J. CHEM. SOC. FARADAY TRANS., 1994, 90(7), 963-968 Triplet of Cyclooctatetraene:Reactivity and Properties Tomi Nath Das* and K. lndira Priyadarsini Chemistry Division, Bhabha Atomic Research Centre, Bombay-400085, India Using 7 MeV electron pulses from a linear electron accelerator, the triplet excited state of cyclooctatetraene (COT), a commonly used triplet scavenger in dye lasers, has been characterised in hydrocarbon solvents. In deoxygenated cyclohexane and benzene, 3COT shows an absorption due to a triplet-triplet (T-T) transition between 300 and 400 nm with a peak at 350 nm in cyclohexane and at 360 nm in benzene, and with a lifetime of 100 ps in cyclohexane. Unlike many other solute triplets, 3COT exhibits comparatively low reactivity towards molecular oxygen and the measured rate constant was 3 x lo7 dm3 mol-' s-'.The triplet nature of the transient was confirmed by T-T energy-transfer processes from triplet donors like biphenyl, p-terphenyl and pyrene in a deoxygenated cyclohexane matrix and the molar absorptivity of 3COT at 350 nm was estimated to be 7000 k300 dm3 mol-' cm-'. In benzene, the T-T energy transfer rate constants using various 7-aminocoumarin laser dyes as donors were estimated. These values (of the order of lo9 dm3 mol-' s-') are 6.5 for Coumarin 47 (C47), 5 for C102, 6 for C120, 2.8 for C152 and 1.5 for C153. Similarly, rate constants were also measured using aromatic triplet donors. These values in the same order are 3.8for p-terphenyl, 13.3 for biphenyl, 3.3for both naphthalene and benzil, 0.74 for pyrene, 0.3 for anthracene and 0.7 for acridine.Based on the results of equilibrium studies in cyclohexane with 3anthracene as the donor, the energy of the first excited 3COT (ET)was estimated to be 41 L-1 kcal mol-'. Conjugated non-aromatic cyclic polyenes (CP) show unique properties in their excited states.lT2 These molecules, in both their singlet and triplet excited states, absorb in the UV region. The respective energy gaps between these states are large, the triplet absorption cross-sections are low and the corresponding energy levels are not yet precisely known, although they are assumed to lie in the region of 50 kcal mol-' (ET).3-5These properties allow these polyenes to be used as quenchers for the triplet excited states of various dyes used in laser systems.During the operation of a dye laser, the triplet excited levels of the dyes are populated along with the singlet states, which has a detrimental effect on their This happens because the fraction of the excited singlet state population responsible for the laser action becomes deacti- vated by the intersystem-crossing mechanism. These triplet dye molecules exhibit broad optical absorptions due to T-T transitions with high molar absorptivities. This results in sig- nificant loss of laser efficiency which becomes prominent when it operates in the CW or long-pulse mode.' Molecular oxygen is considered to be an efficient triplet scavenger, but the resulting singlet molecular oxygen produced by energy transfer from dye triplets is frequently responsible for the fast degradation of the dye molecules and the formation of non- fluorescent product^.^.'^ Hence, it is essential to use a suit- able triplet scavenger, which may preferentially remove the triplet dye molecules without interfering with the laser effi- ciency.Cyclopolyenes such as cyclooctatetraene, cyclohexadiene and cycloheptatriene fulfil these unique triplet properties and have been employed for efficient dye laser operation in the past.' '-I4 Many laser dyes have shown improved per-formance in the presence of these additives. Pappalardo et al.11*12could generate long laser pulses from Rhodamine-6G solutions using mmol dm -quantities of cyclooctatetraene (COT) as triplet scavenger. They observed that COT was as effective as oxygen for this purpose.On the other hand, Marling et a1.13 observed that dye laser output from RhodaminedG in the presence of COT was comparatively more than its output in the presence of oxygen. Thus, COT has been well accepted as an efficient triplet scavenger in dye- laser systems.'' However, the detailed mechanism for the scavenging process of dye triplets by COT is still not well