Site-Selective Benzannulation of N-Heterocycles in Bidentate Ligands Leads to Blue-Shifted Emission from [(P^N)Cu]2(μ-X)2 Dimers

Abstract

Benzannulated bidentate pyridine/phosphine (P^N) ligands bearing quinoline or phenanthridine (3,4-benzoquinoline) units have been prepared, along with their halide-bridged, dimeric Cu(I) complexes of the form [(P^N)Cu]2(μ-X)2. The copper complexes are phosphorescent in the orange-red region of the spectrum in the solid-state under ambient conditions. Structural characterization in solution and the solid-state reveals a flexible conformational landscape, with both diamond-like and butterfly motifs available to the Cu2X2 cores. Comparing the photophysical properties of complexes of (quinolinyl)phosphine ligands with those of π-extended (phenanthridinyl)phosphines has revealed a counterintuitive impact of site-selective benzannulation. Contrary to conventional assumptions regarding π-extension and a bathochromic shift in the lowest energy absorption maxima, a blue shift of nearly 40 nm in the emission wavelength is observed for the complexes with larger ligand π-systems, which is assigned as phosphorescence on the basis of emission energies and lifetimes. Comparison of the ground-state and triplet excited state structures optimized from DFT and TD-DFT calculations allows attribution of this effect to a greater rigidity for the benzannulated complexes resulting in a higher energy emissive triplet state, rather than significant perturbation of orbital energies. This study reveals that ligand structure can impact photophysical properties for emissive molecules by influencing their structural rigidity, in addition to their electronic structure.