7537
J. Phys. Chem. 1995, 99, 7537-7541
Proton Transfers of Aqueous 7-Hydroxyquinoline in the First Excited Singlet, Lowest Triplet, and Ground States Seong-In Lee and Du-Jeon Jang*,t Department of Chemistry, Seoul National University, Seoul 151-742, Korea Received: September 12, 1994; In Final Form: December 23, 1994@
Proton transfers of aqueous 7-hydroxyquinoline are observed in the lowest triplet states as well as in the first excited singlet and ground states by measuring kinetic profiles and spectra of absorption and emission. An excited normal molecule undergoes within 10 ps protonation to the N atom to form an excited cation intermediate or deprotonation from the OH group to form an excited anion intermediate.. The cation’s OH group deprotonates in 30 ps while the anion’s N atom is protonated in 200 ps, either intermediate yielding a zwitterion as the final photochemical product in the first excited singlet states. Excited zwitterions relax in 3 ns into the lowest triplet state and into the ground state. The ground state reverse proton transfer process is completed in 20 ys. Triplet state zwitterions go through triplet state proton transfer in 80 pus, generating cations or anions. The reverse proton transfer triplet state intermediates relax to their ground states in 2 ms, replenishing photon-depleted species immediately without giving birth to transient absorption referable to a cation or anion as ground state proton transfer intermediate. Excitation of zwitterions initiates a proton transfer cycle composed of triplet and ground state proton transfers without excited singlet state proton transfer.
Introduction
SCHEME 1
Proton transfers are among the simplest chemical reactions, but they have provided us with a vast amount of information on equilibria, kinetics, isotope effects, free energy relationships, etc., as compared to any other class of reactions. It has been known since 1924l that the rate constants are correlated with the equilibrium constants, which can be different by many orders of magnitude between excited and ground state^.^,^ An excited aqueous species with a functional group which has a large pK difference between ground and excited states may undergo protonation or deprotonation in excited state^.^ The produced excited protropic species relaxes to the ground state and then goes through reverse reprotonation, completing a four well proton transfer cycle.5 In the case where two groups with opposite pK tendencies in excited states exist in an aqueous molecule, a photon may initiate protonation and deprotonation to yield a zwitterion. Studies on proton transfer cycles of this type with a minimum of six potential wells are interesting since these may serve as experimental models for proton relays and proton pumps in biological A variety of molecular systems have been extensively studied1°-16 for photon-initiated proton transfer cycles involving the first excited and ground singlet electronic states. In some proton transfer occurs in the lowest triplet states as well. Even in these cases triplet state proton transfer process is known to be a minor process compared with the excited singlet state proton transfer process and to proceed always with excited singlet state proton transfer. In the present paper we report a case in which triplet state proton transfer proceeds without an excited singlet state proton transfer. 7-Hydroxyquinoline (7HQ) in aqueous solutions consists of four protropic equilibrium species,22a normal molecule (N), a protonated cation (C), a deprotonated anion (A), and a zwitterion (Z), as drawn in Scheme 1. The OH group and N atom in some hydroxyquinolines are known22323to become more acidic and basic, respectively, in the first excited (n,z*) singlet state than t A member of the Center for Molecular Science, Taejon 305-701, Korea. @Abstractpublished in Advance ACS Abstracts, May 1, 1995.
a a-? Deprotonated Anion (A)
-=
HO
““Q
Normal 7HQ (N)
Zwitterion HO
(z)
H
Protonated Cation (C)
in the ground state. Upon absorption of a photon normal forms in hydroxylic solvents are known24-26to undergo excited state proton transfer process, forming the excited state tautomer form. Following the relaxation, reverse proton transfer in the ground states restores the photochemically depleted normal form. Participations of the solvent molecules were reported"^^^-^^ to be important in the excited and ground state proton transfers. The excitation of the aqueous 7HQ protropic equilibrium species may also lead to an excited and ground state protonation and deprotonation cycle. To our knowledge no report has been made on triplet state proton transfer in 7HQ. In the present paper we report our study on the protonation and deprotonation reactions of the 7HQ protropic equilibrium species in the lowest triplet state as well as in the first excited singlet and ground states by measuring absorption and emission spectra and picosecond fluorescenee and microsecond transient absorption kinetic profiles. Intermediates in the photoconversion reaction between the N and Z species are observed in the excited singlet and triplet states but not in the ground states. Triplet state proton transfer is observed also by exciting the Z species.
