Ultrafast Excited-State Dynamics in a Fluoro-Aluminum Phthalocyanine Thin Film

ABSTRACTUsing femtosecond pump-probe spectroscopy, bleaching of the π-π* absorption Q-band, and photo-induced absorption on the high energy side of the Q-band, have been observed and time-resolved in a nearly-amorphous thin film of fluoro-aluminum phthalocyanine. Following excitation, the induced absorption signal develops as the absorption saturation signal diminishes, suggesting exciton decay into a subgap state. The different bimolecular decay dynamics observed for the absorption saturation (τeff ≈700 fs) and induced absorption (τeff ≈ 2 ps) signals support this conclusion. Possible origins of the subgap state are discussed. In addition, polarization-dependent spectral hole burning is observed at very early times. These results suggest the need for exploration of thin phthalocyanine films which are ordered in three dimensions over distances of at least 200–300 Å. Initial femtosecond results for epitaxially-grown chloro-indium phthalocyanine structures, which meet these criteria, are similar to those for the nearly-amorphous film, but indicate an additional polarization-dependent photo-induced absorption within the Q-band.