Solvent Effects on the Formation and Decay of an Exciplex between

Picosecond laser photolysis reveals the formation of an exciplex between the lowest excited singlet state. [DBA*(S1)] of 9,10-dibromoanthracene (DBA) ...
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J. Phys. Chem. 1996, 100, 18431-18435

18431

Solvent Effects on the Formation and Decay of an Exciplex between the Lowest Excited Singlet State of 9,10-Dibromoanthracene and Ground-State Amine (N,N-Dimethylaniline or Triethylamine) Toshihiro Nakayama, Tetsuya Hamana, Pallabi Jana,† Seiji Akimoto,‡ Iwao Yamazaki,‡ and Kumao Hamanoue* Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan ReceiVed: August 19, 1996X

Picosecond laser photolysis reveals the formation of an exciplex between the lowest excited singlet state [DBA*(S1)] of 9,10-dibromoanthracene (DBA) and ground-state amine in acetonitrile (CH3CN) and ethanol (EtOH) containing N,N-dimethylaniline (DMA) [and n-heptane (HP) containing DMA or triethylamine (TEA)]. Only in CH3CN-DMA, however, can decomposition of the DBA-DMA exciplex into the DBA radical anion (DBA•-) and the DMA radical cation be seen, indicating very small generation of DBA•- from the DBA-amine exciplex formed in EtOH-DMA and HP-amine (DMA or TEA). Interestingly, no DBATEA exciplex is formed in CH3CN and EtOH containing TEA but nanosecond laser photolysis reveals the existence of DBA•- in the former solvent. Furthermore, the rate of DBA f 9-bromoanthracene debromination upon steady-state photolysis in CH3CN-TEA is 1 order of magnitude smaller than that in CH3CN-DMA but 2 or 3 orders of magnitude greater than those in EtOH and HP containing amine. This suggests that diffusion-controlled quenching of DBA*(S1) by TEA in CH3CN and EtOH gives rise to the formation of a nonemissive short-lived encounter complex or ion pair. It thus be concluded that generation of DBA•- from the DBA-amine exciplex or the encounter complex (or the ion pair) is affected by the dielectric constant of a pure solvent.

Introduction It was proposed that the intermediates for the photoinduced dehalogenation of aromatic halo compounds in the presence of amine were the radical anions of halo compounds generated by decomposition of exciplexes formed between the lowest excited singlet states of halo compounds and ground-state amine.1-8 By means of nanosecond laser photolysis and steady-state photolysis, we thus studied photoinduced debromination of haloanthracenes (XA, i.e., the 9-bromo, 9,10-dibromo, 9-chloro, and 9,10-dichloro compounds) in acetonitrile containing N,Ndimethylaniline (DMA) or triethylamine (TEA).9-12 Although no absorption bands responsible for the lowest excited singlet state [XA*(S1)] of XA and an exciplex [1(XA-amine)*] of XA*(S1) with amine could be detectable, following scheme (Scheme 1) was proposed: SCHEME 1

reaction of XA*(S1) with amine; (2) since the lowest excited triplet state [XA*(T1)] of XA in the absence of amine was populated via the indirect XA*(S1) f XA*(Tn) f XA*(T1) intersystem crossing through an adjacent higher excited triplet state [XA*(Tn)],13-15 amine-assisted population of XA*(T1) was ascribed to the intersystem crossing from 1(XA-amine)* to its triplet exciplex [3(XA-amine)*] followed by rapid decomposition into XA*(T1) and amine. In order to understand the solvent effect on the formation and decay of an exciplex or a nonemissive short-lived encounter complex (or ion pair) which generates the radical anion of 9,10dibromoanthracene (DBA) as an intermediate for amine-assisted DBA f 9-bromoanthracene debromination, the present paper deals with quenching of the lowest excited singlet state [DBA* (S1)] of DBA by amine (DMA or TEA) in acetonitrile (CH3CN), ethanol (EtOH), and n-heptane (HP) studied by picosecond laser photolysis as well as steady-state photolysis. Experimental Section

i.e., (1) the intermediate for photoinduced dehalogenation of XA was its radical anion (XA•-) generated by decomposition of 1(XA-amine)* which is formed by a diffusion-controlled * Author to whom correspondence should be addressed. † On leave from the Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 70032, India. ‡ Department of Chemical Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060, Japan. X Abstract published in AdVance ACS Abstracts, November 1, 1996.

