. The polystyrene disc completely absorbs the incident radiation at X < 270mp and converts it into polystyrene emission.The intensity of the specimen fluorescence I', due to this indirect excitation by the polystyrene emission, was compared with the intensity I due to direct excitation, at X < 270 mp. The curves of Ip against concentration at X = 240-270 mp are identical within the experimental error. The variation of Ip and I with c at X = 270 mp is shown in fig. 5. DISCUSSION The energy transfer is independent of wavelength in the region X = 240-270 m,u, which corresponds to excitation into the 1st excited state of polystyrene.The polystyrene fluorescence emission spectrum 3 ~ 4 extends from X = 285-360 mp.J . B . BIRKS AND K . N. KUCHELA 61 This emission is efficiently absorbed by T.P.B. at relatively low concentrations. The indirect excitation curves of Ip against c (fig. 5 ) show that 90 % absorption of the polystyrene emission occurs at about c = 10-4 M, corresponding to an effective molar extinction coefficient E - 104 for a 1-cm specimen. This agrees with the value of E for T.P.B. (fig. 1) averaged over the polystyrene emission spec- trum. The direct excitation intensity 1 is proportional to Ip (allowing for the small component due to unabsorbed polystyrene emission) up to c - 10-4 M, showing that the energy transfer at these concentrations is purely radiative. Similar conclusions have been reached 4s 5 for excitation by ionizing radiations. The radiative transfer coefficient f R = Aqo, where qo is the fluorescence quantum efficiency of polystyrene, and A is the fraction of the emission absorbed by T.P.B., which can be estimated from the Ip against c curve.I 0 -3 0 -2 i il M C FIG. 6.-Energy transfer coefficient f against concentration c (A = 240-270 mp). (a) radiative f R , (b) non-radiative f N R , (c) total f, from (2) and (3) ; I, experimental. indicates that an additional transfer process, which is non-radiative, becomes operative. If the alternative processes competing for the solvent excitation energy are emission, self-quenching, quenching by the solute, and non-radiative transfer to the solute, and their relative probabilities are Pe, pq, psc and ptc respectively, then the non-radiative transfer coefficient will be given by PtC fNR = Pe 3- Pq + (ps + p t ) ~ 1 + (TIC a (TC =- Since the non-radiative transfer competes with the solvent emission, f R becomes A90 1 + (T'c' f R = ~ (3) Fig.6 shows a comparison between the experimental data and the values of f N R , f R and f( = f R + f n R ) from (2) and (3, taking qo = 0.1, (T = (T' = 30.62 Similar equations to (2) and (3), with a = a’ and A = 1 (for c > 10-4 M) have been derived by Swank and Buck.3 Their experimental data on the relative scintillation pulse height S of polystyrene + T.P.B. solutions for p-particle ex- citation are plotted in fig. 4(6). Within the experimental error S is proportional to f over the range of concentrations studied, indicating that when the system is excited by ionizing radiation the energy transfer occurs from the 1st excited state of the solvent. A decrease is observed in the energy transfer coefficient f at h < 240mp, c > 10-4 M.This spectral region corresponds to excitation in the 2nd absorption band of polystyrene. One possible explanation is that additional quenching processes (e.g. singlet-triplet state conversion) compete with the internal conversion of the polystyrene excitation energy from the 2nd to the 1st excited singkt state, and thus reduce f. An alternative, and more probable, explanation is that the ENERGY TRANSFER I N FLUORESCENT PLASTIC SOLUTIONS C FIG. 7.-Excitation at A = 220 and 240 mp : (a) direct ; I against c ; (b) indirect ; I p against c.effect is associated with the reduced depth of penetration x of the incident radiation due to the increase in K. For K = 104, 90 % of the radiation is absorbed within 1 p of the surface. A similar decrease is observed in the scintillation efficiency of organic crystals and solutions,ss 9 excited by ionizing particles which penetrate less than a few p below the surface. This effect 10 is attributed to the surface escape or quenching of the excitation energy. It is found that the scintillation efficiency decreases with x towards a limiting value at the surface, which is 0-5 of that in the interior of the material. In the present experiments, for c > 5 x 10-4 M, f decreases with A, i.e. with decrease in x , from A = 240 mp to 220 mp to about 0-5 of its maximum value.This similarity of behaviour for excitation by short- range ionizing and ultra-violet radiations indicates that the decrease in f is due to a similar surface effect. The shape of the I against c curve at h = 220 mp differs from that at /I = 230-270 mp, at c < 5 x 10-4 M (fig. 7). It appears that, while the non-radiative transfer f n R is unchanged apart from the surface effect, the radiative transfer f R which operates at lower c is modified. The results suggest a change in the spectrum of the polystyrene emission, since it is completely absorbed by lower concentrations of T.P.B. Further experiments are required to elucidate thisJ . B. BIRKS AND K . N. KUCHELA 63 apparent change, which might be due to emission from a triplet state, from the 2nd excited singlet state,ll or from surface impurities. It is interesting to note that at h = 220mp there is a clear discrimination between the alternative energy transfer processes. 1 Koski, Physic. Rev., 1951, 82, 230. 2 Pichat, Pestail and Clement, J . Chim. Physique, 1953, 50, 26. 3 Swank and Buck, Physic. Rev., 1953,91, 927. * 4 Krenz, Trans. Faraday SOC., 1955, 51, 172. 5 Cohen and Weinreb, Proc. Physic. SOC. B, 1956, 69, 593. 6 Birks and Cameron, Proc. Physic. SOC., 1958, 72, 53. 7 Weber and Teale, Trans. Faraday SOC., 1958, 54, 640. 8 King and Birks, Physic. Rev., 1952, 86, 568. 9 Birks and Brooks, Proc. Physic. SOC. B, 1956, 69, 721. IoBirks, Physic. Rev., 1952, 85, 569; 1953, 90, 1131. 11 Birks, Physic. Rev., 1954, 94, 1567.

