Time-resolved studies of the excited-state dynamics of meso-tetra(hydroxylphenyl)chlorin in solution

Meso-tetra(hydroxyphenyl)chlorin (m-THPC) is a new photosensitizer develope d for potential use in photodynamic therapy (PDT) for cancer treatment. In PDT, the accepted mechanism of tumor destruction involves the formation of excited singlet oxygen via intermolecular energy transfer from the excited triplet-state dye to the ground triplet-state oxygen. Femtosecond transient absorption measurements are reported here for the excited singlet state dy namics of m-THPC in solution. The observed early time kinetics were best fi t using a triple exponential function with time constants of 350 fs, 80 ps and greater than or equal to 3.3 ns, The fastest decay (350 fs) was attribu ted to either internal conversion from S-2 to S-1 or vibrational relaxation in St. Multichannel time-resolved absorption and emission spectroscopies w ere also used to characterize the excited singlet and triplet states of the dye on nanosecond to microsecond time scales at varying concentrations of oxygen. The nanosecond time-resolved absorption data were fit with a double exponential with time constants of 14 ns and 250 ns in ambient air, corres ponding to lifetimes of the S-1 and T-1 states, respectively. The decay of the T-1 state varied linearly with oxygen concentration, from which the int rinsic decay rate constant, k(i), of 1.5 x 10(6) s(-1) and the bimolecular collisional quenching constant, k(c), of 1.7 x 10(9) M-1 s(-1) were determi ned. The lifetime of the S-1 state of 10 ns was confirmed by fluorescence m easurements. It was found to be independent of oxygen concentration and lon ger than lifetimes of other photosensitizers.