understood. In addition, other parameters like the T-T absorption spectrum of 3COT and its molar absorptivity and lifetime in different solvents are not yet available in the liter- ature. In this study, triplet excited states of COT were produced and detected in cyclohexane and benzene solvents using nanosecond pulse radiolysis.Its T-T absorption spectra, life- time, reactivity with oxygen, energy-transfer reactions from a variety of donor triplets and the corresponding rate con-stants, as well as the energy level of its first excited triplet state were quantified. Experimental Laser-grade dyes Coumarin 47 (C47), Coumarin 102 (C102), Coumarin 120 (C120), Coumarin 152 (C152) and Coumarin 153 (C153) were obtained from M/s Lambda Physik and were used without any further purification. Scintillation-grade p-terphenyl, made at BARC was used for the studies. Biphenyl and benzil obtained locally from M/s SISCO were recrystallised from methanol. Pyrene, naphthalene, azulene, and anthracene, obtained from M/s Fluka, and acridine, obtained from M/s Sigma, were used as received.Spectro- grade benzene and cyclohexane obtained from M/s Spectro- chem India were purified by passing through an activated alumina column followed by distillation. Iolar grade N, , N,O and 0,used for various studies were obtained from M/s Indian Oxygen Limited. Stabiliser-free working solutions of COT, obtained from M/s Aldrich, were prepared as follows. A fixed volume of the solvent benzene (or cyclo- hexane, 10 cm3) was first deoxygenated in a vacuum line fol- lowing a repeated freeze-pumpthaw technique. Then the required amount (1 x lo-' cm3) of COT was distilled on-line under vacuum onto the frozen solvent matrix. On warming it produced the required stock solution of 1 x lo-' mol dm-3 COT in the solvent.This oxygen-free stock solution was stored inside a refrigerator and was stable for one month. Pulse radiolysis studies were carried out using 7 MeV elec- tron pulses from a linear electron accelerator providing pulses of 50 ns and 2 ps duration. The triplet excited states of various molecules produced were detected using a kinetic J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 spectrophotometric setup. The details regarding this setup have already been p~blished.'~~'~ The absorbed doses were determined with a thiocyanate dosimeter (1 x lo-, mol dm-3 KCNS aerated solution in water (using the value of GE= 2.23 x m2 J-' at 500 nm from the literature.'* The errors encountered in the measurement of various parameters varied between f10 and 15% and are not re- ported separately.Any pulse to pulse variation of the absorbed dose was minimised by averaging of the oscillo- scope traces. Results and Discussion Pulse radiolysis in hydrocarbon solution ultimately results in the formation of solvent excited states. because the primary species produced initially from ionization eventually undergo ion rec~mbination.'~ This happens because of the low rela- tive permittivity of the matrix which does not help the ions to escape from their mutual Coulombic attraction. The excited states of the solvents undergo a number of photophysical processes and sometimes return to the ground state.20 In addition to these reactions, if a solute is present in the solu- tion, these excited species transfer their excitation energy and generate the excited states of the solute.Thus, pulse radiolysis of hydrocarbon solutions provides an excellent and indirect method to produce solute excited states, especially triplets, which cannot be produced by other conventional techniques like flash photolysis. Solvents like benzene and cyclohexane are generally employed for such purposes, and the former is sometimes preferred more over the latter owing to the higher yields of triplets available., ' The typical steps involved in the formation of a solute triplet (3S) in a hydrocarbon (HC) such as cyclohexane or benzene are as follows : HC -*+ HC' + e-HC+ + e-'HC*, 3HC* --+ HC' + S-HC + S+ e-+S-S-* 3 *s++s-,I s, s 3HC* + S -HC + 3S* While in cyclohexane, ion recombination is an important process, in benzene, energy transfer is considered to be mainly responsible for triplet f~rmation.~~.