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J. Phys. Chem. 1980, 8 4 , 2020-2024
Role of Charge-Transfer Interactions in Photoreactions. 2. Exciplexes between Stilbene-like Molecules and Amines G. G. Alolsl, G. Bartocci, G. Favaro, and U. Mazzucato’ Istituto di Chimica Fisica, Universits di Perugia, 1-06100 Perugia, Ita& (Received: August 7, 1979; In Final Form: February 11, 1980) Publication costs assisted by the Consiglio Nationale delle Ricerche (Roma)
Fluorescence quenching of stilbene, styrylnaphthalenes, and some aza analogues by aliphatic amines has been studied in organic solvents. The quenching is associated with exciplex emission, and the Stern-Volmer quenching coefficient increases with a decrease in the amine ionization potential. Trans cis photoisomerization has been followed in parallel as a useful mechanistic probe for the deactivation of the complex. C$c is reduced proportionately less than 4 F as induced intersystem crossing through charge-transfer association leads to isomerization in the triplet manifold. General conditions required for a relevant induction of photoreaction by charge-transfer interaction are briefly discussed.
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Introduction Our interest in donor-acceptor complex formation between fluorescent trans ethylenic compounds (electron acceptors) and amines or other electron donors is in the role that they play in photochemical reactions and in the mechanism of their formation and their radiative and radiationless We now report the results of a photophysical study of the charge-transfer (CT) interaction between stilbene (St), styrylnaphthalenes (StN’s), and some of their aza analogues (dipyridylethylenes (DPE’s) and naphthylpyridylethylenes (NPE’s)), in the first excited singlet state, and aliphatic amines.
NPE’s
StN’s
(* UcJ N
’H
bridge. The amine quenchers were commercial products (Fluka AG puriss.) distilled before use. The solvents were from Carlo Erba, RS or RP grade. The fluorescence spectra and quantum yields were measured with a Perkin-Elmer MPF-44 spectrophotofluorimeter with an accessory for spectrum correction using rhodamine B as a quantum counter. The measurements of the emission yields were carried out in dilute solutions (absorbance 0.07 a t 325 nm) by using a-(l-naphthyl)5-phenyl-l,3,4-oxadiazole(a-NPD) in cyclohexane as standard (& = 0.58).6 The solutions were deaerated by bubbling nitrogen. For quenching experiments, the olefin concentration was in the range 8 X 104-3 X M, and amine concentration was varied up to 0.13 M except for donors with higher ionization potential where it was raised up to 0.5 M. The KQ values reported in the tables were reproducible within 5% * The trans cis photoisomerization quantum yields were determined spectrophotometrically by irradiation at 325 nm using a xenon XBO 150-W lamp coupled with a Hilger-Watts D292 grating monochromator. Irradiations were performed in deaerated solutions under conditions of total absorption of light by the olefins (concentration was of the order of M, depending on the extinction coefficients). The conversion percentage was held below 10% to avoid the competition of the back cis trans photoreaction. In all measurements the irradiation wavelength was kept constant at 325 nm. Some disagreement between the quantum yields given in this work (means of three independent experiments; mean deviation 4 % for C$F and 8% for &) and those from the literature (see Table V) is mainly ascribed to their dependence on the excitation wavelength resulting from the presence of different conformations in s o l ~ t i o n . ~ * ~
-
-
-
3,3’-DPE
Quenching of the excited S1 state of organic molecules by electron donors and acceptors, with formation of CT stabilized exciplexes, has been reported14 to lead often to induced intersystem crossing (isc) to T1 and consequent induced photoreaction in the triplet manifold. The importance of these CT-induced processes in synthetic and mechanistic photochemistry is clear.4 In order to point out the conditions required for a perturber to enhance the photoreaction quantum yield, we have also started a parallel study of the perturbed trans cis photoisomerization of the molecules investigated. The results so far obtained are here reported.
