High N-H Photoacidity of a Nitrogen-Rich Fused Ring Heteroaromatic Scaffold

It is shown that 4-methyl-7-(4-nitrophenyl)-triazolo[3,2-c]triazole exhibits a high photoacidity, unprecedented in neutral molecules in which the proton is delivered by a NH bond, which promotes acetalization of aldehydes under irradiation by a near-UV low power LED. Time dependent transient absorption spectroscopy shows a series of stimulated emission signals attributable to photo-tautomerization and formation of the deprotonated species, followed by broad positive absorption band which does not decay within the investigated timescale, compatible with the formation of a long living species, possibly the anion in the lowest energy triplet state. Computational analyses qualitatively confirm the strong photoacidity of the molecule in all its tautomeric forms, allow to assign transient absorption signals, and show that the first singlet excited state exhibits high spin-orbit couplings with excited triplet states which could promote a fast singlet-triplet transition, thus leading to a long living species able to photocatalyze chemical reactions.