INFRARED AND RAMAN-SCATTERING SPECTROSCOPIC STUDY ON THE STRUCTURES OF LANGMUIR-BLODGETT MONOLAYERS CONTAINING A MEROCYANINE DYE

Infrared reflection-absorption (IRRAS) spectra were measured for Langmuir-Blodgett monolayers containing a merocyanine dye (abbreviated to C22M and C22MH+, see Figure 1) adsorbed on silver surfaces. The stretching frequency of a central C = C bond of the merocyanine dye (nu(C = C)) was found to be a good marker to monitor a shift of the resonance balance between benzenoid and quinoid forms. A one-monolayer LB film of C22M on a thick silver surface gives rise to nu(C = C) at 1574 cm-1, which is appreciably lower than the corresponding frequency observed for C22M in a crystalline state (1589 cm-1); the result indicates that, on direct interaction with the silver surface, the merocyanine shifts the resonance balance to the quinoid side compared to the balance of the dye in C22M in the crystalline state. The IRRAS spectrum of a one-monolayer LB film of C22MH+ on a thick silver surface shows almost identical features to those observed for the IRRAS spectrum of the one-monolayer LB film of C22M, indicating that the dye in the former LB layer is converted to a deprotonated state as a result of interaction with the silver surface. IR transmission spectra were measured for the one- to nine-monolayer LB films of C22MH+ with both a 5-nm silver island overlayer and a 5-nm silver island underlayer. The spectra with an appreciable enhancement due to high electromagnetic field in the vicinity of the islands proved the following: (i) the LB films in direct contact with the silver islands are converted to a deprotonated state; (ii) the deprotonated merocyanine moiety interacting with the silver overlayer gives nu(C = C) at 1570 cm-1 and the merocyanine interacting with the silver underlayer gives the frequency at 1597 cm-1. Thus, the resonance balance of the former dye is appreciably shifted to the quinoid side compared to the balance of the latter.