Photophysical investigations of Langmuir-Blodgett mono- and multilayer films of pheophorbide-a

Absorption, fluorescence and time-resolved fluorescence spectroscopy has been applied to quasi two-dimensional Langmuir-Blodgett monolayers and three-dimensional arrangements of Langmuir-Blodgett films with different numbers of layers. Pure dye films of pheophorbide-a and films in which pheophorbide-a molecules were embedded in a behenic acid matrix have been investigated. Optical spectroscopy is used in order to obtain evidence for intra- and interlayer interaction in the membrane-like environment and distinguish between these different types of interactions. Depending on how the dye molecule is embedded a broad range of ir-system interactions occur. Optical band shifts a: well as band shape changes and the formation of new bands are indicators for these molecular interactions [M.B. Grieve, A.J. Hudson, T. Richardson, Thin Solid Films, 1994, 243, 581.]. Among such interactions, exciton coupling and excimer formation are considered to be possible reasons for the observed major differences between spectra and kinetics of mono- and multilayer Langmuir-Blodgett films. Exciton coupling is indicated by red shifts of the Q-band of about 600 cm(-1) in the absorption spectra of monolayer films. The observed broad red shifted fluorescence in the near-infrared region and their short lifetimes (less than or equal to 100 ps) are assigned to excimer formation. Time-resolved fluorescence spectra of bilayer films as well as polarization dependencies of the Langmuir-Blodgett film absorption are presented. (C) 1998 Elsevier Science S.A. All rights reserved.