S0022-3654(96)02554-3 CCC: $12.00

DBA (Aldrich) was recrystallized three times from EtOH, and GR-grade DMA and TEA from Wako were refluxed over calcium hydride and distilled under a nitrogen atmosphere. The spectral-grade solvents used were CH3CN (Dojin), EtOH (Nacarai), and HP (Dojin); although HP was used without further purification, CH3CN and EtOH were dried using molecular sieves 3A (Wako). All experiments were performed at room temperature, and the sample solution was degassed by several freeze-pump-thaw cycles or deoxygenized by bubbling of Ar gas. The emission spectra due to DBA*(S1) and its exciplex with amine [1(DBA-amine)*] were recorded using a Hitachi MPF-4 spectrofluorometer, and the changes in the emission intensities with time were measured by a single-photon-counting method using a picosecond Ti:sapphire laser.16 For the picosecond © 1996 American Chemical Society

18432 J. Phys. Chem., Vol. 100, No. 47, 1996

Figure 1. Transient absorption spectra obtained by picosecond laser photolysis of DBA in (a) CH3CN and (b) EtOH containing 1 M DMA.

double-beam absorption spectroscopy, a mode-locked Nd:glass laser was used;17 the excitation light pulse was the third harmonic [λ ) 351 nm with a full width at the half-maximum intensity (fwhm) of 7 ps] and the probing light pulse was generated by focusing the fundamental (λ ) 1054 nm) into D2O. The transient absorption spectra due to DBA*(S1) and 1(DBAamine)* were recorded using two polychromator-image sensor detector systems. Steady-state photolysis of DBA in the presence of 0.04 M amine (DMA or TEA) was carried out using the 404.6-nm monochromatic light selected from a USH-500D super-highpressure mercury lamp. Then, using a Hitachi 200-20 spectrophotometer, the relative rate of DBA f 9-bromoanthracene debromination was determined by measurements of the absorbance decrement of DBA at 402 nm. Results Formation and Decay of an Exciplex between the Lowest Excited Singlet State of DBA in CH3CN-DMA, EtOHDMA, and HP-Amine. Figure 1 shows the transient absorption spectra obtained by picosecond laser photolysis of DBA in CH3CN and EtOH containing 1 M DMA. Clearly, band BM with an absorption maximum at 620 nm decreases with time accompanied by the increase and then the decrease of band BE with an absorption maximum at 685 nm. Although a similar result is obtained in HP containing 1 M amine (DMA or TEA), band BE observed in the presence of TEA has an absorption maximum at 680 nm. With regard to the spectral profile and the position of absorption maximum, band BM is very similar to the absorption bands due to the lowest excited singlet states of anthracene and its several derivatives such as the 9-acetoxy, 9-phenyl, 2,9-diacetoxy, 2,9-diacetoxy-3-methoxy, and 2,9diacetoxy-3-methoxy-10-phenyl compounds.18 In the absence of amine, furthermore, only band BM is observed and its decay rate constant (kM) is found to be equal to the fluorescence lifetime (kf), i.e., kM ) (7.7 ( 1.2) × 108 s-1 and kf ) (7.7 ( 0.6) × 108 s-1 in CH3CN, kM ) (7.1 ( 1.0) × 108 s-1 and kf ) (7.1 ( 0.5) × 108 s-1 in EtOH, and kM ) (8.3 ( 1.4) × 108 s-1 and kf ) (8.3 ( 0.7) × 108 s-1 in HP. Hence, band BM is assigned to the singlet-singlet (S′ r S1) absorption originating from the lowest excited singlet state [DBA*(S1)] of DBA.

Nakayama et al.

Figure 2. Plots of DM(t)/DM(max) [at 620 nm (O)] and DE(t)/DE(max) [at 685 nm (b)] against time obtained in (a) CH3CN and (b) EtOH containing 1 M DMA. The solid and dashed curves are the best-fit absorbances calculated assuming a Gaussian intensity function (fwhm ) 7 ps) for both the excitation and probing light pulses, and using the biexponential concentration functions given by eqs 1 and 2, respectively; the rate constants k1 and k2 are listed in Table 2.