~~ In this paper, while the T-T absorption spectra of 3COT were measured in both solvents, its molar absorptivity was estimated in cyclohexane.The energy-transfer studies with laser dyes and other donors were carried out in benzene with an improved signal to noise (S/N) ratio, owing to the higher triplet yields present. The absorption spectra of the transient, generated by the radiolysis of deoxygenated solutions of 1 x lov2mol dmW3 COT in cyclohexane and 2 x lo-* mol dm-3 COT in benzene, are presented in Fig. 1. These spectra show similar broad absorption profiles between 300 and 400 nm, with peaks at 350 nm in cyclohexane and 360 nm in benzene.The inset in Fig. 2 shows the kinetic trace for the first-order decay of the transient in cyclohexane at a low dose of 7 Gy, and its lifetime in cyclohexane was measured as 100 ps (k, = 1 x lo4 s-'). Although, the spectrum obtained in benzene strongly suggests the transient to be 3COT, from these measured parameters alone in cyclohexane, the possibility of formation of a different transient other than the 3COT, such as COT' or COT-, could not be ruled out. In order to confirm the nature of the transient, a cyclohexane solution of COT was 0.040 -lo.012 0.036 3 0.031 I u-0.027 t0.022 290 320 350 380 410 wavelength/nm Fig.1 Absorption spectra of 'COT generated by pulse radiolysis of an N,-saturated solution of 1 x mol dm-' COT in cyclo- hexane (0)at 60 Gy dose and 2 x lo-' mol dm-' COT in benzene (*) at 100 Gy dose saturated with N20, a frequently used electron scavenger, and the transient behaviour compared with cases where COT solutions were either deoxygenated or oxygen saturated. In Fig. 2, the transient decays under these conditions are pre- sented. The transient was observed to decay at a faster rate (k, = 2 x lo5 s-') in the presence of dissolved molecular oxygen (under saturation), while in the presence of dissolved N20,although the yield of the transient at 350 nm decreased by ca. 40%, the decay characteristics (k, = 2 x lo4 s-') did not differ appreciably from the transient decay obtained in the case of deoxygenation (nitrogen saturation).Under similar experimental conditions in the presence of dissolved N,O in cyclohexane, studies with a standard system such as anthracene revealed that the triplet anthracene yield decreased by a similar value. In both of these cases, since the decay kinetics of the transient remained unaffected and remained as under N,-saturated conditions, radical anion nature of the transient was ruled out. The absorption spectra of COT anion and dianion are reported in the literat~re~~" and these are distinctly different from the spectra presented in Fig. 1. Similarly, the possibility of formation of the radical cation of COT under these experimental conditions was neg- ligible, because in the presence of an electron scavenger like N,O, the yield of cation should have increased to a large extent and its decay slowed down. The decrease in the yield in the presence of 0, (electron scavenger) and the formation of a similar transient in benzene (cation scavenger) thus confirm its triplet nature.0.01 5 I _1 200a 0.005 0.ooop -0.005l I I I I I -0 20 40 60 80 100 120 time/ps Fig. 2 Oscilloscope traces of 'COT decay at 350 nm under (a)N, saturation, (b) N,O saturation and (c) 0, saturation at low dose. Inset: First-order decay of 'COT in cyclohexane under N, saturation and 7 Gy dose. J. CHEM. SOC.FARADAY TRANS., 1994, VOL. 90 The reactivity of 3COT with oxygen was determined by mixing different volumes of aerated cyclohexane in deaerated COT solutions at a fixed concentration, using the value of oxygen concentration in cyclohexane under air-saturated conditions of 2.3 x lop3mol dmP3 from the literat~re.