FT-IR study of Langmuir-Blodgett films of a merocyanine dye:: effect of 13C-substitution and influence of J-aggregation

An amphiphilic merocyanine dye, C18DS, forms J-aggregates in its Langmuir-Blodgett (LB) films. In order to investigate the influence of the J-aggregation upon the electronic state of the dye, IR absorption spectra were observed for LB films of C-13-substituted C18DS, in which a carbon atom in the central conjugated system of the dye is replaced by C-13. Clear red shifts due to the C-13-substitution were observed for the IR bands that exhibit red shifts upon the J-aggregation. This result shows that these bands originate from stretching of central conjugated system, and then suggest an enhancement of the intramolecular charge transfer upon the J-aggregation. Frequency calculation using the density functional method was performed for C4DS, in which the octadecyl group of C18DS is truncated to a butyl group. The calculation results coincide with both the IR spectrum of C18DS and the C-13-substitution effect on it. The observed IR bands were assigned on the basis of this calculation. (C) 2003 Elsevier B.V. All rights reserved.