Fluorescence of Hydroxyphenyl-Substituted “Click” Triazoles

The
structural and optical properties of hydroxyphenyl-substituted-1,2,3-triazole
molecules (“click” triazoles) are described. “Click”
triazoles are prepared from the copper­(I)-catalyzed azide–alkyne
cycloaddition reactions. The alkyne-derived C4 substituent of a “click”
triazole engages in electronic conjugation more effectively with the
triazolyl core than the azide-derived N1 substituent. Furthermore,
triazolyl group exerts a stronger electron-withdrawing effect on the
N1 than the C4 substituent. Therefore, the placement of an electron-donating
group at either C4 or N1 position and the presence or the absence
of an intramolecular hydrogen bond (HB) have profound influences on
the optical properties of these compounds. The reported “click”
triazoles have fluorescence quantum yields in the range of 0.1–0.3
and large apparent Stokes shifts (8000–13 000 cm–1) in all tested solvents. Deprotonation of “click”
triazoles with a C4 hydroxyphenyl group increases their Stokes shifts;
while the opposite (or quenching) occurs to the triazoles with an
N1 hydroxyphenyl substituent. For the triazoles that contain intramolecular
HBs, neither experimental nor computational results support a model
of excited state intramolecular proton transfer (ESIPT). Rather, the
excited state internal (or intramolecular) charge transfer (ICT) mechanism
is more suitable to explain the fluorescence properties of the hydroxyphenyl-substituted “click”
triazoles; specifically, the large Stokes shifts of these compounds.