~~ The rate constant for the reaction of 3COT with oxygen was esti- mated to be 3 x lo7 dm3 mol-' s-'.In the absence of dis- solved oxygen, at a higher absorbed dose (60 Gy or higher), the decay of 3COT followed second-order kinetics [2k/ ~l=5.qk2.5)x lo6 s-'1 which changed to pseudo-first-order kinetics (k, = 1 x lo4 s-') at low doses (7 Gy). At higher doses, the larger concentration of 3COT produced probably decayed by self-quenching or disproportionation processes, which became insignificant at low doses with matrix de-excitation processes taking over.Therefore, a change in the decay kinetics from second order to pseudo- first order, as observed above, is expected. Energy-transfer Studies In order to confirm further the triplet nature of the transient absorbing at 350 nm and estimate its E,, triplet energy, the following energy-transfer studies were carried out. The triplet-triplet energy-transfer process is known to proceed by an exchange mechanism.25 The donor (D) and acceptor (A) molecules collide with each other and form a collision complex. The overlap of their electron clouds leads to an exchange of electrons which finally results in excitation energy transfer. The necessary condition for an efficient energy transfer is that the energy level of 3Dmust be higher than that of the 3A by ca.3 kcal mol-'. The decay mecha- nisms of the 3Dmay be represented as follows: 3D*klD (1) 3D+ALD+ 3A* (11) where k, is the rate constant for the decay of donor triplet in the absence of acceptor and k, is the energy-transfer rate constant. The observed pseudo-first-order decay constant (k,) of 3Don account of energy transfer is given by: where [A] is the concentration of the acceptor. Therefore, a plot of observed decay rate constant against acceptor concen- tration is linear and the resulting slope gives the value of k, for the donor-acceptor combination. In order to keep the energy-transfer process predominant over the other decay modes, the reactivity of this process (k,[A]) was always kept sufficiently higher than that of all the other processes.Addi- tionally, sufficient care was taken to minimise any direct for- mation of 3A. Some representative plots of measured k, against [A] are presented in Fig. 3 for selected triplet donors and COT as the acceptor in the benzene matrix. The T-T energy-transfer rate constants obtained from similar plots with different donors are listed in Table 1. The T-T energy- transfer rate constants from the standard donors 3biphenyl and 3pyrene to COT were measured as 3.3 x lo9 and 7.4 x lo8 dm3 mol-' s-l, respectively, and were used to confirm that the transient produced from COT was indeed its triplet. The triplet energy levels, E,, of these donors are biphenyl (65.6 kcal mol- ') and pyrene (48.7 kcal mol- ').The concentration of each of the donors was kept constant at 1 x lo-' mol dm-3 in deoxygenated cyclohexane and the concentration of COT was varied from 5 x lod5to 6 x mol dm-3. Under these conditions, the concentration of donor was sufficiently high to prevent any direct formation of 3COT. In other words, under these experimental conditions c I v) v1 0 5 -Y [COT]/10-5 mol dm-3 Fig. 3 Linear plots of T-T energy transfer from various donors to COT in deoxygenated benzene solution at 7-10 Gy dose. Donors: p-terphenyl (0); C153 (A); C47 (*); C152 (+) and acri- benzil (0);dine ( x ). donor triplets were produced first, which subsequently trans- ferred the energy to the acceptor molecules.Fig. 4(a) shows oscilloscope traces for the decay of 3biphenyl at 360 nm in the absence and presence of COT. From this figure it can be seen that in the presence of COT, the initial decay due to 3biphenyl became faster than when COT was absent. However, in the presence of COT, with the expected formation of 3COT with its A,,, at 350 nm, the later part of the trace clearly shows such a behaviour and sub- sequent very slow decay (due to 3COT) in this timescale. This mixed behaviour was due to the overlap of the absorption spectra of the two triplets, viz. 3biphenyl and 3COT (with different molar absorptivities). The results in the case of pyrene (triplet absorbing at 415 nm) in Fig. 4(b)similarly show an increase in the decay rate of 3pyrene in the presence of COT.At 350 nm, where 3COT has absorption maxima, the initial part of the oscilloscope trace showed an overall mixed behaviour (due to 3pyrene decay and 3COT formation with different respective molar absorptivities) and remained virtually invariant in the time- scale of 3pyrene decay. The initial mixed behaviour was a result of the non-zero molar absorptivity of 3pyrene at 350 nm. In order to confirm the T-T spectrum of COT, time- resolved energy-transfer spectra were measured using p-terphenyl as donor in cyclohexane. p-Terphenyl