Table of Contents
Laser Chemistry
Volume 4 (1984), Issue 1-6, Pages 305-310

Fluorescence Quantum Yield and Free Rotor Effect

Department of Chemistry, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt

Received 6 June 1983; Accepted 15 September 1983

Copyright © 1984 Hindawi Publishing Corporation. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


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.