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Experimental Section The ethylenic compounds were synthesized for previous work, and their preparation is described el~ewhere.~ In the notation for styrylnaphthalenes and naphthylpyridylethylenes, the Greek letters indicate the isomeric positions of the naphthyl group, and the numbers indicate those of the pyridyl group with respect to the ethylenic 0022-3654/80/2084-2020$01 .OO/O
Results and Discussion Fluorescence Measurements. The general features of the absorption and emission spectra of stilbene-like molecules (M) in the presence of amines as quenchers (&) are consistent with exciplex (lE*) formation by CT interaction. This interpretation is also consistent with the results of Lewis’ extensive work on stilbene-amine system^.^^,^ NO changes were observed in the absorption spectra with increasing amine concentration, showing the complexes to be dissociated in the ground state. The quenching of the acceptor fluorescence is associated with the appearance of a structureless exciplex fluorescence band a t longer 0 1980 American Chemical Society
The Journal of Physical Chemistty, Vol. 84, No.
Charge-Transfer Interactions in Photoreactions
IS, 1980 2021
1
,*-. \ \ \
,, ,, \ \ \ \
\ \ \ \ \
+ 350
400
450
500
I 0 01 0 03 0 05 0 07 o 09
2. (nm)
Figure 1. Fluorescence spectra of trans-P-StN in n-hexane, alone (1) and in the presence of tributylamine: [TBA] = 5.4 X lo-* M (2), 5 X IO-' M (3),and 5 X lo-' M, with higher (XIO) instrumental sensitivity
[a]
(M)
Figure 2. SV plots for fluorescence quenching of P,CNPE in nhexane by amines of different Ionization potential (see symbols in Table I).
(4).
TABLE I: Stern-Volmer Coefficients for Fluorescence Quenching of p,Q-NPE by Amines in Deoxygenated ( K Q , M-I 1 and Air-Eauilibrated [ K o a ,M-') n-Hexane quencher tri-n-butylamine (TBA) triethylamine (TEA) di-n-butylamine (DBA) piperidine (PIP) diethylamine (DEAM) cyclohexylamine (CHA) n-butylamine (MHA) Data from a From ref 10. of ref 1 0 by interpolation.
IP,aeV
KQ
Koa
6.98 7.11 7.50b 7.85 7.85 8.37 8.50b
83 85 69 58 48
47 47 39 32 28 -0.6
1 0
0 ref 11 correlated to those
TABLE 11: Solvent Effect on the Wavenumber V F the Exciplex Fluorescence Peak for Some Ethylenic Compounds + TBA at 20 "C V F E (lo4 ~ ~cm-') solvent n-hexane n-heptane cyclohexane methylcyclohexane dioxane benzene toluene diethyl ether chlorobenzene ethyl acetate dichloro ethane isobutyl alco h ol
dielectric constanta
St
a-StN p-StN
E
of~
P,3-
P,4-
NPE
NPE
1.89 1.92 2.02 2.02
2.17 2.17 2.17 2.17
2.15 2.15 2.15 2.15
2.22 2.22 2.22 2.22
2.17 2.17 2.17 2.17
2.08 2.08 2.08 2.08
2.21 2.28 2.39 4.34 5.71
1.90 2.02 2.04 2.00 1.94
2.02 2.04 2.02 2.00
2.00 2.08 2.10 2.06 2.06
2.04 2.04 2.00 2.00
1.96 1.96
wavelengths. Typical fluorescence spectra for the (P-StN TBA) system are reported in Figure 1 as an example. 6.09 1.88 1.92 10.65 1.85 1.92 The quenching coefficients, K = k$rM, were obtained as slopes of the Stem-Volmer ?SV) plots of ~ $ M / ~ F M 1.90 17.70 against [Q] where the ratio of the fluorescence quantum yields in the absence and in the presence of Q was apa From ref 12. proximated to the intensity ratio at the analytical wavelength. KQ decreases with increasing ionization potential of the amine quencher, as shown in Table I and Figure 2. TABLE 111: Oxygen Effect on the Monomer and Exciulex Fluorescence of O-StN and 0.4-NPE in n.Hexanea A typical characteristic of the CT emission bands is the sensitivity of their position (vFEmsX) to the polarity of the solvent. The red shift of the bands when the polarity of the solvent increases io shown in Table 11. It can be used 6,4-NPE 83 47 1.77 1.71 1.2 1 2.1 to estimate the dipole moment of the exciplex when the p-StN 28 18 1.55 3.12 3.4 0.3 26.0 dielectric constant, the refractive index of the solvent, and a Q = TBA. the equivalent sphere radius of the complex volume are known.13 To compensate specific solute-solvent effects, The exciplex fluorescence was directly followed as