A Quantum-Chemical Study on the Photochemical Properties of Azides of a Series of Heteroarenes from Pyridine to Dibenzoacridine in Neutral and Protonated Forms

Abstract

Molecules of a series of heteroaromatic azides in the ground (S ₀) and the lowest excited singlet (S ₁) states were calculated by the PM3 semiempirical method. It was shown that in the S ₀ state, the azide group in all the azides has quasi-linear geometry and a significant positive charge on the two terminal nitrogen atoms. The azide photoactivity is determined by the population of the σ ^*_NN orbital in the excited state, which is unoccupied in the ground state. The population of this orbital was found to depend on the size and charge of the aromatic π system. For the initial members of this azide series, the σ ^*_NN orbital is populated in both neutral and protonated forms. This is consistent with the experimental data and means that these azides are photoactive. With an increase in the size of the aromatic system, the energetic gap between the σ ^*_NN orbital and LUMO increases. As a result, the σ ^*_NN orbital is not populated in the S ₁ state when a particular threshold size of the π system is achieved, and the azide becomes photo-inactive.