Abstract

Using femtosecond pump-probe spectroscopy, we have studied the excited-state dynamics of thin films of highly ordered, “epitaxial” chloro-indium phthalocyanine (CllnPc) and nearly amorphous fluoro-aluminium phthalocyanine (FAlPc). The highly-ordered, ≈200-Å thick CllnPc film was grown using van der Waals molecular beam epitaxy (MBE) onto a transparent tin disulfide substrate, while the ≈0.2-μm thick FAlPc film was vapor sublimed onto a silica substrate. The absorbance spectra of the samples (Figs. 1 and 2) show that, as expected, the Q-band of the epitaxial thin film is narrower than that of the disordered film. The Q-band spectral width for the disordered film results from the different local molecular environments present in the thin film. For the epitaxial film, however, two optically-allowed transitions appear to be responsible for the Q-band width. The π- π^* optical transitions responsible for the Q-band in metallophthalocyanines are assigned to the phthalocyanine ring and are practically independent of the central metal. Thus, although our experiments to date have forced us to compare disordered FAlPc films with highly ordered CllnPc films, the differences in the central metal cations are expected to be much less significant than the differences in ordering of the two films. Both films were pumped with ≈ 620-nm, ≈ 100-fs pulses and the differential transmission (DT) spectra measured by probing with a weak, broadband pulse about 300 fs after transmission of the pump pulse.

© 1992 IQEC