Excited state dynamics of anthraquinones and electron transfer from ground-state triethylamine to the second and/or lowest excited triplet states of anthraquinones

Excited state dynamics of anthraquinones and electron transfer from ground-state triethylamine to the second and/or lowest excited triplet states of anthraquinones

K U M A O H A M A N O U E * a n d T O S H I H I R O N A K A Y A M A

Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan

Abstract. Picosecond laser photolysis of 1,8-dibromoanthraquinone (I,8-DBAQ) and 1,8-dichloroanthraquinone (1,8-DCAQ) in solutions at room temperature has revealed the existence of the second excited nn* triplet [ T2(nn*)] states with a localized charge-transfer character between the halogen and oxygen atoms. The internal conversion (IC) times from the T2 (nn*) states to the lowest excited ltn* triplet [ 1"10rn* )] states are 70-110 ps for 1,8-DBAQ

and 700-750 ps for 1,8-DCAQ. In toluene and ethanol, an electron transfer from ground-state triethylamine (TEA) to triplet XAQ (anthraquinone and halogenoanthraquinones) forming the exciplexes [3(XAQ - TEA)*] has been found to occur via the T 1 states of XAQ; these 3(XAQ- TEA)* change to the contact ion pairs between the XAQ radical anions (XAQ ~) and the TEA radical cation. Neither free XAQ ~ nor 3(XAQ -TEA)* are produced via the T 2 states of XAQ in toluene and ethanol. In acetonitrile, however, free XAQ ~ and 3(XAQ - TEA)* or the ion pairs (or the contact ion pairs) are produced via the T2 and T1 states of XAQ, respectively.

Keywords. Excited state dynamics of anthraquinones; photoinduced intermolecular electron transfer; triethylamine; the second and lowest excited triplet states of anthraquinones.

1. Introduction

T h e photophysical and photochemical behaviour o f quinones has received considerable attention o n a c c o u n t of its i m p o r t a n c e in u n d e r s t a n d i n g the action o f light u p o n biological materials. W h e n the nn* a n d nn* triplet states o f a r o m a t i c c a r b o n y l c o m p o u n d s are very close to each other, however, it is possible to shift the relative energy of the triplet states by substitution in the molecule itself, this being particularly true of n o n - p l a n a r a n t h r a q u i n o n e (AQ) derivatives such as 1 - c h l o r o - A Q (1-CAQ), 1 - b r o m o - A Q , 1,5-dichloro-AQ (1,5-DCAQ), 1 , 5 - d i b r o m o - A Q , 1,8-dichloro-AQ (1,8- D C A Q ) , 1,8-dibromo-AQ (1,8-DBAQ), 1,2-di-tert-butyl-3-trimethylsilyl-AQ and 1,2,3- tri-tert-butyl-AQ ( H a m a n o u e et a11983, 1987a; N a k a y a m a et a11988b): (1) The lowest excited triplet ( T t ) states o f these n o n - p l a n a r a n t h r a q u i n o n e s are of mixed nn*-nn*

or nn* character, while the excited nn* triplet states are the lowest for p l a n a r a n t h r a q u i n o n e s such as A Q , 2 - c h l o r o - A Q (2-CAQ), 2 - b r o m o - A Q , 1-tert-butyl-AQ a n d 1,2-di-tert-butyl-AQ; the very short triplet lifetimes o f n o n - p l a n a r a n t h r a q u i n o n e s with mixed m t * - n n * o r nn* character are ascribed to the distortion of the geometrical

* For correspondence

219

220 Kumao Hamanoue and Toshihiro Nakayama

structure of molecules; for ~-halogenoanthraquinones, the internal heavy-atom effects of halogen atom(s) also affect the triplet lifetimes. (2) Steady-state photolysis of

~-halogenoanthraquinones with 313 nm light in ethanol at room temperature yields the corresponding ~=halogenoanthrahydroquinones, while 366 nm photolysis gives rise to the following sequence: the formation of ~-halogenoanthrahydroquinones is followed by photochemical dehydrohalogenation yielding the corresponding 0t-halogenoanthraquinones with one less halogen atom than the original ones; the final photoproduct is anthrahydroquinone (Hamanoue et al 1986a, d); for fl-halogeno- anthrahydroquinones, howeve.r, no photochemical dehydrohalogenation is observed.