TABLE 1: Rate Constants (k1, k2) Determined from Biexponential Changes of Absorption and Emission Intensities with Time Obtained for DBA*(S1) and 1(DBA-Amine)* in Solvents Containing 1 M Amine (DMA or TEA) from absorption

from emission

solvent

k1/1010 s-1

k2/109 s-1

k1/1010 s-1

k2/109 s-1

CH3CN-DMA EtOH-DMA HP-DMA HP-TEA

4.2 ( 1.5 7.7 ( 3.5 4.5 ( 1.3 1.3 ( 0.2

1.5 ( 0.1 1.5 ( 0.1 1.4 ( 0.1 3.6 ( 0.5

4.2 ( 0.5 7.7 ( 1.2 4.5 ( 0.8 1.3 ( 0.1

1.5 ( 0.1 1.5 ( 0.1 1.4 ( 0.1 3.6 ( 0.5

The time-dependent intensity changes of bands BM [DM(t)/ DM(max), open circles] and BE [DE(t)/DE(max), closed circles] shown in Figure 2 can be reproduced by the solid and dashed curves, respectively. These best-fit curves are calculated as follows; (1) a Gaussian intensity function (fwhm ) 7 ps) is assumed for both the excitation and probing light pulses; (2) at time t, the concentration [CM(t)] of a transient species responsible for band BM and that [CE(t)] of another transient species responsible for band BE are expressed by biexponential functions given by eqs 1 and 2, respectively,

CM(t) ) c1 exp(-k1t) + c2 exp(-k2t)

(1)

CE(t) ) c3[-exp(-k1t) + exp(-k2t)]

(2)

where k1 and k2 are the rate constants listed in Table 1, and c1, c2, and c3 are found to be positive experimental constants. As shown in Figure 3, addition of a high concentration of DMA (1-2 M) in CH3CN or EtOH causes the appearance of a broad emission band at wavelengths longer than those for the fluorescence band [the monomer emission band due to DBA*(S1)]. A similar result is also obtained in HP containing 0.5-1 M DMA or 1-2 M TEA. In these cases, the intensity of the monomer emission band decreases continuously but that of the broad emission band increases at first and then decreases with time. Furthermore, the time-dependent intensity changes of emission bands can be reproduced by their calculation using

Exciplex between DBA*(S1) and Ground-State Amine

J. Phys. Chem., Vol. 100, No. 47, 1996 18433

Figure 3. Monomer and exciplex emission bands observed for DBA in (a) CH3CN and (b) EtOH containing 1 or 2 M DMA. The spectral intensities are normalized at the emission peaks indicated by arrows.

SCHEME 2

the biexponential concentration functions of transient species responsible for the monomer emission band and the broad emission band given by eqs 1 and 2, respectively. Naturally, as listed in Table 1, the rate constants (k1 and k2) thus obtained are found to be equal to those obtained for the time-dependent intensity changes of transient absorptions. If quenching of DBA*(S1) by amine gives rise to the formation of an exciplex [1(DBA-amine)*] (cf. Scheme 2),19 the concentrations of DBA*(S1) and 1(DBA-amine)* at time t can be expressed by eqs 3 and 4, respectively:

[DBA*(S1)]t ∝

1 [(k + kM - λ2) exp(-λ1t) + λ1 - λ2 (λ1 - k - kM) exp(-λ2t)] (3)

[1(DBA-amine)*]t ∝

k [-exp(-λ1t) + exp(-λ2t)] λ1 - λ2

(4)

where the rate constants λ1 and λ2 are given by

Figure 4. Plots of k1 + k2 against DMA (O) or TEA (b) concentration in CH3CN, EtOH, and HP.

