Deep blue emitting Cu(<scp>i</scp>) tripod complexes. Design of high quantum yield materials showing TADF-assisted phosphorescence

Abstract

In a previous investigation, it was shown that [Cu(tpym)(PPh₃)]PF₆1 with tpym = tris(2-pyridyl)methane represents a deep blue emitter (λ_max = 466 nm) though with a low emission quantum yield Φ_PL if doped in a polymer (7%) or dissolved in a fluid solvent (≪1%). In this study, we present new tripod compounds with sterically demanding ligands: [Cu(tpym)(P(o-tol)₃)]PF₆2 and [Cu(tpym)(P(o-butyl-ph)₃)]PF₆3 with P(o-tol)₃ = tris(ortho-tolyl)phosphine and P(o-butyl-ph)₃ = tris(ortho-n-butylphenyl)phosphine. These compounds show high emission quantum yields even in a fluid solution (dichloromethane) reaching a benchmark value for 3 of Φ_PL = 76%. This becomes possible due to the specific design of rigidifying the complexes. Importantly, the deep blue emission color is maintained or even further blue shifted to λ_max = 452 nm (compound 3 powder). Compound 2 is characterized photophysically in detail. In particular, it is shown that the lowest excited triplet state T₁ experiences very efficient spin-orbit coupling (SOC). Accordingly, the phosphorescence decay rate is as large as 5 × 10⁴ s^-1 (20 μs) belonging to the fastest T₁→ S₀ transition values (shortest decay times) reported so far. Investigations down to T = 1.5 K reveal a large total zero-field splitting (ZFS) of 7 cm^-1 (0.9 meV). Although thermally activated delayed fluorescence (TADF) grows in at T≥ 160 K, the phosphorescence of 2 still dominates (60%) over TADF (40%) at ambient temperature. Thus, the compound represents a singlet harvesting-plus-triplet harvesting material, if applied in an OLED.