MOLECULAR-ORIENTATION AND AGGREGATION IN MIXED LANGMUIR-BLODGETT-FILMS OF -N-OCTADECYLPYRIDYL)-10,15,20-TRI-P-TOLYLPORPHYRIN AND STEARIC-ACID STUDIED BY ULTRAVIOLET-VISIBLE, FLUORESCENCE, AND INFRARED SPECTROSCOPIES

Molecular aggregation and orientation in mixed Langmuir-Blodgett (LB) films of -N-octadecylpyridyl)-10,15,20-tri-p-tolylporphyrin (porphyrin 338a) and stearic acid have been investigated by means of ultraviolet -visible (UV-vis), fluorescence, and infrared (IR) spectroscopies. It has been found that for the multilayer mixed LB films deposited onto g lass and gold-evaporated glass substrates, the formation of porphyrin aggregates is facilitated by adding stearic acid, evidenced by a 3-6 n m red shift of the Soret band in the W-vis spectra and a 5 nm red shif t of the Q(x)(0,0) band in the fluorescence emission spectra, compared to the corresponding LB films of porphyrin 338a. This result and that for the one-layer mixed films suggest that fatty acid molecules impos e their effect on promoting the porphyrin aggregation in the mixed fil ms via interlayer interactions. Comparison of IR spectra between the m ixed films and those of porphyrin 338a leads to the conclusion that th e alkyl chains of porphyrin 338a molecules in the mixed LB films becom e ordered and closely packed and more perpendicular to the substrate s urface compared to those in the LB films of porphyrin 338a. The compar ison of ratio of intensities of CH2 antisymmetric and C=N (pyridyl gro up) stretching bands in the IR spectra between mixed LB films and the LB films of porphyrin 338a also supports this conclusion.