Solvent dependence of cyanoindole fluorescence lifetime

Abstract

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.

Structures, dipole moments and excited state lifetime of isolated 4-cyanoindole in its ground and lowest electronically excited singlet states

“…15 The fluorescence lifetime of the different cyanoindoles in solution has been found to depend considerably on the solvent. 16,17 The reason for these differing photophysical properties, depending on the position of the cyano substituent is due to the relative energetic positions of the lowest excited pp* singlet states. These states can be classified in the nomenclature of Platt for cata-condensed aromatics as 1 L a and 1 L b states 18 a scheme, which was later extended to indole by Weber.…”

“…Interestingly, the fluorescence lifetime of 3-cyanoindole in water solution is the shortest of all cyanoindoles. 16 Other 4-substituted indoles show a different behavior concerning their lifetimes, compared to indole. 4-Hydroxyindole has an unusually short excited state lifetime of 200 ps.…”

Synthesis, Crystal Structure, DFT studies, Docking Studies and Fluorescent Properties of 2-(Adamantan-1-yl)-2H-isoindole-1-carbonitrile

“…The strong emission can be explained by comparing the fluorecent properties of the 1-nitrile substituted compound 1 with that of a reported unsubstituted N-methylisoindole compound (emission = 345 nm) [15]. When the electronwithdrawing nitrile group is present the charge separation of the molecule is enhanced due to an internal Stark effect [29,30] which, in turn, results in a red-shift in the fluorescence spectrum and increase in fluorecence intensity [31]. Therefore, the nitrile moiety is of significant importance in order for compound 1 to show strong fluorescent properties.…”

Synthesis, Crystal Structure, DFT Studies, Docking Studies, and Fluorescent Properties of 2-(Adamantan-1-yl)-2H-isoindole-1-carbonitrile

“…When the electron-withdrawing nitrile group is present, a red-shift in the emission maximum and increase in fluorescence intensity is observed. This is because the charge separation of the cyanoisoindole moiety is increased due to an internal Stark effect [29][30][31]. Therefore, the nitrile moiety is of significant importance in order for compound 1 to show strong fluorescent properties.…”