Fluorescence lifetime study of intermolecular coupling between biphenyl–ester moieties of a thermotropic liquid crystal

Using steady-state and time-resolved fluorescence spectroscopy, we investigated the intermolecular coupling between mesogenic biphenyl–ester moieties of a thermotropic liquid crystal under phase transition. In the crystalline-to-smectic C transition, a broad excitation band centered at 280 nm and a fluorescence redshift by 25 nm, followed by a drastic change in the fluorescence time-decay constant (from ∼90 to ∼230 ps), are consistently observed. These changes result from the intermolecular configurational change from biphenyl–biphenyl to biphenyl–ester. Emissions from intermolecular complexes such as J-aggregated biphenyls in crystalline and biphenyl–ester exciplexes in smectic C, smectic A, and isotropic phases are characterized.