ISSN:0366-9033    

DOI:10.1039/DF9592700057

出版商:RSC    

年代:1959

数据来源: RSC

摘要:

ON LUMINESCENCE DECAY TIMES AND THEIR RELATION TO MECHANISMS OF ENERGY TRANSFER IN RADIATION CHEMISTRY OF LIQUIDS* BY MILTON BURTON AND HERBERT DREESKAMP Dept. of Chemistry, University of Notre Dame, Notre Dame, Indiana Received 2nd April, 1959 Luminescence against time curves, obtained by a newly developed technique, are given. Decay times of luminescence processes in mixed cyclohexane + benzene scintil- lator systems (with air present) lie in the range around 2 x 10-9 sec. Addition of benzene to a cyclohexane solution of p-terphenyl causes a sharp rise in the decay time to a maximum at low benzene concentrations. The decay time is attributed tentatively to the energy transfer process from the lowest excited state. The estimated fraction of such low excited states transferring excitation to the scintillator is only 0-04 to 0.08.On the other hand, the scintillator appears also to quench the excited solvent. With the preliminary data at hand, the luminescence decay times are used as a tool for interpretation of the mechanism of protection in radiolysis of mixed liquid systems. 1. INTRODUCTION 1.1. ON PROTECTION Numerous experiments on radiolysis of liquid mixtures containing cyclo- hexane and benzenel-4 indicate that the 100eV yield of hydrogen from cyclo- hexane may be decreased by two effects attributable to the benzene. One may be true free-atom scavenging. The other appears to be a true protection, in the sense that energy primarily absorbed in a cyclohexane molecule is transferred to a benzene molecule, where it is there dissipated. One of the first suggestions of the nature of the energy-transfer process in this case was that both ionization- transfer and excitation-transfer may be involved, because both the ionization potential and the lowest (singlet) excitation potential of cyclohexane, determined for the gas, lie higher than the respective quantities for benzene.1 There are numerous difficulties in establishment that a protective effect in a liquid mixture is properly attributable to ionization transfer.Not the least is the difficulty in following experimentally the behaviour of an ion in such cases. Evidence does exist of ionization transfer in gases. However, in view of the various possible complexities of ion-molecule behaviour in liquids as seen, in part, from behaviour of gases, and, in part, from pure speculation, extrapolation of such evidence to liquid cases can be a quite unjustifiable procedure.5 A further difficulty in the assumption of an ion-transfer mechanism seems t o be wholly theoretical ; for pure hydrocarbon systems like cyclohexane + benzene the ion may have too short an independent life to participate in an ionization- transfer mechanism representable by such a simple reaction as Cf + B -+ C + B+, (1) if we think of the two ions as having separate existence.On the other hand, there is a possibility that the combination of initial positive-hole and electron, * Contribution from the Radiation Project operated by the University of Notre Dame and supported in part under Atomic Energy Commission contract AT(11-1)-38.64M. BURTON AND H. DREESKAMP 65 produced in a spur, may not be localized but may itself move very rapidly so that independently of what is initially ionized the neutralization process does involve a benzene molecule. In such a sense an ionization transfer process does occur but the mechanism has connotations somewhat different from what the chemist ordinarily means by reaction (1). According to an alternative view, the neutralization process does involve a truly primarily-ionized molecule. For a cyclohexane + benzene mixture rich in cyclohexane, this means that the predominant neutralization process would be C+ + e -+ C+ where C+ represents a highly excited state of cyclohexane, either singlet or triplet.5 In any event a significant number of excited cyclohexane molecules are produced primarily in a variety of excited states (2) c -c*.(3) According to Kasha,6 in most liquid cases the more highly excited states initially produced (i.e. either by reactions (2) or (3)) internally convert quite rapidly (i.e. in about 10-13 sec) to the lowest excited state permitted by the selection rules or by considerations of stability. Thus, if the initial highly excited state is singlet, the final excited state is singlet. On the other hand, there is a possibility (not discussed in the paper by Kasha), for triplet states particularly, that the course of internal conversion may lead into a repulsive state and directly into decomposition, thus interrupting the excitation degradation at an intermediate point. Such a mechanism may be the path of radiolysis of benzene which decomposes with reasonable yield 1 (e.g.G(H2) = 0.037) in spite of the fact that the photochemical quantum yield of decomposition, in contrast, is negligible in the absorption region near 2500 A and detectable only on irradiation 7 9 8 near 1800 A. The mechanism of internal conversion suggested by Kasha has not yet been elucidated nor has it been established whether a presumed protective process like C* + B -+ C + B* can occur during the time of the excitation degradation or is necessarily delayed until the degradation to the lowest excited state of cyclohexane is complete. Furthermore, in a certain sense, mechanisms have been suggested for the simul- taneous resistance of benzene to radiolysis and its failure to show any large fluorescence.They involve the notion of an extremely rapid internal conversion from the lowest excited state to the ground state. However, such an interpretation appears contrary to the known great effectiveness of benzene as a scintillator solvent 9 and to the data of the present paper. A major difficulty in an attempt to solve the problems involved in mechanisms of protection results from the various possible effects of trace quantities of im- purity.5 They may act as scavengers, as negative-ion formers, as protective agents and as inducers of internal conversion. When an effort is made to establish the behaviour of a pure substance as a reference material, the question of the possible determinative effect of an unknown impurity enters. While this method of approach to the solution of the problems of energy transfer cannot be ignored and must continue to be pressed, the most recent endeavour in our laboratories has been to avoid such complicating features wherever we can.