Photoreactivity and emission properties of liquid-crystalline twin dimers containing cinnamic acid moieties at both ends of ethylene glycol spacers. II. Emission properties

Emission properties of liquid-crystalline twin dimers containing 4-methoxy-cinnamic acid moieties at both ends of various ethylene glycol derivatives (nEGMC), where n denotes the number of the ethylene glycol units in the spacers, have been explored by steady-state and time resolved fluorescence spectroscopy in relation to the morphology of the system. Characteristic emissions were observed in the twin dimers, particularly in 2EGMC and 5EGMC, which were ascribed to excimer emission, and the energy gap between the locally excited state (monomer emission) and the excimer state and could be correlated to the difference in the spatial orientation of the mesogenic 4-methoxycinnamate groups adopted in nEGMC in the ground state. The picosecond time resolved fluorescence spectroscopy revealed no observed rise in the excimer emission in any sample at any temperature, indicating that the excimer is most probably formed by direct excitation of the pair wise arrangement of the 4-methoxycinnamate moieties in the ground state; thus, the excimer formation behaviour reflects well the ground state morphology of the twin dimers.