Toward the Design of Phosphorescent Emitters of Cyclometalated Earth-Abundant Nickel(II) and Their Supramolecular Study

A series of cyclometalating tridentate N<^>C<^>N and tetradentate N<^>C<^>N<^>O ligand-containing complexes of earth-abundant nickel(II) has been designed and synthesized. Among them, the carbazolylnickel(II) complex demonstrates, for the first time, an orange color room-temperature luminescence. Such a complex is also found to exhibit intense luminescence with excited state lifetimes in the submicrosecond regime at 77 K, suggesting the triplet nature of the emissive state. Meanwhile, the self-assembly property of the tetradentate ligand-containing nickel(II) complex in solution has been investigated. Owing to its nearly perfect square planar geometry, as evidenced by X-ray crystal structure determination, it is found to exhibit self-assembly properties with the aid of pi-pi interactions and possibly weak Ni center dot center dot center dot Ni interactions, which have been supported by DFT calculations and NCI plot. Indeed, the ground-state aggregation behavior of this complex has been confirmed by concentration-dependent UV-vis absorption spectroscopy. Moreover, in the solution-induced aggregation studies, upon the addition of nonsolubilizing solvents, the emergence of low-energy absorption bands has been realized, and such a complex is found to demonstrate interesting self-assembly behaviors to offer well-defined and highly ordered supramolecular architectures.