Temperature and isotope effects on the rates of photoinduced iridium(I)-pyridinium electron-transfer reactions

Intramolecular electron-transfer (ET) rates have been measured as functions of reaction driving force (0.0-2.0 eV) and temperature (200-280 K) for a series of iridium dimers (Ir-2) covalently bound to substituted pyridinium (py(+)) groups. We found that the rates of recombination reactions (py(.) --> Ir-2(+)) at high driving forces are strongly dependent on temperature and that Ir-2* --> py(+) and py(.) --> Ir-2(+) rates in both the normal and inverted regions are slower on deuteration of the pyridinium accepters. Analysis of the rates in terms of semiclassical theory gives the following parameter values: H-AB = 52(9) cm(-1), lambda = 1.05(2) eV (280 K); H-AB = 22(2) cm(-1), lambda = 1.04(1) eV (210 K). 4-Phenyl-N-ethylpyridinium iodide, chosen as a model for the accepters in the Ir-2* --> py(+) reactions, exhibited resonance Raman scattering that implicated a 1565-cm(-1) mode as dominant for inner-sphere reorganization.