Quenching of excited states of xylenols due to hydrogen bonding with triethylamine in different solvents

Journal of Luminescence 36 (1987) 339—346 North-Holland, Amsterdam

339

QUENCHING OF EXCITED STATES OF XYLENOLS DUE TO HYDROGEN BONDING WITH TRIETHYLAMINE IN DIFFERENT SOLVENTS T.K. PAL, G.K. MALLIK, T. GANGULY and S.B. BANERJEE

*

Optics Department, Indian Association for the Cultivation of Science, Jadavpur, Calcutta — 700032, India Received 23 May 1986 Revised 30 September 1986 Accepted 4 November 1986

Hydrogen bonding between xylenols in the ground state and triethylanune (TEA) brings about red shift and increase of intensity of the absorption bands of the xylenol molecules. TEA quenches fluorescence of the xylenols at 300 K in varying degrees, possibly through a very short-lived CT species formed by strong hydrogen bonding interaction between excited xylenols and 1’EA, which leads to large charge transfer. At 77 K, restricted solvent relaxation or orientation impedes formation of such CT complex and consequently impedes the quenching process. Quenching is weak and absent in polar aprotic and alcoholic solvents, respectively. The monoexponential nature of phosphorescence decay of xylenols indicates establishment of very rapid equilibrium between hydrogen-bonded complexes in the triplet state and free molecules.

1. Introduction Quenching of fluorescence of aromatic molecules in nonpolar or polar protic and aprotic solvents by amines has evoked a great deal of interest in recent years [1—6]. In highly polar solvents, formation of free solvated radical ions enhances quenching of fluorescence by an amine [5]. Tertiary amines are reportedly the most efficient quenchers among the amines [5]. Charge transfer (CT) complexes between tertiary amine and naphthalene compounds in solution have been reported [7], and the transfer of charge has been shown to occur from the electron donor amine molecules to the excited state of the acceptor naphthalene and its derivatives. It has been shown by Beecroft and Davidson [8] that in cyclohexane solution the primary process for quenching of benzene fluorescence by tertiary amine is energy transfer. Weller [9,10] observed that photoexcited aromatic hydrocarbons form emissive exciplexes with tertiary amines in nonpolar solvents. In solvents of high dielectric constant the quantum *

To whom all correspondence should be sent.

yield and lifetime of exciplex emission decrease due to formation of radical ions. Triethylamine (TEA), a tertiary amine which has no ~-e1ectrons, has been widely used as quencher, and quenching of fluorescence by TEA through CT interaction or hydrogen bonding has been discussed by different authors [4,11—17].Kuzniin and Guseva [14] reported that TEA quenches fluorescence of hydrocarbons in nonpolar and polar solvents, though it does not produce any change in their absorption spectra. In addition a new broad structureless band, assigned by the authors to CT, appears in the 450 mn region. In connection with fluorescence of $-naphthol in cyclohexane and TEA, Mataga and Kaifu [15] expressed the view that TEA might not be a good quencher in spite of a large amount of charge transfer to the —OH antibonding orbital of excited naphthalene because the interaction is of a-type. Leonhardt and Weller [11] showed that TEA quenches fluorescence of perylene through a very short-lived charge transfer complex. According to Mataga and Kaifu [15] the n-orbital of TEA and the it-electron system of perylene might be involved in a direct interaction. Interaction between the n-orbital of TEA and

0022-2313/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

340

T. K. Pal et a!.

/

Quenching of excited states of xvlenols due to hydrogen bonding with TEA

excited THQ has been discussed in a previous paper [17]. TEA can act as a strong electron donor capable of strong hydrogen bonding interaction with a molecule having an electron acceptor or proton donor group. There is thus a distinct possibility of large charge transfer, which may lead to the formation of a transient complex involving TEA and the electron acceptor molecules, through which fluorescence quenching may occur. It was therefore thought worthwhile to measure fluorescence emission of some xylenols in polar and nonpolar solvents at 300 K and 77 K with and without addition of TEA, with the object of de-

excitation wavelength selected for the xylenols was 280 nm and the emission wavelength for studying phosphorescence decay was 400 nm. After the phosphorescence intensity achieved a steady state, the exciting radiation was cut off to obtain the dark decay on the recorder. Phosphorescence decay time for 2,6-xylenol (— milliseconds) was measured with a TS-8123 storage scope. The fluorescence lifetimes of the systems were measured with an Applied Photophysics Nanosecond Spectrophotometer employing the time-correlated single photon counting technique. The source was a N2-filled nanosecond flash lamp with one

termining the kinetic behaviour due to hydrogen bonding with TEA and the probable routes for nonradiative energy loss from the excited xylenol molecules. This paper is concerned with the observed results and their interpretation.

atmospheric pressure. The FWHM of the pump function was of the order of 1.5 ns. The decay functions were deconvoluted with the Applied Photophysics Decon programme.

