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.
2035
(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(