a it is preferable to plot the vFErnm values of two different function of [Q] for the systems investigated. When eq 1 exciplexes against each other. The values of Table I1 were was used,l where $ F E ~is ~the ~ quantum efficiency of the thus plotted against the corresponding values for the (PStN + diethylaniline, DEA) system.' The dipole moments ~ F $ ~ / $ F E = 1 ~/KQ[$] ( 1) were close to 10 D for the systems investigated, indicating 60-70% of charge transferred to the excited acceptors. exciplex (referred to [Q] m ) and $FE is the quantum In polar solvent, the exciplex fluorescence was drastically linear plots of 1 / + F E yield at intermediate values of [Q], quenched as expected. In fact, it is known14 that polar against l/[Q]were obtained (see Figure 3 as an example, solvents enhance the radical ion pair formation from lE*, for the system (P-StN + TBA) in n-hexane). The SV which could be followed by free-radical p r o c e s ~ e s . ~ ~ coefficients ~~ obtained by this independent method were in Therefore, it is probable that the reduction in 4FEis due reasonable agreement with those previously calculated by to competitive formation of solvated ions which could take the quenching of the acceptor fluorescence. By extrapoplace from the nonrelaxed exciplex state or directly from lation to 1/[Q] = 0, the fluorescence quantum efficiencies the collisional complex bypassing the e~cip1ex.l~ of lE* were obtained (see Table V).
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~
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TABLE IV Stern-Volmer Coefficients and Rate 7 Parameters for Fluorescence Quenching Stilbene and Related Molecules by TBA and Oxygen in n-Hexane :
of
excited St a-StN 0-StN 0,3-NPE 0,4-NPE 3,3'-DPE a Data in
/
-1
10
20
30
co3-' ,M-)
40
Figure 3. Dependence of the fluorescence quantum yiekl of the (0-StN 4- TBA) exciplex on quencher concentration in deaerated n-hexane following eq 1.
Oxygen Effect and Kinetic Parameters. The oxygen effect on the quenching was studied in detail for two compounds, the 0-StN and its aza analogue the 0,CNPE. Table I11 collects the parameters obtained for the two excited acceptors (Q = TBA). In Table I, the oxygen effect on the fluorescence quenching of the aza derivative by several amines is reported. The SV coefficient in the absence of oxygen is always higher than KQ" measured in air-equilibrated solutions (superscript a indicates the aerated solution) according to eq 2. For the different = 1 + Kaa[QI = 1 + ~ Q ~ M " [ Q I (2) amines of Table I (excluding the two last donors with higher ionization potentials) with the same acceptor, p,4NPE, the ratio K/Ka has a constant value of 1.77 f 0.03. Practically the same value (1.71 f 0.03) was found for the ratio between the fluorescence quantum yields in the absence of amine (d''FM/d'FM)a
~''FM/~"FM* =
1 + K0,[021 = 1 + ko2T~[021 (3)
This result indicates that for the system (P,4-NPE + amine) the exciplex formation is not reversible'J5 and that both perturbers, oxygen and amine, interact with practically the same rate parameter with the excited olefin. The efficiency of exciplex formation between lM* and Q in the presence of oxygen is thus reduced as the singlet lifetime TM is reduced to TM' by oxygen. On the other hand, reversibility can be operative in other systems investigated. In fact, for p-StN in the presence of DEA (IP 6.8 eV)16in n-hexane, the rate parameter for the back dissociation of the complex had been found to be kME= 1.07 x lo7 s-l and p = 0.74.l The fraction p = kQ/kEMis the quenching reaction probability per collision between the excited monomer IM* and Q while kEM is the diffusion-controlled collisional rate parameter1,15
-
lM* + Q SlE* km
km
In the present system (P-StN + TBA), the value of p obtained1 from the oxygen effect on the fluorescence of lM* and 'E*, was found to be even smaller (see Table 111), consistent with the slightly higher (-0.2 eV)16ionization potential of the amine donor. It is known that reversible exciplex formation can be a source of curvature in the lifetime SV plots ( T O / , vs. [Q]) due to the coupling between
