Synthesis, Structure, Photophysics, and a DFT Study of Phosphorescent C*N∧N- and C∧N∧N-Coordinated Platinum Complexes

The reaction of N,N-dipheny1-2,2'-bipyridin-6-amine (L1) and N,N-diphenyl-6-(1H-pyrazol-1-yl)pyridin-2-amine (L2) with K2PtCl4 produced C*(NN)-N-boolean AND-coordinated cycloplatinated compounds with a five-six fused metallacycle la and 2a, respectively, which were then converted into their phenylacetylide derivatives 1b and 2b, respectively. Similar reactions starting from 2-phenyl-6-(1H-pyrazol-1-yl)pyridine (L3) produced (CNN)-N-boolean AND-N-boolean AND-coordinated platinum complexes 3a and 3b with a five-five-fused metallacycle. The structures of la, lb, 2b, 3a, and 3b were determined by X-ray crystallography. The C*(NN)-N-boolean AND-coordinated platinum complexes are closer to a square geometry, whereas the (CNN)-N-boolean AND-N-boolean AND-coordinated complexes display a nearly perfect planar geometry. The pi center dot center dot center dot pi interactions were revealed in the crystal packing for la, 2b, and 3a with a pi center dot center dot center dot pi contact of 3.450, 3.422, and 3.414 angstrom, respectively. Two conformers were revealed in the crystal structure of 2b, one with the phenyl ring of the phenylacetylide being approximately parallel with the coordination plane and the other with the phenyl ring being approximately perpendicular to the coordination plane. Both la and lb are weakly emissive in the red region. Complexes 2a and 3a are also weakly emissive, but their acetylide derivatives 2b and 3b emitted strongly green light at room temperature with quantum yields of 43 and 62%, respectively. DFT/TDDFT calculations were performed to elucidate the nature of their electronic transitions. The calculations suggested that lowest singlet and triplet excited states are characteristic of a mixed state involving one or more charge-transfer transitions such as ILCT, MLCT, and LLCT.