J . Phys. Chem. 1987, 91, 4269-4273 States of NCP,
C248 E
BA-
T=48ps
4269
308-nm laser light, there is apparently insufficient energy to populate state C since the subsequent emission demonstrates no detectable rise time. The decay time is also different from that observed for the banded emission following the 248-nm photolysis, indicating the emitting state (state B) is not the same. State B must be at a higher energy than state A since the banded emission is shifted to shorter wavelengths compared to the banded emission originating from state A. However, since the band separation observed is the same for both photolysis energies, the banded emission for both must be ending in the same lower state. The proposed (C A) transition following photolysis at 248 nm is likely to be radiative in view of the observed 1-ps lifetime of state C and the pressures at which we carried out these experiments. During 1 pus, at a total pressure of 0.2 Torr, there would be 1-2 collisions. In our system, the collisions must not have been effective in quenching state C because we saw no pressure dependence on the rise times of the emission observed from state A. The fact that we did not observe any emission that could be assigned to the (C A) transition, even though the energy difference appears to lie in the visible region of the spectrum, is probably because more than a single step is involved in going from state C to state A. This would place the emission in the infrared. The coupling of the C state of NC13 with the excited state of the other emitter was also not induced by collisions, since again, the rise time of the NClz emission showed no pressure dependence. However, this interaction could take place without collisions or radiation if a dissociative state of NCI, exists at about the same energy as state C. In the future we hope to obtain excitation spectra of NCl, using a pulsed tunable UV source. This will enable us to study the excited states of NC1, and the photolysis channels open to them in more detail. If there is a large amount of ground-state NCl2 produced in our experiments, we should be able to detect it through optical means. Finally, visible laser-induced fluorescence studies of NC1, will allow us to probe the low-lying excited states which we believe are the sources of the banded emission spectra that we have observed. -+
nresolved emission
308 nm
1’ ‘Bonded Emission XFigure 6. A model of the photochemistry of NC13 following photolysis at 248 and at 308 nm. cm-l, agrees with this value within our error limits. A strongly harmonic potential also exists along the v2 coordinate in NH3. On the basis of these arguments, we assign the banded spectra to transitions from excited states of NCl, to the ground state with long progressions in up. Based on the emission spectra obtained and the corresponding time profiles of the emission, we propose the model shown diagramatically in Figure 6. Following the 248-nm laser pulse, the NCl, is initially excited to a state (labeled C in Figure 6) which has a lifetime that is equal to the rise time of the emission time profiles (Figure 4) of about 1 ps. From state C, two exit channels exist. One channel is to a lower excited state (state A), followed by emission to the ground state of NC13, producing the banded spectrum. State A, then, has a lifetime given by the decay time of the banded emission. (However, the fact that the lifetime of the bands appeared to increase as the wavelength of the band increased is not explained by this model.) The other channel is dissociation to NC12, some of which is produced in different vibrational levels of an excited state. Emission from this state of NCI2, which has a lifetime of a few microseconds, produces the unresolved emission spectrum. When NCl, is irradiated with
-
Acknowledgment. This work was supported by the U S . Air Force Office of Scientific Research under Grant no. AFOSR84-003 1 . Registry No. N U 3 , 10025-85-1.
Triplet-Triplet Energy Transfer in a Copper( I I ) Porphyrin-Free-Base Porphyrin Dimer Osamu Ohno, Yoshiharu Ogasawara, Motoko Asano, Yoshizumi Kajii, Youkoh Kaizu, Kinichi Obi, and Hiroshi Kobayashi* Department of Chemistry, Tokyo Institute of Technology, 0-okayama, Meguro-ku, Tokyo 152, Japan (Received: December 30, 1986; In Final Form: April 1 , 1987)
Energy transfer from the excited triplet (trip-doublet and trip-quartet) state of copper porphyrin to the lowest excited triplet state of its free-base partner was detected by measurements of T-T transient absorption of a -(CH2)3- covalently linked porphyrin dimer. The excited dimer with free base in the excited singlet is partly quenched by the copper counterpart. On the other hand, the excited dimer with copper porphyrin in the trip-multiplet state is dissipated not only by the fast electronic relaxation within the copper porphyrin but also by the intramolecular energy transfer to the free-base partner.
