2378
J. Phys. Chem. 1991, 95, 2378-2381
Free Enthalpy Dependence of Free-Radical Yield of Photoinduced Electron Transfer In Acetonitrile Koichi Kikuchi,* Yasutake Takahashi, Masato Hoshi? Taeko Niwa, Tomoharu Katagiri, and Tsutomu Miyashi Department of Chemistry. Faculty of Science, Tohoku University, Aoba, Aramaki, Aoba- ku, Sendai 980, Japan (Received: June 29, 1990; In Final Form: October 18, 1990) The free enthalpy dependence of the free-radical yield aR of the electron-transfer (ET) fluorescence quenching was studied in acetonitrile by using anthracenecarbonitriles as the electron-accepting fluorescer and 1,4-diphenyl-1,3-butadienes as the electron-donating quencher. eR decreases, passes through a minimum, increases with increase of AGf, the free enthalpy change involved in the actual ET process, and then suddenly falls when AGf goes beyond -0.25 eV. Switchoverof the quenching mechanism was suggested for the ET fluorescence quenching: The radical pairs are exclusively produced by the full ET in the encounter state between the fluorescer and the quencher when AGf is smaller than -0.4 eV, but in contrast through the partial ET, Le., the exciplex formation as the primary quenching products when AGf is larger than -0.4 eV.
Introduction In a previous work1 we studied the heavy-atom effects on the free-radical yield aRand the triplet yield aT of the electrontransfer (ET) fluorescence quenching in acetonitrile using 9,lOdicyanoanthracene (DCA) as the electron-accepting fluorescer and para-halogenated anisoles (I), anilines (II), and N,N-dimethylanilines (111) as the electron-donating quenchers. It was found that (i) in system I % is extremely enhanced by the halogen atom substitution into the quencher whereas aRis diminished and (ii) in systems I1 and 111 both @T and 9, are quite small and are scarcely affected by the halogen atom substitution. From these findings it was suggested that the primary quenching product is the geminate radical pairs in systems I1 and 111 as shown in Scheme 1, whereas the exciplex in system I as shown in Scheme
SCHEME I
lA' +
kar_
encounter
geminate
complex
radical pair
(lA.+
k-.n
D)
ku_ 1(A e3A
+ D
+
De+)
k,
+
De+
A + D
11.
On the other hand, we have studied the free enthalpy dependence of aRin acetonitrile using anthracenecarbonitriles as the electron-accepting fluorescers and 1,Cdiphenyl- 1,3-butadienes (XY-BD) as the electron-donating quenchersS2 On the assumptions that the ET fluorescence quenching is induced by the full ET in the encounter state between the fluorescer and the quencher, and that the free radicals are produced according to Scheme 1, the result was used to establish the bell-shaped relationship between the rate k b of return ET within the photoproduced geminate radical ion pairs and the free enthalpy change AGb (= El - E , / 2 0 X )of the return ET. The values for AGb were cakulated by using the reduction potentials El 2RED of the electron acceptors and the oxidation potentials of the donors in acetonitrile. A typical bell-shaped relationship was found in the range AGb = -1.35 to -2.56 eV, and it was satisfactorily interpreted with a semiclassical t h e ~ r y on ~ - the ~ ET reaction, which assumes a weak electron exchange interaction between the electron donor and acceptor. Therefore, Scheme I has been considered to be valid for this system, and also it may be concluded that the return E T is induced by the weak electronic interaction. It was noticed, however, that the kb value at AGb = -2.79 eV for the couple of 9-cyanoanthracene (CA) and 1&diphenyl- 1,3-butadiene (PP-BD) deviates upward from the bell-shaped relationship. Such a phenomenon cannot be explained by Scheme I. Since the energy gap (0.25 eV) between the locally excited (LE) state and the contact radical pair (RP) state for this couple is so small as that (0.18 eV) for system I, the LE state may mix with the contact RP state to form an exciplex. In the case of this couple, therefore, the fluorescence quenching might be induced by the partial ET, Le., the exciplex formation as the primary quenching product. If the radical pairs are produced from the exciplex according to Scheme 11, %R may be diminished by the internal conversion ( k l c ) 'Present address: Biological Laboratory, Kao Co., 2606 Akabane Ichikawamachi, Haga-gun, Tochigi 321-34, Japan.
