Effect of Solvent and Temperature on Proton TransferReactions of Excited MoleculesBY H. BEENS, K. H. GRELLMANN, M. GURR AND A. H. WELLERChemisch Laboratorium der Vrije Universiteit, Amsterdam, NetherlandsLaboratorium fur physikalische Chemie der Techn. Hochschule, Stuttgart, GermanyReceived 12th February, 1965The spectral difference in fluorescence emitted from the forms (I), (11) and (III) has been usedto study proton transfer reactions of electronically excited molecules :* * *o 0(I) (10 (111)AH+B+AH.. . B-tA . . . HE3by fluorescence measurements. With intermolecular hydrogen bonds, pronounced effects ofsolvent and of temperature have been observed which indicate that in this case proton transferis strongly facilitated by an increase in polarity of the surrounding medium.In polar solventsconsiderable changes in solvent orientation are required and can become rate-determining, par-ticularly at low temperatures. With intramolecular proton transfer equilibria the solvent effect isweaker and in the opposite direction. This is attributed to compensation by intramolecular elec-tron migration of the dipole moment of form (111). An upper limit of 0.12 kcal/mole has beenobtained for the activation energy of the intramolecular proton transfer.Proton transfer in solution from an acid AH to a base B, to yield the conjugatebase A- and the conjugate acid HB+, quite generally, can be considered to occurin three steps :diffiusion reaction diffusion AH+B 4 A H . . . B --, A - . . . HB+ + A-+HB+.The rates of the first and third step, both being diffusional, depend upon the diffusioncoefficients of the species involved, and on the dielectric constant of the mediumwhen both partners are ions.The second step is the actual proton transfer reactionin which solvent molecules may participate as transferring agent and/or becauseof the reorganization of the solvation spheres involved. The investigation describedbelow was undertaken as part of a programme having the aim to elucidate themechanism of the internal proton transfer process in a number of typical cases.In the first part of this series 1 the rate of protolysis of ammonium derivatives ofpyrene (cf. table 1) in the excited state was examined in alcoholic solvents betweenroom temperature and 100°K.It was found that the reaction velocity and itstemperature coefficient depended on the nature of the solvent, but not on the strengthof the acid which varied from 102 to 106 moIe/l. This could be explained by theassumption that the rearrangement of surrounding solvent molecules, prior to theproton transfer is rate-determining, which implies proton-tunnelling between Franck-Condon states.The fluorescence method which is also applied in this study makes use of theinformation, which can be obtained from fluorescence spectra and fluorescenceintensities, about the kind and relative amount, respectively, of any particular(1) (11) (IV)181 84 PROTON TRANSFER REACTIONS OF EXCITED MOLECULESspecies occurring in the excited (fluorescent) state.The experimental methods havebeen described.1-3 With few exceptions the concentration of the fluorescing substancenever exceeded 2 x 10-4 mole/l.1.0I 30.5020 25 30v (103 cm-1)FIG. 1 .-Fluorescence spectra of ,%naphthol in various solvents : methylcyclohexane, - ;dioxane, - - . --f triethylamine, ---a ; alkaline alcohol, . . . . . . . . ; n-butylamine, ---aI I I1.0-4' 0.50175 20 2 2.5 25 2 75v (103 m-1)FIG. 2.-Fluorescence spectra of 3-hydroxypyrene in various solvents : methylcyclohexane, - ;dioxane, - - ; triethylamine, -.-a ; alkaline alcohol, . . . . . . . . ; n-butylamine,- - aThe fluorescence spectra of P-naphthol and 3-hydroxypyrene in a number ofdifferent solvents are shown in fig. 1 and 2. The absorption spectra of these com-pounds show little variation in these solvents ( < 700 cm-1 with /?-naphthol anH .BEENS, K . H . GRELLMANN, M. GURR AND A. H . WELLER 185< 500 cm-1 with 3-hydroxypyrene) except in alkaline alcohol, where the absorption(and also the fluorescence) is due to the ionized form, which can be described asthe hydrogen bond complex ArO- . . . HOR. On the other hand, in methyl-cyclohexane, the free hydroxycompound ArOH is present, so that the spectra ofTABLE l.--CHANGE OF pK ON EXCITATION (ROOM TEMP.)