Introduction Currently several groups are studying the energy transfers between two different porphyrin moieties which are covalently linked with a -(CH2),,- chain.’* In the zinc and free-base hybrid
dimers, an energy transfer from the lowest excited singlet (S,) state of zinc porphyrin to that of the free-base counterpart has been ~ b s e r v e d . ~A*free-base ~ dimer forms a hybrid dimer of free base and diacid in benzene containing an appropriate concentration
( I ) Kaizu, Y.; Maekawa, H.; Kobayashi, H. J . Phys. Chem. 1986, 90, 4234. ( 2 ) Anton, J. A.; Loach, P. A.; Govindjee Photochem. Photobiol. 1978, 28,
(4) Brookfield, R. L.; Ellul, H.; Harriman, A. J . Chem. Soc., Faraday Trans. 2 1985, 81, 1837. ( 5 ) Schwarz, F. P.; Gouterman, M.; Muljrani, Z.; Dolphin, D. H. Bioinorg. Chem. 1972, 2, 1 . (6) Mialoco, J. C.; Glannotti, C.; Maillard, P.; Momenteau, M. Chem. Phys. Lett. 1984, 23, 87.
235.
(3) Brookfield, R. L.; Ellul, H.; Harriman, A,; Porter, G. J . Chem. Soc., Faraday Trans. 2 1986, 82, 219.
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of CF3COOH. This particular hybrid dimer exhibits a very fast energy transfer from the second excited singlet ( S , ) state of the free base to the corresponding S, state of the diacid moiety.’ The singlet-singlet energy transfer is attributable to a dipole-dipole interaction in the excited porphyrin dimer and thus the yield of energy transfer depends not only on the distance and the mutual orientation of the two component porphyrins but also on the energy gap between the donor and acceptor excited states. The lifetime of the excited triplet of zinc porphyrin moiety in a variety of hybrid dimers is reduced in comparison with that of monomeric zinc porphyrin when the partner porphyrin has a paramagnetic metal This may be attributable to a fast energy transfer from the zinc-excited triplet to the excited multiplet of the paramagnetic partner porphyrin, which is of porphyrinexcited triplet orgin. Without sound evidence, however, one cannot discriminate intramolecular energy transfer from the electronic relaxation within zinc porphyrin accderated by the paramagnetic perturbation of the counterpart. The energy transfer to zinc porphyrin from the higher excited states of the paramagnetic partner porphyrin seems much less likely, since the relaxation within the paramagnetic porphyrin is very fast.’,* In fact, the fluorescence of zinc porphyrin sensitized by paramagnetic partner porphyrin is not observed. However, it has been suggested that an energy transfer may occur between the thermal equilibrium state of trip-doublet and trip-quartet of copper moiety and the excited triplet of the zinc counterpart in the hybrid dimer, because the transient absorption of the copper trip-quartet decays in the hybrid dimer too fast to be detected in contrast with that of the monomeric copper t r i p - q ~ a r t e t . ~In the present paper, we report an efficient production of the excited triplet of free-base porphyrin by an energy transfer from the thermally equilibrated trip-doublet and tripquartet states of copper partner porphyrin in the hybrid dimer. Experimental Section
Preparation qf the Dimer. The -(CH2)3- covalently linked porphyrin dimer of copper(I1) and free base, [5-[2-[3-[2[ 10,15,20-tris(4-methylphenyl)-2 lH,23H-porphyrin-5-y1]phenoxy]propoxy]phenyl]-lO,l5,20-tris(4-methylphenyl)porphyrinat~]copper(II)-@’,fl~,fl~,@~ (o,o-C3(TTP),CuH2), was synthesized by the analogous method to the preparation of the vanadyl-free-base dimer described by Little and his co5- [ 2 4 3-Bromopropoxy)phenyl] - 10,15,20-tris(4methylpheny1)-2 lH,23H-porphyrin (TTBrPH2) was prepared by stirring a mixture of 5-(2-hydroxyphenyl)- 10,15,20-tris(4methylpheny1)-2 1H,23H-porphyrin (TToHPH2), anhydrous potassium carbonate, and excess l .3-dibromopropane in N,N-dimethylformamide (DMF) at room temperature for 25 h. TTBrPH2 was chromatographed on a column of alumina (Merck alumina 90, activity 11-111) using chloroform as eluent and then recrystallized from chloroform/methanol. TToHPH, was prepared by refluxing pyrrole and a 1:3 mixture of 2-hydroxybenzaldehyde and 4-methylbenzaldehyde in propionic acid’, and the chlorin contamination was oxidized by use of 2,3-dichloro-5,6-dicyanop-benzoquinone.13 The product was purified by chromatography on an alumina column and a silica gel (Wakogel C-200) column using chloroform as eluent. TToHPCu was made from TToHPH2 by the method of Adler et al.I4 and purified by chromatography and then by recrystallization from chloroform/hexane. TToHPCu thus obtained has no contamination of TToHPH2detectable by emission measurements The dimer o,o-C,(TTP),CuH, was ( 7 ) Kobayashi, T.; Huppert, D.; Straub, K. D.; Rentzepis, P.M. J . Chem. Phys. 1979, 70. 