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 π–π
interactions and possibly weak Ni···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.