Temperature Dependence of Molecular Orientation and Hyperpolarizability in Hemicyanine LB Films Studied by Second-Harmonic Generation

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Abstract
The molecular orientational structure and the optical nonlinearity in mixed monolayers of hemicyanine and arachidic acid were investigated as a function of temperature by means of second-harmonic generation, linear optical absorption and X-ray diffraction. The average molecular tilt angle with respect to the surface normal has an inflection point in the temperature dependence for the films with low dye concentration. This phenomenon is explainable by the melting of arachidic acid. The molecular hyperpolarizability, β, shows a maximum at about 60°C in the films with a high dye fraction. This originates from the competition between the increase of β due to the dissociation of H-aggregates and the decrease of β due to protonation effect, the isomerization and decomposition.