Highly Luminescent Tridentate N∧C*N Platinum(II) Complexes Featured in Fused Five–Six-Membered Metallacycle and Diminishing Concentration Quenching

A series of cyclometalating ligands, N-phenyl-N-(3-(pyridin-2-yl)phenyl)pyridin-2-amine (L1), N-(3-(1H-pyrazol-1-yl)phenyl)-N-phenylpyridin-2-amine (L2), N-phenyl-N-(3-(quinolin-2-yl)phenyl)pyridin-2-amine (L3), N-phenyl-N-(3-(pyridin-2-yl)phenyl)quinolin-2-amine (L4), N-(3-(isoquinolin-1-yl)phenyl)-N-phenylpyridin-2-amine (L5), and N-phenyl-N-(3-(pyridin-2-yl)phenyl)isoquinolin-1-amine (L6), were synthesized, which reacted with K2PtCl4 in glacial acetic acid to produce N∧C*N-coordinated platinum(II) complexes featured in a fused five–six-membered metallacycle, 1–6, respectively. The structures of 1, 3, 4, and 6 were determined by single crystal X-ray crystallography. The square geometries of the complexes are improved when compared with those of the N∧C∧N-coordinated complexes as the bite angles for the platinum in N∧C*N-coordinated complexes 1, 3, and 4 are increased. The Pt–C bonds (1.94–1.95 Å) are shorter than those of C∧N∧N-coordinated platinum complexes but longer than those found for N∧C∧N-coordinated platinum complexes. With the increase of the steric interaction, the distortion of the molecules from a planar coordination geometry becomes more and more severe from 1 to 3 to 4 and 6, and in 6, the N-phenyl ring has to stand up on the coordination sphere to minimize the steric interaction with the N-isoquinolyl ring. The photophysical properties of the complexes were studied, and their absorption and emission spectra were interpreted by relating to the structural features revealed by the X-ray crystal structures and the orbital characters predicted by DFT calculations. All complexes are emissive in fluid at room temperature, and the quantum yields (up to 0.65) are comparable to those of highly emissive N∧C∧N-coordinated platinum complexes. Self-quenching was not observed in the concentration range of 10–6 to 10–4 M. Large rigidochromic shifts for the emissions of 2, 4, and 6 upon cooling from room temperature to rigid glass (77 K) were observed. Two different triplet states that control the emissions were proposed to account for the photophysical properties of 6.