TABLE 2: Rate Constants for the Formation (kq) and Decay (k′, kE) of 1(DBA-Amine)* solvent

kq/1010 M-1 s-1

k′/109 s-1

kE/109 s-1

CH3CN-DMA EtOH-DMA HP-DMA HP-TEA

3.8 ( 0.2 2.7 ( 0.1 3.7 ( 0.2 0.91 ( 0.03

2.7 ( 0.3 11 ( 2 2.1 ( 0.2 1.0 ( 0.1

4.8 ( 0.5 40 ( 4 7.3 ( 0.7 5.1 ( 0.4

the rate constants kq and kE′ ) k′ + kE can be obtained from the slope and the intercept of eq 6, respectively; kM ()kf) is the decay rate constant of DBA*(S1) obtained previously in the absence of amine. By measurements of the time-dependent intensity changes of monomer and exciplex emissions at various amine concentrations, therefore, the rate constants (k1 and k2) are determined and a plot of k1 + k2 against DMA (open circles) or TEA (closed circles) concentration gives a straight line (cf. Figure 4). As listed in Table 2, however, the quenching rate constants (kq) thus obtained in CH3CN, EtOH, and HP containing DMA are 2.0-4.4 times greater than those estimated from the viscosities (η) of solvents20 using the Debye-Smoluchowski equation (kD ) 8RT/3000η) given for a diffusion-controlled reaction, i.e., kD ) 1.9 × 1010 M-1 s-1 in CH3CN, 6.1 × 109 M-1 s-1 in EtOH, and 1.7 × 1010 M-1 s-1 in HP at 25 °C. Since kq obtained in HP-TEA is 0.5 times smaller than kD, no explanation can be given for the difference between kq and kD. 2. The intensity ratio (I0/IA) of monomer emissions in the absence (I0) and presence (IA) of amine should be expressed by

I0/IA) λ1λ2/kM(k′ + kE)

λ1,2 ) (1/2) [k + k′ + kE + kM ( {(k + kM - k′ - kE) + 4kk′} ] (5) 2

1/2

Hence, one can conclude that eqs 1 and 2 are identical with eqs 3 and 4, respectively; i.e., both the absorption band (BE) shown in Figure 1 and the broad emission band shown in Figure 3 are responsible for 1(DBA-amine)*. In order to determine the rate constants for the formation (kq) and decay (k′, kE) of 1(DBA-amine)* shown in Scheme 2, the following measurements and calculations are performed. 1. Since the sum of rate constants λ1 and λ2 is expressed by

λ1 + λ2 ) k1 + k2 ) kq[amine] + k′ + kE + kM

(6)

) 1 + kq[amine] (1 - γ)/kM ) 1 + s′[amine]

(7)

where γ ) k′/(k′ + kE) is the efficiency for repopulation of DBA*(S1) from 1(DBA-amine)*. Hence, I0 and IA are calculated by integration of the corresponding fluorescence spectra over wavenumbers and I0/IA is plotted against amine concentration. Typical examples obtained in CH3CN-DMA are shown in Figure 5, where (a) the fluorescence spectrum due to the emission of DBA*(S1) decreases with increasing DMA concentration and (b) a plot of I0/IA (open circles) against DMA concentration gives a straight line with a slope s′ indicated. Similar results are obtained in EtOH-DMA and HP-amine

18434 J. Phys. Chem., Vol. 100, No. 47, 1996

Nakayama et al.

Figure 6. Plot of I0/IA (O) against TEA concentration in (a) CH3CN and (b) EtOH. s is the slope of a straight line drawn for I0/IA. Figure 5. (a) Intensity decrease of the monomer emission band upon addition of DMA and (b) plot of I0/IA (O) against DMA concentration in CH3CN. In (b), s′ is the slope of a straight line drawn for I0/IA.