(4) 1.2. O N LUMINESCENCE OF MIXTURES An obvious approach to the problems of the behaviour of excited states in radiolysis is through the study of luminescence as a function of the composition of multicomponent mixtures. Some of our results have already been reported 10-13 and the evidence can be interpreted to indicate (as might have been expected) C66 LUMINESCENCE DECAY TIMES that in certain mixtures, like cyclohexane + benzene + 0.6 mole % p-terphenyl, the path of excitation transfer may be represented by C* + B 3 C + B*, B* + T --f B + T*, (4) (5) T * + T + h , (6) followed by light emission from the scintillator and accompanied by a number of other energy degradation and quenching processes.One rather significant result obtained in such cyclohexane + benzene + p-terphenyl mixtures containing oxygen, phenyl bromide or methyl bromide as quencher is the existence of a luminescence minimum at benzene concentration near 1 volume percent. Existence of such a minimum is not a general property of quaternary scintillator solutions and is even affected by the nature of the quencher. We have made repeated efforts at understanding of these results by both chemical and physical experiments. I0 r I 0. I 0 10 20 t, units of 10-9 sec FIG. 1.-Luminescence intensity as a function of time for an aerated solution containing 5 g p-terphenyl per litre of benzene.I is in arbitrary units. The present paper represents a brief report of current work on a number of physical features of luminescence studies and their possible significance for radi- ation chemistry. Only the general features of the preliminary results are related. Details will be published in forthcoming papers. 2. TIME STUDIES OF LUMINESCENCE A technique developed in our laboratory 14 makes possible the study of decay processes which go to completion in only a few millimicroseconds. Fig. 1 shows the relative luminescence yield from a liquid solution containing benzene +M. BURTON AND H. DREESKAMP 67 terphenyl + air, as a function of time after switching on the beam from a 30 kV X-ray tube and switching it off about 1 1 millimicroseconds later.The rise-time of the X-ray beam and the fall-off time are each estimated to be about 10-9 sec. Attention is called to the fact that, after the beam is turned off, the decrease of luminescence fails to follow an exponential law for ca. 1 x 10-9 sec ; thereafter the fall-off is exponential over about two decades of intensity and the decay time, T [defined by I = I0 exp (- t / ~ ) ] , in this case appears to be 2.2 x 10-9 sec. A more careful calibration of the time scale may change the latter value by f 5 %. I I t, units of 10-9 sec FIG. 2.-Luminescence intensity as a function of time in various crystals and plastics. I is in arbitrary units. The samples are: (1) anthracene, (2) stilbene, (3) Sintilon, (4) an unidentified plastic.A variety of plastic scintillators, some of unknown composition, has also been tested by Dreeskamp, together with Ghosh and Yguerabide, in our laboratory and it has been found that no such scintillators show a simple first-order decay law. Fig. 2 displays the nature of some of the results. To be sure, all the plastic scintillators were characterized by the fact that the bulk of the energy absorption must have been in the major, essentially non-luminescent, component and that energy must have been transferred at some time to the luminescent component (perhaps, in some cases, components). However, fig. 2 also indicates that the processes measured by luminescence decay in this way, were not first-order decay processes of a single component. Fig. 3 shows the type of luminescence curve obtained for a quaternary solution of cyclohexane + benzene + p-terphenyl + oxygen.13 Luminescence rise and decay curves, such as shown in fig.1, have been obtained for the complete range of cyclohexane/benzene ratios shown in fig. 3. In their general features they rescmble fig. 1 but the decay times are different. The results are summarized in68 LUMINESCENCE DECAY TIMES fig. 4. Several facts are noteworthy. The decay times are functionally related to the relative concentrations of the two solvents but remain always first-order. 5 4 3 I 2 I I I I I 1 I 1 I I I 0 20 40 60 00 100 vol. % B FIG. 3.-Co6o-gamma-induced luminescence in air-saturated cyclohexane + benzene + p-terphenyl according to Nosworthy.13 I is luminescence intensity in arbitrary units.21 1 1 I I I I I I I I 0 50 100 vol. % benzene FIG. 4.-Luminescence decay time in aerated solution of cyclohexane + benzene + p-terphenyl. The decay time rises sharply in the region where the luminescence goes through a minimum and attains a maximum at low benzene concentration. The decay time in such solutions is about twice that in cyclohexane solutions.M. BURTON AND H. DREESKAMP 69 3. POSSIBLE INTERPRETATION OF LUMINESCENCE AGAINST TIME CURVES As already stated, this paper is concerned only with preliminary interpreta- tions. The region A-B in fig. 1 seems characteristic of all the luminescence- time curves and may be representative exclusively of the cut-off time of the X-ray tube. If it is, two alternative interpretations of the result lead to the opposed conclusions that the measured T is representative (a) of the decay time of the scintillator or (b) of the time involved in energy transfer.The first interpretation is that the time required for the processes C* + T -+ C + T* (7) i n cyclohexane solution and in benzene solution, as well as the possible succession of processes, B* -t T + B + T* C* + B -+ C + B* B* + T - t B + T* in cyclohexane + benzene solutions does not exceed 10-9 sec.* A change of T of the scintillator itself (as might be measured by u.-v. excitation with light which is absorbed by the scintillator alone) may be the result of one of two effects. (i) A change in T in such case could be the result of different solvent surroundings and would be revealed by a shift in the emission spectrum; un- published results of S.F. Pensabene indicate that small concentrations of benzene such as cause a sharp rise in T (cf. fig. 4) are without significant effect on the fluorescence spectrum. (ii) On the other hand, change in T of the scintillator might be the result of quenching of the scintillator. However, it has been shown 10 that, in the range of ca. 10-3 M quencher, only the solvent is effectively quenched. The result is confirmed by the data of Knau 15 for the system benzene + anthracene +nitrobenzene as quencher, irradiated with 3600A u.-v. or with less than 50 keV electrons, which indicate an effect on the solvent (or the energy transfer system) appearing at < 10-3 M quencher. An effect on the solute itself begins at 6 x 10-3 M quencher concentration.In contrast, benzene (present as second solvent) does not act as a quencher at all, as shown for low concentrations in fig. 4 by the sharp rise in T and for high concentration in fig. 3 by the increased luminescence. We are, therefore, impelled to an interpretation leading to the conclusion (b), much more interesting from the viewpoint of radiation chemistry. It is that the decay times shown by curves such as represented by fig. 1 (and given in fig. 4) are characteristic of the excitation transfer processes shown by reactions (7) or (9, or the combination (4), (3, and not of the light-emission process, Of course, this interpretation states nothing about the time required for the initially ionized and excited molecules to degrade to the low-excited states, C* or B*, represented in these equations; if the views already summarized (e.g.those of Kasha) are applicable, the latter time is ca. 10-13 sec. A more convincing demonstration that a quencher (to be distinguished from a second solvent like benzene) acts to decrease the life of excited solvent molecules is given by some unpublished results of S. F. Pensabene and A. Ghosh. The former has preliminary evidence that azulene cannot quench u.