(3) The hydrogen-atom abstraction by ~t-halogenoanthraquinones from ethanol originates from the 7"1 states in spite of their mixed nn*-rcn* or r~rc* character, and the rate of hydrogen-atom abstraction decreases with increasing nz* character of the T~ states (Hamanoue et al 1984, 1986a, c); this result is consistent with that reported for the triplet nn* states of substituted ketones having 10-2-10 -4 times lower intrinsic reactivity for hydrogen-atom abstraction than that of the nn* triplet states (Porter and Suppan 1964, 1965; Formosinho 1978).

In connection with the interest in photophysics and photochemistry of non-planar anthraquinones, this review deals with the triplet-state dynamics of 1,8-DCAQ and 1,8-DBAQ as well as electron transfer from ground-state triethylamine to triplet anthraquinones (XAQ, i.e. AQ and the halogeno compounds) in several solutions at room temperature.

2. The existence of the second excited triplet states of anthraquinones in solutions at room temperature

Figure 1 shows the time-resolved absorption spectra obtained by picosecond laser photolysis of 1,8-DCAQ in toluene (Hamanoue et a11986b, 1987b). Clearly the spectra in the first picosecond time regime have three absorption band (bands A1, B and A2) and band A I decreases with accompanying increase of the band B which is characteristic of the triplet-triplet ( T ' ~ T1) absorption spectrum (Hamanoue et al 1983, 1987a). Moreover, the existence of two isosbestic points can be clearly seen;

plots of intensity changes of bands A ~ (open circles) and B (closed circles) with time are shown in figure 2. Assuming that the lowest excited singlet state is produced

0~

o A2 B i 60ps

\ \ \

~ . \ ~ . " ~ .

t

J / / 1 ns

450 500 550 600

Wavelength Into

Figure L Time-resolved absorption spectra obtained by picosecond laser photolysis of 1,8-DCAQ in toluene.

0 ~

OO6

u t- O

. D

0D2

3.12

"~" / ~'~ " ~

o

.;-~~

/ ~ ' 9 0

.. , " ' "

3.08

)04

0 9

I i ~ ~ j [ , ~ , , I I I I I I I

0 05 1.0 1.5 2.0

Belay time / ns

Figure 2. Intensity changes of bands A t (O) and B (0) with time. Full and dashed curves are the theoretical absorption changes with time for the P and T states, respectively.

instantaneously after the excitation of the second excited singlet state, followed by a single-exponential decay to a precursor (called the P state hereafter), the concentrations of the P and TI states at time t are given by

Cr(t)= f'_ l'(t')klk lk {exp[-k2(t-t')]-exp[-kl(t-t')]}dt',

⁽¹⁾

f ' _ oo/'(t'){1 k_~2k~ e x p [ - k l ( t kl t')]

Cr(t) = +

^-

k~ exp[-

^{k 2 ( t -}

t')] }dt', (2)

kl k2

where k I and k 2 are the first-order rate constants for the buildup and decay of the P state, respectively, and

l'(t')

is the Gaussian intensity of the excitation-light pulse.

When the concentration of a given transient species at time t is denoted by

C(t),

the theoretical absorbance at delay time ~, D(O, can thus be obtained by

D ( , ) = - l o g [ f : l(t-,)lO-yC(~ I(t-Odt],

⁽³⁾

where

l(t - ~)

is the Gaussian intensity of the probing-light pulse, and y is chosen such that D(~) is equal to the absorbance actually observed at a given delay time. By the best fit of the theoretical absorbances (full curve) to the experimental ones (open circles) in toluene, ethanol, carbon tetrachloride and EPA (diethyl ether/isopentane/

ethanol = 5:5:2 in volume ratio), the rate constants have been found to be kl = 2.9- 3"3 x 101~ s- 1 and k 2 = 1.3 - 1-4 x 10 9 s - 1, indicating that the rate constants obtained from the intensity change of band A1 are consistent with those obtained from the intensity change of band B (cf. closed circles and dashed curve): The change of band A 2 with time can well be interpreted in terms of superposition of the absorption due to the P state on that due to the T 1 state.

