Diarylethene-Containing Cyclometalated Platinum(II) Complexes: Tunable Photochromism via Metal Coordination and Rational Ligand Design

The synthesis, characterization, electrochemistry, photophysics and photochromic behavior of a new class of cyclometalated platinum(II) complexes [Pt(C^∧N)(O^∧O)] (1a–5a and 1b–5b), where C^∧N is a cyclometalating 2-(2′-thienyl)pyridyl (thpy) or 2-(2′-thienothienyl)pyridyl (tthpy) ligand containing the photochromic dithienylethene (DTE) unit and O^∧O is a β-diketonato ligand of acetylacetonato (acac) or hexafluoroacetylacetonato (hfac), have been reported. The X-ray crystal structures of five of the complexes have also been determined. The electrochemical studies reveal that the first quasi-reversible reduction couple, and hence the nature of lowest unoccupied molecular orbital (LUMO) of the complexes, is sensitive to the nature of the ancillary O^∧O ligands. Upon photoexcitation, complexes 1a–3a and 1b–3b exhibit drastic color changes, ascribed to the reversible photochromic behavior, which is found to be sensitive to the substituents on the pyridyl ring and the extent of π-conjugation of the C^∧N ligand as well as the nature of the ancillary ligand. The thermal bleaching kinetics of complex 1a has been studied in toluene at various temperatures, and the activation barrier for the thermal cycloreversion of the complex has been determined. Density functional theory (DFT) calculations have been performed to provide an insight into the electrochemical, photophysical and photochromic properties.