Substrate-Dependent Aggregation and Energy Transfer in Langmuir−Blodgett Films of 5-(4-N-Octadecylpyridyl)-10,15,20-tri-p-tolylporphyrin Studied by Ultraviolet−Visible and Fluorescence Spectroscopies

Substrate-dependent energy transfer and aggregation in Langmuir−Blodgett (LB) films of 5-(4-N-octadecylpyridyl)-10,15,20-tri-p-tolylporphyrin (porphyrin 338a) have been investigated by ultraviolet−visible (UV−vis) and fluorescence spectroscopies. It has been found that the porphyrin molecules form different kinds of aggregates in the LB films on the glass and gold-evaporated glass substrates, evidenced by red shifts of about 10 and 25 nm, respectively, of the Soret bands compared to that of monomeric species in the chloroform solution. The two kinds of aggregates show different fluorescence characteristics. An additional feature of absorption around 480 nm of the LB films on the gold-evaporated glass substrate and marked quenching of fluorescence by the excitation at this wavelength suggest the formation of two different kinds of head-to-tail type J-aggregates in the LB films. Drastic quenching of fluorescence of the porphyrin LB films on gold-evaporated glass substrate and enhanced luminescence with increasing distance between the porphyrin and the gold-evaporated glass substrate by imposing arachidic acid LB films of known thickness may be explained due by efficient radiationless energy transfer from excited porphyrin molecules to the metal surface. An interlayer energy transfer has been observed for the multilayer LB films on both glass and gold-evaporated glass substrates.