Abstract
It was found that the weakly fluorescent trans-2(4′-N,N-dimethylaminostyryl)- pyridinium betaine dyestuff (1) becomes efficiently fluorescent in very viscous media. The fluorescence quantum yield (φF) for dyestuff (1) approaches unity in ethanol–ether (1 : 1) glass at 98 K. Moreover, the fluorescence wave-length is blue shifted (1874 cm−1) when the temperature is lowered to 98 K. The results are rationalized as being due to solvent viscosity dependent rate constant for a radiationless internal conversion process, involving rotation of the different parts of the molecule, a free rotor effect. The rate of internal rotation was estimated to be 3.4 × 1011 s−1. The results suggest that this molecule can be used as a fluorescent probe to study microscopic structural changes in surrounding media occurring on the picosecond time scale.