Experimental Section Samples. 7HQ, purchased from Eastman Kodak, was purified by vacuum sublimation. Deuterated 7HQ (7DQ) solutions were prepared by equilibrating 7HQ in 99.9% deuterated water. Neutral solutions were prepared simply by dissolving sublimed 7HQ in distilled water since extremely slow residual bleaching is observed to increase in buffered samples. The measured
0022-3654/95/2099-7537$09.00/0 0 1995 American Chemical Society
7538 J. Phys. Chem., Vol. 99, No. 19, 1995
Lee and Jang
actual pHs of neutral solutions mentioned here were always lower than 7, probably due to dissolved C02. The pHs of nonneutral solutions were adjusted by using an aqueous HC1, HC104, or NaOH solution. The typical 7HQ concentration was M. Solution temperature was controlled by using a refrigerated bath circulator (Jeio Tech, RC-1OV). Results presented hereafter were obtained from undeuterated, unbuffered, neutral, aqueous 7HQ solutions at room temperature if not otherwise specifically indicated. Static Measurements. Absorption spectra were measured by using a homemade spectrometer which was assembled with a WPH2 lamp, a 0.25-m monochromator (Kratos, GM 252), and a photomultiplier tube (Hamamatsu, R376). For measurements of emission spectra, samples were excited by a wavelengthselected beam of a 1-kW Xe lamp (Schoeffel, LPS 255 HR) using a 0.275-m monochromator (ARC, Spectrapro-275). Luminescence was collected from the front surface of the sample excitation and focused to a 0.25-m monochromator (Kratos, GM 252) which was attached with a photomultiplier tube (Hamamatsu, R374). Emission spectra reported here were not corrected for the wavelength-dependent variation of detector sensitivity. Phosphorescence was checked with a homemade phase modulation phosphorimeter which was additionally equipped to the above emission spectrometer with an optical chopper (SRS, SR540) and a lock-in analyzer (Ithaco, 393). The phase phosphorimeter detects the phase shift and demodulation of luminescence emitted after excitation by an intensity-modulated beam. Kinetic Measurements. Picosecond time-resolved fluorescence kinetic profiles were measured by using a previously described31time-correlated single photon counting system of a 70-ps-fwhm response time with a Coherent Antares YAGpumped, hybrid mode-locked and cavity-dumped dye laser. Samples had to be excited only by frequency-doubled R6G dye laser pulses of 290 nm because of limited laser availability. Microsecond transient absorption kinetic profiles and spectra were obtained by monitoring transmittance changes of the above Xe lamp beam passing a sample, which was excited by a 0.6ns N2 laser (Laser Photonics, LN1000) or its dye laser (LN102). The wavelength of the Xe lamp beam was selected by using a 0.25-m monochromator (Kratos, GM 252), and it was separated again from laser scattering and luminescence by using filters and a 0.2-m double monochromator (Kratos, GM 200). The probe beam was detected with a photomultiplier tube (RCA, 1P28; or Hamamatsu R928), digitized with a transient recorder (General Research, PCTR-160) or an oscilloscope (Tektronics, TDS 350) and accumulated with an IBM compatible 486 PC computer. Delays between laser and digitizer were adjusted by using a digital pulse/delay generator (SRS, DG535). Samples were flowed during transient absorption measurements by using a peristaltic pump (ISCO, 1612) to avoid sample decomposition.