2.4 12
1.0 1.26 3.12 2.15
2.4 4.5 0.64 28 (22.8) (3.0)d 0.13 76 9.5e (12.3) 4.0 0.80 83 1 . 7 1 8 . 5 e (7.6) 2.8 0.97 12 1.14 (1.5) (3.0)d (0.8) parentheses refer t o lifetimes calculated from @'FM/@I'FM~ by assuming ko = kdiff = 3 X 10" M-'s-' and [ O , ] = 3.1 X M. F r o h ref 20. ' From ref 1. d Assumed equal t o kd8f. e From ref 21. O.lb
1.9' (2.8)
the decays of 'M* and 'E* when the feedback step is operative.17 However, the fluorescence intensity SV plots ( P / I vs. [Q]) remain linear, their slopes being only modified by the factor p (KQ= pkQ7M). In fact, a good linearity was found even when back dissociation of the complex is operative, at least up to the highest [Q] employed. From the oxygen effect on the exciplex fluorescence intensity, the exciplex lifetimes were evaluated (by using the analogue of eq 3), by assuming a diffusion-controlled 3 X lo1' M-' s-l)18and using the known quenching (k, oxygen concentration in n-hexane ([O,] = 3.1 X M).18 The 73 value for (P-StN + TBA) was found noticeably longer than that for (6,CNPE + TBA), in agreement with the observed reversibility in the formation of the longerlived complex. A similar behavior has been reported for the complexes of St with tertiary aminesag Table IV shows the SV coefficients and the rate parameters for the fluorescence quenching of six stilbene-like molecules by TBA and oxygen in n-hexane. Experiments with the amines were carried out in deaerated solution. The fluorescence of St is practically unquenched by oxygen so that degassing was unnecessary in this case. The k values were obtained from the KGs and the experiment3 fluorescence lifetimes, except for 3,3'-DPE, whose lifetime is not available, and for 0-StN (see later). In two cases, St and P-StN, the T~ values of Table IV are not without question as two exponential decay components have been reported.'J9 The nature of these components is still a matter of controversy in the case of St. Although recent studies independently performed in different laboratoriesm have failed to uncover the long lifetime ( 7 2 = 1.5 ns) reported in ref 19 (attributed to reversibility of the twisting process), a careful investigation, reported in a very recent paper,22has detected both components again. In any case, since most of the steady-state emission would be associated with the short lifetime,22i23we have used r1 = 0.1 ns to calculate the kQrate parameter. In this way, a rather high kQ value, close to the diffusion limit, was obtained. This was unexpected on the basis of the electron acceptor properties of St, which should be weaker, at least compared to the corresponding aza aromatics. We believe that the calculated kQ value represents an upper limit because the longer-lived component was neglected in the calculation. In the case of p-StN, the two decay components, which seem due to conformeric species, have lifetimes r1 = 5.2 and 7 2 = 27.6 ns8 (to be compared with 4.0 and 24.7 ns reported by F i ~ c h e r ) .The ~ analysis of the differential quenching of the two species leads to a complicated expression since the behavior depends on both A,, and &,,.' To avoid this complication, we have utilized our experimental data (obtained at fixed A,, = 325 nm and A,, = 380 nm) in the following way. From the oxygen effect on the monomer fluorescence, an average TM of 22.8 ns was obtained. This value is closer to the long lifetime 72, thus
The Journal of Physical Chemistry, Vol. 84, No. 16, 1980 2023
Charge-Transfer Interactions in Photoreactions
TABLE V : Fluorescence and Trans -+ Cis Phot_oisomerization Quantum Efficiencies for Some Stilbene-like Compounds and Their Exciplexes with TBA in n-Hexane at 20 "Ch excited acceptor St a-StN p-StN p,3-NPE p,4-NPE 3,3'-DPE
1
2
3
5 [Q].1O2
I
(MI
Figure 4. SV-type plots for fluorescence(F) and photoisomerization (R) quenching of trans-NPE's by TBA in deaerated n-hexane.
