LASER-INDUCED LIGHT ATTENUATION IN SOLUTIONS OF PORPHYRIN AGGREGATES

ZnTPP and free base H2TPP (meso-tetraphenylprophyrin) form aggregates in methanolic aqueous media. Picosecond laser excitation of these aggr egates showed a pulse-width limited (less than or equal to 30 ps) blea ching of the porphyrin absorption. Recovery of the dye's ground-state absorption occurs in subnanosecond time. The laser light is strongly a ttenuated by the sample, the efficiency of-which grows to a maximal va lue and then decays. Characteristic times of this growth and decay are of the order of nanoseconds. The efficiency of light attenuation decr eases at longer wavelengths. Laser excitation of the ZnTPP-aggregates adsorbed on the surface of small TiO2 particles leads to electron tran sfer from the excited porphyrin to TiO2. Light attenuation is also obs erved with (qualitatively) the same kinetic pecularities as in the cas e of porphyrin aggregates in solution. Transient light attenuation in the nanosecond time domain originates from light scattering by tempera ture inhomogeneities in the solvent which occur because of thermal rel axation of the excited porphyrin aggregates.