PHOTOACID SWITCHING IN FUSED-RING N-RICH AROMATICS

[1,2,4]triazolo[3,2-c][1,2,4]triazole is a simple fused-ring N-rich aromatic system, with remarkable properties with respect to acid-base behavior and tautomerism1-3. Neutral heterobicycles (HL) exhibit amphoteric behaviour (they can deliver the N−H proton forming the conjugated base L- and can accept up to two protons, forming the species H2L+ and H3L++) and show an unprecedented tautomeric switching upon protonation, as revealed by single crystal X-ray analysis and confirmed by theoretical calculations4 . By varying the groups attached at the heterocycle, a remarkable shift of pKai values, up to 5-6 units, is observed. The studies of the acid-base properties in the excited state show that the N-H group in neutral triazolo-triazoles has an intrinsic photobasic behavior. Singly protonated forms of the system have instead a photoacid behavior. If a photoacid phenol group is attached at position 7 of the intrinsically photobasic triazolo-triazole unit, the interaction between the two complementary photoactive functions in the same molecule is additive, in such a way that in the ground state N-H is more acid than O-H while the opposite holds in the excited state. So, between ground and excited state, the acid proton is delivered by different sites of the same molecule (photoacid switching). [1] C-H Zhou, Y. Wang,Current Medicinal Chemistry,19 (2012) N.2, 239- 280. [2] Y. Murti at all, American Journal of Chemistry1 (2011) 42-46. [3] R. Centore, S. Fusco, A. Capobianco,V. Piccialli, S. Zaccaria, and A. PelusoEuropean Journal of Organic Chemistry18 (2013) 3721-3728. [4] R. Centore, C. Manfredi, S. Fusco, C. Maglione, A. Carella, A. Capobianco, A. Peluso, D. Colonna, A. Di Carlo,Journal of Molecular Structure 1093 (2015) 119–124.