Excited state dynamics of phthalocyanine films

Femtosecond pump-probe transient absorption measurements were performed for thermally evaporated polycrystalline vanadyl and lead phthalocyanine (VOPc and PbPc) films in order to obtain information about the excitation energy migration and relaxation. The films were shown to be composed of phase II and amorphous material. Fast excitation localization in phase II was concluded from measurement and analysis of the ground and excited state spectra. Comparison of the ground state, difference absorption, and luminescence spectra suggests a small oscillator strength of the electronic transition from the lowest excited state to the ground state. The influence of local heating on the transient spectra is discussed, and the possibility to obtain the excitation decay kinetics free from this influence is proposed. Exciton-exciton annihilation with a time dependent rate (proportional to t(-0.5)) is observed in both films. This is explained by one-dimensional diffusion-limited annihilation. Linear relaxation times are equal to 28 +/- 6 and 42 +/- 8 ps and approximate intermolecular excitation hopping times of 0.1 divided by 0.4 and 0.02 divided by 0.08 ps were determined for VOPc and PbPc, respectively.