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A comprehensive study of the Rayleigh–Brillouin scattering spectra of o-terphenyl in the liquid and supercooled liquid state has been carried out. Anomalies in the Brillouin frequency and linewidth data are observed at the glass transition temperature (Tg) and at a temperature (TK) about 36 K higher than the melting point. The anomaly at Tg is due to the absence of free volume associated with the freezing-in of the density fluctuation. The longitudinal hypersonic frequency kink as well as the linewidth maximum and the dimunition of the transverse sound frequency at TK are interpreted as due to the relaxation of translational order which disappears above TK. The result of the depolarized Rayleigh scattering spectral study shows that cluster formation as previously proposed is unlikely. Hydrodynamic slip boundary conditions for molecular reorientation are found to be appropriate in liquid o-terphenyl. Rotation about the short and long axes lying in the plane of the parent phenyl ring is found to be consistent with the experimental data, whereas rotation about the axis perpendicular to the plane of the parent phenyl ring gives too high a hydrodynamic volume. Also observed is quasidiscontinuity at Tg in the integrated intensity versus temperature plot for the central Rayleigh component of the VV scattering spectral density. The Brillouin intensity is found to follow the temperature dependence of the configuration entropy, suggesting the occurence of the adiabatic configurational changes. The configuration changes are found frozen in below Tg, leading to a large residual entropy.

The optical properties of monomolecular layers at an air–water interface and also of Langmuir–Blodgett (LB) films of several photochromic spiropyrans with a single hydrophobic long chain and either a hydroxyl or a hydroxymethyl group have been investigated. Changes in the optical spectra of these films were induced by either UV irradiation or heating. UV-visible, and IR absorption spectra showed that spiropyran SP145 formed J-aggregates and SP147 formed H-aggregates. The half-decay period of the J-aggregate to the spiropyran (SP) form was longer than that of the colored monomeric photomerocyanine (PMC) to the SP form. It was concluded that hydrogen bonding between neighboring colored PMC molecules via hydroxyl, hydroxymethyl, and nitro groups stabilizes the aggregate state.

The formation of J-aggregates in Langmuir monolayers of a neat merocyanine dye (3-carboxymethyl-5-[2-(3-octadecyl-2-benzothiazolinyldene)ethylidene]-2-thioxo-4-thiazolidinone) has been examined by measuring the normal incidence absorption spectra. Five types of J-aggregates were found to be formed, depending on the temperature and the concentration and type of cations present in the subphase. The shift in the absorption peak wavelength on the J-aggregates formation is accounted for in terms of the number and the geometrical arrangement of the chromophores in each J-aggregate. The spectral shifts were analyzed numerically using both a point dipole model and a transition density model. These calculations indicate that the five different J-aggregates are linear ones with aggregation numbers ranging from 2 to 6.

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