0022-3654191 12095-2378$02.50/0
A + D
A + D
and the intersystem crossing (klsc). Hence, an apparently larger value of kb may be deduced for this couple, if kb is evaluated according to Scheme I instead of Scheme 11. When the full ET (keJ and the exciplex formation (k,) are the competitive processes in the encounter complex, aR may depend on the relative rates such as kc,/kexc,kcsl(kcs + klc + klsc), sind k / k b . kb depends on AGb, whereas k,, depends on AGf (= El 28i - E1lZRED - Eo,o),the free enthalpy change of the full ET, where Eo,ois the excitation energy of the fluorescer. In such case, therefore, it is necessary to study the dependence of @R on both AGf and AGb. In the present work we extended AGbto a more negative region and AGf to a more positive region to know how aRdepends on a wide spread of AGb and AGP In spite of a small extension of AGb from -2.79 to -2.89 eV and AGf from -0.25 to -0.15 eV, a sharp decrease in aRfrom 0.32 to 0.017 or an apparently drastic increase in k b from 0.1 1 x 1Olo to 2.8 x 1O'O s-l was found. It is suggested that (i) when the energy gap JAGflbetween the LE (1) Kikuchi, K.; Hoshi, M. Niwa, T.; Takahashi, Y.;Miyashi, T. J . Phys. Chem. 1991, 95, 38. (2) Kikuchi, K.; Takahashi, Y.;Koike, K.; Wakamatsu, K.; Ikeda, H.; Miyashi, T. Z . Phys. Chem. (Munich) 1990, 167, 27. (3) Ulstrup, J.; Jortner, J. J . Chem. Phys. 1975, 63, 4358. (4) Efrima, S.;Bixon, M. Chem. Phys. 1976, 13, 447. ( 5 ) Miller, J. R.; Beitz, J. V.; Huddleston, R. K. J. Am. Chem. Soc. 1984, 106, 5057.
0 1991 American Chemical Society
Photoinduced Electron Transfer in Acetonitrile
The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2379 TABLE I: Values for k,, OR, AG,, AGn and k,
acceptor donor CA CNPP-BD CA PP-BD' CA CIPP-BD BrPP-BD CA CNPP-BD DCA CIPP-BD DCA BrPP-BD DCA PPPP-BD DCA "Reference 1
%,
-AGk, eV
-Act,eV
kk,loLos-I
0.017 0.32 0.087 0.069 0.15 0.13 0.11 0.055
2.89 2.79 2.79 2.78 2.26 2.16 2.15 2.10
0.15 0.25 0.25 0.26 0.63 0.73 0.74 0.79
2.8 0.11 0.52 0.67 0.28 0.33 0.40 0.86
0 0
-1 AGt f eV
-2
10
15
20 x .lo
0
0
Wavenumber / cm-' Figure 1. Transient spectra of XY-BD'+ in acetonitrile: (a) 0, CNPPBD"; 0 , PPPP-BD". (b) 0 , BrPP-BD'+; 0 , CIPP-BD'+.
Figure 2. Free enthalpy (AG,) dependence of free-radical yield of electron-transfer fluorescence quenching in acetonitrile: 0,ref 2; e, this work.
and the R P state is smaller than 0.3 eV, the ET fluorescence quenching is dominantly due to the exciplex formation even in a highly polar solvent such as acetonitrile, and (ii) when the energy gap is larger than 0.5 eV, it is exclusively due to the full ET. The switchover of the quenching mechanism is supposed to occur around ACr = -0.4 eV.
