ULTRAFAST PHOTOINDUCED CHARGE-SHIFT REACTIONS IN ELECTRON DONOR-ACCEPTOR 9-ARYLACRIDINIUM IONS

Charge-shift species arising from intramolecular electron transfer have been identified for 9-aryl derivatives of the 10-methylacridinium ion through observation of red-shifted fluorescence emission bands and the appearance of radical(ion) transients (CH3CN and CH2Cl2 solvents) using laser flash photolysis. Rates of forward and return charge-shift processes have been determined, the former occurring in the 1 ps time domain with small effects associated with changes in solvent and the electron donor substituent. Rates for return electron transfer were a strong function of the donor oxidation potential and showed acceleration with decreased driving force (inverted behavior). The lifetimes for charge-shift intermediates could be 'tuned' over three orders of magnitude as the result of changes in acridinium structure and solvent.