1720. (8) Kim, D.: Holten, D.; Gouterman, M. J . A m . Chem. Sot. 1984. 106, 2193. (9) Deleted in proof. ( I O ) Deleted in proof. ( I 1 ) Little, R. G . J . Heterocycl. Chem. 1978, 15, 203. (12) Little, R. G.: Anton, J.; Loach, P. A,; Ibers, J. B. J . Heterocycl. Chem. 1975. 12. 343. (13: Barnett, G . H.; Hudson, M. F.: Smith, K. M . J . Chem. Sac., Perkin Trans. I 1975, 1401. (14) Adler. A. D.; Longo, F. R.; Varadi. V . Inorg. Synth. 1976, 16, 213.
Ohno et al.
0 H*$’
R = @CH3
Figure 1. A twisted conformation of the hybrid dimer
prepared by stirring an equimolar mixture of TTB,PH, and TToHPCu with anhydrous potassium carbonate in DMF at room temperature for 85 h. The product was loaded onto an alumina column and eluted by chloroform. The earlier-eluted fraction was a mixture of the dimer and unreacted TTBrPH2,while the later-eluted fraction was unreacted TToHPCu. Purification of the dimer was carried out by gel permeative chromatography on a column (50 mm diameter X 1000 mm length) of styrene-divinylbenzene (BioRad Biobeads S-X2 200-400 mesh). The earlier-eluate was further purified by chromatography on a styrene-divinylbenzene column and then on a silica gel column by chloroform elution. The dimer thus obtained was recrystallized from chloroform/hexane. The final sample was confirmed with a single peak obtained by mass spectroscopy. Anal. Calcd for C 9 7 H 7 4 N 8 0 2 (mol C ~ wt 1447.3): C, 80.50; H, 5.15; N 1.74. Found: C, 80.88; H, 4.95; N, 7.66. MS, m / e 1446. Figure 1 shows a twisted conformation of the hybrid dimer. Measurements. Absorption and second-derivative absorption spectra were taken on s Hitachi 330 spectrophotometer. Second-derivative spectra were obtained by measurements of the difference spectra for two different wavelengths with an interval Ah = 2 nm using a slit width of 2 nm. Luminescence emission as well as excitation spectra were measured on a Hitachi spectrofluorometer 850 equipped with a Hamamatsu Photonics photomultiplier R928. The emission and excitation intensities were calibrated by use of a concentrated ethylene glycol solution of rhodamine B (8 g/cm3).I5 Emission spectra were also corrected by using standard solutions of 4-dimethylamino-4’-nitrostilbene (800-600 nm) and m-(dimethy1amino)nitrobenzene (630-480 nm).16 Fluorescence lifetimes were measured by the single-photoncounting method on a PRA nanosecond fluorometer system. The sample (a typical concentration of -2 X lo-’ M) was excited by 400-nm pulses of 2 ns duration from a hydrogen-gas lamp (PRA Model 510B) through a monochromator (Jobin Yvon Model H10). The lifetimes were determined by fitting the decay curve exponentially using an iterative least-squares method. Quantum yields of emission were measured with reference to the yield of the S , emission of (5,10,15,20-tetraphenylporphyrinato)zinc(TPPZn) in benzene (& = 0.033)’’ by using optically dilute solutions.” T-T absorption spectra were measured through a Nikon P-250 monochromator by a Hamamatsu R928 photomultiplier using a pulse of 5 0 0 - c ~duration ~ from a xenon flash lamp (Ushio UXL150DS, 150 W) synchronously fired by the excitation pulse. The lowest excited triplet population was generated in a rectangular quartz cell of 4 cm optical path by irradiation with 308-nm pulses (15) Melhuish, W. H. J . Res. Natl. Bur. Stand., Sect. A 1972, 76, 547. (16) Lippert, E.; Nagele, W.; Seibolt-Blankecstein, I.; Staiger, U.; Voss, W . Z . Anal. Chem. 1959, 170, I . (17) Quimby, D. J.; Longo, F. R. J . Am. Chem. SOC.1975, 97, 5 1 1 1. (18) Demas, J. N.; Crosby, G . A . J . Phys. Chem. 1971, 75. 991.