/3-napht hol3 -hydr oxyp yrene4-ammoniumpyrene (in ethanol)3-ammoniumpyrene-5,8,1O-trisulphonatebenzoic acid cationacetophenoncationPK PK-~"K9.5 6.78.7 5.03.5 5.52.6 9.7- 7.2 - 6.0- 6.0 - 9.0fig. 1 and 2 can be ascribed to the following excited species, with the wavelength offluorescence decreasing from left to right :* ocC2 ">; A o H*o H *o *o 0ArO .. .HN; ArO . . .HOR; ArO . . . HNR3; ArOH.. .a c2 lH4The gradual red shift in this series is a direct consequence of the gradual changeof the whole substituent,? whose ionization potential decreases in going from -OH0 Ht o - 0 . . . HN.RThis assignment, together with the negligible change in absorption, implies thatin amines as solvents the ionization of the hydroxy-compound occurs only in theexcited state. This is due to the well-known 3-5 change on excitation of acid-baseproperties of substituted aromatic compounds. In many cases the pK (pK valuein the excited state) differs substantially from the ground-state pK.An estimateof the difference, pK-pK, can be obtained on the basis of eqn. (1)***AH -AH (kcal/mole) = 2.86 x Av (cm- '), (1)which relates the reaction enthalpies of the acid-base equilibria in the ground andexcited states to the frequency shift Av of the fluorescence or long wavelength ab-sorption band in going from the conjugate base to the acid. With the assumptionthat the corresponding entropy changes are equal (AS = AS), one obtains**pK-pK = (0*625/T)Av (cm-l);some values relevant to this study are given in table 1.INTERMOLECULAR PROTON TRANSFERIn inert solvents a hydrogen-bond complex is formed between acid and base:KassAH+Br=tAH.. . B. (3)Nagakura and Baba 6 have shown that the absorption spectra of 1 : 1 hydrogen-bond complexes of phenols with suitable bases are shifted to longer wavelengthsi including base (or acid) attached to it by hydrogen bonds186 PROTON TRANSFER REACTIONS OF EXCITED MOLECULEScompared with the absorption spectra of the unbonded species.The shift Avass istypically about 500 cm-l and indicates a stronger hydrogen bond in the excited state.Thus additional complex formation according to* kiz *AH+B+AH.. . B (4)can occur in the excited state. The rate constant klz (or, rather, the relative rateconstant klzz, where z is the lifetime of the excited acid AH) can be obtained fromfluorescence measurements at different concentrations of B making use of the spectraland intensity differences of the fluorescence of complexed and uncomplexed acid.The method has been outlined 3 ~ 7 for hydrogen-bond complexes with pyridine anda-chloropyridine, where, on account of the hydrogen-bond complexes being non-fluorescent, the procedure is particularly simple.As the fluorescence behaviourof the hydrogen-bond complexes, considered here, was more complicated, as canbe seen, for example, from fig. 3 and 6, a modified method had to be applied, detailsof which will be given elsewhere.8 In any case, knowledge of Kass (the associationconstant in the ground state) as well of &I/&, the ratio of the extinction coefficientsat the excitation wavelength of AH . . . B and AH, respectively, is required, inorder to calculate the fraction*of directly excited complexes. Fluorescence measurements yield the quantity &the degree of association that eventually, during the lifetime of the excited acid,will be reached and which according to eqn.(6),also depends on the rate of hydrogen bond formation in the excited state. Eqn. (6)has been derived 3 with the assumption that dissociation of the excited hydrogen-bond complex is negligible.? Corresponding values of k12~ (cf. table 2) of P-naphtholTABLE L-DATA ON HYDROGEN-BOND FORMATION IN GROUND AND EXCITED STATESAT 22"acid base Avms (cm-1)cB (M), wherea% 0.99in methylcyclohexane (q = 0.712 cpoise)/I-naphthol triethylamine 120 700 110 0.078n-butylamine 170 680 (> 50) 0.0963-hydroxypyrene triethylamine 180 360 205 0.05 1n-butylamine 230 320 (> 100) 0.07 1(0.010) 0in toluene (q = 0.572 cpoise)fl-naphthol t riethylamine 42 670 45 0.2073-hydroxypyrene triethylamine 60 350 115 0,113in o-chlorotoluene (q = 1.02 cpoise)3-hydroxypyrene triethylamine 1 10 350 145 0.0750 estimate for 230°K.t An estimate with the aid of eqn.