Cyclometalated 6-Phenyl-2,2′-bipyridyl (CNN) Platinum(II) Acetylide Complexes: Structure, Electrochemistry, Photophysics, and Oxidative-and Reductive-Quenching Studies

Three cyclometalated 6-phenyl-4-(p-R-phenyl)-2,2'-bipyridyl (CNN-Ph-R) Pt(II) acetylide complexes, Pt(CNN-Ph-R)(CCPh), where R = Me (1), COOMe (2), and P(O)(OEt)(2) (3), have been synthesized and studied. Compounds 1 and 3 have been structurally characterized by single crystal X-ray crystallography and are found to exhibit distorted square planar geometries about the Pt(II) ions. The electrochemical properties of the compounds, as determined by cyclic voltammetry, have also been examined. Complexes 1-3 are brightly emissive in fluid CH2Cl2 solution and in the solid state with lambda(max)(em) of ca. 600 nm and lifetimes on the order of ca. 500 ns in fluid solution. The emissions are assigned to a (MLCT)-M-3 transition. The complexes undergo oxidative quenching by MV2+ with quenching rates near the diffusion-controlled limit (k(q) similar to 1.4 x 10(10) M-1 s(-1)) in CH2Cl2 solution. Reductive-quenching experiments of complexes 1-3 by the amine donors N,N,N',N'-tetramethylphenylenediamine (TMPD), phenothiazine (PTZ), and N,N,N',N'-tetramethylbenzidine (TMB) follow Stern-Volmer behavior, with very fast quenching rates on the order of 10(9)-10(10) M-1 s(-1) in CH2Cl2 solution. When the complexes are employed as the sensitizer in multiple component systems containing MV2+, TEOA, and colloidal Pt in aqueous media, approximately one turnover of H-2 (TN vs mol of chromophore) is produced per hour upon irradiation with lambda > 410 nm but only after at least a 2 h induction period.