SOLVENT AND TEMPERATURE-DEPENDENCE OF ELECTRON-TRANSFER IN THE INVERTED REGION

The solvent and temperature dependences of the mte constant for the el ectron transfer reaction Re(I)(byp.--PTZ.+)(CO)3Cl --> Re(I)(bpy-PTZ)( CO)3Cl (bpy-PTZ is 10-[4'-methyl-2,2-bipyridin-4-yl]phenothiazine) hav e been studied by transient absorption measurements. This reaction, wh ich occurs in the inverted region, was induced following Re(I) --> bpy excitation of Re(I)(bpy-PTZ)(CO)3Cl and intramolecular-PTZ --> Re(II) electron transfer. On the basis of electrochemical measurements of DE LTAE1/2 [DELTAE1/2 - E1/2(PTZ0/+) - E1/2(bpy0/-) and dielectric contin uum theory, the variation of ln k in seven relatively polar solvents ( k = 4.33 X 10(6) s-1 in benzonitrile to 6.67 x 10(6) s-1 in propylene carbonate) can be accounted for quantitatively by the energy gap law. A linear correlation between ln k and T for electron transfer in propy lene carbonate over the range 223 K (k = 6.33 X 10(6) s-1) to 303 K (k = 7.11 X 10(6) s-1) is also consistent with the energy pp law. From t emperature-dependent MeasureMents Of DELTAE1/2, -partial derivative DE LTAE1/2/partial derivative T approximately DELTAS = 12 cal mol-1 K-1. From this value, the quantity gamma/HBARomega(M) = 3.4 eV-1 obtained f rom an earlier free energy dependence study, and the variation of In k with T, a solvent reorganizational energy (lambda0') of approximately 0.4 eV can be calculated. This quantity also includes a contribution from low-frequency vibrations treated classically.