(2) using data of table 2 gives k217'<@0511. REENS, K . H . GRELLMANN, M . GURR AND A . H . WELLER 187and 3-hydroxypyrene, respectively, differ by about a factor of two. This is due toa similar difference in lifetime.From the data of table 2, a-values at room temperature can be obtained. Inthe last column, base concentrations have been calculated at which practically allacid molecules are present as hydrogen-bond complexes in the excited state.The change of fluorescence spectra of P-naphthol in methylcyclohexane at roomtemperature on addition of triethylamine is shown in fig.3. Up to about 0.1 h4*F i I I30.5020 25 30FIG. 3 .-Fluorescence spectra of j?-naphthol in methylcyclohexane at different concentrations of trie-thylamine 1,O.OOO M ; 2,0-002 M ; 3,0404 M ; 4,0408 M ; 5,O-020 M ; 6,0*10 and 0.1 5 M ; 7, assolvent.triethylamine the change undoubtedly is due to hydrogen-bond formation. How-ever, the broadness of the spectrum observed at and above 0.1 M triethylamine,when practically all excited naphthol molecules are complexed, indicates that thisspectrum is due to more than one emitting species only. Comparison with thefluorescence spectrum in pure triethylamine suggests the contact ion-pair (In) tobe the other component.Fluorescence quenching experiments with oxygen showthat both components are equally quenched. This indicates that the proton-transferequilibrium (eqn. (7)) :between the forms (II) and (111) is fully established within a time which is shorterthan the mean lifetime of the excited molecules.Lowering of the temperature, however, changes the fluorescence spectrum ofthe hydrogen-bond complex in favour of the ion-pair (111), as shown in fig. 4. Theintensity ratio at two sufficiently separated frequencies (22,500 cm-1 and 28,000 cm-1)can be used as a relative measure of the equilibrium constant K23. Values of188 PROTON TRANSFER REACTIONS OF EXCITED MOLECULES*log K23 obtained in this way from the spectra of fig. 4 (and from others at intermediatetemperatures) yield AH23 = -0.9 kcal/mole when plotted against T-1. Thisexothermicity is further evidence for a proton-transfer equilibrium (7) which ispractically established.Fig.4 also shows the fluorescence spectrum in toluene of the hydrogen-bondcomplex P-naphthol-triethylamine. Again, the amine concentration is high enough*1.00.5 301 I IToluene Met h y L c yc lohexa n e20 25 30v (103 cm-1)FIG. 4.-Fluorescence spectra of 8-naphthol in methylcyclohexane+ 0-14 M tritthylamine at differenttemperatures, and in toluene+ 0.20 M triethylamine.to prevent fluorescence of uncomplexed P-naphthol to be observed. The same istrue in pure triethylamine (cf. fig. 3). In both solvents, the long wavelength com-ponent due to the ion-pair (III) is much more pronounced than in methylcyclo-hexane.Relative values of the equilibrium constant K23 in these three solvents,obtained from the intensity ratio of the two components, are given in table 3.*TABLE 3.-RELATIVE EQUILIBRIUM CONSTANTS FOR INTER- AND INTRA-MOLECULAR PROTON TRANSFER M THE EXCITED STATE.(relative to toluene, for which all equilibrium constants arbitrarily were put equal to 10)solvent*K intranu nOH 0OH 0 I II II C*K23 with triethylamineconstant diel. % 20 k:Etrol f-hydroxy- ArOH = ((AoR (>'\ORpyreneOGHSmethylcyclohexane 2.02 1.4241 1.5 t l 18 12toluene 2.38 1.4968 10 10 10 10triethylamine 2.42 1.4003 7.7 7.6o-chlorotoluene 4.45 1.5255 90acetonitrile 37.4 1.3441 4.5 H. BEENS, K.H. GRELLMANN, M. GURR AND A . H. WELLER 189The fluorescence spectra in methylcyclohexane and toluene of the hydrogen-bond complex 3-hydroxypyrene-triethylamine are shown in fig. 5. The changecaused by lowering the temperature is much less significant than with /3-naphtholFIG. 5.-F1MethylcyctohexaneI 20.5020 25 30v (103 m-1)uorescence spectra of 3-hydroxypyrene in methylcyclohexane+ 0.14 M triethylaminein toluene+ 0.126 M triethylamine.and- 0.5 4n 0020 24 28v (103 cm-1)FIG. 6.