Experimental and Theoretical Studies of N ² –Substituted 1,2,3-Triazoles: Synthesis, Photochromism, Luminescence, and Air-Stable Diradicals

A new series of N2-substituted 1,2,3-triazole-based organic molecules has been synthesized via copper(I)-catalyzed N-arylation. These molecules generate stable solid-state radicals upon irradiation in air, accompanied by distinct color changes. Furthermore, all four compounds exhibited broad emission bands in the ranges of 400–550 and 750–800 nm under UV excitation. Theoretical calculations suggest that photoexcitation promotes electrons from the HOMO to the LUMO, giving rise to an open-shell singlet excited-state configuration. The very small singlet–triplet energy gaps in all four compounds indicate the presence of accessible low-lying triplet states, which may promote population of the triplet state at room temperature and thus account for the observable EPR signals. However, stronger intermolecular antiferromagnetic coupling in compounds 2 and 4 results in more efficient spin pairing and thus suppresses the EPR response, whereas compounds 1 and 3 exhibit ultralong-lived charge-separated states.