Energy Transfer in Cu( 11)-Free-Base Porphyrin Dimers EilO‘
The Journal of Physical Chemistry, Vol. 91, No. 16, 1987 4271
EllO‘
I
Figure 2. Absorption spectra of o,o-C3(TTP),CuH2(A, -), a 1:l mixture of TTPCu and TTPH, (A, .-), TTPH, (B, -), and TTPCu (B, .-) in toluene.
E”
Wavenumber/103cm-‘
n
Wavenumber/103cm~‘
Figure 4. Emission spectra (A) of toluene solutions of TTPH2(a), 1:l mixture of TTPCu and TTPH2 (b), and ~ , O - C ~ ( T T P ) ~ C (c)Uwith H~
equal concentrations of the free-basemoiety (excitation wavelength, 515 nm). Excitation spectra (B, -) in the Q and B bands of the dimer emission (monitored at 720 and 650 nm, respectively) reproduce the absorption spectrum of the free base but not with that of the dimer (B,
p------
.-).
kE‘lok 4
lo 0 22 2
24
26
Wavenumber/IO3cm~’
2022
2&
26
Wavenumber/103cm-l
Figure 3. Second-derivativeabsorption spectra (d’) in the B (Soret) band of o,o-C,(TTP),CuH, (A) and a 1:l mixture of TTPCu and TTPH, (B) in toluene.
(15 ns duration, 70 mJ pulse-]) from a xenon chloride excimer laser (Lambda Physik EMG52 MSC). The lifetimes of the triplet state were determined in the porphyrin solutions (-2.5 X M) with excitation by 540-nm pulses from a dye head (coumarin 540) operated by the excimer laser. Signals from the photomultiplier were measured by a digital memory (Iwatsu DM-901 with highest time resolution of 10 ns/channel) on line to an NEC computer PC9801-F2 and were typically accumulated for 100 shots. The decay lifetime and the T-T absorption spectra at various stages of time delay were also calculated on the computer. Solvents, toluene and acetonitrile, used for the measurements were commercially available and purified and dried by distillation on CaH2. Solutions of the porphyrins were sealed in cuvettes after being purged with nitrogen gas just before the measurements. Solutions used for the T-T absorption and triplet lifetime measurements were degassed by freeze-pump-thaw cycles.
