Steady-State and Time-Resolved Fluorescence Analysis for a Cyanobiphenyl Mesogen in Polymer-Dispersed Liquid Crystal Films

Steady-state and time-resolved fluorescence behavior of 4-cyano-4‘-pentylbiphenyl (5CB) in polymer-dispersed liquid crystal (PDLC) films sandwiched between two quartz substrates was characterized by the surface-limited excitation (SLE) and through-film excitation (TFE) methods, which were used to discriminate the emissions from the interface layer with the substrate and from the interior domain. Although PDLC films showed both the monomer and excimer emissions of 5CB, SLE gave a substantially greater contribution of the former in the fluorescence than TFE. By contrast, a neat nematic film of 5CB showed exclusively the excimer emission independently of the excitation methods. Time-resolved analysis of the excimer emission from PDLC films revealed double-exponential decay profiles with shorter-lived (1.5−1.8 ns) and longer-lived (12−17 ns) components, in which the former is dominant for the excimer emission obtained by SLE, while the latter takes a major part for that obtained by TFE. By contrast, the excimer emission from a neat nematic film of 5CB showed a single-exponential decay with a ~17 ns lifetime independently of the excitation methods. The unique fluorescence behavior of the PDLC films has been discussed in terms of different molecular aggregation features of 5CB in the interface layer and in the interior domain.