Modulation of acid-base propertiesin fused-ring n-rich aromatics

Heterocycles are key compounds of synthetic chemistry. This class of compounds shows many relevant applications in biochemical field[1], but recently they are playing a fundamental role as building blocks of active molecules in many fields of advanced materials: conducting polymers[2], organic field-effect transistors[2], Dye-sensitized Solar Cells (DSSC)[3] , nonlinear optically active and liquid crystalline compounds[4], high energy compounds[5] . The [1,2,4]triazolo[3,2- c][1,2,4]triazole systemis a fused-ring N-rich aromatic system with remarkable acid-base properties and tautomerism. It is possible to exercise a fine control of the tautomeric forms present in solution or in the solid state and of the acid-base properties in the fundamental and excited state, by acting on the electronic character of the group at position 7 of the heterobicycle. In fact the relative amount of the two singly protonated tautomeric forms (1H-3H and 2H-3H) can be finely tuned in such a way that only 1H-3His present, or only 2H-3Hor both. In the case of the phenol substituent (7-phydroxyphenyl), both tautomeric forms are present in comparable amount in solution; the two tautomers can also be selectively precipitated in different crystalline salts using suitable counterions[6,7]. In this work, we propose a study of acidbase properties of the structural isomers of the 4-methyl-7-(hydroxyphenyl)-2H-[1,2,4]triazolo[3,2,-c][1,2,4]triazole (Fig. 1). The hydrolytic equilibria have been studied at 25 °C in constant ionic medium NaCl 0.5 M/ethanol 4% (v/v) by recording UV-VIS spectra (absorption and emission) at different pH. The experiments have been performed as acid-base titrations at constant concentration of TT5, without varying the 0.5 M level of [Cl]. The pH investigated spans between 0.3 and 11. For each experimental point, the equilibrium free proton concentration was evaluated from the measured electromotive force at the ends of the galvanic cell G.E. /T.S. /R.E., where T.S.indicates the Test Solution, G.E.is the glass electrode and R.E.is a reference electrode. 1. V. Piccialli, S. D’Errico, N. Borbone, G. Oliviero, R. Centore, S. Zaccaria, Eur. J. Org. Chem. (2013) 1781-1789. 2. A. J. Heeger, Chem. Soc. Rev. (2010) 39, 2354-2371. 3. J. Li, A. C. Grimsdale, Chem. Soc. Rev. 39 (2010) 2399-2410. 4. S. Fusco, R. Centore, P. Riccio, A. Quatela, G. Stracci, G. Archetti, H.-G. Kuball, Polymer 49 (2008) 186-191. 5. Klapötke, T. M.; Schmid, P. C.; Schnell, S.; Stierstorfer, J. Chem. Eur. J. 2015, 21, 9219−9228. 6 R. Centore, C. Manfredi, S. Fusco, C. Maglione, A. Carella, A. Capobianco, A. Peluso, D. Colonna, A. Di Carlo J. Mol. Struct. (2015), 1093, 119-124. 7. C. Manfredi, A. Capobianco, S. Volino, M.V. Ferrara, A. Carella, S. Fusco, A. Peluso, J. Org. Chem. (2017) DOI: 10.1021/acs.joc.7b00380.