As shown in figure 3, the time-resolved absorption spectra obtained for 1,8-DBAQ

222 Kumao Hamanoue and Toshihiro Nakayama

Q2 L ~ 0 ps

Ol i i - i I t , ,.i, 1 I~, , i I , ~ ' r ' , ~ , ~

i

I-.- , ^! , ⁱ , ⁱ

60 ps

" : '

I

c

,

^'

I

^{100 ps}

~

QO.2

o r - , ,

,!,, ,,'.,, ! , ,

< l- : I ! 200ps

~ 450 500 550

Wavelength / nm

Figure 3. Time-resolved absorption spectra obtained by picosecond laser photolysis of 1,8-DBAQ in toluene (full curves). The dotted curve is the simulated absorption spectrum of the P state at 60 ps delay.

clearly indicates that band A builds up and decays in the first picosecond time regime, while bands B, and B 2 increase continuously (Nakayama et al 1988a; Hamanoue et al 1991b). Since bands B1 and B 2 are characteristic of the T'*-- T 1 absorption spectrum (Hamanoue et al 1983, 1987a), band A can safely be ascribed to a superposition of the absorption due to the P state on that of the T 1 state of 1,8-DBAQ.

Thus, the concentration of the P state at delay time ~ can be expressed by the following equations;

flCp(~) = D(2a) - aD(2b), (4)

fl = ~ ( ~ ~ - ~e~(X~), (5)

~t = e T ( 2 a ) / e T ( 2 b ), (6)

where D(2) is the absorbance of the transient absorption spectrum at wavelength Z(= 2, and 2b) and delay time r and e~, and er are the molar absorption coefficients of the P and T1 states, respectively. Since the value of ct can be calculated from the pure T',-- 7"1 absorption spectrum and ,8 is independent of time ~, the absorption change due to the P state can be found by plotting the terms on the right-hand side of (4) against time as shown in figure 4 (open circles); by the best fit of the theoretical absorbances (full curve) to the experimental ones (open circles) in toluene, ethanol and carbon tetrachloride, the rate constants are determined to be k 1 = 5.0-6.7 x 101~ s- 1 and k 2 = 0.91-1.4 x 101~ s- 1.

Since the spectrum at 2 ns delay in figure 3 is due to the absorption of the pure

?'1 state and its theoretical concentration at time t is given by (2), the intensity of the

,-- 0.1 _ca

,<

"6

1

,< O

9 0.I

. , r o o-'--...~ I o_

0 50 I00 150 200 250 300 ~ Deloy time I ps

Figure 4. Changes of D( 2o) - oeD( 2b ) ( O ) and D(2c) - ee' D(2d) ( O ) with time. Ful] and dashed curves are the theoretical absorption changes with time for the P and T~ states, respectively.

pure T ' ~ T 1 absorption spectrum at delay time ~ can be calculated by (3) using the values of kl and k2 obtained above. Subtracting the calculated T ' ~ TI absorption spectrum from the observed spectrum, one can simulate the absorption spectrum of the pure P state at delay time of ~. In figure 3, we also display the simulated absorption spectrum (dotted curve) due to the pure P state at a delay time of 60 ps. From this spectrum, one can evaluate the values of ~' ⁼

F,p(2c)/F,p(•d)

and the following equation can easily be derived:

fl'Cr(~) = D0.c) - ~'O(2d), (7)

fl'

= ~ T ( 4 ) -- ~ ' ~ T ( 4 ) - (8)

Thus, the relative concentration of the T~ state at delay time ~ can be calculated from the terms on the right-hand side of (7) and the values obtained are also shown by closed circles in figure 4, together with the best-fit ones (dashed curve) calculated by (2) and (3); the rate constants obtained from the absorption change of the T1 state are consist.ent with those obtained from the absorption change of the P state.

Since the absorption and phosphorescence spectra of 1,8-DCAQ and 1,8-DBAQ reveal that the lowest excited singlet ($1) and triplet (T1) states are of nrr* and nn*

character, respectively (Hamanoue et al 1983, 1987a), we conclude that the P state is the second triplet (T2) state of 1,8-DCAQ (and 1,8-DBAQ) with nrr* character.