3. Results and discussion 2. ExperimentaL The samples 3,5-, 2,3- and 2,6-xylenol (Fluka, AG. Switzerland) were used after vacuum sublimation, and 3.5-dimethylanisole (Aldrich Chemical Ltd., USA) was distilled under reduced pressure. Triethylamine (TEA) obtained from Fluka was distilled at atmospheric pressure. Solvents cyclohexane, acetonitrile, methanol (MeOH), ethanol (EtOH) (E. Merck) were of spectroscopic grade. They were distilled under vacuum and tested for absence of any emission in the wavelength regions studied. Methylcyclohexane (MCH) obtamed from Fluka was purified by the same procedure described elsewhere [18]. At 77 K MCH was used as an inert solvent to obtain a glassy matrix. The emission spectra were recorded with a Perkin—Elmer model MPF 44A fluorescence spectrophotometer attached with corrected spectra unit and the absorption spectra with a Shimadzu UVVIS 210 A spectrophotometer. The concentration of 3,5-, 2,3- and 2,6-xylenols and 3,5-dimethylanisole in different solvents at 300 K and 77 K were in the range iO~—iO~M. In ternary mixtures, the concentration of TEA in different solvents varied from 10—a to 10—1 M for fluorescence and phosphorescence experiments and from 0.1 to 0.4 M for absorption study. The

3.1. Absorption spectrum and hydrogen bonding in the ground state The electronic absorption bands of 3,5- and 2,3-xylenol in cyclohexane at 300 K undergo red shift and are intensified when TEA is added (figs. 1(a) and 1(b)). The minimum concentration of TEA necessary for the changes being about 0.1 M. The presence of an isosbestic point in the absorption curves indicates formation of a 1 : 1 complex between the xylenols in the ground state and TEA due to specific interaction, which in all probability is hydrogen bonding involving the —OH group of xylenol and the lone pair electrons on the N atom of TEA. In favour of this inference, it may be noted that TEA does not produce any change in the absorption spectrum of 3,5-dimethylanisole (3,5-DMA) in cyclohexane. The absorption spectral shift (table 1) is maximum for 3,5-xylenol and is minimum for 2,6-xylenol, in which two methyl groups are close on two sides of the hydroxyl group and hinder hydrogen bonding interaction. The broad bands were scanned slowly to find the peak OD values. The bands were not very asymmetric but there is some uncertainty in the precise peak position. The absorption spectra of xylenols in EtOH, MeOH or acetonitrile are not at all

/ Quenching of excited states of xylenols due to hydrogen bonding with

T.K. Pal et aL

TEA

341

1.81

H

3C,1:~1~.CH3

/

~ 1.2

~ 0.6k

I

0.0 250

0

I

I

270

I

I

0.6

0.6

0.3

0.3

0.0 250

290

3

/ \ ~4

Il

0.2

H3C_~J.CH3

1.2

0.9F

0.4

11..

o\\

00 250

I

270 290 7\(nm)

270

I

290

Fig. 1. (a) Ultraviolet absorption curves for 3,5-xylenol in CH (1.75 x iO~ M) at 300 K. Concentration in M of TEA in: (0) 0; (1) 0.134; (2) 0.40; (3) 0.66. (b) Ultraviolet absorption curves for 2,3-xylenol in CH (7.04 x iO~ M) at 300 K. Concentration 4 M) at in300 M K. of TEA: (0) 0; (1) 0.106; (2) 0.212; (3) 0.424. (c)in Ultraviolet absorption for 0.212; 2,6-xylenol in CH (5.93X10 Concentration M of TEA in: (0) 0; (1) curves 0.106; (2) (3) 0.634.

