Electron transfer from axial ligand to S1- and S2-excited phosphorus tetraphenylporphyrin

Photoinduced processes of a series of phosphorus tetraphenylporphyrin (PTPP) derivatives ([PTPP-(NHC6H4X)(2)]Cl-+(-), X = OCH3, CH3, H, Cl, CF3, and CN) have been investigated by using femtosecond laser flash photolysis mainly. PTPP with OH as an axial ligand showed S-2 fluorescence upon excitation of the Soret band. The S-2 fluorescence lifetime was estimated to be 1.5 ps. On the other hand, both S-2 and S-1 fluorescence bands of PTPP- (NHC6H4X)2 were difficult to observe, indicating the existence of an additional deactivation process such as charge separation (CS). From MO calculation and cyclic voltammetry, PTPP and the axial ligand are expected to act as an acceptor and a donor, respectively, upon excitation of PTPP. CS via the S-2 state was confirmed during the femtosecond laser flash photolysis by observing the transient absorption of radical anion of PTPP. Furthermore, CS via the S, state of PTPP was also observed. The CS rate via the S-1 state was faster than that from the S-2 state. The free energy dependence of the electron-transfer rates was discussed on the basis of Marcus theory.