Observations of merocyanine J-aggregate domains in mixed molecular monolayers using SHG/fluorescence and atomic force microscopes

Using a newly developed microscope, which enables us to obtain the optical second-harmonic and fluorescence (FL) images and the FL and absorption spectra of the same site of specimen, the monolayer of merocyanine dye (MD) mixed with arachidic acid (AA) was observed at the air–water interface. It was revealed that the cigar-shaped J-aggregate domains have a non-centrosymmetric structure with the polar axis inclined by ∼30° from the long axis of the domain. Complemental AFM observations of the Langmuir–Blodgett monolayer clarified that the monolayer consists of three topographically different phases: the MD J-aggregate domain, monomer MD, and AA.