Cd. Clark et Mz. Hoffman, SOLVENT REORGANIZATION ENERGY IN EXCITED-STATE ELECTRON-TRANSFER REACTIONS - QUENCHING AND GEMINATE-PAIR BACK ELECTRON-TRANSFER, Journal of physical chemistry, 100(35), 1996, pp. 14688-14693
The temperature dependencies of the quenching rate constants (k(q)) an
d cage escape yields of the redox products (eta(ce)) from the electron
-transfer reaction of Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine) with nin
e aromatic amines in deaerated 1:1 (v/v) CH3CN/H2O solutions have been
determined. Values of lambda, the solvent reorganization energy for e
lectron-transfer quenching and back electron transfer within the solve
nt cage, have been extracted from plots of log(k(q)T(1/2)) vs 1/T and
log((eta(ce)(-1) - 1)T-1/2) vs 1/T, respectively. For the quenching pr
ocess, lambda is not a constant value for the series of quenchers; in
general, higher values of lambda are exhibited by primary amines and l
ower values by tertiary amines. The structure and size of the quencher
s and the nature of the ring substituents contribute to the value of l
ambda. For the back-electron-transfer reaction within the geminate red
ox pair formed in the quenching process, the more sterically hindered
two-ring amines exhibit a higher value of lambda (1.1 +/- 0.08 eV) tha
n do the majority of the one-ring amines (0.82 +/- 0.04 eV). A Marcus
plot of log(eta(ce)(-1) - 1) vs Delta G degrees(bt) shows a correlatio
n within only the inverted region for systems with the same lambda; th
e earlier identification of the results for the same photosensitizer a
nd quenchers as a bell-shaped curve is due to the coincidental overlap
of two independent segments within the inverted region.