gives the transient absorption spectrum, which is the superposition of the spectra of the fluorescer radical anion and the quencher radical cation. No triplet-triplet absorption was observed. Figure 1 shows the transient absorption spectra obtained for the aerated solutions containing DCA as the fluorescer and either CNPP-BD, CIPP-BD, BrPP-BD, or PPPP-BD as the quencher. These spectra are assigned to the radical cations XY-BD'+ of XY-BD, because the radical anion of DCA is rapidly quenched with molecular oxygen.z The molar extinction coefficients tR of the transient absorption due to XY-BD'+ were determined by means of the ET from diphenylamine (tR = 1.9 X lo4 M-' cm-' at 670 nml) to XY-BD'+: 101 000 M-' cm-' at 533 nm for CNPP-BD, 92000 M-' cm-' a t 545 nm for ClPP-BD, 89 000 M-' cm-' a t 550 nm for BrPP-BD, and 54000 M-l cm-l at 560 nm for PPPP-BD. The free-radical yields aR were determined by use of the emission-absorption flash photolysisZas listed in Table I. Then the aerated sample solutions were used, because XY-BD'+ was found to be stable for molecular oxygen. In Figure 2 the determined values for aR were plotted against AGf together with the data obtained in a previous work.z The first excited singlet energies Eo,owere determined from the mirror-image relationship between the absorption and fluorescence spectra: 3.04 eV for CA and 2.89 for DCA. It is noteworthy that aR decreases with increasing AGf, increases after passing through the minimum around -1.2 eV, and sharply falls as AGf goes beyond -0.25 eV. Such a specific free enthalpy dependence of aRhas probably been found for the first time. In order to understand such an interesting phenomenon, we have evaluated the rate kb of the retum ET within the photoproduced geminate radical pairs from the 0,values listed in Table I. If we assume Scheme I, aR may be described by eq 1, because the rate constant ki, of spin-forbidden radical-pair recombination is usually much smaller than the rate constant kb of spin-allowed radical-pair recombination
Experimental Section 1,1,4,4-Tetraphenyl-1,3-butadiene (PPPP-BD; Aldrich, scintillation grade) was used as received. 1-(4-Cyanopheny1)-4phenyl-1,3-butadiene (CNPP-BD), 1-(4-chlorophenyl)-4phenyl-l,3-butadiene (ClPP-BD), and 1-(4-bromophenyl)-4phenyl-I ,3-butadiene (BrPP-BD) were synthesized from cinnamylphosphonium chloride and 4-substituted benzaldehyde according to a reported method for the synthesis of diarylbutadienes! The other reagents were the same as previously reported.z The free-radical yields were determined by means of a newly developed flash photolysis.z This method measures the transient absorption and the time-integrated fluorescence intensity during a flash a t the same time. The former is used to know the initial concentration of free radicals, and the latter is used to evaluate the amount of light absorbed by a sample solution. The oxidation potentials EIlzox for CNPP-BD, ClPP-BD, BrPP-BD, and PPPP-BD were determined in acetonitrile to be 1.3 1, 1.21, 1.20, and 1.15 V versus SCE, respectively. The reduction potentials E112RED in acetonitrile have been reported to be 1.58' and 0.95 Vz versus SCE for 9-cyanoanthracene (CA) and 9,lO-dicyanoanthracene (DCA), respectively. All measurements were made a t 298 K. Results and Discussion The spectral shapes of both absorption and fluorescence of CA and DCA were not changed with the addition of 0.5-2 mM XY-BD. Thus, the fluorescence quenching rate constants k, were determined from the Stem-Volmer plots for the fluorescence yield: 1.6 X 1O'O M-' s-I for the CA-CNPP-BD system and (2.2 f 0.2) X IOio M-I s-' for all the other systems. Flashing of the deaerated solution containing 0.1 mM fluorescer and 0.5-2 mM quencher (6)M+onald, R. N.;Campbell, T. W.J. Org. Chem. 1959,24, 1969. (7) Eriksen. J.: Foote, C. S.J . Phys. Chem. 1978,82,2659.
