2007 Volume 56 Issue 11 Pages 579-586
Absorption and fluorescence spectra, fluorescence lifetimes, and steady-state and time-resolved fluorescence anisotropies were measured for 4-methyl-4’-cynanobiphenyl (1CB) and 4-hexyl-4’-cynanobiphenyl (6CB) in glycerol at various temperatures. The energies and intensities of the absorption and emission spectra for 1CB and 6CB were almost equally observed, and the fluorescence lifetimes around 1.0 ns were also almost equally obtained and did not depend on temperature. However, the time-resolved fluorescence anisotropies measured in glycerol solution indicated that the molecules of 1CB rotate more rapidly than those of 6CB: the rigid-sphere radii for 1CB and 6CB obtained by using the pulsed exciting light were evaluated as 1.3×10-10 m (1CB) and 1.8×10-10 m (6CB) in the low temperature region (2-10°C), whereas those evaluated in the high temperature regions (20-60°C for 1CB and 30-60 for 6CB) were 2.7×10-10 m (1CB) and 3.6×10-10 m (6CB). These radii evaluated in the high temperature region were in good agreement with those obtained by using steady-state exciting light, 2.9×10-10 m (1CB) and 3.4×10-10 m (6CB). It is noted that the increase of radius was observed across an intermediate temperature region (10-30°C for 1CB or 10-40°C for 6CB), and was more remarkable for 6CB than for 1CB. This implies that the movements of hexyl-chain are much more enhanced at high temperature, and are effectively reflected to the increase in radius of rotational diffusion. The changing phenomenon of movements of peripheral alkyl-chain in liquid-crystal molecules is important for the phase transition between liquid crystal and isotropic liquid phases: the changing to less vigorous movements of the alkyl-chain in 6CB is thought enough to contribute to the liquid-crystal formation of 6CB molecule.