-Fluorescence spectra of 3-hydroxypyrene in o-chlorotoluene at different concentrationsof triethylamine.1, 000 M ; 2, OOOO9 M; 3, 0.0036 M ; 4, 0.0113 M ; 5, 0.21 190 PROTON TRANSFER REACTIONS OF EXCITED MOLECULES*and can be interpreted as a slight sharpening of the structure. Thus, AH23 is eitheraround zero or strongly positive.The latter assumption implies that the spectrumin methylcyclohexane is solely due to the form (11). In toluene, evidently, the ion-pair (111) is favoured over form (II) and the same is true in pure triethylamine (cf.fig. 2). This effect is still more pronounced in o-chlorotoluene, as shown in fig. 6,where at triethylamine concentrations high enough for complete association in theexcited state, almost exclusively the emission due to the ion-pair is observed.Relative values of the equilibrium constants K23 in these solvents, calculated fromthe intensity ratio at 21,000 cm-1 and 25,600 cm-1, are given in table 3. Resultsvery similar to these with triethylamine have also been obtained with N-dimethyl-benzylamine.9 Whenever tertiary amine was added, the changing fluorescencespectra had at least one isosbestic point in common, the spectra in fig. 3 and 6 beingtypical examples.*17.5 22-5v (103 cm-1)27.5FIG.7.-Fluorescence spectra of 3-hydroxypyrene in methylcyclohexane+ n-butylamine at 230°K.1, 0.01 M ; 2, 0.20 MDifferent results, however, were obtained with primary amines. Since the effectcan be demonstrated more clearly at low temperatures, fluorescence spectra obtainedat 230°K are shown in fig. 7. At this temperature the viscosity of methylcyclo-hexane is 2-27 cpoise, i.e., only about three times larger than at room temperaturewhereas the association constant Kass is increased by more than a factor of 30.Thus at a concentration of 0.01 M n-butylamine practically all 3-hydroxypyrenemolecules are present in the complexed form (11).This is borne out by the fluor-escence spectrum showing a red shift of some 400 cm-1 from that in pure methyl-cyclohexaiie (cf. fig. 2). Increase of the butylamine concentration causes the gradualappearance of a new fluorescence component at longer wavelength at the expenseof the original one. The close similarity of this long wavelength emission with thaH . BEENS, K . H. GRELLMANN, M. GURR AND A . H. WELLER 191obtained in pure n-butylamine, combined with the failure of tertiary amines to bringabout this same effect, suggests an interpretation according to reaction (8) :* H k24 * @ €3 @HArOH.. . NHi-RNH,-+ArO .. . H N . . . HNH (8)R R R(11) W’>From the fluorescence spectra of fig. 7 and others at intermediate base con-centrations, k24~11=30 M-1 could be obtained, where ZII is the lifetime of the com-plexed form (11) which probably does not differ very much from that of the excitedmolecule in pure methylcyclohexane. This result is compatible with a diffusion-controlled reaction rate in methylcyclohexane at 230°K.INTRAMOLECULAR PROTON TRANSFERThe fluorescence spectra of compounds with intramolecular hydrogen bonds,like those given in fig. 8 have been explained previously 3 9 10 by the assumption ofthe protomeric isomerization equilibrium (9)Q OOH 0 0 HOI II * I IImet hy Icy clohexanen-butylchloride --toluene __ .-acetonit rileethanol _-- .... -VOH 01 M inmet h y lc yclo hexanen-butylchloride - - -toluene - -_acetonitrile __ . -OC2H5-__FIG. 8.-Fluorescence spectra of salicylic ester and 5-ethoxysalicylic ester in diserent solvents atroom temperature192 PROTON TRANSFER REACTIONS OF EXCITED MOLECULESwhich is established during the lifetime of the excited state. The intramolecularproton transfer is made possible by the mutually opposite effect of excitation on theacid-base properties of the two substituents. As indicated in table 1, the hydroxy-group becomes more acidic and the carbonyl group more basic on excitation.The fluorescence spectra shown in fig. 8 consist of two components. The shortwavelength component, whose spectral position is very similar to that of the cor-responding o-methoxy derivative, is evidently due to form (V), whereas the othercomponent can be ascribed to the form (VI).Relative values of the equilibriumconstant in various solvents or at different temperatures can be obtained from theintensity ratio of the two fluorescence components. Intensities measured at21,800 and 27,500cm-1 have beenusedfor salicylic ester and at 20,000 and 25,300 cm-1for the ethoxy derivative. Relative Kintra-values in some solvents at room temper-ature as obtained from the fluorescence spectra in fig. 8 are given in table 3 . Valuesof AHinka, obtained through van't Hoff-plots from fluorescence measurements be-tween 300 and 200°K in methylcyclohexane are :***AHintra = -0.7 kcal/mole for salicylic ester,AHintra = + 0.9 kcal/mole for 5-ethoxy-salicylic esterThus, lowering of the temperature shifts equilibrium (9) to the right-hand side forsalicylic ester and in the opposite direction with the 5-ethoxy derivative.Fluorescence measurements with salicylic ester in methylcyclohexane have beenextended 'f to still lower temperatures.It has been found that the spectra at liquid-nitrogen and at liquid-helium temperatures were identical within the precision limitsof the measurement. The almost complete absence in these spectra of the shortwavelength component is of particular interest in view of the activation energy involvedwith proton transfer across hydrogen bonds.