Thus the rate constants kl and k2 obtained should correspond to those for the

S l ( n r t * ) ~ Z 2 (n/~*) intersystem crossing (ISC) and the T 2 ( n • * ) ~ T1 (rcn*) internal conversion (IC), respectively. The shorter ISC time (15-20 ps)for 1,8-DBAQ compared with that (30-35 ps) for 1,8-DCAQ, therefore, may reflect the larger heavy-atom effect of the bromine atom compared with that of the chlorine atom, because it is generally accepted that the singlet nrr* state undergoes inherently weak spin-orbit coupling with the triplet nn* state. As a consequence, the magnitude of the spin-orbit coupling induced by the internal heavy-atom effect may be a cause of the slow S~(nn*)~ T2(nrr*) ISC. Also, the conformational distortion of molecules may accelerate the ISC rate based on the following facts: (1) The short Sl(nn*)--* Tl(nn*) ISC time (Sps) for benzophenone is ascribed to the non-planarity of the molecule, resulting in coupling between the nn* and ~trr* states in both the singlet and triplet manifolds (Damschen et al 1978). (2) By the method of M N D O calculation, we have confirmed that the molecular structures of ~-halogenoanthraquinones are remarkably different from those of planar compounds (AQ and fl-halogeno-AQ) owing to steric hindrance between the oxygen and halogen atoms, and 1,8-dihalogeno compounds have a

224 Kumao Hamanoue and Toshihiro Nakayama

dish-like structure; since the conformational distortion of the bromo compounds is expected to be greater than that of the corresponding chloro compounds, the faster ISC rate for 1,8-DBAQ compared with that for 1,8-DCAQ is reasonable.

Although no clear absorption bands characteristic of the T2 states have been observed for AQ, 1-CAQ, 2-CAQ and 1,5-DCAQ, analysis of the change of the transient absorptions with time has led us to the conclusion that the T2 states also exist below the S~ states and the T 2 ~ Tt IC time is less than 70ps (Nakajima 1983).

3. Electron transfer from ground-state triethylamine to the Tt states of anthraquinones in toluene and ethanol at room temperature

Figure 5 shows the time-resolved absorption spectra obtained by picosecond laser photolysis of 1,8-DCAQ in toluene/TEA(1 M) (Hamanoue et al 1985a, 1991a). The spectrum at 100 ps delay is identical to that observed in the absence of TEA, where the spectrum is assigned to a superposition of the T" ~ T2 and T' ~-- T1 absorptions of 1,8-DCAQ. At delay times longer than lOOps, a new absorption (band D) grows in with accompanying decrease of the T' ~ 7"1 absorption and a clear isosbestic point can be seen at 517nm. This indicates that a new intermediate is produced at the expense of the T1 state of 1,8-DCAQ.

As shown in figure 6, the time-resolved absorption spectrum (with band D) obtained at the end of nanosecond pulse excitation (0 ns delay) in toluene/TEA(0.2 M) is almost identical with that obtained by picosecond laser photolysis at 2 ns delay, while the

-- ~ & 2 n s

cO.2 " " L)/ Tins

e"

~ Cl 0 L - - ' J u

~0.2 x . A 2 B A I

<I: ~ lOOps

I , i , I, ' , i I , ' ; , 1 1 ~

0 450 500 550 600

Wavelength ^{I n m}

Figure 5. Time-resolved absorption spectra obtained by picosecond laser photolysis of 1,8-DCAQ in toluene/TEA.

0.3 002

~

^0.I

~ o

00.3

~ o2

0.1 0

~I$ jlOus

400 500 600 700

Wavelength / nm

Figure 6. Time-resolved absorption spectra obtained by nanosecond laser photolysis of 1,8-DCAQ in toluene/TEA.

spectrum (with band E) obtained at 1 #s delay is shifted to shorter wavelength by

