Temperature-dependent photophysical properties of a liquid-crystalline random copolyester

We have investigated the temperature-dependent time-resolved fluorescence of Vectra A910, a commercially produced main-chain liquid-crystalline copolyester composed of 4-hydroxybenzoic acid (HBA) and 6-hydroxy-2-naphthoic acid (HNA). The experimental temperature range was from -180 to +400 degreesC, in which all four previously reported phase transitions were observed. At all observed temperatures, the time-resolved fluorescence was found to fit first-order stretched exponential decays. These decays are indicative of inherent disorder based on the random intrachain sequencing of the HBA and HNA subunits and the distribution of torsional angles between adjacent HNA chromophores causing variations of the pi -orbital conjugation lengths. The kinetics of the fluorescence emission is rotationally coupled to the HNA chromophores, which can be associated with the observation of two separate temperature realms. The first realm, in which the HNA subunits are rotationally frozen, is characterized by a vibrationally coupled emission only. This realm is distinguished by static disorder within the stretched exponential envelope. In the second-higher temperature realm, the HNA torsional rotations feature both vibrationally and rotationally coupled excited-state relaxation; The rotational coupling manifests as dynamic disorder in the stretched exponential decays. The dynamic disorder is indicative by a broadening of the rotational distribution with increased temperature. The second temperature-dependent realm can be further; characterized by a classical Arrhenius behavior leading to an activation energy of 1.357 kcal/mol (474.9 cm(-1) or 0.05885 eV).