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

Abstract

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 CaF2 plate are similar to each other.

Effects of a central metal on the organization of 5,10,15,20-tetra-(p-chlorophenyl)–rare earth porphyrin hydroxyl compound at the air/water interface and in Langmuir–Blodgett films

“…This difference is related to the orientation of porphyrin molecules at the air/water interface. Many studies show that the orientation of porphyrin rings in monolayers or LB films are dependent on the interactions between the hydrophilic substituted groups on the periphery rings and subphases or substrates and those between the porphyrin rings [28,47,50]. Apparently, in the present case, the different central metal ions caused the different macrocycle orientations.…”

Strong nonlinear optical refractive effect of self-assembled multilayer films containing tetrasulfonated iron phthalocyanine

“…Since the spectra do not change with the polarization mode at 908, but, have a polarization dependence at 458, we conclude that the macrocycles of phthalocyanine molecules lie referentially in the plane of the substrate. Visible polarized spectra, together with UV-Vis spectra, indicate that the macrocycles of FePcTsNa 4 molecules lie flat relative to the plane of the substrate, taking on face-to-face configuration [13][14][15][16].…”

Dicyanopyrazine-linked porphyrin Langmuir–Blodgett films

“…Note that the 2-DCPP LB film shows only two Q bands. This phenomenon is due to aggregation of porphyrin rings as the LB film assembles or a conformational change of the porphyrin macrocycle in the film [13,15,33,34]. Fig.…”