Molecular Aggregation, Orientation, and Structure in Langmuir-Blodgett Films of 2,4-Bis[(3,3-dimethyl-1-octadecyl-2,3-dihydro-2-indolylidene)methyl]-1,3-cyclobutadienediylium-1,3-diolate Studied by Visible Absorption and Infrared Spectroscopies

Abstract

Molecular aggregation, orientaiton, and structure in Langmuir-Blodgett (LB) films of a squarylium dye,2,4-bis[(3,3-dimethyl-1-octadecyl-2,3-dihydro-2-indolylidene)methyl]- 1,3-cyclobutadienediylium- 1,3-diolate (SQ-VI), have been investigated by using visible absorption and infrared (IR) spectroscopies. The visible absorption spectra show that SQ-VI forms H-aggregates as well as monomer in the LB films and that the proportion of the H-aggregates increases significantly with surface pressure. IR spectra of the LB films of SQ-VI are largely different from those of SQ-VI in solid and cast film. The former show bands at 1718 and 1660 cm -1 assignable to C=O and C=N + stretching modes, respectively, while the latter do not give such bands, indicating that SQ-VI takes the structure shown in Figure 1, parts b and a, respectively, in the LB films and the solid and cast film. Vibrational frequencies of CH 2 stretching bands are also different between the former and the latter ; it is suggested from the frequencies that the hydrocarbon chains of SQ-VI take a nearly trans-zigzag conformation in the solid and cast film, but a few parts of the chains assume a gauche conformation in the LB films. Comparison of the IR transmission and reflection-absorption (RA) spectra of the LB films of SQ-VI suggests that the chromophoric part is fairly perpendicular to the substrate surface and that both the hydrocarbon chains and the molecular axis of the chromophore are tilted considerably with respect to the substrate normal. The molecular orientation and structure of the LB films of SQ-VI are also discussed in comparison with those of the LB films of another type of squarylium dye, 2,4-bis[4-(methyloctadecylamino)-2-methylphenyl]cyclobutadienediylium-1,3-diolate (SQ-II), which we previously investigated.