A theoretical exploration of the nonradiative deactivation of hydrogen-bond complexes: isoindole–pyridine and quinoline–pyrrole

Abstract

The second order approximate Moller–Plesset (MP2) and coupled cluster (CC2) methods have been employed to investigate the geometry, electronic transition energies and photophysics of the isoindole–pyridine and quinoline–pyrrole complexes. The most stable geometry of both isoindole–pyridine and quinoline–pyrrole complexes has been predicted to be a perpendicular structure. It has also been found that the first electronic transition in both complexes is responsible for UV absorption owing to its ¹ππ* nature, while a charge transfer ¹ππ* state governs the nonradiative relaxation processes of both complexes. In this regard, excited state intermolecular hydrogen/proton transfer (ESHT/PT) via the charge transfer electronic states plays the most prominent role in non-radiative deactivation. In the HT/PT reaction coordinate, the minimum potential energy profile of the lowest CT-¹ππ* state predissociates the local ¹ππ* state, connecting the latter to a curve crossing with the S₀ state. At the region of this curve crossing, the S₀ and CT state become degenerate, enabling the ¹ππ* state to proceed as the predissociative state and finally direct the excited system to the ground state.