J. Phys. Chem. 1987, 91, 2033-2035
2033
Back Electron Transfer to the Excited State In Photoinduced Electron-Transfer Reactions of Ruthenium( I I ) Complexes Noboru Kitamura, Ritsuko Obata, Haeng-Boo Kim, and Shigeo Tazuke* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku. Yokohama 227, Japan (Received: January 7, 1987)
The rates of both reductive and oxidative quenching of excited cis-dicyanobis(1,lO-phenanthroline)ruthenium(II) by aromatic amines and nitroaromatics in acetonitrile (ionic strength = 0), respectively, exhibited a negative temperature dependence. Comparing the present results with those for Ru(bpy)?+ (bpy = 2,2’-bipyridine), we concluded that the electrostatic interaction (repulsive or attractive) within product ion pairs decided the quenching mechanism and, consequently, the temperature dependence of the quenching.
Introduction Enhancement of the efficiencies of photoinduced electron transfer and suppression of the subsequent dark back electron transfer to the ground-state reactants have been of central research interest over the years to achieve efficient charge separation to photoredox products.’ Our recent studies, however, provided unequivocal evidence that back electron transfer to the excited state had to be considered as ell.^,^ Although the importance of back electron transfer to the excited state has been demonstrated on the basis of analyzing the free energy changerate relationship for oxidative quenching of tris(2,2’-bipyridine)ruthenium(II), R ~ ( b p y ) , ~ by + , neutral electron acceptors,“ we recently observed the bell-shaped Eyring plots for the several Ru(bpy)$+-neutral electron acceptor systems which enabled us to calculate the rate constants and the activation parameters of both forward and back electron transfer proce~ses.~Now we are presenting the first example for the reductive quenching of the Ru(I1) complex to be subject to back electron transfer to the excited state. Experimental Section cis-Dicyanobis(1,lo-phenanthroline)ruthenium(II), RuCN, was prepared and purified by reported procedures’ and was identified by elemental analysis, electrochemical, and spectroscopic measurements. All the quenchers and acetonitrile as a solvent were purified by accepted procedures.6 The activation controlled quenching rate constants, kq, at various temperatures were determined by Stern-Volmer plots by both emission intensity and lifetime. The diffusional effect on observed quenching rate constant, k,, was corrected by the equation4 k;’ = k,,-I - k12-’, where k I 2is the diffusion rate constant in acetonitrile. The activation enthalpy (AH$)and entropy (AS*)of the quenching were determined by an Eyring plot (In (kq/7‘)vs. 1/T) assuming the reaction to be adiabatic. The temperature dependence of the emission lifetime of RuCN and k12in acetonitrile necessary to calculate kq at each temperature is summarized in Table I.’ The sample solutions were deaerated by several freeze-pumpthaw cycles. (1) Kavarnos, G. J.; Turro, N. J. Chem. Rev. 1986,86,401.
(2) (a) Kitamura, N.; Okano, S.; Tazuke, S . Chem. Phys. Leu. 1982, 90, 13. (b) Tazuke, S.; Kitamura, N. Pure Appl. Chem. 1984, 56, 1269. (c) Tazuke, S.;Kitamura, N.; Kawanishi, Y. J. Photochem. 1985, 29, 123. (3) (a) Kitamura, N.; Obata, R.; Kim, H.-B.; Tazuke, S . Book of Abstracts, 6th International Conference on Photochemical Conversion and Storage of Solar Energy; Paris, 1986; A-29. (b) Kim, H.-B.; Kitamura, N.; Kawanishi, Y.; Tazuke, S . J. Am. Chem. Soc., in press. (4) Bock, C. R.;Connor, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K. J . Am. Chem. SOC.1979, 101, 4815. ( 5 ) Demas, J. N.; Turner, T. F.; Crosby, G. A. Inorg. Chem. 1%9,8,674. (6) Perrin, D. D.; Armargo, A. L. F.; Perrin, D. R. Purification of Laboratory Chemicals, 2nd ed.; Pergamon: New York, 1980. (7) A detailed discussion on the temperature dependence of the emission lifetime will be described in a separate publication. Kitamura, N.; Sato, M.; Kim, H.-B.; Obata, R.;Tazuke, S., manuscript in preparation.
