Photophysical Processes Occurring in a Zn-phthalocyanine in Ethanol Solution and on TiO₂ Nanostructures

The photophysics of 9(19),16(17),23(24)-tri-tert-butyl-2-[ethynyl-(4-carboxymethyl)phenyl] phthalocyaninatozinc(II) (ZnPc) in solution and adsorbed on TiO₂ and ZrO₂ nanoparticle films is characterized by stationary and time-resolved spectroscopies in the subpicosecond to nanosecond time interval. The comparison between the solution and the solid substrate data allows us to identify different pathways of the energy and electron relaxation. On the solid substrate, the presence of H-aggregates adds a further nonradiative deactivation channel competing with the charge injection into the TiO₂-conducting band, thus providing an explanation of the reduced efficiency of the charge transfer processes. The comparison between the kinetics recorded after excitation of the S₀-S₂ transition and those recorded after excitation of the S₀-S₁ transition provides an estimate of the internal conversion between S₂ and S₁ which occurs very efficiently and on an ultrafast (<50 fs) time scale. The ground-state recovery characterized by the decay of the bleaching band in the transient spectra slows down in TiO₂ samples and is taken as evidence of the charge injection in this kind of sample.