-v.-induced fluor- escence of p-terphenyl in cyclohexane solvent. The latter finds, also in preliminary experiments, that 1 /T varies approximately linearly (see fig. 5 ) with azulene * It should be noted that the possibility of an ionization-transfer mechanism being involved in the luminescence is specifically omitted on the basis of results reported in a forthcoming paper by S.Lipsky and M. Burton. T*-+T+hv. (6)70 LUMINESCENCE DECAY TIMES (quencher) concentration in such a solution, irradiated with Co6O-y, just as is required for normal quenching action. The reasonably admissible conclusion is that quenching itself involves the excited solvent directly. The results of fig. 5 show that the decay time of the excited state of p-terphenyl in this work is definitely less than 2 x 10-9 sec. It is a reasonable inference that quencher fails to act directly on the scintillator, in this case at least, because of its very short life. If such very short life is a general property of scintillators excited indirectly by high-energy radiation, a legitimate next question concerns the mechanism of two different effects: (a) the difference in luminescence efficiencies of various scintillators ; (6) the effect of concentration of scintillator on total luminescence.0.50 4 I 8 OI 4 ru 0 0.46 0.42 0 2.5 5.0 azulene, units of 10-4 M FIG. 5.-Reciprocal decay time as a function of azulene (quencher) concentration in a solution of 1.2g p-terphenyl per litre of cyclohexane, according to A. Ghosh. The quenching constant y appears to have a value in the range 320 to 440 mole-1. 3.1. EFFECT OF CHANGE OF SCINTILLATOR Preliminary data obtained by A. Ghosh for a solution of diphenylhexatriene in cyclohexane irradiated with 30 kV X-rays are similar to those shown in fig. 1 . However, the decay time, T, in this case is ca. 8 x lo-gsec, in contrast with T N 2.2 x 10-9 sec shown for p-terphenyl scintillator at the r ime concentration.It is known from studies of Co60-y induced luminescence that p-terphenyl is a more efficient scintillator than is diphenylhexatriene.16 The fact that the less efficient luminescence emission is associated with the longer decay time indicates that T represents the rate controlling (i.e. slowest step) in transfer of energy from excited state of solvent to scintillator. Although r might, on a naive basis of arithmetic alone, be representative of a longer life of excited cyclohexane, there is no apparent process by which the intrusion of a second component can prolong the life of a state. A simpler interpretation is that in the presence of a less effective energy acceptor, the energy transfer process itself is prolonged.The longer r is in such cases, the greater is the probability of interception of the excited state of the solvent by some type of energy degradation process. 3.2. EFFECT OF SCINTILLATOR CONCENTRATION According to Swank, Phillips, Buck and Basiie,l7 increase of p-terphenyl concentration in toluene solvent results in decrease of r to a limit determined byM. BURTON AND H. DREESKAMP 71 the decay time of the scintillator itself ; this figure as given by them is about 2.2 x lO-9sec. Fig. 1 and 4 of this paper taken together show likewise that T decreases with increasing scintillator concentration in benzene solution. The lower value shown is also 2.2 x 10-9 sec. However, the experiments with quencher present (cf. fig. 5 ) give T as low as 1.8 x 10-9 sec, which value must be 2 T for excited p-terphenyl. Thus, the effect of p-terphenyl is two-fold : to decrease T of the excitation transfer process and simultaneously to increase luminescence output," just as found by Swank et aZ.17 If the fraction of excited states of the solvent which terminate in luminescence of the scintillator is large, this effect is readily understood because decay time and luminescence yield would appear just on the basis of very simple kinetics to be reciprocally related.However, if that fraction is small, the effect of scintillator concentration or type on decay time must be explained for the most part as quenching of the excited solvent by the scintillator, as described by Birks and Cameron.18 In $4, a rough calculation indicates that but a relatively small fraction of excited molecules transfer their energy to the scintillator.Tests of the effect of p-terphenyl concentration on T in cyclohexane solution will assist in elucidation of the mechanism and in a test of Birks and Cameron's suggested quenching mechanism; in this case, inter- pretation of the results will be simplified because p-terphenyl is known not to protect cyclohexane from radiolysis.19 4. SIGNIFICANCE FOR RADIATION CHEMISTRY For multicomponent systems of the type here discussed, a variety of factors determines the probability that an initial excitation or ionization will result in an eventual emission of a photon as luminescence. For scintillator concentrations such as employed in this work we can assume all the initial energy absorption to be in solvent molecules; we can neglect also the trivial contribution to lumin- escence made by such molecules.ne = the number of scintillator molecules which emit luminescence, per unit of energy primarily absorbed in the solvent ; no = the number of solvent molecules primarily ionized or excited, per unit of energy primarily absorbed in the solvent ; #d = fraction of primarily ionized or excited molecules which do not complete the course of internal conversion to the lowest excited (singlet or triplet) state. The intercepting processes can include both chemical reaction and, otherwise undefined, physical quenching. The fraction which reaches the lowest excited state is then = 1 - $ d ; #t = fraction of molecules, in lowest excited state of solvent, which transfer energy to scintillator molecules.The fraction 1 - $t, which do not transfer energy in this way, may lose energy both in chemical reaction and in physical quenching. Thus, the nature and the quantity of impurities present affect the value of #t ; #e = probability that an excited scintillator molecule emits its energy as luminescence. The probability that an initial excitation or ionization results in a photon Let emission is, on this very simple model, given by An equation as simple as the above is restricted for useful application to one- solvent systems. In $ 1.1, we have seen that that portion of the Kasha excitation * The effect on luminescence output is, of course, an old and well-known effect in these t Note that ne/no 2: 2 x the fraction of absorbed energy re-emitted as luminescence.cases.72 LUMINESCENCE DECAY TIMES degradation 6 which can involve