~ 5 nm; after then, band E decreases accompanied by the appearance of a new absorption (band F) which is identical with the absorption band of 1,8-dichloroanth- rahydroquinone. Similar results are obtained not only for 1,8-DCAQ in ethanol/TEA, but also for AQ, 1-CAQ, 2-CAQ, 15-DCAQ and 1,8-DBAQ both in toluene and ethanol containing TEA (Hamanoue et al 1985a, c, 1988, 1991a, b), where the transient absorptions grow in within the duration of nanosecond pulse excitation and then increase slowly with time following first-order reaction kinetics accompanied by a shift of bands D to bands E by 3-5 nm in toluene/TEA and 5-10 nm in ethanol/TEA, and the decay of bands E (accompanied by the formation of anthrahydroquinones) can also be analyzed by first-order reaction kinetics. Since no transient photocurrents are detected, we have concluded that the intermediates with bands D are the exciplexes [3(XAQ-TEA)*] of the lowest triplet anthraquinones [3XAQ*( i/'1)] with ground- state TEA, and those with bands E are the contact ion pairs [3(XAQ--TEA t ) ] between the radical anions (XAQ .+) of XAQ and the radical cation (TEA t ) of TEA;

these contact ion pairs finally disappear by proton transfer from TEA -+ to XAQ-, generating the anthrasemiquinone radicals (XAQH.) and the triethylamine radical (TEA.).

Except for 1,8-DCAQ in toluene/TEA, the decay constants of the exciplexes increase with increasing TEA concentration, but the decay constants of the contact ion pairs are almost independent of TEA concentrations. We thus have proposed the existence of a rapid interconversion between 3(XAQ-TEA)* and the triplexes [3(XAQ-TEA2)*]

of 3(XAQ - TEA)* with TEA. For 1,8-DCAQ in toluene/TEA, however, not only the decay constant of the exciplex but also that of the contact ion pair decrease with increasing TEA concentration, indicating that there also exists a rapid interconversion between 3 ( X A Q - - T E A t ) and a complex [a(XAQ--TEA .+ )] of the 1,8-DCAQ radical anion with the TEA dimer radical cation. The generalized kinetic scheme is as follows (scheme 1, Hamanoue et al 1991a):

TEA3 ,

XAQ + TEA +- a ( X A Q - T E A ) * ~- ( X A Q - T E A 2) --, XAQ + TEA,-- 3 ( X A Q _ _ T E A .+ )T~3 (XAQ_: TEA2 t ) - ,

X A Q H - Scheme 1.

4. Electron transfer from ground-state TEA to the T 2 and Tt states of anthraquinones in acetonitrile at room temperature

Figure 7 shows the time-resolved absorption spectra obtained by picosecond laser photolysis of 1,8-DCAQ in acetonitrile/TEA(1M) (Asada et al 1990). A new absorption (band X with 2max = 590 nm) grows in at first and then another absorption (band Y with 2max = 550 nm) appears gradually up to 2 ns delay; the overall spectral profile at this delay time looks like a superposition of bands X and Y. Since band X can be observed even at a shorter delay time where the concentration of the T 1 state of 1,8-DCAQ is negligibly small, the intermediate with band X may be produced via the T2 state.

226 Kumao Hamanoue and Toshihiro Nakayama

I' 2

_ 1 , 5

z~50 500 550 600 650

Wavelength / nm

Figure 7. Time-resolved absorption spectra obtained by picosecond laser photolysis of 1,8-DCAQ in acetonitrilefrEA.

~O,E

u C

g

0.4

0 n

9 ~ a21

t

400 450 500 550 600 650 700

Wavetength t nm

Figure 8. Time-resolved absorption spectra obtained by nanosecond laser photolysis of 1,8-DCAQ in acetonitrile/TEA.

As shown in figure 8, the time-resolved absorption spectrum obtained at the end of nanosecond pulse excitation of 1,8-DCAQ in acetonitrile/TEA(0.2 M) is rather similar to that obtained by picosecond laser photolysis at 2 ns delay, but the spectral intensity increases with time up to 1 #s delay accompanied by the increase in intensity ratio of band X to band Y. This indicates that the intermediate with band Y changes to that with band X in the submicrosecond time regime. Similar results are also obtained for AQ and other halogenoanthraquinones, indicating the shift of bands Y by 34-47 nm to bands X at longer wavelengths (Hamanoue et al 1985b, 1991b; Asada et al 1990). Since the single-exponential rise of transient absorptions (and the red-shift of bands Y to band X) are accompanied with the rate-matching increase of transient photocurrents, and since the decay of bands X [accompanied with the formation of the anthrahydroquinones (XAQH2) and their monoanions (XAQH-:)] can be analyzed by second-order reaction kinetics, we have assigned bands X to the absorptions of the free radical anions (XAQ-:) produced by the direct electron transfer from the ground-state TEA to 3XAQ*(T2). In fact, bands X are very similar to absorptions of the free radical anions (XAQ: ) produced by the direct electron transfer I, 8-DCAQ (Asada et al 1990) observed by nanosecond pulse radiolysis in acetonitrile without TEA. Since picosecond laser photolysis reveals the increment of bands Y with accompanying decrement of the T ' ~ T 1 absorptions and since bands Y have

