Vibrational modes of merocyanine dyes softened upon J-aggregation of the dyes in their Langmuir–Blodgett films

Head-to-tail molecular arrangements of J-aggregates modify not only the excited state, but also the ground state of the dye chromophore therein. To clarify such modification of the ground state, which is related with the aggregation mechanism, infrared absorption spectroscopy has been applied to J-aggregates of two alkylated merocyanine dyes, 6MeDS and DO, formed in their pure or mutually mixed Langmuir–Blodgett films. Some vibrational modes have been found to be softened upon the J-aggregation. The nature of these vibrational modes has been determined by density functional calculations. Then, it has been concluded that the J-aggregation induces an enhancement of intramolecular charge transfer both in the 6MeDS and DO cases, although these two dyes are largely different in their tendency to form J-aggregates.