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 aggregates showed a pulse-width limited (less than or equal to 30 ps) bleaching of the porphyrin absorption. Recovery of the dye's ground-state absorption occurs in subnanosecond time. The laser light is strongly attenuated by the sample, the efficiency of-which grows to a maximal value and then decays. Characteristic times of this growth and decay are of the order of nanoseconds. The efficiency of light attenuation decreases at longer wavelengths. Laser excitation of the ZnTPP-aggregates adsorbed on the surface of small TiO2 particles leads to electron transfer from the excited porphyrin to TiO2. Light attenuation is also observed with (qualitatively) the same kinetic pecularities as in the case of porphyrin aggregates in solution. Transient light attenuation in the nanosecond time domain originates from light scattering by temperature inhomogeneities in the solvent which occur because of thermal relaxation of the excited porphyrin aggregates.