transition to repulsive states may terminate in decomposition before the lowest excited state is attained. Thus, $d can be a rather large quantity. In the specific case of pure cyclohexane, we can roughly estimate the lower limit of the yield of molecules initially ionized or excited from the relationship 20 50 50 Ic Ec’ G * & - - + - where Ic N 11 eV,21 the ionization potential, and Ec = 7.1 eV,22 the excitation potential-both in the gaseous state-are used in lieu of information regarding the liquid. Thus G* 2 11.5. Using the 100 eV yield of total gas,l G(gas) N 6, as representative of the decomposition of cyclohexane, it follows that $d can be as large as 0.5 if all the decomposition processes occur before the lowest excited states are reached.On the other hand, it is interesting to note that the value of gc(H2), the yield of H2 per 100 eV of energy actually absorbed in the cyclohexane, goes to a limiting value of 0.5 in mixtures containing increasing concentration of benzene.3 The yield of cyclohexane molecules decomposed per 100 eV of energy actually absorbed by the cyclohexane is then approximately given by where the last term, the yield of H atoms scavenged (in the same units) is estimated on the assumption 3*4 gc(H) - G(H) = 2.2, (4) and the value 2-2 is estimated from results with iodine or other scavenger.39 49 23 Thus, gc(-C6H12) N 2.7 and $d - 2.7/115 = 0.23 on the unlikely assumption that decomposition is complete before the lowest excited state is reached.On the other hand, $d - 0.5/11.5 = 0.04 if rupture into H atoms does not contribute at low cyclohexane concentration. Under Co60-y irradiation conditions ne/no I 0.02 for the systems employed in this work.* Also, evidence from u.-v. studies indicate that $e can equal ca. 0.5. for solutions of p-terphenyl in cyclohexane.24 Assuming that this $e value should be used and using $d = 0.04 to 0-5 and $e - 0.4 to 0.8 for Co60-y irradiation conditions, must lie in the range 0.04 to 0.08. Obviously, some future efforts in the study of these systems can well be addressed to better establishment of the value of $d but it is nevertheless apparent that $t is a small quantity. The presently most reasonable explanation of the results of fig.4 appears to be that it is the rate of the excitation-transmission process as reflected by t,ht which is affected by change in the solvent. The assumption in the use of eqn. (1) for estimation of $l is that the transmission process involves exclusively lowest excited states of the solvent. It is a reasonable conclusion from the Kasha effect and the considerations of the previous paragraphs that the same lowest excited states make the principal contribution to the chemical processes; i.e. to the actual radiolysis. It would simplify interpretation for radiation chemistry if one might didactically state that change in half-life of luminescence decay is the same thing as change in half-life of the excited state involved.Actually, the effects shown in fig. 4 probably represent a more complicated situation. Addition of a very small amount of benzene to a cyclohexane + p-terphenyl mixture vauses a sharp increase in T. The inference for radiation chemistry is that the observed effect is the result of transfer of excitation represented by reaction (4). The state produced has T greater than that in benzene + p-terphenyl. The implication is that isolated excited benzene molecules have larger half-lives than those in the immediate *This figure is based on an estimate 10 for the system benzene + terphenyl. It is definitely on the high side.M. BURTON AND H. DREESKAMP 73 environment of other benzene molecules. The mechanism and consequences of a presumed energy transfer, involved in such interaction, for radiation chemistry remain to be established.4.1. THE SCINTILLATOR AS AN INDICATOR It has been shown 19 that the presence of p-terphenyl in benzene has no de- tectable effect on the 100 eV yield of H2 when the liquid is irradiated with Co60-y rays. According to the suggested interpretation of the results in the present paper, the explanation is that only a small fraction of the excited states (repre- sented by i,!~t = 0.04 to 0.08) transfers energy to the p-terphenyl. Thus the latter is without effect on the major course of events (non-radiative deactivation, decom- position, etc.) in which the lowest excited states of the solvent can be involved. In other words, the scintillator, in luminescent solutions of the type here discussed, may be used as an indicator by which one may examine the life of such states.4.2. CHEMICAL EFFECTS OF THE QUENCHER A related conclusion refers to the effect of quencher on the decay times, a factor which we have not yet been able to study in detail. It may be anticipated that a quencher will affect T and, in mixed solvents, will affect 7 of each solvent in- dependently. While we are not prepared to say what the effect will be on the radio- lysis process, it appears reasonable to expect that such an effect (i.e. as a quencher, not merely as a chemically reactive agent) will ultimately be found-if not on the overall results, certainly on the detailed elementary processes. It is interesting in this connection that Bach and Sorokin 25 report that oxygen, a typical quencher, increases yield of H2 in radiolysis of ethanol. 1 Manion and Burton, J, Physic. Chern., 1952, 56, 560. 2 Burton and Patrick, J . Physic. Chem., 1954, 58, 421. 3 Burton, Chang, Lipsky and Reddy, Radiation Research, 1958, 9, 203. 4 Meshitsuka and Burton, Radiation Research, 1959, 10,499. 5 Burton, Hamill and Magee, Proc. Second Int. Congress Peacefitl Uses of Atomic 6 Kasha, Faraday SOC. Discussions, 1950, 9, 14. 7 Wilson and Noyes, Jr., J. Amer. Chem. SOC., 1941, 63, 3025. 8 Krassina, Acta physiochim., 1939, 10, 189. 9 Kallmann and Furst, Physic. Rev., 1952, 85, 816. 10 Burton, Berry and Lipsky, J. Chim. Physique, 1955, 52, 657. 11 Berry and Burton, J. Chem. Physics, 1955, 23, 1969. 12Berry, Lipsky and Burton, Trans. Faraday SOC., 1956, 52, 311. 13 Nosworthy, Magee and Burton, Radiation Research, 1958, 9, 160; also, forth- 14 Dreeskamp and Burton, Physic. Rev. Letters, 1959, 2, 45. 15 Knau, Z. Naturforschung, 1957, 12a, 881. 16 P. J. Berry, Thesis (University of Notre Dame, 1955). J. L. Kropp, unpublished 17 Swank, Phillips, Buck and Basile, IRE Transactions Nuclear Science, 1958, NS-5, 18 Birks and Cameron, Proc. Physic. SOC., B, 1958, 72, 53. 19 Burton and Patrick, J. Chem Physics, 1954, 22, 1150. 20 cf. Burton and Kurien, J. Physic. Chem., 1959, 63, 899. 21 Hustrulid, Kusch and Tate, Physic. Rev., 1938, 54, 1037, 22 Pickett, Muntz and McPherson, J. Amer. Chem. Soc., 1951, 73, 4862. 23 Dewhurst, J. Physic. Chem., 1959, 63, 813. 24 Bowen and Williams, Trans. Faraday SOC., 1939, 35, 765. 25 Bach and Sorokin, Sbornik Rabot Radiatsonnoi Khimi, Akad. Nauk., U.S.S.R., 1955 1, 163 ; English translation : Symposium on Radiation Cheniistry, Acad. Sci. U.S.S. R., 1955, 1, 135. Energy, 1958. coming publication. work. 183.