with the following slopes of straight lines: s′ ) kq(1 - γ)/kM ) 29.3 M-1 in EtOH-DMA, 33.9 M-1 in HP-DMA, and 9.0 M-1 in HP-TEA. Hence, a combination of γ ) k′/(k′ + kE) derived from s′ with kE′ ) k′ + kE gives the decay rate constants (k′ and kE) of 1(DBA-amine)* as listed in Table 2. Although the decay rate constants obtained in CH3CN-DMA and HPamine are on the order of 109 s-1, those obtained in EtOHDMA are on the order of 1010 s-1; no reason for this discrepancy is presently understood. Quenching of the Lowest Excited Singlet State of DBA by TEA in CH3CN and EtOH. No emission band due to 1(DBA-TEA)* can be seen in CH CN and EtOH containing 3 e4 M TEA, although such an emission band is observed in neat TEA. Also, no exciplex absorption (band BE) is observed by picosecond laser photolysis of DBA in CH3CN and EtOH containing 1 M TEA, and band BM decreases with time following a single-exponential function. In neat TEA, however, both bands BM and BE can be seen and the rate constants obtained are k1 ) 1.0 × 1011 s-1 and k2 ) 2.5 × 109 s-1. Although the decay rate constant of band BM, i.e., kA ) 2.2 × 1010 s-1 in CH3CN-TEA (1 M) or 5.3 × 109 s-1 in EtOHTEA (1 M), is found to be equal to that obtained from the emission decay curve, kA is very large compared with the decay rate constant (kM ) 7.7 × 108 s-1 in CH3CN or 7.1 × 108 s-1 in EtOH) obtained in the absence of TEA. Hence, the decay rate constant (kA) of DBA*(S1) in CH3CN or EtOH containing e1 M TEA can be expressed by

kA ) kM + kq[TEA]

(8)

Then, the intensity ratio (I0/IA) of monomer emissions obtained in the absence (I0) and presence (IA) of TEA is given by

I0/IA ) 1 + kq[TEA]/kM

(9)

This indicates that the slope (s ) 29.2 M-1 in CH3CN-TEA or 6.5 M-1 in EtOH-TEA) of the straight line shown in Figure 6 should be equal to kq/kM. A choice of kM ) kf ) 7.7 × 108 s-1 in CH3CN or 7.1 × 108 s-1 in EtOH obtained in the absence of TEA gives the quenching rate constants of kq ) 2.3 × 1010 M-1 s-1 in CH3CN-TEA and 4.7 × 109 M-1 s-1 in EtOHTEA which are comparable with those (kD) estimated previously using the Debye-Smoluchowski equation, i.e., kD ) 1.9 × 1010 M-1 s-1 in CH3CN and 6.1 × 109 M-1 s-1 in EtOH. Hence,

Figure 7. Absorption spectral change upon steady-state photolysis of DBA and plot of Dt/D0 against photolysis time in CH3CN containing 0.04 M DMA.

quenching of DBA*(S1) by TEA can be attributed to a diffusioncontrolled reaction. In fact, the rate constant [kA ) 2.2 × 1010 s-1 in CH3CN-TEA (1 M) or 5.3 × 109 s-1 in EtOH-TEA (1 M)] obtained for the decay of DBA*(S1) with time is almost equal to that estimated by kcal ) kM + kq[TEA], i.e., 2.4 × 1010 s-1 in CH3CN-TEA (1 M) or 5.6 × 109 s-1 in EtOH-TEA (1 M). Relative Rates of Amine-Assisted DBA f 9-Bromoanthracene Debromination upon Steady-State Photolysis. As shown in Figure 7a, steady-state photolysis of DBA in CH3CN-DMA (0.04 M) causes the decrease of reactant absorption peaks (R1, R2, and R3) with time accompanied by the increase of product absorption peaks (P1, P2, and P3) which are identical with those of 9-bromoanthracene.9 Measuring the 402-nm absorbance of DBA at photolysis times 0 (D0) and t (Dt), Dt/D0 is plotted against photolysis time as shown in Figure 7b. Since similar results are obtained in CH3CN-TEA, EtOH-amine and HPamine, the relative rate (VR) of debromination is estimated by dividing the initial slope of absorbance decrement obtained in a given solvent by that obtained in CH3CN-DMA (cf. Table 3). Discussion Figures 1a and 2a indicate that the disappearance of band BE in CH3CN-DMA (1 M) results in the existence of a residual

Exciplex between DBA*(S1) and Ground-State Amine

J. Phys. Chem., Vol. 100, No. 47, 1996 18435

TABLE 3: Relative Rates (VR) of DBA f 9-Bromoanthracene Debromination in Solvents Containing 0.04 M Amine (DMA or TEA) and Dielectric Constants (E) of Pure Solvents VR (relative)

a

solvent

DMA

TEA

a

CH3CN EtOH HP

1.0 5.8 × 10-3 4.4 × 10-4

2.9 × 10-1 3.7 × 10-3 4.4 × 10-3

37.5 24.6 1.92

Reference 24.

