Luminescent alkynylplatinum(II) complexes of 2,6-bis(N-alkylbenzimidazol- 2′-yl)pyridine-type ligands with ready tunability of the nature of the emissive states by solvent and electronic property modulation

Abstract

A new class of luminescent alkynylplatinum(II) complexes of tri-dentate bis(N-alkylbenzimidazol-2′-yl)-pyridines (bzimpy), [Pt(R,R′-bzimpy)- (C≡C-R″)]X (X = PF6, OTf), and one of their chloro precursor complexes, [Pt(R,R′-bzimpy)Cl]PF6, have been synthesized and characterized; one of the alkynyl complexes has also been structurally characterized by X-ray crystallography. Electrochemical studies showed that the oxidation wave is alkynyl ligand-based in nature with some mixing of the metal center-based contribution, whereas the two quasi-reversible reduction couples are mainly bzimpy-based reductions. The electronic absorption and luminescence properties of the complexes have also been investigated. In solution, the high-energy and intense absorption bands are assigned as the π-π* intraligand (IL) transitions of the bzimpy and alkynyl ligands, whereas the low-energy and moderately intense absorptions are assigned to an admixture of metal-to-ligand charge-transfer (MLCT) (dπ(Pt) →π*(R,R′-bzimpy)) and ligand-to-ligand charge-transfer (LLCT) (π-(C≡C-R″) →* (R,R′-bzimpy)) transitions. Upon variation of the electronic effects of the arylalkynyl ligands, vibronic-structured or structureless emission bands, originating from triplet metal-perturbed intraligand (IL) or an admixture of triplet metal-to-ligand charge-transfer (MLCT) and ligand-to-ligand charge-transfer (LLCT) excited states respectively, were observed in solution. Interestingly, two of the complexes showed a dual luminescence that was sensitive to the polarity of the solvents. Upon cooling from 298 K to 155 K, drastic color, UV/Vis, and luminescence changes were observed in a butyronitrile solution of 1, and were ascribed to the formation of aggregate species through Pt ⋯ Pt and π-π stacking interactions. DFT and time-dependent DFT (TD-DFT) calculations have been performed to verify and elucidate the results of the electrochemical and photophysical properties. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.