Solvent dependence of cyanoindole fluorescence lifetime

Highlights: • We report the photophysical properties of 6 cyanoindoles. • The absorption spectra of 4-, 6-, and 7-cyanoindoles are significantly red-shifted from that of indole. • 4-cyanoindole has the longest fluorescence lifetime in water. Several cyanotryptophans have been shown to be useful biological fluorophores. However, how their fluorescence lifetimes vary with solvent has not been examined. In this regard, herein we measure the fluorescence decay kinetics as well as the absorption and emission spectra of six cyanoindoles in different solvents. In particular, we find, among other results, that only 4-cyanoindole affords a long fluorescence lifetime and hence high quantum yield in H₂O. Therefore, our measurements provide not only a guide for choosing which cyanotryptophan to use in practice but also data for computational modeling of the substitution effect on the electronic transitions of indole.