Two-step photoactivation of hematoporphyrin by excimer-pumped dye-laser pulses

The selective excitation of high lying singlet or triplet states of hematoporphyrin has been achieved using high peak-power nanosecond pulses generated by excimer-pumped dye lasers. The interaction involves two steps: a pulse at 630 nm raises the molecules to the S1 state and a second one, at 481 nm, further excites them either to a higher singlet state if shed simultaneously or to a triplet state higher in energy than T1 if it arrives delayed with respect to the pulse at 630 nm by a time interval longer than the S1 lifetime. Photodegradation of L-tryptophan (100 microM in 30vol.%methanol-70vol.% buffer, pH 7.4) sensitized by 21 microM hematoporphyrin is reported. While a pure type-II mechanism, which obeys the time-intensity reciprocity law up to peak-intensity values of about 20 MW cm-2, is photosensitized by pulses at 630 nm, strong non-linearities are found for pulsed irradiation at both 630 nm and 481 nm, i.e. when the sensitizer is pumped to high lying singlet states and when it is pumped to high lying triplet states. The dependence of the subsequent reactions on the presence of oxygen and their competition with the photodynamic action has been investigated; in particular, a pathway was observed in which an electron was photoejected from a hematoporphyrin high energy triplet, showing maximum efficiency when the pulses were delayed by 16.4 ns.