Formation of two-dimensional herringbone aggregates in Langmuir films of amphiphilic merocyanine dye derivatives

Doublet absorption bands are seen in visible spectra of Langmuir films of amphiphilic merocyanine dyes, DS, DO and DSe, prepared on pure water. In the DS and DO cases, the both maxima of the doublet bands exhibit red shifts upon the compression of the films. These phenomena cannot be explained by assuming that isolated monomers and/or dimers are the majority in the films. Contrastively, they can be well explained by assuming that large herringbone aggregates are the majority. In the latter model, the surface pressure-induced spectral change is interpreted as a structural phase transition. A numerical simulation for herringbone aggregates of DS reproduces the observed excitation wavenumbers. Infrared absorption spectra have been observed for Langmuir-Blodgett films of the dyes transferred from pure-water surfaces. They indicate the formation of mutual hydrogen bonds between the carboxylic groups in adjacent dye molecules and then suggest that the interaction between those groups is a part of the driving force of the aggregation.