Luminescence of 1,3,6-trisubstituted pyrazolo[3,4-d][1,2,3]triazoles

The photophysical properties of four 1,3,6-trisubstituted pyrazolo[3,4- d][1,2,3]triazoles were studied by a combination of UV–vis and fluorescence spectroscopy as well as density functional theory (DFT) and time-dependent DFT calculations. All four compounds are weakly fluorescent. Upon protonation with trifluoroacetic acid, all compounds display a drastic luminescence turn-on. The addition of excess trifluoroacetic acid further enhances the luminescence intensity, which is tentatively attributed to the formation of extended hydrogen bonding network rather than the formation of doubly protonated 1,3,6-trisubstituted pyrazolo[3,4- d][1,2,3]triazoles. All new compounds were characterized by nuclear magnetic resonance and X-ray crystallography.