Highly luminescent dinuclear platinum(II) complexes incorporating bis-cyclometallating pyrazine-based ligands: a...

A series of luminescent dinuclear platinum(II) complexes incorporating diphenylpyrazine-based bridging ligands (L(n)H2) has been prepared. Both 2,5-diphenylpyrazine (L(2)H2) and 2,3-diphenylpyrazine (L(3)H2) are able to undergo cyclometalation of the two phenyl rings, with each metal ion binding to the two nitrogen atoms of the central heterocycle, giving, after treatment with the anion of dipivaloyl methane (dpm), complexes of formula {Pt(dpm)}2L(n). These compounds are isomers of the analogous complex of 4,6-diphenylpyrimidine (L(1)H2). Related complexes of dibenzo(f,h)quinoxaline (L(4)H2), 2,3-diphenyl-quinoxaline (L(5)H2), and dibenzo[3,2-a:2',3'-c]phenazine (L(6)H2) have also been prepared, allowing the effects of strapping together the phenyl rings (L(4)H2 and L(6)H2) and/or extension of the conjugation from pyrazine to quinoxaline (L(5)H2 and L(6)H2) to be investigated. In all cases, the corresponding mononuclear complexes, Pt(dpm)L(n)H, have been isolated too. All 12 complexes are phosphorescent in solution at ambient temperature. Emission spectra of the dinuclear complexes are consistently red shifted compared to their mononuclear analogues, as are the lowest energy absorption bands. Electrochemical data and TD-DFT calculations suggest that this effect arises primarily from stabilization of the LUMO. Introduction of the second metal ion also has the effect of substantially increasing the molar absorptivity and, in most cases, the radiative rate constants. Meanwhile, extension of conjugation in the heterocycle of L(5)H2 and L(6)H2 and planarization of the aromatic system favored by interannular bond formation in L(4)H2 and L(6)H2 leads to further red shifts of the absorption and emission spectra to wavelengths that are unusually long for cyclometalated platinum(II) complexes. The results may offer a versatile design strategy for tuning and optimizing the optical properties of d-block metal complexes for contemporary applications.