(1) = ke,/(kesc + kb) where k , is the rate constant for the geminate radical separation into the free radicals. In this study we have used anthracenecarbonitriles as the electron acceptors, 1&diphenyl- 1,3-butadienes as the donors, and acetonitrile as the solvent. It is supposed that the variation in the k , value is not so large among these electron donor and acceptor systems. Assuming k , = 5 X lo8 s-l, which was evaluated for the oppositely monocharged radical ion pairs @R
2380 The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 "
' 0
-2 A G b I eV
-1
-3
Figure 3. Free enthalpy (AGb) dependence of the rate of return electron transfer within geminate radical ion pairs: 0 , ref 2; 0 , this work.
in acetonitrile,* the k b values were calculated as listed in Table I. The plot of kb versus AGb is shown in Figure 3, where a bell-shaped curve was calculated according to the equation with the following fitting parameters: k, =
where S = XV/hu
the electronic coupling matrix element V = 8 cm-I, the solvent reorganization energy As = 1.5 eV, the reactant vibrational reorganization energy Xv = 0.3 eV, and the average energy of active vibrational mode hu = 1500 cm-I. It is obvious that the free enthalpy dependence of kb in the range AGb > -2.6 eV (or AGf C -0.5 eV) is satisfactorily interpreted with eq 2. Therefore, it may be concluded for the systems in this range of AGb that the return ET within the geminate radical pairs is induced by a weak electronic exchange interaction. Furthermore, it is suggested that the geminate radical pairs are directly produced by the full E T in the encounter state between the fluorescer and the quencher. However, in the range AGb < -2.7 eV (or AGf > -0.3 eV) the values for k b are 2 or 3 orders of magnitude larger than the values expected from the theoretical curve. As stated in the Introduction, such apparent deviation of k b may be interpreted in terms of the exciplex formation. If the fluorescence is quenched by the exciplex formation, either the appearance of the exciplex fluorescence or the enhanced triplet formation may be expected. We measured the fluorescence spectrum for the solution containing 10 pM CA and 50 mM 1,4-diphenyl-1,3-butadiene(PP-BD), but we were not able to detect the exciplex fluorescence. Flashing of the deaerated solution containing CA alone gives only a trace of T-T absorption owing to the very low yield of intersystem crossing, ca. 0.003.9 Flashing of the deaerated solution containing CA and 1-50 mM PP-BD gives only the transient absorption due to CA' and PP-BD'+. The triplet energy of CA (1.80 eV9) is very close to that of PP-BD (1.82 eV'O). The lifetime of triplet PP-BD has been reported to be 1.5 ps in cyclohexane.I0 Even if the triplet CA is produced from the exciplex, therefore, it may rapidly be quenched with (8) Welter, A. 2.Phys. Chem. (Munich) 1982, 130, 129. (9) Kikuchi, K.; Hanaoka, K.; Hoshi, M.; Takahashi, Y.;Miyashi, T. To
be submitted for publication. (IO) Yce, W. A.; Hug,S. J.; 2164.
Kliger, D. S . J . Am. Chem. Soc. 1988,IlO.
Kikuchi et al.