ISSN:0366-9033    

DOI:10.1039/DF9592700064

出版商:RSC    

年代:1959

数据来源: RSC

摘要:

ENERGY TRANSFER IN POLYETHYLENE AND POLYETHYLENE-POLYBUTADIENE MlXTURES DURING GAMMA IRRADIATION * BY MALCOLM DOLE AND T. F. WILLIAMS Dept. of Chemistry, Northwestern University, Evanston, Illinois Chemistry Division, A.E.R.E., Harwell Received 14th January, 1959 Disappearance of initial vinyl or vinylene unsaturation on gamma-ray irradiation of highly crystalline linear polyethylenes follows a first-order decay law in the liquid state, but only over a limited dose range in the solid. Evidence is given that this decay must be the result of transfer of energy of excitation within localized spurs. Breakdown of the first-order law in the solid state when a certain concentration of radiolytically and randomly produced vinylene groups has been attained is explained on the basis of a pro- tective action of the latter.Concentrations of cis- or trans-1 : 4-polybutadiene up to 5 % have little effect on the radiolysis of crystalline polyethylene in the polyethylene + polybutadiene mixture, but in the liquid state the polybutadiene must interact both with ions and excited states of the polyethylene because G(H2) and k l , the vinyl group decay constant, are both lowered markedly. cis-Polybutadiene exerts less of a protective action than trans-polybutadiene. The difference between the behaviour in the liquid and solid states is explained on the basis of a more uniform distribution of the polybutadiene in the liquid and a greater frequency of collisions between the polybutadiene segments and ions or excited groups of the polyethylene. In 1953 Dole and Keeling1 noted that vinylidene-type double bonds -CH2CCH2-, about one per molecule, in a low density polyethylene disappeared II CHZ on irradiation in a heavy-water pile much more rapidly than would have been expected on a purely statistical basis.Thus, after an irradiation dose sufficient to liberate from -CH2- units 3.6 molecules of hydrogen per number average polyethylene molecule of molecular weight 32,000, essentially a11 of the vinylidene groups had disappeared. In a later more accurate paper, Dole, Milner and Williams 2 found that during gamma-ray irradiation, the initial rate of hydrogen evolution in molecules evolved per 100 eV of energy absorbed, G(H2), was com- parable to the initial G-value of vinylidene group elimination, despite the fact that the concentration of vinylidene groups, 0.414 x 10-4 mole/g was almost 2000-fold smaller than the concentration of CH2 groups, 7-1 x 10-2mole/g.It was also demonstrated 2 ~ 3 that a similar effect was observed for the disappear- ance of the vinyl double bonds in a high density polyethylene, Marlex-50. The decay of the latter initially followed the first-order law, - dPi]/dD = kiwi], (1) where P i ] represents the vinyl group concentration, kl the first-order constant and D the dose. The first-order constant was the same for two different poly- ethylenes whose initial vinyl group concentration differed five-fold. From the data of Lawton, Balwit and Powell 3 it appears that kl is also independent of dose rate. * The research described here will be submitted by one of us (T.F. W.) to the Uni- versity of London in partial fulfilment of the requirements leading to the Ph.D. degree. 74M. DOLE AND T. F . WILLIAMS 75 The selective reactivity of the vinyl and vinylidene groups has been inter- preted 2 in terms of transfer of excitation energy. The present paper deals with a further study of this phenomenon, not only with respect to pure Marlex-50, but also to Marlex-50 containing up to 5 "/o of either trans- or cis-1 : 4-polybutadiene. In addition a polyethylene containing initially only vinylene unsaturation, -CH= CH-, was studied. EXPERIMENTAL The trans-vinylene group concentrations were determined in this laboratory by the infra-red method while the percentage compositions were given by the supplier. Table 1 contains a list of materials studied and their composition.TABLE 1 .-COMPOSITION OF MATERIALS STUDIED Y.3 yo trans-1 : 4- Yo cis-1 : 4- [ trans-Vl] Marlex-50 polybutadiene polybutadiene moles/p x 104 supplier Phillips Petroleum Co. 100 0 0 0.06 98-5 1-35 0.15 2.1 95 4.5 0.5 7.0 95 0.2 4.8 0.3 Standard Oil Co. of Indiana *75 The vinyl group concentration in the pure Marlex-50 varied slightly from sample to sample, but was of the order 1 x 10-4 moleslg. The Standard Oil Co. of Indiana (SOI) material and the Marlex-50 were linear high- density polyethylenes. The Marlex-50 component of the Phillips Petroleum Co. materials was antioxidant free, but the polybutadiene contained about 1.5 % of phenyl-/3-naphthyl- amine antioxidant. As far as the experiments of this paper are concerned no difference could be detected in the behaviour of pure Marlex-50 with or without antioxidant.The radiation source, Co-60, associated equipment and dosimetry methods were the same as previously described.*, 4 The technique of measuring the concentration of the different unsaturated groups by the infra-red method was also the same as previously described 2 except for the cis-vinylene concentration determinations. Relative changes in the concentration of the latter were estimated by measuring changes in the optical density of the infra-red spectrum at 6.05 p. Inasmuch as the vinyl group absorbs at 6.1 p, the cis-vinylene group absorption had to be detected and measured from the height of the '' step " in the curve of the recording of the infra-red absorption in this spectral region.Measurements of changes in the height of this step enabled estimates to be made of relative changes in the cis-vinylene con- centration. Other techniques such as that used for the evaluation of G(H2) were the same as described previously.29 4 RESULTS The observed changes in concentration of the cis- and trans-vinylene groups at 142" as brought about by the gamma irradiation in all of the different systems studied are illustrated in fig. 1 and 2. If the concentrations of the vinylene groups in pure Marlex-50 at the same total dose at 142" are subtracted from the measured vinylene concentrations of the Standard Oil Co. of Indiana (SOI) polyethylene, the residual vinylene concentra- tions given in fig. 3 can be seen to decline with dose according to the first-order law.The method of plotting the data in fig. 3 is mathematically equivalent to assuming the validity of the zero-order growth and first-order decay law given in a previous publication.2 At room temperature (r.t.) the first-order decay law no longer holds for the disappearance of the vinylene groups that were initially present in the SO1 polyethylene. Instead, the semi-empirical expression used 2 to describe vinyl decay at room temperature in Marlex-50 was again found to be valid, see fig. 4. In eqn. 2, [Vl] represents the vinylene group concentration in units of moles/g, k2, the first-order decay constant for the disappearance76 ENERGY TRANSFER IN POLYETHYLENE FIG. 2.-Decay o f vinylene unsaturation in 5 % polybutadiene + Marlex-50 mixtures at 142".Open circles, trans-po 1 y b u t a d i e n e mixture ; solid circles, cis-polybutadiene mix- ture, The cis-curve is uncorrected for isomer- ization which accounts for only 11 % of the total decay. The truns- curve has been correct- ed for normal vinylene growth in the Marlex- 50 f r a c t i o n of the mixture . FIG. 1 .