was chosen because the Amax due to its triplet absorption lies at 460 nm and its absorption at 350 nm, where Lmax3COT lies, is very low. Fig. 5 shows the time-resolved spectra of T-T energy Table 1 Rate constants for T-T energy transfer between acceptor COT and various donors (D) in benzene rate constant 3D(L/nm) E,/kcal mol-' /lo8 dm3 mol-' s-' biphenyl (360) 65.6 133 naphthalene (4 14) 61 33 p-terphenyl (460) 58.3 38 benzil (480) 53.7 33 C47 (565) 57.5 65 C102 (570) 57.5 50 C120 (505) 58 60 C152 (600) 52 28 C153 (590) 50 15 pyrene (415) 49 7.4 acridine (440) 45.3 7 anthracene (422) 42 3 966 0.07 I H i 0.05 - 3 0.03 - 0.01 - ' (b) -0.01 0 I 10 I 20 I 30 I 40 I 50 ti me/p CJB 0.01 to-Ba#@0'0°1 I I I I J -0.01; 2 4 6 8 10 time/ps Fig.4 A, Oscilloscope traces showing the decay of biphenyl triplet in cyclohexane at 360 nm: (a) in the absence of COT and (b) in the presence of 4 x mol dm-3 COT (A x 2.5).B, Oscilloscope traces showing the decay of pyrene triplet at 415 nm: (a) in the absence of COT, (b) in the presence of 2 x mol dm-3 COT and (c) oscilloscope trace at 350 nm in the presence of 2 x mol dm-3 COT. transfer from p-terphenyl to COT. It is seen that with the passage of time, as absorbance due to triplet p-terphenyl decreased, the simultaneous formation of a new peak match- ing that in Fig. 1 appeared. These results once again con- ) : B wavelength/nm Fig. 5 Time resolved transient absorption spectra in deoxygenated cyclohexane by the energy-transfer mechanism from p-terphenyl donor to COT acceptor at a dose of 10 Gy: after pulse (0)and after 8 PS (0) J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 350 7-1 290 ,/' c 2F I I I J 1500 3000 4500 6000 (1/[COT])/dm3 mot-' Fig. 6 Linear plots of reciprocal 3COT absorbance at 350 nm against reciprocal [COT] in cyclohexane solution. Donors : biphenyl(0)and pyrene (*). firmed the earlier assumption that the transient spectrum shown in Fig. 1 was due to ,COT. Molar Absorptivity of 3COT It is necessary to know the molar absorptivity of the triplet accurately for laser gain measurements. This value can be determined by techniques such as the singlet depletion method or energy-transfer method.26 In this study, the latter method has been followed, the details of which are given below. From eqn. (I) and (11), the fraction of 3D transferring energy to A to give ,A can be written in terms of their respec- tive absorbances (A) and molar absorptivities (8) to give the following relation16 €3AA3D where A3D is the absorbance due to the donor triplet at its known A,,, (Table l), and A,, is the absorbance of acceptor triplet at its A,,, where its molar absorption coefficient is to be determined (350 nm for 3COT).&3D and &3A are the respec- tive molar absorptivities of the triplet donor and acceptor. From eqn.(2), the intercept of the linear plot of l/A3,, us. l/[A] gives the value of &3A by substitution of known values of A,, and &3A. For estimation of &(,COT), both biphenyl and pyrene were used as donors. Fig. 6 shows linear plots of l/A3A against l[A] with these two donors in the presence of COT in cyclohexane.From the respective measured inter- cepts and using values of E for biphenyl at 360 nm of 42800 dm3 mol-' cm-' and for pyrene at 415 nm of 30400 dm3 mol-' cm-' from the literature,26 the molar absorption coef- ficient, &(,COT),at 350 nm in cyclohexane was estimated to be 7000 f300 dm3 mol-' cm-'. Reactivity and Energy Levels of 3COT These studies were carried out in benzene where the solvent triplet yield is fairly large (Gtriplet= 4.2)27and the high energy level of ,benzene (84 kcal mol-') ensures formation of solute triplets in the ns timescale. As a result, sufficiently high con- centrations of triplets of many molecules can be produced by radiolysis of benzene solutions, especially in those cases where the intersystem-crossing efficiency is low.However, as the benzene triplet lifetime is short (few ns), high concentra- tions of solutes are required to generate measurable yields of solute triplets. It was observed that although the yield of J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3COT in benzene was higher than that in cyclohexane, its molar absorptivity (at A,,, = 350 nm) was lower [%COT(~~~~)/&~COT(~~H12)x 0.51. We have reported the triplet characteristics of many laser dyes of the 7-aminocoumarin lass.