Synthesis and Luminescent Properties of Zwitterionic Platinum Bis-Phosphine Complexes Based on a Pendant Closo-Monocarborane Cluster

The synthesis, structures and photophysical properties of a series of zwitterionic platinum complexes are reported. These cyclometalated complexes have the general structure Pt(C^N)(P^P), where C^N is a cyclometalating ligand with a pendant monocarborane cluster and P^P is a bis-phosphine ligand (dppe or dppb). Their solid-state structures have been characterized by x-ray diffraction, and no Pt–Pt interactions are observed. All complexes are emissive at room temperature in solid states with quantum yields of 0.06–0.58 in PMMA films. The emission energy of these complexes is governed by the nature of the C^N cyclometalated ligands, allowing the emission to be tuned from blue to green. In particular, the introduction of an anionic carborane unit provokes a more than twofold improvement in the photoluminescence quantum yield of [Pt(CB₁₁-ppy)(dppe)] (3b) compared to the parent cationic complex [Pt(ppy)(dppe)]⁺. In addition, density functional theory (DFT) calculations were performed on the ground states and first triplet excited states of 1a, 2a, and 3a for interpretation of the observed photophysical properties.