PP-BD when the PP-BD concentration is higher than 1 mM. To confirm this, we determined the quenching rate constant of the triplet CA with PP-BD using 9-cyanophenanthrene as a triplet sensitizer. It was 7 X IO8 M-I s-I in ethanol? Accordingly, it is impossible to observe the T-T absorption of CA in the above sample solution as long as we use a microsecond flash photolysis even if the triplet formation is enhanced by the exciplex formation. Although the enhancement of the intersystem crossing through the exciplex formation was not confirmed, the marked decrease of aR upon a halogen atom substitution into PP-BD suggests the exciplex formation, because klsc may strongly be enhanced by the halogen atom substitution into the quencher. It was found that the experimental values for kb are well reproduced by a theoretical curve as long as the structurally similar analogues are used as the quenchers.2JlJ2 k , may be assumed to be nearly constant for the oppositely monocharged radical pairs. If the fluorescence quenching is entirely due to the full ET, therefore, aRcan be calculated via eqs 1 and 2. In the range AGb C -2.7 eV (or AGf > -0.3 eV) it is evaluated to be larger than 0.5, which is quite larger than the experimental values. Thus, it may be concluded that the fluorescence quenching in the range AGf > -0.3 eV is predominantly due to the exciplex formation and the exciplex is supposed to deactivate exclusively through the internal conversion (kIC)and the intersystem crossing (kIsc). This conclusion is consistent with the results obtained in the study of the heavy-atom effects on aRand the triplet yield aTof the DCA fluorescence quenching in acetonitrile. When para-halogenated anisoles are used as the quenchers, AGf is -0.18 eV. In this case @T increases from 0.023 for anisole to 0.56 for iodoanisole as the atomic number of the halogen substituent increases, whereas aR decreases from 0.055 for anisole to 0.006 for iodoanisole, indicating the fluorescence quenching due to the exciplex formation. When para-halogenated anilines and N,N-dimethylanilines are used as the quenchers, AGf is -1.07 and -1.23 eV, respectively. In these cases both aTand aR are much smaller than unity and are not so much affected by the halogen atom substitution, indicating the fluorescence quenching due to the full ET. Consequently, Figure 2 may indicate that the switchover of the quenching mechanism occurs around AGf = -0.4 eV. The reason why the exciplex formation becomes important in the E T fluorescence quenching for the system with AGf > -0.4 eV is considered to be as follows: When the energy gap lAGA between the LE and the contact R P state is small, the exciplex state may be produced by the mixing of these two state. The energy of the exciplex state is lower than that of the LE state as a result of the resonance stabilization, so that the rate k , of the exciplex formation is considered to have a finite value. In contrast, the rate k,, of the full ET strongly depends on AGf and decreases with increase of AGf in the normal Therefore, it is expected that k,, becomes smaller than k,, when AGf increases beyond a certain point of AGf in the normal region. In the vicinity of this crossing point the full E T reaction and the exciplex formation may compete with each other. If the above explanation is applied to the E T fluorescence quenching in the highly exothermic region (AGf C -1 eV), the disappearance of the inverted region in the Rehm-Weller (RM) relation~hip'~might be attributed to the short-lived exciplex formation as already pointed out by Weller.8 However, we cannot yet exclude the possibility of the formation of electronically excited radical ions as the primary quenching product. The free enthalpy dependence of aR has already been studied by Iwa et a1.I6for the system of the excited singlet oxonine and ( 1 1) Gould, I. R.; Moser, J. E.; Ege. D.; Farid, S.J. Am. Chem. Sor. 1988, 110, 1991. (12) Gould, I. R.; Ege, D.; Moser, J. E.;Farid, S.J . Am. Chem. Sor. 1990, 1 1 2,4290. (13) (a) Kakitani, T.; Mataga, N. J . Phys. Chem. 1986, 90, 993. (b) Mataga, N.; Asahi, T.; Kanda. Y.; Okada, T. Chem. Phys. 1988, 127, 249. (14) (a) Marcus, R. A. J. Chem. Phys. 1956,24,966, (b) Marcus, R.A. Annu. Rev. Phys. Chem. 1964, 1 5 , 155. (15) Rehm, D.; Weller, A. Isr. J . Chem. 1975, 63, 4358. (16) Iwa, P.;Steiner, U. E.; Vogelmann, E.; Kramer, H. E. A. J . Phys. Chem. 1982,86, 1211.
