Molecular Orientation and Aggregation in Langmuir−Blodgett Films of 5-(4-N-Octadecylpyridyl)-10,15,20-tri-p-tolylporphyrin Studied by Ultraviolet−Visible and Infrared Spectroscopies

The molecular orientation and aggregation behavior of 5-(4- N -octadecyl-pyridyl)-10,15,20-tri- p -tolylporphyrin (porphyrin 338a) in mono- and multilayer Langmuir−Blodgett (LB) films have been investigated by employing ultraviolet−visible (UV−vis) and infrared (IR) spectroscopies. It has been found that the porphyrin planes assume a nearly flat orientation with respect to the surface of the solid substrates in the LB films, irrespective of the number of layers. This orientation remains unaltered even after aging of the LB films for 1 month or heating up to 150 °C, indicating high chemical and thermal stabilities of molecular arrangement in the porphyrin LB films. On the other hand, the porphyrin cores are distorted to some extent, and the attached long hydrocarbon chains are loosely packed in the LB films, as deduced from a comparison of the IR spectra of the LB and cast films with that of the solution. The porphyrin molecules form head-to-tail type (J-type) aggregates in the mono- and multilayer LB films, evidenced by significant red shift of the Soret bands in the UV−vis spectra of the LB films, compared to solution. The comparative and complementary investigations by the UV−vis and IR techniques also suggest that the structural features of the porphyrin mono- and multilayer LB films on CaF 2 plate are similar to each other.