absorption band. In EtOH-DMA (1 M) and HP-amine (1 M DMA or TEA), however, no residual absorption band can be seen after the disappearance of band BE (cf. Figures 1b and 2b). Since we have confirmed that this residual absorption band is identical with the absorption band of the radical anion (DBA•-) observed at the end of nanosecond pulse excitation of DBA in CH3CN-DMA (1 M),11 decomposition of 1(DBAamine)* into DBA•- and the amine radical cation in EtOHDMA (1 M) and HP-amine (1 M) may be extremely slow compared with that in CH3CN-DMA (1 M). In fact, nanosecond laser photolysis of DBA in the former solvents gives rise to no appreciable appearance of the absorption band responsible for DBA•-. As shown in Figure 1, the intensity of a weak band (with an absorption maximum at ∼475 nm) in CH3CN-DMA is greater than that in EtOH-DMA; although this band is very similar to that of the DMA radical cation,21-23 no such a band is observed in HP-amine. Since a chargetransfer character (χ) sometimes enhances the absorption intensity, χ of 1(DBA-amine)* may decrease in the order of χ (in CH3CN-DMA) > χ (in EtOH-DMA) . χ (in HP-amine) and this may be a cause for observation of the absorption band due to DBA•- [generated from 1(DBA-DMA)*] only in CH3CN-DMA (1 M) upon picosecond laser photolysis. By nanosecond laser photolysis of DBA in CH3CN-TEA (1 M), the absorption band of DBA•- can be clearly seen with an absorbance of ∼0.03.11 Since this absorbance is nearly equal to the baseline fluctuation ((0.02 absorbance unit) of the present picosecond laser photolysis system,17 no observation of the absorption band due to DBA•- by picosecond laser photolysis of DBA in CH3CN-TEA (1 M) may be reasonable. For DBA in EtOH-TEA (1 M), however, no absorption band of DBA•can be seen even by nanosecond laser photolysis. Also no emission originating from 1(DMA-TEA)* can be seen in both CH3CN and EtOH containing TEA as stated previously. In spite of these circumstances, Table 3 indicates that the relative rate (VR) of DBA f 9-bromoanthracene debromination upon steadystate photolysis in CH3CN-TEA is 1 order of magnitude smaller than that in CH3CN-DMA but 2 or 3 orders of magnitude greater than those in EtOH and HP containing amine; however, VR in EtOH-TEA is nearly equal to that in EtOH-DMA. All the facts stated above suggests that diffusion-controlled quenching of DBA*(S1) by TEA in CH3CN and EtOH gives rise to the formation of an encounter complex or an ion pair; i.e., this short-lived complex with no detectable emission and absorption intensities is different from an exciplex proposed in Scheme 1. Since the dielectric constant (37.5) of CH3CN is much greater than those of EtOH (24.6) and HP (1.92),24 it can be concluded that generation of DBA•- is essential for amineassisted debromination of DBA and that decomposition of

1(DBA-amine)* or an encounter complex (or an ion pair) into DBA•- and the amine radical cation is affected by the dielectric constant of a pure solvent. This is supported by the well-known fact in radiation chemistry,25-29 where organic halides are frequently used as effective electron scavengers and a so-called dissociative electron attachment reaction occurs; i.e., a solvated electron in polar solvents easily attaches to the organic halides generating the radical anions followed by dissociation into the dehalogenated neutral radicals and the halogen anions.