Results and Discussion The lowest absorption bands of aqueous 7HQ protropic equilibrium species are spectrally well distinguishable. The absorption bands with the peaks at -330 and -400 nm in Figure 1 are mainly due to the lowest electronic transitions of the N and Z species, respectively. C and A species have the lowest absorption peaks at -350 and -360 nm, respectively. The relative Gibbs free energies in kilojoules per mole of the four protropic species at pH 7 are calculated to be N = Z - 2 = C - 8 = A - 10 in their respective ground states and N* = C* 10 = A* i20 = Z* 70 in their respective first excited singlet states, using pK and pK* values reported by Mason et a1.22 Equilibrium molar percentages of N, Z, C, and A species in
+
+
1
ABSORPTION
EMISSION
/ 5
L
300
400
\ "
500
600
WAVELENGTH ( n m ) Figure 1. Absorption, emission, and difference emission spectra of the aqueous 7HQ solution. The solid emission spectrum is obtained after exciting the N species at 330 nm and the dotted one after exciting the 2 species at 380 nm while the dotted-dashed one is the 23 times
multiplied difference between the solid and dotted ones. the ground states at pH 7 are 67%, 29%, 3%, and 1%, respectively. Selective excitations of particular equilibrium species by adjusting sample pH and excitation wavelength, however, give us emission spectra resembling each other in contrast to speciesdependent characteristic absorption spectra. Excitations of the N and Z species at 330 and 380 nm, respectively, show very akin emission spectra with both peaks at -510 nm, as presented in Figure 1. Excitations of C species at 350 nm at pH 3 and A species at 360 nm at pH 13 also reveal spectra alike those in Figure 1, with the peaks at -510 and -490 nm respectively, though the A-excited emission spectrum at pH 13 is spectrally blue shifted as compared to other species-excited emission spectra and it shows a left shoulder at 450 nm. Irradiation at 330 nm may excite not only the N species but also other species, especially the Z species to its S, state. Nevertheless, the N species is very selectively excited by excitation of the neutral aqueous 7HQ solution at 330 nm. The facts that excitation to the N*, C*, or A* species leads to emission very much resembling the Z*-excited emission and that the emission is enormously Stokes-shifted from the absorption indicate that any of the N*, C*, and A* species undergo photochemical reactions to yield the Z* species as the final product in the first excited singlet state, unless the emission is mostly phosphorescence which happens to be spectrally analogous to fluorescence from Z*. The phosphorescence possibility was checked with a phosphorescence-sensitive phase phosphorimeter. The absence of a phase shift or demodulation demonstrates that the phosphorescence contribution to emission is negligible, if not null, in the emission spectra of any species excitations. Transformation of N* into Z* involves protonation to the N atom and deprotonation from the OH group. Then which process of protonation and deprotonation occurs first, or do two processes occur simultaneously? The difference spectrum between the N-excited emission and the Z-excited emission in Figure 1 is spectrally broad with its peak at -460 nm and seems to contain emission from both the A* and C* species, hinting that conversion from N* to Z* proceeds in a stepwise manner and that protonation and deprotonation processes are competing. More direct evidences can be found from picosecond fluorescence kinetic profiles. After excitation of the N species, the emission decay at the wavelength of the C* and A* fluorescence (430 nm) and the rise at the wavelength of the Z* fluorescence (520 nm) match well temporally. The 430 nm profile shows 30-ps (92%), 230ps (7%),and 2.6-11s (1%) decays, and the 520-nm one shows
hoton Transfers of Aqueous 7-Hydroxyquinoline
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TIME (ns) Figure 2. Fluorescence kinetic profiles of the aqueous 7HQ solution. The dotted and solid profiles were excited at 290 nm and collected at 430 and 520 nm, respectively. 30-ps (90%) and 200-ps (10%) rises and a 2.6-11s decay. The decay time at the expected wavelength of the N* fluorescence is too short to be resolved with our current temporal resolution. This implies that the N* species undergoes protonation to the N atom or deprotonation from the OH group within 10 ps, bearing protonated C* or deprotonated A* extremely rapidly as the reaction intermediate. The 30-ps and 200-ps times must be for conversion from the C* and A* intermediates to the Z* species. We have measured the decay time of C* and the rise time of Z* at pH 0 as 30 ps and the decay time of A* and the rise time of Z* at pH 13 as 90 ps. We assign the 30-ps decay and rise times at neutral pH to the OH deprotonation time of the C* intermediate; for deprotonation, the bond breaking reaction is thought to be pHindependent. The slower 200-ps decay and rise times are considered to be the N atom protonation time of the A* intermediate since proton abstraction from the solvent with a low H30+ concentration of M results from an OH bond breakage of a H20 molecule and since the relative free energy of A* to Z* is lower than that of C* at neutral P H . ~ *On the basis of this argument, the protonation time may also be pHindependent above a certain threshold pH where protons are supplied mostly by H20 bond breakages rather than by hydronium ions. A shorter A* fluorescence decay time of 90 ps at pH 13 than the decay time of 200 ps in Figure 2 is probably because other faster decay channels such as intersystem crossing are competing with the protonation reaction. This suggestion is supported by a lower Z* fluorescence quantum efficiency of the A excitation at pH 13 than that of the N excitation, although the Z* fluorescence lifetime of 4 ns is longer at pH 13 than that of 2.6 ns from Figure 2. The relatively longer lifetime of A*, compared to those of N* and C*, and the relatively smaller Z* formation efficiency at pH 13, compared to those at lower pHs, explain the left shoulder and blue shift of A-excited emission spectrum at pH 13. The relative amplitude of the faster rise component in the Z* fluorescence kinetic profile of Figure 2 is about 10 times larger than that of the slower one. At a glance this could imply that most of the N* molecules are protonated to the N atom first to yield C* instead of A* as the reaction intermediate, which then deprotonates from its OH group to result in Z* as the final photoproduct in the f i s t excited singlet state. However, this suggestion does not go along with the argument employed earlier to explain the transformation reactions of C* and A* to Z*. Relatively much smaller conversion efficiency of A* into Z* rather recommends that intersystem crossing of A* is much faster than the protonation time and that the relatively smaller amplitude of the A*-originated rise component in Figure 2 is as a result of rapid intersystem crossing rather than the relatively
0 m m Q
h
n
0
0.00 3 3
450 WAVELENGTH (nm)
0.0081
550
I
WAVELENGTH (nm) Figure 3. Transient absorption spectra of the aqueous 7HQ solution. Sample excitation was made at 337 nm, and the delay time in seconds between excitation and measurement is indicated in each spectrum.
