PHOTOPHYSICAL INVESTIGATIONS OF LANGMUIR-BLODGETT MONO-LAYER AND MULTILAYER FILMS OF PHEOPHORBIDE-A

Absorption, fluorescence and time-resolved fluorescence spectroscopy h as been applied to quasi two-dimensional Langmuir-Blodgett monolayers and three-dimensional arrangements of Langmuir-Blodgett films with dif ferent numbers of layers. Pure dye films of pheophorbide-a and films i n which pheophorbide-a molecules were embedded in a behenic acid matri x have been investigated. Optical spectroscopy is used in order to obt ain evidence for intra- and interlayer interaction in the membrane-lik e environment and distinguish between these different types of interac tions. Depending on how the dye molecule is embedded a broad range of ir-system interactions occur. Optical band shifts a: well as band shap e changes and the formation of new bands are indicators for these mole cular interactions [M.B. Grieve, A.J. Hudson, T. Richardson, Thin Soli d Films, 1994, 243, 581.]. Among such interactions, exciton coupling a nd excimer formation are considered to be possible reasons for the obs erved major differences between spectra and kinetics of mono- and mult ilayer Langmuir-Blodgett films. Exciton coupling is indicated by red s hifts of the Q-band of about 600 cm(-1) in the absorption spectra of m onolayer films. The observed broad red shifted fluorescence in the nea r-infrared region and their short lifetimes (less than or equal to 100 ps) are assigned to excimer formation. Time-resolved fluorescence spe ctra of bilayer films as well as polarization dependencies of the Lang muir-Blodgett film absorption are presented. (C) 1998 Elsevier Science S.A. All rights reserved.