EXCITED-STATE DYNAMICS OF PHTHALOCYANINE FILMS

Femtosecond pump-probe transient absorption measurements were performe d for thermally evaporated polycrystalline vanadyl and lead phthalocya nine (VOPc and PbPc) films in order to obtain information about the ex citation energy migration and relaxation. The films were shown to be c omposed of phase II and amorphous material. Fast excitation localizati on in phase II was concluded from measurement and analysis of the grou nd and excited state spectra. Comparison of the ground state, differen ce absorption, and luminescence spectra suggests a small oscillator st rength of the electronic transition from the lowest excited state to t he ground state. The influence of local heating on the transient spect ra is discussed, and the possibility to obtain the excitation decay ki netics free from this influence is proposed. Exciton-exciton annihilat ion with a time dependent rate (proportional to t(-0.5)) is observed i n both films. This is explained by one-dimensional diffusion-limited a nnihilation. Linear relaxation times are equal to 28 +/- 6 and 42 +/- 8 ps and approximate intermolecular excitation hopping times of 0.1 di vided by 0.4 and 0.02 divided by 0.08 ps were determined for VOPc and PbPc, respectively.