smaller production. We cannot prove our idea since the alteration times of N* into C* and A* are too short to be measured with our current temporal resolution. The first excited singlet state of photochemically produced Z* is depopulated in 2.6 ns. Conversion of one species into another by photon-triggered excited proton transfer breaks protropic equilibria between species. The relaxation of the equilibrium perturbation is accomplished as the extra amount of the Z species returns to the photochemically depleted N species by reprotonation and redeprotonation. Subsequent slower phenomena were studied with a microsecond transient absorption spectrometer to understand the dynamics and mechanism of reprotonation and redeprotonation, the roles of triplet states, and the relaxation rate and mechanism of ground state equilibrium perturbation. Figure 3 shows time-resolved transient absorption spectra measured after the N species is excited. At least four different transient absorption bands can be found. The transient absorption band at 400 nm decays in a few tens of microseconds, turns into an absorption bleach, and recovers in a slower time scale. This band comes from SOabsorption of Z species. The transient absorption at 450 nm decays in about half of a hundred microseconds, while the transient absorption at 550 nm rises with the decay of the 450-nm transient and decays in a millisecond time scale despite the fact that the 450-nm absorption also shows a slow decay component as well, which decays with the 550-nm transient. The isosbestic point between the two bands can be noticed at 490 nm. We assign the fast decaying 450-nm transient to T1 absorption of the Z* species and the 550-nm transient to T1 absorption of the C* and/or A* species. We can observe an absorption bleach band at 300 nm which recovers in a few tens of microseconds with the fast decay of the 400-nm Z transient and turns into transient absorption, though the absorption spectrum cannot be presented properly
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7540 J. Phys. Chem., Vol. 99,No. 19, I995
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TIME (ms) Figure 4. Transient absorption kinetic profiles, excited at 337 nm and monitored at 400 nm, of the aqueous 7HQ (solid) and 7DQ (dotted) solutions. in a figure because of strong excitation scattering at 337 nm. This slow 300-nm transient absorption acts like the 550-nm transient absorption in time. Its behavior can be noted from transient absorption at 350 nm in Figure 3b. The fast recovering 300-nm bleach results from an SObleach of the N species and the slow decaying transient from TI absorption of the C* or A* species to a high lying triplet state, T,. The 400-nm transient absorption kinetic profile in Figure 4 shows that the surplus amount of the Z species produced by excited state protonation and deprotonation returns to the photochemically depleted N species in 20 pus. Observation of 80 ps in 7DQ indicates that the tunneling effect is significant in ground state proton transfer. The bleach recovery time of 20 ps was measured at 300 nm, and this convinces us that 20 ps is the relaxation time of the ground state equilibrium perturbation. The relative amplitudes in Figures 3 and 4 designate that this relaxation is the major process of reprotonation and redeprotonation in 7HQ proton transfer cycle. We could observe a fast ( 0) turns into an absorption bleach (AA < 0) temporarily. The bleach of the Z species recovers with a 550nm triplet state decay even though an extremely slowly recovering component with an extremely small amplitude can be observed. The extremely slow residual recovery component seems to be due to trapping by photochromic species which are temporarily generated from 7HQ protropic equilibrium species by photon-initiated side reactions. Samples were flowed during transient absorption experiments to avoid photodecomposition or a side reaction, though absorption bleaching produced by a pump pulse is observed to recover completely with our
0.8
TIME (ms) Figure 5. Transient absorption kinetic profdes of aqueous 7HQ, excited at 337 nm and probed at 470 nm (dotted) and 550 nm (solid).