the absorption maxima at the wavelengths rather similar to those for bands D (due to the absorptions of the exciplexes) and E (due to the absorptions of the contact ion pairs) produced in toluene, we have concluded that bands Y are the absorptions of the exciplexes [3(XAQ-TEA)*] or the ion pairs (or the contact ion pairs) [a(XAQ-- TEA +. )] produced by the reaction of 3XAQ*(7"1) with ground-state TEA, and have proposed the following kinetic scheme (scheme 2).

X A Q - % 3 X A Q * (T 2) TE-~-*A X A Q - + T E A "+ - * X A Q H " + TEA" o X A Q H 2 + X A Q H -

1' i

i

, by picosecond laser photolysis . . . . , b y nanosecond laser p h o t o l y s i s

Scheme 2.

In fact, the intensity ratios ofbands X to bands Y calculated for the spectra obtained by picosecond laser photolysis at 2 ns delay, i.e. 1-01 (2-CAQ), 1"04 (AQ), 1.05 (1-CAQ), 1"08 (1,5-DCAQ), t-27 (1,8-DBAQ) and 1.54 (1,8-DCAQ), increase with increasing T z ~ T~ IC times, i.e. less than 70ps for AQ, I-CAQ, 2-CAQ and 1,5-DCAQ, 70-110 ps for 1,8-DBAQ and 700-750 ps for 1,8-DCAQ.

5. Conclusions

The behaviour of the T2(nn*) states observed for 1,8-DBAQ and 1,8-DCAQ differs greatly from that of the usual nx* triplet states in the following ways: (1) T2 ~ T1 IC rates are unusually slow, suggesting that the T2 states are quite different from the Ta states with respect to electronic character and geometrical structure (Schlag et al 1971; Long et al 1973; Khalil et al 1977); (2) no intermolecular exciplex formation between ground-state TEA and the T 2 states of 1,8-DBAQ and 1,8-DCAQ is observed in toluene, ethanol and acetonitrile, while direct electron transfer from the ground-state TEA to the T2 states of 1,8-DBAQ and 1,8-DCAQ yielding their radical anions is observed only in acetonitrile.

We believe that the T" ~ T~ absorption bands observed here are characteristic of 1,8-DBAQ and 1,8-DCAQ, because no such absorption bands are observed for non-planar tert-butyl-anthraquinones (the 1, 2-di-tert-butyl-3-trimethylsilyl and 1,2, 3- tri.tert-butyl compounds), although the behaviour of their T1(nn*) states is very similar to that of 1,8-DBAQ and 1,8-DCAQ (Nakayama et al 1988b). Based on the differences in the electronegativities of the oxygen, chlorine and bromine atoms, we have interpreted the unusual behaviour of the T2(n~*) states of 1,8-DBAQ and 1,8-DCAQ in terms of the transfer of charge from the halogen to oxygen atoms, resulting in a localized charge-transfer character with no reactivity for the exciplex formation between the T 2 states of 1,8-dihalogenoanthraquinones and ground-state TEA. Since the localized charge-transfer character in the T 2 state of 1,8-DBAQ is expected to be greater than that of 1,8-DCAQ, the T 2- T1 energy gap for 1,8-DBAQ may be smaller than that for 1,8-DCAQ. Thus, the shorter T2 ~ TL IC time for

228 Kumao Hamanoue and Toshihiro Nakayama

1,8-DBAQ (70-110ps) compared with that for 1,8-DCAQ (700-750ps) may be ascribed not only to the smaller T2-T1 energy gap but also to the greater conformational distortion of 1,8-DBAQ than that of 1,8-DCAQ.

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