Photoinduced Electron Transfer in Acetonitrile aromatic compound in methanol. They found that aPR decreases monotonously with increase of AG, in contrast to our results. This apparent discrepancy may be explained as follows: The excited singlet energy (2.07eV) of oxonine is not so large as that (3.04 eV) of CA. The maximum rate of the retum ET is located around AGb = -1.8 eV,z1I-l3which corresponds to AGf = 4 . 3 eV for the case of oxonine and AGf = -1.2 eV for the case of CA. Hence, aRfor oxonine may decrease with increase of AGf up to AGf = -0.3 eV owing to the increase of kb If the switchover of the quenching mechanism occurs around AGf = -0.4eV even in the case of oxonine and if kcs -0.3 eV. Quite recently, this group has extended the range of AGf up to +I .4eV for the same system and furthermore studied the effects of halogen atom substitution on aPT and aR.17 The results have been explained in terms of the exciplex formation as the primary quenching product. The free enthalpy dependence of aPT in the range AGf = -0.07 to +0.7 eV was well related to the chargetransfer character within the exciplex.
+
Concluding Remarks In the present work it was suggested for the anthracenecarbonitrile-l,4-diphenyl-l,3-butadienesystems that the switchover of the quenching mechanism occurs around AGf = -0.4eV: In the range AGf > 4 . 4eV the fluorescence quenching is predominantly due to the partial ET, i.e., the exciplex formation, whereas the full ET in the range AGf < -0.4eV. In the theoretical studies to explain the RW relationship, hitherto, the theoretical curve has been depicted in such a way that the free enthalpy dependence of k, around AGf = 0 eV is well r e p r o d ~ c e d . 4 ~ ~As ~ *a~result, ~ + ’ ~ a small reorganization energy (-0.4 eV4*’s)has been adopted for the charge separation (CS) reaction, compared with that (1.8 eV2u1I-l3)for the charge recombination (CR) reaction. This is inconsistent with the result of the molecular dynamics c a l c u l a t i ~ nwhich , ~ ~ ~ predicts ~~ that the reorganization energies of the CS and C R reactions are close to each other. If the switchover of the quenching mechanism occurs even in the electron donor and acceptor systems employed for the R W plot, we have to be careful when we compare the ET theory with the R W relationship. The E T theory, which assumes a weak electronic exchange interaction between the electron donor and (17) Foll, R. E.; Kramer, H. E. A.; Steiner, U. E. J . Phys. Chem. 1990, 94, 2476. (18) Yashimori, A.; Kakitani, T.; Enomoto, Y.; Mataga, N. J. Phys. Chem. 1989, 93, 8316. (19) Carter, E. A,; Hynes,J. T. J . Phys. Chem. 1989, 93, 2184. (20) Tachiya, M. J . Phys. Chem. 1989, 93, 7050.
The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2381 acceptor, might not be available for interpreting the rate of fluorescence quenching due to the exciplex formation. Recently, Yoshimori et a1.18 have elaborated their ET theory and apparently succeeded to fit their theory to the R W relationship. However, there remained an ambiguity on either the solvent reorganization energy or the anharmonicity parameter. Such ambiguity seems to be caused by overlooking the switchover of the quenching mechanism. If the fluorescence quenching is regarded as the competitive processes of the full E T and the exciplex formation and if k,,, -0.4eV.21 For example, when 9-cyanophenanthrene (9-CP) was used as the quencher, an intense exciplex fluorescence was observed for the systems DCA-9-CP (AGf = +0.03 eV) and 2,9,1O-tricyanoanthracene9-CP(AGf = -0.22 eV) but not at all for the system of 2,6,9,10-tetracyanoanthracene-9-CP(AGf = -0.47eV). This result strongly supports the switchover mechanism of the E T fluorescence quenching. kq
+
Acknowledgment. We are greatly indebted to Professor H. Kokubun for his interest in this work and for generous support. We are also grateful for Miss C. Iwanaga and Mr. K.Azumi for the purification of chemicals. R e t r y NO. CA, 12 10- 12-4; DCA, 12 17-45-4; PPPP-BD, 1450-63- 1 ; CNPP-BD, 1552-40-5; CIPP-BD, 2733 1-25-5; BrPP-BD, 131545-72-7. (21) Kikuchi, K.; Niwa, T.; Takahashi, Y.; Ikeda, H.; Miyashi, T.; Hashi, M. Chem. Phys. Lett. 1990, 173,421.
