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](μ-X). 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 CuX 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.

Luminescent Platinum(II) Complexes of N^N–^N Amido Ligands with Benzannulated N-Heterocyclic Donor Arms: Quinolines Offer Unexpectedly Deeper Red Phosphorescence than Phenanthridines

“…We also observed a similar effect in the luminescence of dinuclear Cu(I) complexes [(P^N)Cu] 2 (µ-X) 2 with P^N-coordinating phosphine-pyridine ligands based on quinolines and phenanthridines (X = halide): the luminescence of the more conjugated phenanthridine-based complexes was blue-shifted relative to the quinoline analogues. 43 Here, the absorption maxima are unchanged by benzannulation, while emission is starkly affected. We return to the likely origin of this effect in the next section.…”

Deep-Red Luminescence from Platinum(II) Complexes of N^N–^N-Amido Ligands with Benzannulated N-Heterocyclic Donor Arms

“…Similarly exceptional behaviour was observed for phenanthridinyl and quinolinyl derivatives of (P^N) 2 CuX 2 dimers. 47 In that case, emission was shifted to higher energy for complexes of phenanthridinyl ligands despite a red-shift in absorption. The library of proligands presented here enables both further insight into the impact of π-extension and also how substitution patterns of benzannulated ligands affect absorption and emission.…”

Exploiting synergy between ligand design and counterion interactions to boost room temperature phosphorescence from Cu(i) compounds

“…The π--extended P^N donor L3 was synthesized similarly to L1 25 and L2. 26 The tricyclic frame of the phenanthridine moiety was prepared via a one--pot, Pd--catalyzed cross--coupling/condensation of the appropriately substituted aniline with 2-acetylphenylboronic acid (Scheme 1). L3 was then accessed via lithium--halogen exchange between 4--bromo--2,6--dimethylphenanthridine and sec--butyllithium, followed by quenching with Ph 2 PCl.…”

“…) are all 'head--to--tail', with longer Cu--Cu distances. 25,26 The presence of ortho methyl groups adjacent to the phenanthridine nitrogens thus appears to override the steric preferences of the PPh 2 units to avoid each other in the solid--state. Addition of a second equivalent of the P^N ligand, followed by metathesis with NaPF 6 or NaBPh 4 in tetrahydrofuran, gave bright yellow/green suspensions, from which the targeted [(L) 2 Cu]X salts could be isolated as light yellow solids.…”