-Growth and decay of trans-vinylene unsatura- tion at 142°C : solid circles, Mar lex - 5 0 ; ha 1 f -filled circles, SO1 polyethylene ; open circles, Marlex-50 + adiene mixture. 1.5 % trans-1 : 4-polybut- dose, eV g-1 x 10-20 I I 8 I2 I 4 dose, eV g-1 x 10-20M. DOLE AND T. F. WILLIAMS 77 I I 1.8 FIG. 3.-Vinyleneunsaturation 0 decay in SO1 polyethylene at t~ 142" after correction for nor- 3 ma1 vinylene growth. + \o 1.7 dose, eV g-1 x 10-20 I I I I I I 0.2 05 1.0 1 - exp (- k2D) FIG.4.-Decay of unsaturation at room temperature plotted according to eqn. (2) and (3). Open circles, vinyl groups in pure Marlex-50 ; solid circles, trans-vinylene groups in SO1 polyethylene.78 ENERGY TRANSFER I N POLYETHYLENE of vinylene groups randomly produced by the irradiation, and the subscripts zero, infinity and M refer to zero and infinite dose and to the pure Marlex-50 respectively. For SO1 polyethylene, eqn. (2) takes the form where k2 represents the first-order decay constant for the vinylene groups initially present. For polyethylene, the value of kZ given in table 2 for the decay of the vinylene groups TABLE z.-FIRST-ORDER DECAY CONSTANTS FOR DISAPPEARANCE OF UNSATURATION g/eV x 1021 material Marlex-SO so1 5 % polybutadiene trans cis group r.t . 142 O r. t. 142' 142 O 142 O vinyl 1.61 2 . 0 9 1 *42 1 -92 time time In Marlex-50 trans-vin ylene 0 . 5 2 0.64 0.46 0-65 1.2 initially present was calculated to be 0.46 X 10-21 g/eV. In this calculation, [Vllo, was necessarily taken to be zero. At low doses where the exponential term of eqn. (2) can be replaced by (1 - kzD), eqn. (2) reduces to the first-order decay law, eqn. (1). I I 5 1 0 dose, eV g-1 x 10-20 FIG. 5.-Decay of trans-vinylene unsaturation in polybutadiene + Marlex-50 mixtures at 142". Open circles, 5 % trans-1 : 4-polybut- adiene ; half-filled circles, 1.5 % trans-1 : 4-polybutadiene. Both curves corrected for normal trans- vinylene growth in the Marlex-50 fraction of mixture. If the trans-vinylene growth curve in the Marlex-50 fraction of the mixture as estimated from the data on pure Marlex-50 was subtracted from the measured trans-vinylene con- centrations of the 1.5 and 5 % trans-1 : 4-polybutadiene + Marlex-50 mixture, the residual vinylene group concentration at 142" decreased with dose according to the first-order law, fig.5. Fig. 6 illustrates the influence at 142" of 1.5 and 5 % trans- and 5 % cis- polybutadiene on the decay of vinyl groups in the Marlex-50 + polybutadiene mixtures. Finally the effect of the cis- and trans-polybutadiene on the vinyl group decay constant kl and on G(H2) is illustrated in fig. 7.M. DOLE AND T. F. WILLIAMS I I I I I I 5 10 15 dose, eV g-1 x 10-20 79 FIG. 6.-Vinyl unsaturation decay in Marlex-50 at 142" as affected by polybutadiene.Solid circles, pure Marlex-50 ; horizontally half-filled circles, 5 % cis-1 : 4-polybutadiene ; vertically half-filled circles, 1 -5 % trans-1 : 4-polybutadiene ; open circles, 5 % trans- 1 : 4-polybutadiene. 1 I I 4 5 yo polybutadiene FIG. 7.-Effect of added polybutadiene on G(H2) and kl for vinyl decay in Marlex-50 at 142". Open circles, trans-1 : 4-polybutadiene ; half-filled circles, cis-1 : 4-polybutadiene.80 ENERGY TRANSFER I N POLYETHYLENE The various decay constants are collected in table 2. Data for the decay of the cis- vinylene groups are not given for reasons to be discussed below. Actually, the observed rate of decay of the cis-vinylene groups in the 5 % cis-mixture was the same within the limits of experimental uncertainty as the rate of decay of trans-vinylene groups in the 5 % trans-mixture, see fig.2. Data for the decay constants of the trans- or cis-vinylene groups at room temperature in the 5 % polybutadiene + polyethylene mixtures are also not given because the cis- and trans-vinylene concentrations changed so little with dose that such changes could not be accurately determined. DISCUSSION For pure Marlex-50 at room temperature the high G-values for vinyl decay were probably the result of activation of the vinyl groups by excitation energy released in localized regions corresponding to the spurs of fast electron tracks followed by reaction to form end-links, as previously postulated.2 Processes resulting mainly from ionization were probably evolution of hydrogen, vinylene double-bond formation and cross-linking.It was also postulated that as vinylene groups were produced by the irradiation in the Marlex-50, mostly in the crystalline regions since Marlex-50 film is about 85 % crystalline, these randomly created vinylene groups served to protect partially the vinyl groups from further decay. It was estimated 2 that each sphere of excitation corresponding to the dissipation of 100 eV of energy was large enough to contain approximately 10 vinyl groups at the initial concentration of the latter. The concentration of vinylene groups at the dose when the vinyl decay began to deviate from the first-order law was about two per sphere of excitation. As far as could be told from the data, the decay of the randomly created vinylene groups followed the first-order law over the whole dose range.A Ziegler-type polyethylene was studied in which the initial vinyl concentration was about 0.2 of that in Marlex-50 ; nevertheless despite this low vinyl concentration the initial decay followed the first-order law until the same dose was reached at which vinyl decay deviated from the first-order law in the Marlex-50 case. These doses in the two cases corresponded to approxim- ately equal vinylene concentrations. Similar effects were observed in the studies on the SO1 polyethylene in which the initial unsaturation was all vinylene. In this case the decay of the initially present vinylene groups was first-order until the concentration of the randomly created vinylene groups had again reached a value of about two per excitation sphere.The similarity in behaviour of the vinyl groups of Marlex-50 and the initially present vinylene groups of SO1 polyethylene suggests that the latter must have existed either near the ends of the molecular chains or in the amorphous regions. If they had been randomly located throughout the crystalline mass of the polyethylene, one would have expected no difference in resistance to the gamma radiation between the vinylene groups initially present and those randomly created by the radiation. For the Marlex-50 + polybutadiene mixtures at room temperature, the very little influence of the trans- or cis-vinylene groups of the polybutadiene on the vinyl decay, hydrogen evolution, and cross-linking in the Marlex-50 fraction, indicates that transfer either of electric charges or of excitation energy from crystalline Marlex-50 to the amorphous regions containing the polybutadiene has a low efficiency.This is probably partly due to the segregation of the polybuta- diene in the amorphous regions ; on a sub-microscopic scale its distribution must have been far from random. The behaviour of the pure polyethylenes and the polyethylene + polybuta- diene mixtures in the molten phase at 142" was considerably