affected by TEA. Quite possibly, solvation of both donor and acceptor molecules in the highly polar solvents prevents hydrogen bonding. 3.2. Hydrogen bonding in the excited state and fluorescence quenching at 300 K The fluorescence spectra of xylenol molecules in cyclohexane (CH) at 300 K change remarkably Table 1 Absorption spectral shift (8 v) due to hydrogen bonding inter-

when TEA is present at a low concentration (— iO~ M), though for absorption spectral changes of xylenols a high concentration of TEA (— 0.1 M) is needed. With gradual addition of TEA to the binary mixture of xylenol—CH, the intensity of fluorescence emission gradually falls (figs. 2(a) and 2(b)), the position and shape of fluorescence emission curve remaining unchanged. In table 2 is shown dynamic fluorescence quenching rate constants (Kq) of the systems determined from the well-known equation, f 0/f

action between xylenols and TEA in cyclohexane solution at 300 K 1) ~b (cm’) ~ = i’,, — Molecules Vf (cm (cm’) 3,5-xylenol 2,3-xylenol

35640 36619 35703 36753

35139 36025 35325 36352

(—501) (594) (—378) (—401)

2,6-xylenol

__________________________________________

=

1

+

Ks,, [Q]’

(1)

which is applicable in the cases of the systems concerned. Here f~ and f are the relative integrated fluorescence intensities without and with the quencher concentration [Q] and ~ (Stern—Volmer constant) KqT~ where T is the fluorescence decay time in the absence of a quencher. Plots of fe/f versus [Q]for xylenols in CH and TEA are reproduced in fig. 3. Values of K,~and =

342

T. K. Pa! et a!.

/

Quenching of excited states of xy!eno!s due to hydrogen bonding with TEA

0

4

CH

_i:~i.. OH

o a

H3C

H3cL.~. OH

3

1,2H3C

b

CH3

OCH3CH3

3

c

2 ~‘

2

.1-I

3 4

>

~

4

~

5 6

Ix

0.001

6

I

280

0.01

310

340

2~0

I

310

I

I

340

I

280

I

310

340

~(n m) Fig. 2. (a) Fluorescence spectra of 3,5-xylenol in CH (1.75 < i0~ M) at 300 K (excitation wavelength: 3 280 (2) nm). 8.04 X Concentrai0~3 (3) 1.34x10~2 (4) 2.68X10~2(5) 1.34X10~1. tion in M of TEA in: (0) 0; (1) 5.358.04X10~2(6) X i0 (b) Fluorescence spectra of 2,6-xylenol in CH (1.10 X i0~3~ at 300 K (excitation wavelength: 280 nm). Concentration in M of TEA in: (0) 0; (1) 7.15 X10~2 (2) 9.54X10~2(3) 1.14x 10_i; (4) 1.43 X10~’; (5) 1.90X10~1 (6) 2.38X10~’. (c) Fluorescence spectra of 3,5-dimethylanisole in CH (1.33 x 10~ M) at 300 K (excitation wavelength: 280 nm). Concentration in M of TEA in: (0)0; (1) 1.75 X 102; (2) 1.14 X lOt.

Kq for isomeric xylenol—TEA systems determined from the slope of the curves are given in table 2. It is seen that the Kq values have the right order of

magnitude for a diffusion-controlled bimolecular process and the quenching efficiency Kq decreases in the that xylenol, orderis, 3,5-xylenol the quenching> is2,3-xylenol directly related > 2,6to the strength of hydrogen bonding [19]. As a corroborative evidence it may be pointed out that the emission of 3,5-DMA in CH is not quenched by TEA (fig. 2(c)). In the polar protic solvents EtOH or MeOH no quenching of fluorescence of xylenols occurs with TEA (table 2, fig. 4). A logical conclusion is that in alcoholic solution the nonbonding lone pair on TEA is engaged in hydrogen bonding of R—OH N-type with the alcohols, and this prevents charge transfer or electron transfer process in the xylenol—TEA system which is responsi. .

Table 2 Bimolecular fluorescence quenching rate constants (Kq) in different solvents for xylenols—TEA systems at 300 K Xylenols

Solvents

r (ns)

~

3,5-

CH EtOH MeOH Acetonitrile CH EtOH MeOH Acetonitrile CII EtOH MeOH Acetonitrile

2.82 (±0.12)

33 No quenching No quenching 6.3 35 No quenching No quenching 6.6 10 No quenching No quenching 5.9

2,3-

2,6-

0.1

Fig. 3. Plot of fo/f versus [Q][a]InM for a xylenol—TEA system at 300 K in CH: (1) 3,5-xylenol; (2) 2,3-xylenol; (3) 2,6-xylenol.