@OFMU
@FElirn
0.036 (0.04 0.61 (O.69Jb 0.65 (0.75)d 0.67 (0.51)d 0.38 (0.26)d 0.12 (0.04, 0.13)e
0.09 0.14 0.17 0.03 0.00 0.00
@OCU
@CElim
0.41 (0.50)' -0.1 0.16 (0.16)d 0.2 0.12 (0.13)d 0.10 0.15 (0.15)d 0.09 0.26 (0.24)d 0.06 0.48 (0.48)' 0.009
O-StN t 0.72 0.12 DEA' 0.00 0.43 p-StN + I- g a Data in parentheses refer to literature values in the same solvent (where not otherwise specified). From ref 24 (n-pentane). ' From ref 26 (methylcyclohexane-isohexane). From ref 5. e From ref 27 (benzene). f From ref 28. g In acetonitrile-water 40/60 (v/v). he,= 325 nm.
confirming that the longer-lived species predominates under 325 nm excitation7,*and giving an explanation for the observed linearity of the fluorescence intensity SV plots. The average T~~ was then used to calculate hQ for hexane. The SV-type plots of 4 ' ~ / 4 c against [Q] show the quenching by amine (followed at the same wavemore or less strong deviations from linearity, and their lengths). gradients KQc are always less than KQ, indicating that The rate parameters for the quenching by oxygen, calamines do not quench fluorescence and photoreaction culated from eq 3 andl reported in Table IV, reach pracproportionately, as already found in similar cases.lI2 The tically the diffusional limit in n-hexane at room temperphotoisomerizafion is less quenched than fluorescence ature, as expected. On the other hand, the rate parameters because the complex itself can contribute to the reaction, for quenching by TBA, neglecting the value for St, approbably by formation, during its decay path, of the reproach the diffusional value only for the aza compounds, active triplet state of the olefin. In other words, is the as the presence of the heteroatom makes them stronger sum of two contributions, #CM + &E. The first is the electron acceptors. For the StN's, the kQ values remain contribution by the acceptor molecules which react without substantially lower, also compared with hQ previously reinteracting with Q and can be evaluated at any [Q] from ported1 for the exciplexes with DEA, a more powerful 4OCin the absence of Q and K obtained from fluorescence electron donor. This probably reflects, in addition to steric quenching measurements (in act, ~ O C / ~ C M= 1 + KQ[Q]). hindrance by TBA, an increase in the rate parameter for The second term is the contribution of the molecules which the back dissociation of the weaker exciplex 'E* to lM* Q in the present case. From data of Tables I11 and IV, react through the complex lE* and can be simply evaluated by 4c - 4CM. Table V reports the fluorescence and phokm was in fact estimatsd as 9 X lo7s-l. From the p value, k E M gives 4.3 X lo9 M-l s-l, giving an equilibrium constant toisomerization 4 O values in the absence of perturber for the quenching process, KE, of 47.8 M-l. compared to the corresponding4h values for the complex Photoisomerization Quantum Yields. The presence of lE*, measured at high [Q] and/or obtained by extrapolathe amine also affects the trans cis photoisomerization tion to [Q] ~0 of the plots 1/&E against 1/[Q] (where X = F or C). quantum yield C$C of the ethylenic compounds. It is well-known that both singlet and triplet excited It can be noted that, contrary to the case of the system states are reactive in the direct geometrical photoisom(P-StN + DEA),l where the limiting value of the complex erization of stilbene-Bike compounds.24 As far as the fluorescence (Table V) reached the high inherent value of molecules investigated are concerned, St itself is believed the uncomplexed molecule M, these exciplexes are less to isomerize in the singlet manifold,24while the presence fluorescent and have probably shorter lifetimes (see also of the naphthyl group seems to increase the inherent c $ ~ ~ ~ Table 111). This is in agreement with the results reported and makes possible a prevalent triplet m e c h a n i ~ m . ~ ~for~ the ~ ~ systems ~ ~ ~ ~ (St ~ + tertiary amine^)^^^^ where longer It has been reported that the singlet excited St unlifetimes had been found with amines of lower ionization dergoes photoaddition reactions by tertiary amines in polar potential. The complex with a weaker electron donor has aprotic solvents and by secondary amines in all aprotic probably a higher chance to deactivate through internal solvents? These photoreactions, which have been reported conversion or isc. to occur through a C-H and N-H homolysis of the tertiary Data in Table V show also that the photoreaction and secondary amine, respectively, could also take place quantum yield of the complex, with the exception of StNs, with the other olefins studied in the present work. Howis substantially lower than the inherent yield 4OC. This ever, if one considers that the addition and reduction means that in these molecules, where the inherent yield quantum yields for the system (St + TEA) in acetonitrile9 of isc is relatively small (because S1 fluoresces