Results and Discussion Figure 2 shows the absorption spectrum of the dimer in toluene. Absorption intensity in the Q band of the dimer is in good agreement with that of a 1:l mixture of TTPCu and TTPH2, while the B band of the dimer is rather broad and the observed absorption maximum is 25% reduced compared with that of the 1:l mixture. In fact, the second-derivative absorption spectra yield double minima corresponding to the split B bands of the dimer, while they only a single minimum for the superposing B bands of the 1:l mixture as shown in Figure 3. The porphyrin dimer used in the present work is conformationally flexible. The split B peaks indicate that the porphyrin dimer is in two different conformations. An exciton coupling gives rise to a blue shift of
the B band when two porphyrin moieties are stacked, while the interaction is much reduced in a twisted conformation. The red component band is ascribed to the B band of the “open (twisted) conformation”, while the blue component band is that of the “closed (stacked) conformation”.’ Figure 4A shows the emission spectra of TTPH2 (a), a 1:l mixture of TTPCu and TTPH2 (b), and o,o-C3(TTP)&!uH2 (c) in toluene, and Figure 4B presents the excitation spectra of the dimer emission. The emission band of the dimer has a profile similar to that observed with monomeric free-base TTPH2. The emission spectrum of the 1:1 mixture has the profile and intensity, which correspond to those from a solution of monomeric free base of equal concentration ( 5 X lo4 M) in the same solvent, regardless of the excitation wavelengths within the Q band. On the other hand, the emission spectrum of the dimer measured upon excitation in the Q band is reduced in its intensity as compared with that of the 1:l mixture with equal concentration of the free-base moiety. The excitation spectrum of the dimer emission reproduces fairly well the absorption spectrum of the monomer free base but not the dimer absorption spectrum. This suggests that the emission arises from the SIstate of the free-base moiety in the dimer, which is directly excited on the free-base moiety but not excited by sensitization due to the excited copper counterpart. Decay lifetimes of emission of the dimer are 3.1 and 1.9 ns in toluene and acetonitrile, respectively, while those of the monomeric free base are 11.7 and 11.7 ns. The lifetime of the dimer varies with solvents. In addition the lifetime of the free base (2 X lo-’ M) in mixtures containing TTPCu up to 2 X M was invariantly 11.7 ns. The excited free-base porphyrin is not appreciably quenched by the diffusion-controlled encounters with copper porphyrin in such dilute solutions of the mixtures as used in the present work; however, the copper porphyrin moiety in the dimer quenches the excited free-base counterpart. The quantum yield (&) of the dimer emission in toluene was determined with 5 15-nm excitation by measuring the integrated emission intensity of an optically dilute solution relative to the corresponding intensity of TPPZn in benzene: & = 0.026 (the dimer); c $ ~ = 0.12 (TTPH2). The ratio between the yields of the dimer and the monomeric free base is the same (1/ 5 ) as observed with the ratio of the corresponding lifetimes. The porphyrin dimer shows split B bands. The red component band is the B band of the “open conformation” and the blue component band is that of the “closed conformation”. The second-derivative absorption spectra indicate that the blue component increases in acetonitrile compared to that in toluene while the red component decreases. Despite the fact that there are different conformers in solution, the best fits of fluorescence decays are obtained by single exponentials but not double exponentials. The reduced lifetime observed in acetonitrile is ascribed to an enhanced participation of the closed conformation preferred in this solvent.
4272
The Journal of Physical Chemistry, Vol. 91, No. 16, 1987 A30
.
p
c*t
P
’\
6%
C --I
soc
-30
6CC
Wavelength /nm
Figure 5. T-T absorption spectra taken with varied time delays after the 308-nm pulse excitation of o,o-C3(TTP),CuH, (A), TTPH, (B), and TTPCu (C) in toluene.
The observed lifetime is determined by rate constants not only of excitation-energy dissipation processes in two different conformers but also of conformational change. However, what is observed in the present work indicates that the relaxation within the closed conformer is much accelerated compared with that in the open conformer. Figure 5 presents the T-T absorption spectra of the dimer, TTPHz and TTPCu, in toluene as a function of time delay after the 308-nm pulse excitation by a xenon chloride excimer laser. The lifetime of TTPCu in toluene at ambient temperature is 25 ns. Thus the excited triplet of copper moiety in the dimer may be populated only in a short period after excitation. In fact, the spectrum of the dimer is very similar to that of the free base and thus the decay of the dimer excited triplet is governed by the decay of excited triplet of the free-base moiety. The decay of porphyrin T-T absorption a t 446 nm was measured in toluene solutions of TTPH2, the 1:l mixture, and the dimer with equal concentrations of the free-base moiety excited by 540-nm pulses from a dye head operated by the excimer laser.