Acknowledgment. This work was supported by a Grantin-Aid for Priority-Area-Research on Photoreaction Dynamics from the Ministry of Education, Science, Sports and Culture of Japan (No. 06239101). References and Notes (1) Ohashi, M.; Tsujimoto, K.; Seki, K. J. Chem. Soc., Chem. Commun. 1973, 384. (2) Tsujimoto, K.; Tasaka, S.; Ohashi, M. J. Chem. Soc., Chem. Commun. 1975, 758. (3) Bunce, N. J.; Pilon, P.; Ruzo, L. O.; Sturch, D. J. J. Org. Chem. 1976, 41, 3023. (4) Bunce, N. J.; Kumar, Y.; Ravanal, L.; Safe, S. J. Chem. Soc., Perkin Trans. 2 1978, 880. (5) Chittim, B.; Safe, S.; Bunce, N.; Ruzo, L.; Olie, K.; Hutzinger, O. Can. J. Chem. 1978, 56, 1253. (6) Davidson, R. S.; Goodwin, J. W. Tetrahedron Lett. 1981, 22, 163. (7) Saeva, F. D. Topics Current Chem. 1990, 156, 61. (8) Fulara, J.; Latowski, T. Polish J. Chem. 1990, 64, 369. (9) Hamanoue, K.; Tai, S.; Hidaka, T.; Nakayama, T.; Kimoto, M.; Teranishi, H. J. Phys. Chem. 1984, 88, 4380. (10) Hamanoue, K.; Nakayama, T.; Ikenaga, K.; Ibuki, K. J. Phys. Chem. 1992, 96, 10297. (11) Hamanoue, K.; Nakayama, T.; Ikenaga, K.; Ibuki, K.; Otani, A. J. Photochem. Photobiol. A: Chem. 1993, 69, 305. (12) Nakayama, T.; Ibuki, K.; Hamanoue, K. Proc. Indian Acad. Sci. (Chem. Sci.) 1993, 105, 567. (13) Tanaka, M.; Tanaka, I.; Tai, S.; Hamanoue, K.; Sumitani, M.; Yoshihara, K. J. Phys. Chem. 1983, 87, 813. (14) Hamanoue, K.; Hidaka, T.; Nakayama, T.; Teranishi, H. Bull. Chem. Soc. Jpn. 1983, 56, 1851. (15) Hamanoue, K.; Nakayama, T.; Ikenaga, K.; Ibuki, K. J. Photochem. Photobiol. A: Chem. 1993, 74, 147. (16) Ohta, N.; Tamai, N.; Kuroda, T.; Yamazaki, T.; Yamazaki, I. Chem. Phys. 1993, 177, 591. (17) Ushida, K.; Nakayama, T.; Yuhara, Y.; Ito, M.; Hamanoue, K.; Teranishi, H.; Matsui, T.; Nagamura, T. Radiat. Phys. Chem. 1989, 34, 465. (18) Szczepan´ski, J.; Heldt, J. Z. Naturforsch. 1985, 40a, 849. (19) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: New York, 1970; Chapter 7. (20) Weast, R. C. Handbook of Chemistry and Physics, 62nd ed.; CRC Press: Boca Raton, FL, 1981. (21) Hamil, W. H. Radical Ions; Kaiser, E. T., Kevan, L. Eds.; Wiley: New York, 1968; Chapter 9. (22) Shida, T. Electronic Absorption Spectra of Radical Ions; Elsevier: Amsterdam, 1988. (23) Iwamura, H.; Eaton, D. F. Pure Appl. Chem. 1991, 63, 1003. (24) Riddick, J. A.; Bunger, W. B. Organic SolVents, 3rd ed.; Techniques of Chemistry 2; Wiley-Interscience: New York, 1970. (25) Arai, S.; Tagawa, S.; Imamura, M. J. Phys. Chem. 1974, 78, 519. (26) Neta, P.; Behar, D. J. Am. Chem. Soc. 1980, 102, 4798; J. Am. Chem. Soc. 1981, 103, 103. (27) Behar, D.; Neta, P. J. Phys. Chem. 1981, 85, 690; J. Am. Chem. Soc. 1981, 103, 2280. (28) Kigawa, H.; Takamuku, S.; Toki, S.; Kimura, N.; Takeda, S.; Tsumori, K.; Sakurai, H. J. Am. Chem. Soc. 1981, 103, 5176. (29) Hamanoue, K.; Kimoto, M.; Nakayama, T.; Teranishi, H.; Tagawa, S.; Tabata, Y. Radiat. Phys. Chem. 1984, 24, 445.

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