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.

本文报道了一类新型环金属化铂(II)配合物[Pt(C∧N)(O∧O)]（1a–5a和1b–5b）的合成、表征、电化学、光物理及光致变色行为，其中C∧N为含有光致变色二噻吩乙烯（dithienylethene, DTE）单元的环金属配体2-(2'-噻吩基)吡啶（2-(2′-thienyl)pyridyl, thpy）或2-(2'-噻吩并噻吩基)吡啶（2-(2′-thienothienyl)pyridyl, tthpy），O∧O为乙酰丙酮根（acetylacetonato, acac）或六氟乙酰丙酮根（hexafluoroacetylacetonato, hfac）类β-二酮根配体。此外还测定了其中5个配合物的X射线晶体结构。电化学研究表明，该类配合物的首个准可逆还原电对，即其最低未占据分子轨道（lowest unoccupied molecular orbital, LUMO）的性质，对辅助配体O∧O的结构敏感。在光激发下，配合物1a–3a和1b–3b会发生显著的颜色变化，该变化归因于可逆光致变色行为，且该行为对吡啶环上的取代基、C∧N配体的π共轭程度以及辅助配体的种类均具有敏感性。本文还在不同温度下的甲苯溶液中研究了配合物1a的热褪色动力学，并测定了该配合物热环逆转反应的活化能垒。此外，通过密度泛函理论（Density functional theory, DFT）计算，为阐明该类配合物的电化学、光物理及光致变色性质提供了理论依据。