Fragmentations by photoinduced electron transfer. Fundamentals and practical aspects

Abstract

Removal of an electron from a bonding orbital or addition of an electron to an antibonding orbital of a diamagnetic molecule activates the resulting radical ion for fragmentation. Such reactive radical ions may be generated by photoinduced electron transfer (PET). There are two alternative ways to accomplish the transfer of an electron: (1) the local excitation of a donor or an acceptor which is well described by the empirical Weller equation and (2) the excitation of the charge-transfer complexes according to the Mulliken theory. The fragmentation reaction competes with back-electron transfer (BET) within the photogenerated radical ion pairs. The back electron transfer is well described by the Marcus theory. In most PET systems the rate of BET decreases with the increasing exergonicity and the rate is faster within contact ion pairs than with solvent separated ion pairs. The exergonicity of BET as well as ion pair solvation and spin multiplicity are predetermined by the method of ion-pair generation. These factors, in addition to the rate of cleavage, are critical in determining the overall efficiency of the PET fragmentation.