different from the room-temperature behaviour. In the first place, the vinyl group decay followed the first-order law over practically the whole dose range. Although the initial G-value and the kl constant rose somewhat with temperature, it is possible that energy transfer was less efficient, but that the probability of end-linking followingM .DOLE AND T . F . WILLIAMS 81 activation was greater so as to give an overall greater decay constant. G(H2) and C (cross-linking), G(X), both rose with temperature.4 It is unlikely that such an increase was due to less energy transfer to the vinyl groups, because the rate of hydrogen evolution, at room temperature at least, was independent of vinyl group concentration. We attribute the greater G(H2) and greater G(X) at 142" com- pared to the room temperature values as due to greater segmental mobility of the CH2 chains at 142" which enabled neighbouring -CH2- groups or -CH*- free radicals to approach closely enough for C-C bond cross-links to form, and to a smaller caging effect of liberated hydrogen, either atomic or molecular.In a research only partly published,s Arvia and Dole demonstrated that increase of dissolved hydrogen in crystalline polyethylene lowered the hydrogen yield, pre- sumably because of a back reaction involving molecular hydrogen. This idea finds confirmation in the work of Soviet workers 6 who have shown that deuterium is radiologically built into the polyethylene when deuterium gas and polyethylene are irradiated together. At 142" the polybutadienes, either cis or trans, but trans- greater than cis-, had a marked effect in lowering G(H2) and the vinyl-group decay. In the liquid phase there is greater mobility of the polybutadiene segments as well as of the methylene groups. This greater mobility must increase the frequency of random collisions and make possible charge transfer or transfer of excitation energy.The ionization potential of olefins is of the order 9.2 V as compared to about 10.2 V for long chain normal paraffinic hydrocarbons. The reduction in G(H2) we at- tribute to charge transfer while the reduction in kl for vinyl decay probably resulted from partial transfer of excitation energy from the bulk of the Marlex-50 to the polybutadiene molecules rather than to the vinyl groups of the Marlex-50. In other words the vinylene groups of the polybutadiene competed with the saturated paraffinic chains for positive ions and with the vinyl groups for the energy of excitation. It should also be pointed out that in the liquid phase a more uniform dis- tribution of the polybutadiene must exist than in the solid mixture where both crystalline and amorphous regions can be recognized.The concept of localization of chemical effects has been applied with con- siderable success to a study of the direct effect of ionizing radiation on macro- molecules of biological importance.8.9 Although the actual mechanism of inactivation is in doubt, estimates of molecular size have been made with fair precision based upon the empirical assumption that each primary ionization and excitation zone occurring at random within a single macromolecule is sufficient to cause its inactivation. Recent work 10 on the protective effect of glutathione on the enzyme catalase when irradiated in the dry state bears a strong similarity to the results on the protective action of polybutadiene on the vinyl decay of Marlex-50 in the liquid state given here.Calculation of the decay constant for pure catalase yielded a value of 5 x 10-21 g/eV which is of the same order as that observed for the decay constants of unsaturated groups in polyethylene. Although the presence of the polybutadiene depressed the vinyl decay constant of polyethylene in the liquid state, the polybutadiene itself showed considerable damage in terms of the disappearance of its vinylene groups. This type of pro- tective action in which the protector exhibits sacrificial damage is similar to the effect noticed by Alexander and Charlesby 11 on mixtures of polymethyl meth- acrylate and aniline. They also showed that aromatic ring systems built into polymers or hydrocarbons lowered the radiation damage in terms of either main chain breakage or cross-linking between chains.All the evidence would suggest that migration of chemical activity in large molecules ensuant upon irradiation in the solid state is largely the result of excitation or ionization transfer processes. The effect of added cis- and trans-1 : 4-polybutadienes in lowering G(H2) for polyethylene in the liquid state is qualitatively similar to the results obtained by82 ENERGY TRANSFER I N POLYETHYLENE Manion and Burton 12 for mixtures of cyclohexane + cyclohexene and cyclo- hexane + benzene. Their data were explained in terms of ionization and ex- citation transfer from the component of higher ionization or excitation potential to that of the lower. Benzene has lower values than cyclohexane and exhibits a form of self-protection in which the excitation energy is degraded to heat by internal conversion without accompanying chemical effects. It is clear that although polybutadiene participated in significant energy transfer from poly- ethylene, the subsequent activation of the polybutadiene resulted in extensive chemical changes within the molecule.Thus, although the net G(H2) was lowered there was a much increased disappearance of total unsaturation for the mixture. Energy transfer processes are therefore not always beneficial in terms of a decrease in total decomposition values, but depend on the inherent molecular properties and the variety of processes leading to the depopulation of excited states. Other possible mechanisms for vinyl and vinylene decay such as isomerization, scavenging by free radicals or atomic hydrogen, activation by ionic migration, etc., have been considered, but space does not permit a detailed explanation of why these mechanisms are not considered to be significant in the radiation chemistry of polyethylene and polyethylene + polybutadiene mixtures. Support of this research by the U.S. Atomic Energy Commission and the gift of materials by the Phillips Petroleum Company and the Standard Oil Com- pany (Indiana) are gratefully acknowledged. 1 Dole and Keeling, J. Amer. Chem. SOC., 1953, 75, 6082. 2 Dole, Milner and Williams, J. Amer. Chem. Soc., 1958, 80, 1580. 3 Lawton, Balwit and Powell, J. Polymer Sci., 1958, 32, 257. 4 Williams and Dole, submitted for publication. 5 Dole, Williams and Arvia, Proc. Int. Conference Peaceful Uses of Atomic Energy 4 Varshavskii, Vasilev, Karpov, Lazurkin and Petrov, Doklady Akad. Nauk. S.S.S. R., 7 Field and Franklin, Electron Impact Phenomena and the Properties of Gaseous Ions 8 Lea, Actions of Radiations on Living Cells (Cambridge Univ. Press, 2nd ed., 1955), 9 Pollard, Adv. Biol. Med. Phys., 1953, 39, 53. 10 Norman and Ginoza, Radiation Res., 1958, 9, 77. 11 Alexander and Charlesby, Nature, 1954, 173, 578. 12 Manion and Burton, J. Physic. Chem., 1952, 56, 560. 13 Miller, Lawton and Balwit, J. Physic. Chem., 1956, 60, 599. (Geneva, 1958), paper 818. 1958, 118, 315. (Academic Press, N.Y., 1957), p. 122. p. 69.

ISSN:0366-9033    

DOI:10.1039/DF9592700074

出版商:RSC    

年代:1959

数据来源: RSC