Excitation energy transfer in Langmuir-Blodgett films of 5-(4-N-octadecylpyridyl)-10,15,20-tri-p-tolylporphyrin on gold-evaporated glass substrates studied by time-resolved fluorescence spectroscopy

The fluorescence decays in mono- and multilayer Langmuir-Blodgett (LB) film s of 5-(4-N-octadecylpyridyl)-10,15,20-tri-p-tolylporphyrin (porphyrin 338a ) deposited on gold-evaporated glass substrates were measured to explore va rious channels for excitation energy transfer processes. The decays in the I-layer LB films of porphyrin 338a separated from gold surface by LB film o f arachidic acid were also measured as a function of the thickness of the s pacer, i.e. thickness of the LB films of arachidic acid. Consistent with ou r previous steady-state fluorescence study, the expectation values of the l ifetime obtained by fitting the decay curves with two exponential functions for the LB films indicate that at the short separation distance, the energ y transfer from the porphyrin molecules to the gold surface overshadows oth er processes, including intra- and interlayer energy transfers. The deviati on of energy transfer rate from Chance et al, (CPS) prediction of inverse d istance cubed in our case may be interpreted by assuming the presence of ad ditional excitation energy decay pathways in the LB films of porphyrin 338a on the gold-evaporated glass substrates. (C) 1998 Elsevier Science S.A. Al l rights reserved.