Synthesis, structural characterisation and photophysics of anionic cyclometalated bis(alkynyl)(benzo[h]quinolinate)platinate(ii) species

Abstract

The alkynylation of [Pt(bzq)(μ-Cl)]₂ (bzqH = benzo[h]quinoline) with excess of LiCCR in diethyl ether leads to novel (7,8-benzoquinolinate)bis(alkynyl)platinate(II) monoanionic species [Pt(bzq)(CCR)₂]⁻ (R = ^tBu 1, SiMe₃2, Ph 3, Tol 4, C₆H₄CF₃-4 5, C₅H₄N-2 6, C₆H₄–CCPh 7) which have been isolated as tetrabutylammoniun salts. The molecular X-ray structures of 3 and 6 reveal the presence of discrete anions with bond lengths and angles similar to those of related cyclometalated Pt compounds and no evidence of π–π or Pt–Pt stacking interactions. The influence of the R substituent on the photophysics of the complexes has been examined by UV-visible absorption, emission spectroscopy and cyclic voltammetry. Complexes 1–6 show a similar strong luminescence in both the solid state (structureless emission) and frozen solutions (well-resolved vibronic structures) while the emissions in solution at room temperature are red shifted (relative to the glass) and weak. For these complexes, on the basis of TD-DFT calculations which reproduce the experimental structure of anion 3, [Pt(bzq)(CCPh)₂]⁻, the emissions are proposed to arise from mixed [π CCR/Pt d/π (bzq) → π* bzq] transitions. In contrast, complex 7 which contains the extended p-phenylethynylphenylacetylide ligand, CCC₆H₄–CCPh, displays a structured emission both at 298 K and in glassy CH₂Cl₂ solutions. In this case the lack of a significant rigidochromism and DFT theoretical calculations support the involvement of a low-lying, metal perturbed acetylenic ³ππ* (CCC₆H₄–CCPh) excited state.