On photoinduced double-proton transfer reactions: the photophysics of the 9H-imidazo[1,2-a]benzimidazole dimer

Abstract

The proton transfer in the C2h doubly H-bonded 9H-imidazo[1,2-a]benzimidazole (9HIB) dimer has been investigated. From the theoretical point of view, with the aid of density functional theory (DFT) and Møller–Plesset second-order perturbation theory: (i) the dimer formation presents at 298 K a large free energy for dimerization of ΔG0 = −8.92 kcal/mol; (ii) the double-proton transfer (DPT) tautomer of the 9HIB dimer in the ground electronic state (S0) is only slightly less stable (ΔG0 = 2.45 kcal/mol) than the normal tautomer dimer; and (iii) the DPT potential energy curve in S0 exhibits double minima, and a large activation energy barrier of 8.2 kcal/mol for the reverse DPT process. However, the 9HIB dimer does not undergo an excited state DPT reaction, calculated at the time-dependent DFT level and experimentally checked with fluorescence spectroscopy, owing to the unusual decrease of basicity (−16.7 kcal/mol) of the N-imidazole group upon photoexcitation. The UV–Vis spectroscopic experimental evidence (from 298 to 213 K) confirms the ease to generate the 9HIB dimer, and the card-pack aggregates of 1-methylimidazo[1,2-a]benzimidazole in 2-methylbutane and decalin.