*DISCUSSION AND CONCLUSIONS*According to the data in table 1, the pK value of 3-hydroxypyrene is about3.7 compared with 2.8 for P-naphthol. The latter pK has also been obtained frommore exact kinetic measurements.2, 3 In view of the close structural similarityof the two compounds it seems safe to assume that this, for the greatest part, isan intrinsic difference in acid strength, which also exists in solvents other than water.In other words, excited 3-hydroxypyrene may be considered in any solvent as anacid weaker than excited /I-naphthol by some (1.35(3.7 - 2-8) =) 1-2 kcal/mole.This, in fact, is borne out by the results in methylcyclohexane which show thatAH23, the enthalpy difference between ion-pair (111) and hydrogen-bond complex(11) with triethylamine, for 3-hydroxypyrene is at least 0.9 kcal/mole more positivethan for /I-naphthol.Unfortunately, owing to uncertainties in pK-pK, the pK value of 3-hydroxy-pyrene is not known accurately enough to allow mutual comparison of K23 valuesin column 4 and 5 of table 3.Comparison of K23 values is thus confined to withineach column.t The measurements have been carried out in the laboratory of Prof. Dr. H. C. Wolf, whoseco-operation is gratefully acknowledged.*** **H. BEENS, K . H. GRELLMANN, M. GURR AND A . H. WELLER 193This comparison clearly shows that the equilibrium constants for intermolecularproton transfer strongly increase with solvent polarity. This is evidently due tothe considerable increase of the dipole moment when the ion-pair is formed.The presence of hydrogen atoms, available for additional, although weak,hydrogen bonding in primary (and possibly secondary) amines seems to be a relevantfactor as shown by reaction (8).Evidently, the base-shared ion-pair (IV’) is morestable in this case than the contact ion-pair (III’),* @ @H EHArO . . . HNH . . . NHR R(m’)from which a fluorescence spectrum at shorter wavelengths, similar to that in tri-ethylamine would be expected. Actually, fluorescence spectra of this type areobtained from rigid solutions of both P-naphthol and 3-hydroxypyrene in glassyn-butylamine at temperatures below 150°K. It seems that in these rigid solutionsthe dielectric relaxation time, being longer than the lifetime of the excited state,prevents the proton from further migration.With intramolecular proton transfer equilibria a solvent effect opposite in direc-tion to that with intermolecular hydrogen bonds is observed (cf. table 3), indicatinga decrease in dipole moment when the proton is transferred. This strongly suggestsconsiderable intramolecular charge transfer in the excited state of these salicylicesters, SO that the following structures are possibly a more realistic representation :0 0OH 0 0 HO(V‘)As must be concluded from the fluorescence spectra, there is practically completeproton transfer in salicylic ester (dissolved in methylcyclohexane) even at 4~2°Kwithin lo-gsec, the lifetime of the excited state. Thus the transfer rate constantat this temperaturemust be larger than 108 sec-1. With VOH = lO14sec-1 (frequency of the OHstretching vibration) one obtains E I 0-12 kcal/moIe, a result which strongly suggestsproton-tunnelling as the actual transfer mechanism.k = vOH exp (- E/R .4*2) (10)1 Urban and Weffer, Z. Elektrochem., 1963, 67, 787.2 Weller, 2. pliysik. Chem., 1955, 3, 238.3 Weller, Progress in Reaction Kinetics, ed. Porter (Pergamon Press Ltd., London, 1961), vol. I,4 Forster, Z. Elektrochem., 1950, 54, 42, 531.5 Weller, 2. Elektrochem., 1957, 61, 956.6 Nagakura and Baba, J. Amer. Chem. SOC., 1952,74, 5693.7 Grellmann and Weller, 2. Elektrochem., 1960,64, 145.8 Grellmann, Gurr and Weller, in preparation.9 Grellmann, Diss. (Stuttgart, 1960). Gum, Diss. (Stuttgart, 1962).G187.10 Weller, Naturwiss., 1955, 42, 175 ; Z. Elektrochem., 1956, 60, 1144