Anomalous excimer formation of a pyrenyl derivative having a mesogen group of alkoxycyanobiphenyl in its smectic mesophase

4-Cyano-4′-(1-(1-pyrenyl)-n-octyloxy)biphenyl (8OCBPy), where the hydrogen atom of a well-known mesogen 4-cyano-4′-n-octyloxybiphenyl (8OCB) is substituted with one pyrenyl (Py) chromophore, was synthesized. When 8OCBPy was doped in the mesophases of the similar mesogen of 4-cyano-4′-n-decyloxybiphenyl (10OCB), the Py chromophores should be oriented by a constraint in the mesophases as compared with free chromophores dispersed in the mesophases. The binary mixture exhibited smectic A (Sm A) and nematic (N) mesophases. The mesophases and isotropic (I) phase of the binary mixture exhibited excimer as well as monomer fluorescence. The intermolecular excimer formation-dissociation kinetics of 8OCBPy in the Sm and I phases of its binary mixture was investigated in comparison with that for a 1-ethylpyrene binary system. Positive enthalpy and entropy changes for the excimer formation were found in the Sm A mesophase, whereas their usual changes were found in the I phase. These anomalous phenomena observed in the Sm A mesophase were interpreted by the entropy-controlled excimer formation associated with the decrease in ordering of the 10OCB-mesogen molecules around the 8OCBPy arranged in the Sm A layers.