2.75(±O.10) 3.67(±0.11)

2.62(±0.12) 2.78(±0.11)

2.62( ±0.07)

(lM~1)

Kq (1M’ s’) 1.17x 10’°

2.29Xl09 9.54x109

2.52 x iO~ 3.59x109

2.25 x iO~

T. K. Pa! etaL

/

Quenching of excited states of xy!enols due to hydrogen bonding with TEA

343

0,1,2,3,4 2,3

OH

0CM

3

___

280

310

),.(nm)

b

\\ACH3

340

Fig. 4. Fluorescence spectra of 2,6-xylenol in EtOH (1.10 x 10~ M) at 300 K (excitation wavelength: 2802 nm). (2) Concentration 9.54X10~2 (3) in M of TEA in: (0) 0; (1) 1.14x 2.74X10 10_i.

ble for fluorescence quenching [4,20]. In the aprotic solvent acetonitrile the intensity of fluorescence emission of xylenols falls off slowly with addition of TEA (table 2, fig. 5(a)) and the K value is significantly lower than in the nonpol~tr solvent CH. This may be because hydrogen bonding interaction between xylenol and TEA is retarded due to solvation of both molecules by the highly polar acetonitrile. It should be noticed again that TEA does not produce any change in the fluorescence spectrum of 3,5-DMA (fig. 5(b)) in the highly polar solvent acetonitrile. Hence, any other type of CT interaction, as distinct from hydrogen bonding, which is not expected to occur between 3,5-DMA and TEA, may be excluded as a source for an emissive exciplex or deactivation through solvated ion pair formation. In regard to large quenching of fluorescence of xylenols by TEA in CH at 300 K a logical proposition is that hydrogen bonding between an excited xylenol and strong electron donor TEA is accompanied by transfer of a large amount of charge which favours formation of a short-lived CT species. This formation is related to the strength of the hydrogen bonding interaction and is in equilibrium with the hydrogen-bonded complex. This is depicted in scheme I describing a possible fluorescence quenching pathway. In the case of carbazole—TEA—acetonitrile,

280

I

310

I

7~(nm)

340

280

I

310

7~(nm) 340

Fig. 5. (a) Fluorescence spectra of 3,5-xylenol in acetonitrile (5.61 xi0~ M) at 300 K (excitation wavelength: 280 nns). Concentra~iOnmMofT~Am:(O)O;(~) 2O3XiO~’;(2) spectra of 3,5-dimethylanisole in acetonitnle (7.31 X i0~ M) at 300 K (excitation wavelength: 280 mn). Concentration in M of TEA in: (0) 0; (1) 2.03 X10~’;(2) 2.55 x10~5 (3) 3.06X

10~’;(4) 4.06 X10_i.

Mataga et al. [2] reported a new red-shifted fluorescence band attributed to carbazole anion formed by proton transfer from the excited carbazole to TEA. The xylenol—TEA systems do not exhibit any such band. This means either that there is no interaction similar to the one described above or that the CT or proton transfer complex formed is nonemissive [21—23].Chandross and Thomas [24] had observed that fluorescence of the exciplex formed by interaction between an aromatic hydrocarbon and an amine was quenched when amine in excess of that necessary to form the complex was present. The possibility of formation of a three-component excited complex cornprised of the hydrocarbon-amine complex and another amine was hinted at in ref. [24]. In the case of xylenol TEA systems in different solvents under study here, concentration of TEA was too low to satisfy the condition for formation of a

344

T. K Pa! et aL

/ Quenching of excited states of xy!enols due to hydrogen bonding with K

1

1( Mi )* (Kfi4(j)

~ AN

+ Q

________

TEA

K

)*

1( Mi

.j,

K_1 (x~+K1)

*

2

].(

K3

K~’ + K1’ AR

~

0

0

AN

Scheme 1

three component complex. The quenching rate constants Kq for xylenol—TEA systems in different solvents is expressed as Kq K1K2/(K_1 + K2), (2) where K1 KdIff, which is the diffusion-controlled rate constant. The Kq values are given in table 2. Referring to scheme 1, it may be argued that for the xylenol—CH—TEA system forward reaction prevails as Kq is very nearlytheequal to K~ff so that in CH solvent K 2 >> K_1, K_1 being the rate constant for back reaction. In polar aprotic acetonitrile, solvation of both xylenol and TEA molecules prevents close contact of the donor and acceptor molecules and there is rather weak hydrogen bonding and consequently only a weak CT complex. So the contribution of the back reaction rate K_1 may not be negligible and Kq is low. =

contend that the condition for CT or proton transfer complex during the lifetime of the excited state is unfavourable in the rigid glassy matrix at 77 K where molecular orientation is restricted. The reaction mechanism for hydrogen bonding cornplexation at 77 K may be represented by 1* AR