10
0
20
TIME (ms) Figure 6. Transient absorption kinetic profiles, excited at 337 nm, of the aqueous 7HQ solution at 300 nm (solid) and 550 nm (dotted).
=% i
0.00
0.08 TIME (ms)
0.1 6
Figure 7. Decay profile at 440 nm (solid) and rise profde at 300 nm (dotted) of the aqueous 7HQ transient absorption. Pulses of 380-nm
were used to pump the sample. sensitivity before the following laser pulse, operated with a few hertz, excites the sample. Samples decompose slowly32by laser pulses. Figures 3 and 5 show that the 450-nm transient absorption of T I (Z*) decays to feed the 550-nm transient absorption of T I (A*/C*). Laser scattering, fluorescence, and a fast rise component arising from direct intersystem crossing from the intermediate's S1 state during the N* Z* reaction make the 550-nm rise look faster than the 450-nm decay of 80 ps. The 450-nm fast decay is ascribed to the conversion time of the Z* species into the A* or C* species by reprotonation or redeprotonation in the lowest lying triplet states. We cannot determine with our currently available data whether the 550-nm transient comes from T1(A*) or T1(C*) absorption. The observations of the triplet proton transfer process by transient rise as well as by transient decay would be extremely important in understanding the roles of triplet states in proton transfer cycles. The 450-nm slow decay component decays in the same time scale of the 550-nm transient absorption, as shown in Figure 5, so it comes from the high energy vibronic absorption of
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J. Phys. Chem., Vol. 99, No. 19, I995 7541
Proton Transfers of Aqueous 7-Hydroxyquinoline
7HQ solution are summarized in Figure 8. The N* species undergoes protonation and deprotonation to form the Z* species, while protonation and deprotonation processes compete each other, revealing both C* and A* intermediates. The deprotonation time (30ps) of the C* intermediate is shorter than the protonation time (200 ps) of the A* intermediate. Z* relaxes in 3 ns to the ground state by emitting fluorescence or to the lowest triplet state by intersystem crossing. Relaxation of ground state protropic equilibrium perturbation is completed in the time scale of 20 ps, exhibiting a strong isotope effect. Z* in the T1 state transforms into A* or C* in T1 state in the time scale of 80 ps. This process can be initiated also by exciting the Z species so that the proton transfer cycle involving the triplet state without the excited singlet state proton transfer is observed. The Ti state of A* or C* relaxes into the ground state in 2 ms, replenishing the photochemically depleted N and Z species immediately.
N
A/C
Z
Figure 8. Schematic summary for the proposed mechanism and observations of the proton transfer cycle in neutral aqueous 7HQ solutions.
TI(A*/C*) state. The TI(A*/C*) state depopulates in 2 ms. The 300-nmtransient absorption shows a fast bleach recovery (20 ps) of the depleted N species. It becomes positive as the bleach recovers and decays in the same time scale of the 550-nm transient absorption (Figure 6 ) . It comes from the Tl(A*/C*) absorption to T,. Since the decay time of SO is much shorter than that of TI, the A or C species returns to the N and Z species as soon as Tl(A*/C*) relaxes into the ground state. Triplet state proton transfer should be observed also by exciting the Z species instead of the N species if our above interpretations are correct. Transformation of Z* into A*/C* in the lowest triplet states is observed as given in Figure 7. The signal-to-noise ratio is low since the excitation pulse energy of the Nz-pumped dye laser is low. The 440-nm decay time of 60 ps in Figure 7, observed after excitation of the Z species, is comparable to the 470-nm decay time of 80 pus in