^*,^' The triplet energy levels of these dyes (viz. C47, C102, C120, C152 and C153) and their absorption maxima are listed in Table 1. Energy-transfer studies were carried out in benzene using these dyes as donors and COT as the acceptor.In these studies the ratio of donor to the maximum acceptor concentration was always kept >25. The triplet decay of the donor triplet was moni- tored at its absorption maximum at various concentrations of COT (5x 10'-8 x low4 mol dm-3). Additional energy-transfer studies were carried out with naphthalene, p-terphenyl, benzil, acridine, biphenyl and pyrene as donors. The quenching rate constants thus determined are listed in Table 1. In all these cases, T-T energy transfer was moni- tored by following the decay of donor triplet at suitable wavelengths. To determine the energy-transfer rate constants, slopes of the best-fit linear plots of eqn. (1) are used. These values were observed to approach the diffusion-controlled limit when the energy of the donor triplet was higher than ca.50 kcal mol-', as in the case with laser dyes and with p-terphenyl, naphthalene and benzil. For donors like pyrene, acridine and anthracene, with triplet energy below this value, the rates gradually dropped below the diffusion-controlled limit by almost two orders of magnitude, indicating a concur- rent decrease in the energy-transfer efficiency. These observa- tions suggest that the energy level of 3COT lies in the range 40-42 kcal mol-'. In the case of anthracene, the rates are almost 50 times lower than the diffusion limits. This suggests that the energy level of 3COT is close to the anthracene triplet energy level, with the difference between these being less than 2 kcal mol-'.This observation was further con- firmed when azulene, with its triplet energy level at 39-31 kcal mol-', was used as the donor.' In this case no energy transfer could be observed even at a very high concentration of COT. It confirmed that the triplet energy level value of COT was higher than that of azulene. In a separate study, when 3COT was used as the donor, an energy-transfer mechanism to azulene could not be confirmed quantitatively, mainly owing to the short lifetime of 3azulene and its low molar absorptivity. In Fig. 7, a plot of log (keJ against 3D energy level is presented for the various donors used in this study. As discussed above, from the order of the k,, value, the energy level of COT triplet was observed to lie in the range 40-42 kcal mol-l.Since the triplet energy level of COT has been estimated to be close to that of triplet anthracene, there is a possibility of a reverse energy-transfer mechanism from COT to anthracene LA-I 1J-. -0840 46 52 58 64 70 ET(3D)/kcal mol-' Fig. 7 Plot showing log (ke,)(T-T energy-transfer rate constant) as a function of E, (energy level of donor lowest triplet state) in benzene l8 I-----1 / /' 0' 0.0 I 0.1 I 0.2 I 0.3 I 0.4 0.5 [COTI/[ANl Fig. 8 Plot of k+/[AN] against [COT]/[AN] under equilibrium conditions for 3AN -P COT energy transfer in benzene at a dose of 7 GY in the form of an equilibrium established between these two species. It was indeed observed that with the increase of anthracene (AN) concentration the yield of 3COT decreased proportionately, suggesting the existence of such an equi-librium.kf 3AN + COT 3COT + AN (111) kb At a low absorbed dose, as the individual concentrations of the triplets of AN or COT were comparatively less (~500 times) than these solute concentrations, the forward and the backward reaction kinetics in eqn. (111)above could be rep- resented in terms of a pseudo-first-order process as given below. -dC3AN] = kf[COT][3AN] = k4,C3AN] (3a)dt The observed decay rate of the transient signal due to 3AN then could be expressed in terms of these forward and back- ward pseudo-first-order processes or the respective rate constants3' as (4) where k, and kb are the rate constants for the forward and backward reactions, respectively, for equilibrium (111).For the above relationship to be valid, in addition to the low total triplet yield maintained during the experiment, any