0022-3654/87/2091-2033$01.50/0
TABLE I: Temperature Dependence of Viscosity ( T ) , k 12, and Emission Lifetime (T) of RuCN in Acetonitrile temp, K n,“ CP k 17 x 10-10 b M-1 s-l 273 278 283 288 293 298 303 308 313 318 323 328 333 338
0.4457 0.421 1 0.3991 0.3792 0.3613 0.3449 0.3300 0.3163 0.3037 0.2921 0.28 13 0.27 13 0.2620 0.2533
1.439 1.551 1.666 1.785 1.906 2.03 1 2.122 2.288 2.422 2.558 2.698 2.841 2.987 3.136
T, ns
1480 1420 1360 1300 1250 1210 1160 1120 1070 1040 1010 980 940 920
a Janz, G. Z.; Tomkins,R. P. Nonaqueous Electrolyte Handbook; Academic: New York, 1972. k I 2= 2RT/3000q(2 + rR/rQ+ r Q / r R ) . rR and rQ are radii of *RuCN (6.2 A; Nagle, J. K.; Dressick, W. J.; Meyer, T. J. J. Am. Chem. Soc. 1979,101, 3993) and a quencher (3.8 A; see also ref 4), respectively.
SCHEME I
RuCN
+
0.
R~cN+’- + Q-/+
Results Reductive and oxidative quenching of *RuCN by several aromatic amines and nitroaromatics, respectively, were studied in acetonitrile (ionic strength = 0). Typical examples of the temperature dependence of k, (i.e., Eyring plot) are shown in Figure 1, and all results obtained in this study, k,, AH*, AS*,AG* (activation free energy), and AG23 (free energy change of the forward electron transfer process), are summarized in Table 11. Quenching of *RuCN exhibited a negative temperature dependence for all quenchers except TMPD. Inspection of Table I1 reveals that abnormal negative temperature dependence accompanies large and negative activation entropies. The change in AH* by 2.1 kcal/mol from the quenching by p-aminodiphenylamine (-0.24 kcal/mol) to that by TMB (-2.9 kcal/mol) corresponded to the change in TAS* by 3.9 kcal/mol. Phenomenologically, it is concluded that the quenching proceeds by the entropy controlled reaction path. Discussion The most important result is the negative temperature dependence (AH* C 0) observed for both reductive and oxidative quenching of *RuCN (Figure 1). This result is in contrast to the 0 1987 American Chemical Society
2034 The Journal of Physical Chemistry, Vol. 91, No. 8, 1987
Letters
TABLE Ik Quenching of Excited cis-LXcyanobis(l,lO-pbenmthroline)rutbenium(II)by Organic Quenchers in Acetonitrile (at 298 K, Ionic Strength = 0)" quencher (E,,1 V vs. SCE)b
k,, M-'
methyl m-nitrobenzoate (-1.04)d m-nitroanisole (-1 .14)d p-nitrotoluene (-1.21)~ T M P D (0.12)e p-aminodiphenylamine (0.27)r T M B (0.43)e
4.5 1.5 3.4 6.9 1.1 1.4
s-'
AH23,
AH', kcal/mol
AS', euc
AG', kcal/mol
kcal/mol
-0.11 -1 .o -1.8 +0.30 -0.24 -2.9
-17 -22 -21 -14 -20 -33
4.8 5.5 6.4 4.6 5.1 6.9
-1.4 -2.1 -1 .o -10 -4.9 -4.5
X lo9
x 109 x 108 x 109 x 109 X 10'
ASz3,
euc
AG23,
-2.0 -1 1 -1 3 -22 -22 -21
kcal/mol
-0.83 +1.3 +2.9 -3.6 -0.2 +3.5
T M P D and T M B are N,N,N',N'-tetramethylphenylenediamineand N,N,N',N'-tetramethylbenzidine, respectively. Redox potentials of quenchers in acetonitrile. 'cal/(degmol). dDetermined in this study. Reference 4. fMann, C. K.; Barnes, K. K. Elecrrochemical Reacrions in Non-Aqueous Systems; Marcel Dekker: New York, 1970.