Ohno et al. Figure 6 summarizes the decay of T-T transient absorption of TTPH2, the 1:l mixture, and the dimer. All the T-T absorption decays were measured by using laser pulses of reduced power, which greatly reduces possible two-photon processes. The excited triplet of TTPH2 decays in a lifetime of T~ = 1.1 ms. The 1:l mixture (B) shows a fast decay of the excited copper moiety ( T 50 ns) and then a slow decay of the excited free-base partner (TT = 0.8 ms). Quenching of the mixture is partly due to the infrequent encounters of copper and free-base porphyrins in such M. However, a significant difference a dilute solution as 2.5 X of the decay lifetimes in milliseconds cannot accurately be detected by means of the flash lamp used in the present work. The decay of the 446-nm T-T absorption of the dimer (C) is mainly attributable to the relaxation within the excited free-base moiety ( T =~ 1.1 ps). However, the decay (C’) monitored at 475 nm in the T-T absorption maximum of the copper partner shows a fast decay within the excited copper porphyrin in the initial stage and then the decay of excited free-base counterpart (TT = 1.I ps). The slow decay is apparently of a single component. The lifetime of the free-base excited triplet is reduced 3 orders of magnitude. Since the dimer porphyrin can be in a conformation with sufficient interaction between the component porphyrins during the long lifetime of the free-base excited triplet state, the quenching is accelerated. So-S1 absorbance of TTPCu at 540 nm is 4 times greater than that of TTPHz and thus the 540-nm pulse gives rise to excitation of the dimer 80% in its copper moiety. Nevertheless, the T-T absorption spectrum shows that a rather high fraction of the dimer is excited in the free-base moiety. All the T-T absorption decays shown in Figure 6 were measured by means of laser pulses with equal power in the solutions with equal So-S, absorbance at 540 nm of the free-base moiety. The initial optical density obtained by extrapolation of the slow component decay of T-T absorption to time zero is proportional to the concentration of the free-base excited triplet and thus this must be a good measure of relative quantum yield. The initial optical density (at 446 nm) due to the free-base excited triplet in the dimer is about 3 times greater than those obtained for the monomeric free base and the 1:l mixture, while the initial optical density of the 1:1 mixture is nearly equal to that of the monomeric free base. These observations indicate a higher yield of the excited triplet of the free-base moiety in the dimer than expected for direct excitation of the free-base moiety by the 540-nm pulse, provided that the molar absorbance of T-T excitation of the free-base moiety in the dimer is equal to that of the monomer free base. The intersystem crossing of the free-base moiety in the dimer may be enhanced by the paramagnetic perturbation of the copper partner. However, even if this particular perturbation causes 100% intersystem crossing, the yield cannot be expected more than 1.4 times greater than that of the monomer free base since 41sc = 0.7 for the unperturbed free base.I9 The yield of the free-base
C
8
A
C’
A OD
010
005
0.00
0
2
4
0
2
4
0
2
4
0
2
4
Time/,& Figure 6. Decay of the T-T transient absorption of TTPH2 (A,monitored a t 446 nm),a 1:l mixture of TTPCu and TTPH, (B, monitored at 446 nm), and o,o-C,(TTP)2CuH2 (C, monitored a t 446 nm; C’, a t 475 nm) excited by 540-nm pulses.
~
4273
J . Phys. Chem. 1987,91, 4213-4211 excited triplet in the dimer is 3 times higher than expected for direct excitation of the free base. This is most likely due to an efficient intramolecular energy transfer from the excited copper partner porphyrin. Since the lifetime of a thermal equilibrium state of the lowest tripdoublet and trip-quartet of copper porphyrin is as short as 25 ns, the intermolecular energy transfer on diffusional encounters of excited copper porphyrin and free base is less likely in dilute solutions such as used in the present work.
Conclusions The excited dimer with free-base component porphyrin in the excited singlet is partly quenched by the copper partner porphyrin. This may be attributed to an intramolecular electron transfer between copper and free-base porphyrins in the dimer of the closed conformation.20,21 However, the electron-transfer products (cations and anions) were not detected by the measurements we used in the present work. The energy of lowest excited triplet of free-base porphyrin is lower than the thermal equilibrium state of the lowest trip-doublet and trip-quartet of excited copper porphyrin as illustrated in Figure I . The excited dimer with the copper counterpart in the excited triplet disappears not only by (19) Kajii, Y . ;Obi, K.; Tanaka, I.; Tobita, S . Chem. Phys. Lett. 1984, I l l , 341. (20) Netzel, T. L.; Kroger, P.; Chang, C.-K.; Fujita, I.; Fajer, J. Chem. Phys. Lett. 1919, 67, 223. (21) Netzel, T. L.; Bergkamp, . M. A,; Chang, C.-K. J . Am. Chem. SOC. 1982, 104, 1952.