Figure 5, observed after excitation of the N species. In Figure 7, the 300nm transient absorption was monitored instead of the 550-nm transient absorption to avoid possible luminescence scattering since we have already mentioned that both transient absorption bands arise from the same Tl(A*/C*) state. Our observation of the triplet state proton transfer without an excited singlet state proton transfer is the first case in photon-initiated proton transfer cycles to our knowledge. In some c a s e ~ l ~ proton - ~ l transfers are also observed in the lowest triplet states. In these cases the triplet state proton transfer process is known to be a minor process compared to the excited singlet state proton transfer process and that is observed always with the excited singlet state proton transfer. The reason for the latter is probably since ground state tautomers are not stable. Nonetheless, we have observed triplet state proton transfer with blocking excited singlet state proton transfer in neutral aqueous 7HQ solutions. Conclusions Important observations and our proposed mechanism for the photochemical and photophysical phenomena of neutral aqueous
Acknowledgment. This work was supported by the Nondirected Research Fund, Korea Research Foundation, 1993,and the Korea Ministry of Education. References and Notes (1) Bronsted, J. N.; Pedersen, K. Z. Phys. Chem., Stoechiom., Verwandtschaftsl. 1924, 108, 185. (2) Forster, T. Z. Elektrochem. 1950, 54, 43. (3) Weller, A. Z. Elektrochem. 1956, 60, 1144. (4) Weller, A. Progr. React. Kinet. 1961, I, 188. (5) Kasha, M. J . Chem. Soc., Faraday Trans. 2 1986, 82, 2379. (6) Hallen, S.; Brzezinski, P. Biochim. Biophys. Acta-Bioener. 1994, 184, 207. (7) Lanyi, J. K. Biochim. Biophys. Acta 1993, 1183, 241. (8) Kotlyar, A. B.; Borovok, N.; Kiryati, S.; Nachliel, E.; Gutman, M. Biochemistry 1994, 33, 873. (9) Paddock, M. L.; Rongey, S. H.; McPherson, P. H.; Juth, A.; Feher, G.; Okamura, M. Y. Biochemistry 1994, 33, 734. (10) Chen, Y.; Gai, F.; Petrich, J. W. J . Am. Chem. SOC.1993, 115, 10158. (11) Douhal, A,; Sastre, R. Chem. Phys. Len. 1994, 219, 91. (12) Jang, D.-J.; Kelley, D. F. J. Phys. Chem. 1985, 89, 209. (13) Ganguly, T.; Burkhart, R. D.; Nelson, J. H. J . Phys. Chem. 1994, 98, 5670. (14) Jang, D.-J.; Brucker, G. A.; Kelley, D. F. J . Phys. Chem. 1986, 90, 6808. (15) Law, K. Y.; Shoham, J. J. Phys. Chem. 1994, 98, 3114. (16) Tolbert, L. M.; Harvey, L. C.; Lum, R. C. J . Phys. Chem. 1993, 97, 13335. (17) Jang, D.-J. Bull. Korean Chem. SOC.1991, 12, 441. (18) Gormin, D.; Heldt, J.; Kasha, M. J . Phys. Chem. 1990, 94, 1185. (19) Martinez, M. L.; Studer, S. L.; Chou, P. T. J . Am. Chem. SOC.1990, 112, 2427. (20) Mordzinski, A,; Grellmann, K. H. J. Phys. Chem. 1986,90,5503. (21) Nakamura, H.; Terazima, M.; Horota, N. J. Phys. Chem. 1993, 97, 8952. (22) Mason, S. F.; Philp, J.; Smith, B. E. J . Chem. SOC.A 1968, 3051. (23) Itoh, M.; Adachi, T.; Tokumura, K. J . Am. Chem. SOC.1983, 105, 4828. (24) Konijnenberg, J.; Ekelmans, G. B.; Huizer, A. H.; Varma, C. A. G. 0. J . Chem. Soc., Faraday Trans. 2 1989, 85, 39. (25) Thistlethwaite, P. J.; Corkill, P. J. Chem. Phys. Lett. 1982,85, 317. (26) Bardez, E.; Chatelain, A.; Larrey, B.; Valeur, B. J . Phys. Chem. 1994, 98, 2357. (27) Lavin, A.; Collins, S. Chem. Phvs. Lett. 1993, 97, 13615. (28) Lahmani, F.; Douhal, A.; Breherei, E.; Zehnacker-Rentien, A. Chem. Phys. Lett. 1994, 220, 235. (29) Kang, W.-K.; Cho, S.-J.; Lee, M.; Kim, D.-H.; Ryoo, R.; Jung, K.-H.; Jang, D.-J. Bull. Korean Chem. SOC. 1992, 13, 140. (30) Thistlethwaite, P. J. Chem. Phys. Lett. 1983, 96, 509. (31) Park, J.; Kang, W.-K.; Ryoo, R.; Jung, K.-H.; Jang, D.-J. J . Photochem. Photobiol. A: Chem. 1994, 80, 333. (32) Kochany, J.; Maguire, R. J. Chemosphere 1994, 28, 1097. JP942463V