quenching of *RuCN are sufficiently large and negative to explain the observed negative AH*.lo Analogous results were recently obtained for oxidative quenching of *Ru(bpy),*+ as welL3 Comparing the results obtained for *RuCN and * R ~ ( b p y ) ~ ~ + , an interesting conclusion can be d e r i ~ e d ; ~the J electron-transfer reactions producing oppositely charged product ions like eq 5-7 exhibit a negative temperature dependence of kq while the activation enthalpies to produce electrostatically repulsive ion pairs are always positive (eq 8). (In eq 5-8 D and A represent an
I4,O#/ 13.6 13.2 *
*
-
13,&:/
+
A
*RuCN
+
D
*Ru(bpy)?+
12.6 13.0 *Ru(bpy)S
1000/T
Figure 1. Temperature dependence of the quenching of excited cis-dicyanobis( 1,IO-phenanthroline)ruthenium(II) by p-nitrotoluene (a) and N,N,N',N'-tetramethylbenzidine (b) in acetonitrile (ionic strength = 0).
quenching of * R ~ ( b p y ) ~in~which + the negative temperature dependence is confined to oxidative quenching.*J To explain the difference, the following kinetic analysis was made. The activation controlled quenching rate constant, k,, is expressed as eq 1 according to Scheme !I Depending on the relative kq
case I: case 11:
= K12k23k30/(k30 + k32)
k30
k30
>> k32
0. On the other hand, free product ions are likely to form ion radical pairs again in the case of eq 5-7 and consequently, the fate of product ions will be back electron transfer to the excited state (case I1 and AH* C 0) and/or to the ground state. However, if back electron transfer to the ground (10) For the quenching by p-nitrotoluene, the observed AH23value is not sufficiently negative to explain AH*(-1.8 kcal/mol). Since the apparent increases with increasing ionic strength of the medium (Kitamura, N.; Kawanishi, y.; Kim, H.-B.; Tazuke, S . , unpublished results), the difference in ionic strength of the medium between electrochemical (0.1) and quenching experiments (0) may account for this. (1 1) Sutin, N. In Tunneling in Biological Systems; Chance, B., et al., Ed.; Academic: New York, 1979; p 201. (12) Fuoss, R. M. J . Am. Chem. Soc. 1972, 94, 75. (13) rR has been reported to be 7.1 A for Ru(bpy)32C.See also ref 4 and footnote in Table I. (14) Although K34is temperature-dependent reflecting the temperature dependence of l/exp(-w /RTj (eq IO), the fluctuation of K34within the temperature range stuched is very small (0.098-0.084, 0.098-0.084, 0.0064-0.0043, and 1.1-1.3 M-' for eq 5-8, respectively, at 273-338 K) so that the present discussion is acceptable assuming K34to be almost temperature-independent.
J. Phys. Chem. 1987, 91, 2035-2037 state is the major deactivation path, AH*should be positive (i.e., case I). Another important consequence is the change in electrostatic entropy before and after electron transfer. According to the Born model, the change in electrostatic entropy accompanied by back is derived aslS electron transfer to the excited state, AS3ZQ
= 'Q
+ 2 ( r Z R fd ZQ- 1) (11)
where D, is the static dielectric constant of the medium and ZR and ZQare the valences of the Ru(I1) complex and a quencher (15 ) The upper row and the lower row of the sign (*) in eq 11 correspond to oxidative and reductive reactions of ruthenium complex, respectively. See also: Schmitz, J. E. J.; Linden, J. G. M. Inorg. Chem. 1984, 23, 3298. Kawanishi, Y.;Ph.D. Thesis, Tokyo Institute of Technology, 1985.
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(Q),respectively. In acetonitrile at 298 K, Lis32es for eq 5-8 are +4.8, +4.8, +8.9, and +OS2 eu, respectively. It is apparent from AS32@that the quenching reactions in eq 5-7 favor back electron transfer to the excited state, resulting in the negative temperature dependence of k,. On the other hand, although the data are still limited, we recently found that reductive quenching of *Ru(bpy)?+ by N-methylphenothiazine (AG23 = -0.3 kcal/mol), N,N-dimethyl-p-toluidine (-0.84 kcal/mol), and p-toluidine (+0.9 kcal/mol) possessed positive A H 2 3 (0.8-6.0 kcal/mol).16 The positive AH23 clearly requires normal temperature dependence of the quenching by these amines, and therefore AH* should be positive. It can be concluded that,small K34(