K +

~

1( AN

________ ‘

K

‘

1abs

(K~+K~)

All

0

Scheme 2

According to this scheme a stable emissive hydrogen-bonded complex is formed at 77 K. Because of unfavourable conditions for the CT configura-

3.3. Hydrogen bonding in the excited state and fluorescence enhancement at 77 K

a

b

3

The emission spectral features of xylenols at 77 K are different from those observed at 300 K. With addition of TEA to a xylenol—MCH glassy matrix at 77 K the fluorescence emission becomes more intense and the bands undergo a large red shift. The phosphorescence emission is also augmented (fig. 6). As depicted in scheme 1 for fluorescence quenching at 300 K, a nonemissive transient CT complex is formed due to strong hydrogen bonding between a xylenol in the excited singlet state and TEA. A sufficiently small energy gap between the CT state and the ground state of the fluorescence may enhance internal conversion [25] which is the main process for nonradiative energy loss. The intersystem crossing rate constant is believed to be largely uneffected. Many authors [17,26]

.‘~

2 2

C 4,

C

a,

.2 .4-,

0

I

280

310

340 340

I 400

I 460

Fig. 6. Emission spectra of 3,5-xylenol in MCH (1.75 X M) at 77 K (excitation wavelength: 280 nm). Concentration in M of TEA in: (0) 0; (1) 0.4; (2) 0.67; (3) 0.81. (a) Fluorescence emission; (b) phosphorescence emission.

TK Pa! etaL

/

Quenching of excited states of xyleno!s due to hydrogen bonding with TEA

tion and the large energy gap between i(AH Q)* state and ground state of xylenol,

345

1,2

radiative transitions also play a significant role in excitation energy dissipation at 77 K. The appearance of the emission spectra due to the excited state hydrogen bonded species of xylenols in TEA at 77 K is more or less in accord with this explanation. It is again worthwhile to note that both the fluorescence and phosphorescence spectra of 3,5-DMA in MCH, which cannot take part in H-bonding interaction, show no change when TEA is added (figs. 7(a) and 7(b)). It is noted that the absorption or excitation red shift due to hydrogen bonding at 77 K is larger than at 300 K (tables 1 and 3). Generally ir~ i~ absorption bands are red-shifted when hydrogen bonding solvents are added to the binary mixture of the absorbing molecule in a nonpolar solvent due to greater stabilization of the excited i~ir~ state relative to the ground state on complexation. At 77 K the hydrogen-bonded complex is usually more stable. Energies of the ground and excited states are further lowered but the lowering of the excited state is comparatively larger and the energy gap between the excited and ground states at 77 K is smaller compared to that at 300 K. This may explain larger red shift of the excitation spectra due to hydrogen bond formation at 77 K than that at 300 K. At 77 K, the fluorescence red shift (table 3) is larger compared to the excitation red shift which indicates that in the rigid medium the hydrogen bonding interaction in the excited state is stronger than that in the ground state. The phosphoresCence decay times for 3,5-, 2,3- and 2,6-xylenol in MCH glassy matrix at 77 K are 1.95 s, 3.46 s and 575 ms respectively. There is no change in when TEA is mixed with the binary solution of xylenol and MCH.

1

—

b 1,2

I

I

275

I

I

305

335 350 380 4t0 ~ n m) Fig. 7. Emission spectra of 3,5-dimethylanisole in MCH (7.31 4 M) at 77 K (excitation wavelength: 280 nm). Conx10 centration in M of TEA in: (0) 0; (1) 0.67; (2) 0.81. (a) Fluorescence emission; (b) phosphorescence emission.

