Synthesis, characterization and luminescence properties of homoleptic platinum(II) acetylide complexes

A family of homoleptic tetraalkynylplatinate(II) complexes (NBu4)(2)[Pt(C=CR)(4)] containing various arylsubstituted (R = C6H4X), where X is an electron-donating, - withdrawing or - delocalizing substituent, and ethynylpyridine (C5H4N-2, C5H4N-4) ligands have been prepared. The structures of complexes with R = (4- CF3)C6H4 (4 . 2HCCl(3)), (4-CN)C6H4 (6 . H2O) and C5H4N-4 (9b . HC=CC5H4N-4 . 2H(2)O) have been determined by single-crystal X-ray diffraction. While in the case of 4 two molecules of CHCl3 are incorporated to the anion by short CH...pi interactions, for the para-pyridylethynyl derivative one H2O molecule connect different anions through moderate N...H - O - H...N intermolecular interactions, giving an extended one-dimensional chain. The photophysical and electrochemical properties of the complexes have been examined. All of them display emissions at 77 and 298 K in the fluid and solid state, with lifetimes in the microsecond regime. In some cases the emission profile is clearly the envelope of several closely bands, whose relative intensity depends on the excitation energy. On the basis of TD-DFT theoretical calculations on the anion [Pt{C=C(4-CN)C6H4}(4)](2-) of complex 6, it is proposed that the emissions are due to triplet intraligand excited states, with both an aryl (or pyridyl) and acetylenic character, which arise from an admixture of pi-->pi*(C=CR) IL and d(pi)(Pt) --> pi*(C=CR) MLCT transitions.