strongly, as are about 20 times smaller than our 4c values measured in the case of the naphthyl derivatives,lI6or undergoes easy at the highest [Q] used, the bimolecular photoreactions can internal rotation, as in the case of St19*20324), the isc induced be neglected here. The hypothesis was confirmed by the by the complex also remains low. fact that the measurements of trans cis conversion Singlet isomerization of lM* becomes negligible when under irradiation in the presence of amines did not show [Q]is sufficiently high since the reaction is only induced any dependence on the analytical wavelength. by lE*. The geometrical conversion induced by lE* could Figure 4 reports the fluorescence and photoisomerization in principle occur in the exciplex itself or in the radical quenching ratios of NPE's as a function of [TBA] in nion or in the triplet manifold populated by the c o m p I e ~ . ~ t ~ ~
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The Journal of Physical Chemistty, Vol. 84, No. 16, 1980
The low induced yield found when lM* has small inherent (as in the case of St) indicates that lE* has little chance of isomerizing itself. This can be ruled out also by the fact that oxygen quenches exciplex fluorescence while it enhances olefin isomerization. On the other hand, the low induced yield found in acetonitrilel~~~ indicates that the olefin negative ion radicals do not play an important role in the photoisomerization mechanism. Hence, our data seem to indicate that the exciplex isomerizes prevalently through the triplet manifold by crossing to 3E* which normally dissociates to yield 3M*. It is also likely that the decay ratios from the triplet, cy and (1- a ) ,which give the fractions of molecules decaying toward the trans and cis side, respectively, are approximately the same in the absence and the presence of Q. The a value is around 0.5 as found for many stilbene-like molecules in experiments photosensitized by high-energy triplet donors.24 Having made these assumptions, we can estimate the triplet yield of the complex +mlim (or at least its maximum value) from the limiting value of the photoreaction quanN tum yield induced by the complex, that is 0.54TE1im
Aloisi et al.
deactivation pathways of the exciplex. The use of such perturbers is thus particularly suitable when the excited partner is reactive in the triplet manifold.
Acknowledgment. This work was undertaken under NATO project No. 1190. Financial contribution by the Italian Consiglio Nazionale delle Ricerche is gratefully acknowledged.
References and Notes
4CElirn.
It can be noted that the sum c $ + 24CElim ~ ~ is ~markedly different from unity for the exciplexes of Table V, particularly for the aza derivatives whose principal deactivation pathway is internal conversion to the ground state bypassing the triplet manifold. This result is contrary to that obtained using I- as electron donor,2 as there the formation of 3M*is the dominant process in the deactivation of the complex.29 Results previously obtained with p-StN, using DEA and I- as electron donors, are reported in Table V to compare the effects of the donor nature and complex stability on the behavior of lE*. With the more reactive amine, the internal conversion is much slower compared to the system with TBA while the radiative deactivation of lE* is strongly increased. With I-, which displays both good electron donor and heavy atom properties, a complex is formed which does not fluoresce and undergoes easy isc. As a consequence, the induced isomerization yield is markedly increased. These results indicate that, with quenchers which do not contain heavy atoms, the exciplex formation in stilbene-like molecules is not followed predominantly by enhanced isc to the triplet manifold but by direct radiative or radiationless deactivation to the ground state. As a conclusion, some important conditions are pointed out which should be considered in the choice of a perturber in order to obtain a satisfactory enhancement of the photoreaction yield: (a) The inherent photoreaction yield of the photosensitive partner should be low. St, which has a limiting yield of 0.5, was expected not to be an interesting candidate from this point of view, In previous papers,2 we have studied some systems whose quantum yield, less than 0.1% in the absence of perturber, increased by 2-3 orders of magnitude by C T interaction. (b) The exciplex should have low quantum yields of both fluorescenceand internal conversion. In fact, in the system (p-StN + DEA) the emission yield is too high while in the system (P-StN TBA), where the fluorescence is much weaker, the internal conversion is too high. Hence, &Elirn is only 10% in both cases. (c) The presence of a heavy atom in the perturber (or in the quenched partner) assures a high triplet yield in the
+
(25) (26) . . (27) (28) (29) (30)
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