Charge-Transfer and Ligand-Localized Photophysics in Luminescent Cyclometalated Pyrazolate-Bridged Dinuclear Platinum(II) Complexes

Abstract

We present the synthesis, photophysical characterization, and electrochemistry of three series of cyclometalated binuclear platinum(II) complexes, each bridged by two 3,5-disubstituted pyrazolate ligands (μ-R2pz). These neutral compounds have the general formula [C∧NPt(μ-R2pz)]2, where C∧N is a cyclometalating ligand corresponding to 2-(2′-thienyl)pyridine (thpy), 1-phenylisoquinoline (piq), or 7,8-benzoquinoline (bzq) with R = H, Me, iPr, Ph, corresponding to series I–III dimers, respectively. Systematic variation of the cyclometalating ligands in addition to the bridging pyrazolates renders colorful structures exhibiting a range of electrochemical and spectroscopic behavior with absorption and photoluminescence properties tuned over a wide portion of the visible spectrum. Steric bulk introduced into the 3,5-positions on the pz bridges readily modulates intramolecular d8–d8 metal–metal σ interactions strongly affecting the frontier orbitals’ electronic structure, manifested by changes in absorption and emission energy, excited-state lifetime, and photoluminescence quantum yield. Cyclic voltammetry revealed the presence of two very closely spaced reversible C∧N ligand-based reductions ranging between −1.97 and −2.56 V vs Fc+/Fc, and the first metal–metal-centered oxidation wave was found to be reversible in dichloromethane and irreversible in coordinating THF in most instances. All the complexes of series I displayed triplet ligand-localized excited states at all temperatures, while an increase of steric bulk in the pz bridge in the two other molecular series resulted in a variation of photophysical behavior ranging from charge transfer to ligand localized, including admixture behavior.