other decay modes for these triplets were assumed to be negligible. By rearranging eqn. (4) above, a linear relation was obtained for the variation of k,/[AN] with [COT]/[AN], with its slope k, and intercept kb. The linear plot presented in Fig. 8 indicates the validity of this assumption and gives the equi- librium constant value, K x 30. From this estimate of the back energy-transfer rate constant, k, x 1 x lo7 dm3 mol-' s-', the E, of COT was confirmed to be 41 & 1kcal mol-'. Conclusions Although cyclooctatetraene has been in use as a quencher of dye triplets during laser operation, the mechanism responsible for the quenching process and the fate of the resulting 3COT is still not well understood.To study the 968 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 properties of 3COT, it was not possible to generate it by con- ventional methods such as flash photolysis and low-temperature ma trix-isolation techniques. The reasons may be due to the low intersystem-crossing yield of %OT and its small molar absorption coeficient, so that its characterisation 4 5 6 G. S. Hammond, N. J. Turro and R. S. H. Liu, J. Org. Chem., 1963,28,3297. A. A. Lamola, in Techniques of Organic Chemistry, ed. A. A. Lamola and N. J. Turro, International Science Publishers, New York, 1969, vol.14, p. 17. F. P. Schafer, in Dye-lasers, Topics in Applied Physics, ed. F. P. by optical absorption techniques was difficult. From the results obtained from these pulse radiolysis studies, it is amply clear that this indirect method of generation of solute triplets is a helpful tool for such investigations. The T-T absorption spectrum of COT shows that it has negligible absorption in the wavelength region above 400 nm, which is the most commonly used tunable range for various dye lasers. Even in the region where the 3COT has maximum absorption, its molar absorption coefficient is not very high. Therefore, any loss in laser gain due to 3COT absorption will be negligible. A low value of the energy level of %OT should allow its use as an efficient quencher of triplets of polyphenyls 7 8 9 10 11 12 13 14 Schafer, Springer-Verlag, Berlin, 1973, Vol.1, p. 1. D. P. Sorokin, J. R. Lankard, V. L. Moruzzi and E. C. Hammond, J. Chem. Phys., 1968,48,4726. W. Schmidt and F. P. Schafer, 2. Naturforsch., 1967,22,1563. W. H. Winters, H. I. Mandelburg and W. B. Mohr, Appl. Phys. Lett., 1974, 25, 723. A. N. Fletcher, Appl. Phys., 1983,32,9. R. Pappalardo, H. Samelson and A. Lempicki, IEEE J. Quant. EIectr., 1970,6, 716. R. Pappalardo, H. Samelson and A. Lempicki, Appl. Phys. Lett., 1970,16,267. J. B. Marling, D. W. Dregg and L. Wood, Appl. Phys. Lett., 1970, 17, 527. G. Jones 11, S. F. Griffin, C. Choi and W. R. Bergmark, J. Org. and coumarin dyes commonly used in lasers. In this context it needs to be mentioned that initially the probable triplet energy level of COT was assumed to lie in the range 48-50 kcal mol-'.Therefore, it was believed to be inefficient for deactivation of triplets of rhodamine dyes,31 whose triplet energy levels were in the range 40-42 kcal mol-'. This remained a paradox, as on the contrary, COT was used suc- cessfully in dye laser systems employing Rhodamine-6G. Therefore, the observations were considered to be more trivial than simple T-T energy transfer. Our studies clearly demonstrate the capabilities of COT as a quencher of rho- damine triplets by an energy-transfer process, although the actual transfer process is less efficient than that of coumarin dyes. In our studies, the ultimate fate of 3COT in these sol- vents could not be resolved.Its longer lifetime, as measured, may pose some problems during laser operation. One of the possible paths of its deactivation is via the formation of pro- ducts such as benzene and acetylene, which is an eventual degradation process for the polyene-like COT. The compa- ratively slow interaction rate of 3COT with molecular oxygen suggests chemical reaction rather than energy transfer. From the decay trace presented in Fig. 2, it is observed that in the presence of dissolved oxygen, unlike in its absence (N2 saturated), the absorption of 3COT does not decrease to the baseline value. 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