Synthesis, Structural, and Photophysical Studies of Phenylquinoline and Phenylquinolinyl Alkynyl Based Pt(II) Complexes

We present the synthesis, characterization, and photophysical properties of two series of alkynyl-platinum(II) complexes containing the phenylquinoline (Hpq) or the cyclo-metalated phenylquinolinyl (pq) ligand. Unusual monoanionic complexes (NBu4)[Pt(C CR)(3)(Hpq)] (R = Tol 1, C6H4OMe-3 2, C6H4CF3-4 3) were generated in moderate yield by treatment of [Pt(pq)(mu-Cl)](2) with an excess of LiC CR (1:8), followed by subsequent treatment with NBu4 in iPrOH. However, a similar reaction with LiC (CBu)-Bu-t evolves with formation of [Pt(pq)(mu-kappa C-alpha:eta(2)-C (CBu)-Bu-t)](2) 4. Related dimers [Pt(pq)(mu-kappa C-alpha:eta(2)-C CR)](2) (R = Tol 5, C6H4OMe-3 6, C6H4CF3-4 7) were successfully prepared by modification of the reaction conditions. Single-crystal studies were performed on 2, 4, and 5, revealing the presence of intramolecular (H center dot center dot center dot Pt and H center dot center dot center dot C C) and intermolecular O-methoxy center dot center dot center dot H-Hpq contacts in 2 and the formation of butterfly structures in 4 and 5. In the Bu-t derivative 4, a shorter intramolecular Pt center dot center dot center dot Pt separation was found [3.0936(3) angstrom in 4 vs 3.2109(3) angstrom in 5], which is related to the nature of eta(2)-alkynyl linkages (eta(2)-asymmetrical in 4 vs eta(2)-symmetrical in 5). Anionic 1-3 display bright emissions in rigid media (solid, glass) with resolved side vibronic bands at 77 K. The energy of the emissions (supported by computational studies on 2) allows us to ascribe these emissions primarily to a mixed alkynyl-metal to the phenylquinoline (3)[(L' + M)LCT] excited state, being perturbed in 2 (solid) by the intermolecular O-methoxy center dot center dot center dot H-Hpq contacts. Whereas diplatinum complexes 5-7 have no visible emission at 298 K in the solid state, 4 shows a structureless band (578 nm), which is significantly red-shifted at 77 K (595 nm). Theoretical studies suggest a notable distortion in the excited state in relation to S-0 and identify the emission within the dimetallacycle Pt-2(C C)(2) with alkynyl to pq charge-transfer (3)[(MM + L')LCT] [with minor (IL)-I-3 (pq) contribution], whereas, in 5, the emission is tentatively ascribed to a mixture of L-3'LCT with minor (MMLCT)-M-3 character. The emission profiles observed in glassy solution for all 4-7 complexes (556-562 nm, CH2Cl2) and in the solid state for 6-7 are consistent with those exhibited by mononuclear related chromophores (3)[(L' + M)LCT] (L' = C CR, L = pq), suggesting negligible contribution of the Pt center dot center dot center dot Pt interaction.