In fig. 8 is given a plot of I~(t)/I~ against time in a semi-logarithmic curve for the xylenol—TEA—MCH system. Here and I~(t)are the stationary state and time-dependent phosphorescence intensities respectively. The decay is not of biexponential nature and is identical with the unperturbed decay curve. Possibly, equilibrium between the hydrogen-bonded complex in the triplet state and the free molecule is reached t

Table 3 Excitation and fluorescence spectral shift due to hydrogen bonding interaction between xylenols and TEA in MCH glassy matrix at 77 K Molecules

Excitation spectra Pf

3,5-xylenol 2,3-xylenol 2,6-xylenol

(cm~’)

35714 35703 36090

1’b

Fluorescence spectra (cm~’)

35040 35040 35714

1’b

—

—674 —663 —376

1(cm’) 34230 35200 34651

(cm~’)

33379 34472 34073

—

—851 —728 —578

346

T.K Pa! et aL

/

Quenching of excited states of xy!eno!s due to hydrogen bonding with TEA

1.0

[2] H. Masuhara, Y. Tohgo and N. Mataga, Chem. Lett. (1975) 59. [3] V.A. Kuzmin, A.P. Darmanyan and PP. Levin, Chem. Phys. Lett. 63 (1979) 509. [4] J. Gebicki, Acta Phys. Polon. A 55 (1979) 411. [5] K.E. Al-Am and M. Al-Sabti, J. Phys. Chem. 87 (1983) 446. [6] K. Hamanoue, T. Nakayama, K. Sugiura, H. Teranishi, M. Washio, S. Tagawa and Y. Tabata, Chem. Phys. Lett. 118 (1985) 503. [7] Shui-Pong Van and G.S. Hammond, J. Am. Chem. Soc.

0.5

_ 1-4

~ 0.1

100 (1978) 3895. [8] R.A. Beecroft and R.S. Davidson, Chem. Phys. Lett. (1981) 77. [9] A. Weller, Pure. Appl. Chem. 16 (1968) 15. [10] H. Knibbe, K. Rolling, F.P. Schafer and A. Weller, J. Chem. Phys. 47 (1967) 1184. [11] H. Leonhardt and A. Weller, Z. Phys. Chem. N.F. 29 (1961) 277. [12] K. Chatteijee, S. Laha, S. Chakrovorti, T. Ganguly and S.B. Banerjee, Chem. Phys. Lett. 100 (1983) 88. [13] K. Chatteijee, S. Laha, S. Chakrovorti, T. Ganguly and SB. Banerjee, J. Chem. Soc. Perkin Trans. 2 (1986) 79.

I-4

0.05

o.o10

~

4

~

Time in Secs Fig. 8. Phosphorescence 4)without decay function the perturber of 3,5-xylenol TEA molecules in MCH at 77(0); K (5.45 with TEA X10~ molecules of concentration 0.67 M (~)

very rapidly during the triplet excited state lifetime.

Acknowledgement: The authors thank Miss T. Gum of the Physical Chemistry department for lifetime measurement with facilities provided by the D.S.T. project SERC 2B (IP 2/8/ STP 2). —

—

References [1] 5. Arimitsu, H. Masuhara, N. Mataga and H. Tsubomura, J. Phys. Chem. 79 (1975) 1255.

[14] MG. Kuzmin and L.N. Guseva, Chem. Phys. Lett. 3 (1969) 71. [15] N. Mataga and Y. Kaifu, Mol. Phys. 7 (1963) 137. [16] (1968) N. Mataga, 733. F. Tanaka and M. Kato, Acta Phys. Polon 34 [17] K. Chatterjee, S. Laha, S. Chakrovorti, T. Ganguly and SB. Banerjee, Can. J. Chem. 62 (1984) 1369. [18] G.K. Mallik, T.K. Pal, S. Laha, T. Ganguly and SB. Baneijee, J. Lumin. 33 (1985) 37. [19] 5. Laha, S. Chakravorti, T. Ganguly and S.B. Banerjee Chem. Phys. Lett. 85 (1982) 350. [20] J.A. Ibemesi, MA. El-Bayoumi and J.B. Kinsinger, Chem. Phys. Lett. 53 (1978) 270. [21] N. Ikeda, T. Okada and N. Mataga, Bull. Chem. Soc. Japan 54 (1981) 1025. [22] N. Ikeda, H. Miyasaka, T. Okada and N. Mataga, J. Am. Chem. Soc. 105 (1983) 5206. [23] D. Rehm and A. Welles, Isr. I. Chem. 8 (1970) 259. [24] E.A. Chandross and H.T. Thomas, Chem. Phys. Lett. 9 (1971) 397. [25] MM. Martin, N. Ikeda, T. Okada and N. Mataga, J. Phys. Chem. 86 (1982) 4148. [26] N. Mataga and T. Kubota, Molecular Interactions and Electronic Spectra (Marcel Dekker, New York, 1970) p. 444.