Persistent Hole Burning Spectroscopy Applications On Phthalocyanine Langmuir-Blodgett Films

Abstract

Phthalocyanine Lanngmuir Blodgett (LB) films represent ultra thin monomolecular layers with molecules in tight contact. Most of the phthalocyanine molecules in LB films are ordered in domains where they occupy well defined (parallel) positions. There is also a minority of non-ordered molecules in off-domain positions in LB films. This fact is well documented in X-ray diffraction and transmission electron diffraction [1]. Moreover, significant inhomogeneous broadening of absorption spectra of LB film has been observed (see fig. 1), in comparison with absorption spectra of isolated molecules. On the other hand the observed fluorescence is very weak, nevertheless its spectral profile well corresponds to that of isolated molecules. This phenomenon can be well explained on the basis of strong aggregation and/or fast excited energy transfer (EET) [2]. Persistent hole burning (PHB) (in fluorescence excitation spectra) has recently been applied on LB film of related porphyrin - poly-heptyl-cyanoacrylate matrix [3]. The PHB study of tetraaazoporphyrin in LB films suggested this EET interpretation. The role of EET in free base phthalocyanines was also discussed in [4] together with the analysis of off-resonant holes in fluorescence spectra. The aim of this paper is to determine the excited state lifetimes T1 strongly influenced by fast EET from PHB measurements (in fluorescence excitation spectra).