Picosecond time resolved fluorescence lifetimes of the polymethine and related dyes

Abstract

The fluorescence lifetimes of fifteen polymethine dyes in ethanolic solution have been measured by picosecond time resolved fluorescence spectroscopy. These dyes have been selected to illustrate the effects of varying degrees of steric hindrance upon the fluorescence lifetime, since steric hindrance plays a major role in determining the rate of internal conversion to the ground state. The reported lifetimes range from <5 ps to 6100 ps depending upon the molecular structure of the dye. It is concluded that, in the excited state, the heterocyclic rings twist about the polymethine chain towards a 90° conformation. From this conformation, internal conversion to the ground state is the most probable deactivation process due to the large Franck—Condon factor. Relaxation from the vibrationally excited, 90° ground state conformation can result in the formation of photoisomers whose stability is determined by the degree of steric hindrance.