Control of aggregate formation in merocyanine Langmuir-Blodgett films

An H-like band, blue-shifted from an absorption maximum of a monomer band, has been formed in merocyanine-arachidic acid mixed Langmuir-Blodgett (LB) films by adding n-alkane species of different chain lengths (CH3(CH2)(n-2)CH3, n=18 and 28) as a third component. In each ternary system, the blue-shifted band is prominent or is accompanied by a J-band depending on the preparation conditions. The absorption spectra A(parallel to) and A(perpendicular to) were measured using a normal incident of linearly polarized light with the electric vector either parallel or perpendicular to the dipping direction, respectively. The results of surface pressure-area isotherm measurements have indicated that the alkane species remaining in the monolayer and filling empty spaces near surface-active molecules act as the trigger molecules to induce the H-like aggregates. Comparing these results with the predictions from the theory of flow orientation and the extended dipole model, it is suggested that the present blue-shifted H-like band is of an H-aggregate rather than a dimer. (C) 1998 Elsevier Science S.A. All rights reserved.