*T, 47i
2 '
PH,
N
so
PCU
Figure 7. An intramolecular energy transfer is possible in the hybrid dimer from the thermal equilibrium state of the lowest trip-double and trip-quartet of excited copper porphyrin to the lowest triplet state of the free-base partner porphyrin.
electronic relaxation within the copper porphyrin but also by intramolecular energy transfer which yields the free-base moiety in the excited triplet. However, the decay of the excited dimer with free-base-excited triplet is accelerated up to 1.1 ps by the copper counterpart in comparison with 1.1 ms of the excited triplet of the monomeric free base.
An Extended Kramers Equation for Photoisomerization J. Lee,* S.-B. Zhu, Picosecond and Quantum Radiation Laboratory, Texas Tech University, Lubbock, Texas 79409
and G . W. Robinsont Department of Physical Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia (Received: February 4, 1987)
The experimental data from seven different studies of liquid-state cis-trans isomerization over reasonably high barriers are fit to a simple three-parameter form of the Kramers equation. These fits suggest that in all the systems discussed here the memory kernal remains effectively constant over the time scale of the barrier crossing, as in a "frozen solvent" model. The various parameters derived from these fits seem reasonable.
1. Introduction The molecular photoisomerization process in the liquid state has been experimentally investigated as a function of solvent properties by many workers.'-1° The picture is fairly complicated when polar molecules isomerize in a polar environment. For example, the barrier may decrease as a change is made from nonpolar solvents (alkanes) to polar solvents (alcohols), or in some cases the barrier may even be temperature-dependent for the same solvent!JO On the other hand, for nonpolar molecules in nonpolar solvents belonging to a given "family type", it is usually safe to assume that the barrier remains constant as the viscosity of the solvent is changed. However, even in these more simple cases agreement between the experiments and available theory is not assured. The status of current theoretical work on this problem can be briefly summarized. When intramolecular barriers hindering the isomerization process are absent, the Stokes-Einstein equation Permanent address: Picosecond and Quantum Radiation Laboratory,
P.O. Box 4260, Texas Tech University, Lubbock, TX
79409.
0022-3654/87/2091-4273$01 .50/0
is often capable of describing the experimental isomerization rates." However, when there is a barrier, it is necessary to (1) Velsko, S. P.; Waldeck, D. H.; Fleming, G. R. J . Chem. Phys. 1983, 78, 249. Velsko, S. P.; Fleming, G. R. Chem. Phys. 1982, 65, 59. (2) Velsko, S. P.; Fleming, G. R. J . Chem. Phys. 1982, 76, 3553. (3) Schroeder, J.; Troe, J. Chem. Phys. Lett. 1985, 216, 453. Troe, J. J . Chem. Phys. 1986, 90, 351. (4) Keery, K. M.; Fleming, G. R. Chem. Phys. Lett. 1982, 93, 323. (5) Rothenberger, G.;Negus, D. K.; Hochstrasser, R. M. J . Chem. Phys. 1983, 79, 5360. Lee,M.; Bain, A. J.; McCarthy, P. J.; Han, C. H.; Haseltine, J. N.; Smith, A. B., 111; Hochstrasser, R. M. J . Chem. Phys. 1986,85,4341. (6) Courtney, S . H.; Fleming, G. R. J . Chem. Phys. 1985, 83, 215. (7) Flom, S. R.; Brearley, A. M.; Kahlow, M. A,; Nagarajan, V.; Barbara, P. F. J . Chem. Phys. 1985,83, 1993. Flom, S. R.; Nagarajan, V.; Barbara, P. F. J . Phys. Chem. 1986, 90, 2085. (8) Brearley, A. M.; Flom, S. R.; Nagarajan, V.; Barbara, P. F. J . Phys. Chem. 1986, 90, 2092. (9) Millar, D.; Eisenthal, K. B. J . Chem. Phys. 1985, 83, 5076. Eisenthal, K. B. Chem. Phys. (10) Hicks, J.; Vandersall, M.; Babarogic, 2.; Lett. 1985, 116, 18. (1 1) Bagchi, B.; Oxtoby, D. W. J . Chem. Phys. 1983, 78, 2735. Deviations from Stokes-Einstein behavior for the zero barrier case have recently been
discovered. Fleming, G. R., private communication.
0 1987 American Chemical Society
