FEMTOSECOND STUDIES OF HEMATOPORPHYRIN DERIVATIVE IN SOLUTION - EFFECT OF PH AND SOLVENT ON THE EXCITED-STATE DYNAMICS

We report direct femtosecond measurements of the excited state dynamic s of hematoporphyrin derivative (HpD) in solution. The dynamics are fo und to be very sensitive to the solvent and pH of aqueous solutions. T he decay of the excited singlet states is much faster in acidic and pH 7 buffer aqueous solutions (<230 ps) than in basic aqueous solutions or organic solvents(> 10 ns). The dynamical results show strong correl ation with static fluorescence measurements: weaker fluorescence in ac idic and pH 7 buffer solutions corresponding to shorter-lived excited states. A new fast decay component with a time constant around 5 ps is identified both in acidic aqueous solutions and in organic solvents s uch as acetone and attributed to internal conversion from the second t o the first excited singlet state of aggregates or certain oligomers i n HpD, in accord with the observation that the fast decay component is larger at a higher concentration. Oxygen is found to have no effect o n the dynamics on the time scale investigated, 1 ns, indicating that o xygen quenching of the singlet excited states is insignificant on this time scale. The sensitive solvent and pH dependence of the excited st ate dynamics has important clinical implications in the use of HpD as a photosensitizing agent.