Exothermic rate restrictions in long-range photoinduced charge separations in rigid media.

Glycerol/methanol (9:1) mixtures at 255 K behave as rigid media for photoinduced electron transfers that take place within a few hundred nanoseconds. This media also provides enough polarity and plasticity to accommodate charge separations with reaction free energies ranging from +3 to -34 kcal/mol. The distance dependence of the electron transfer rates from electronically excited aromatic hydrocarbons to nitriles in this medium is accurately described by an exponential decay constant of 1.65 per angstrom. These photoinduced electron transfers display, for the first time in charge separations between independent electron donors and acceptors, a free-energy relationship with a maximum rate followed by a decrease in the rate for more exothermic reactions. According to this free-energy relationship, Franck-Condon factors are maximized at DeltaG(0) approximately -15 kcal/mol. It is suggested that the inverted region observed for these first-order photoinduced charge separations originates from a slower increase of their reorganization energies with DeltaG(0) than that of the analogous second-order photoinduced charge separations, for which inverted regions have never been clearly observed.

Temperature dependence of ultra-exothermic charge recombinations.

We measured the temperature dependence (from +32 to -50 degrees C) of charge-recombination rates between contact radical ion pairs in isopropyl ether. In the systems selected for this study, aromatic hydrocarbon cations are the electron acceptors and the fumaronitrile anion is the electron donor. Nearly quantitative electron transfers occur at all temperatures. The charge recombinations have excess exothermicities of -60 kcal mol(-1) and exhibit a very weak temperature dependence. Our observations emphasize the absence of solvent effects and the relevance of nuclear tunneling in charge recombinations.

Electron transfer in supercritical carbon dioxide: ultraexothermic charge recombination at the end of the "inverted region".

Charge-recombination rates in contact radical-ion pairs, formed between aromatic hydrocarbons and nitriles in supercritical CO(2) and heptane, decrease with the exothermicity of the reactions until they reach -70 kcal mol(-1), but from there on an increase is observed. The first decrease in rate is typical of the "inverted region" of electron-transfer reactions. The change to an increase in the rate for ultra-exothermic electron transfer indicates a new free-energy relationship. We show that the resulting "double-inverted region" is not due to a change in mechanism. It is an intrinsic property of electron-transfer reactions, and it is due to the increase of the reorganisation energy with the reaction exothermicity.