New Cu(i) halide complexes showing TADF combined with room temperature phosphorescence: the balance tuned by halogens

Abstract

A series of Cu(I) halide complexes derived from tris(2-pyridyl)phosphine (Py₃P), [Cu₂(Py₃P)₂X₂] (X = Cl, Br, I), have been synthesized by a straightforward reaction in solution or through a mechanochemical route. At room temperature, the solid complexes exhibit bright dual-mode photoluminescence (λ_max = 520–550 nm, τ = 14.5–20.0 μs, and Φ_PL ≈ 53%), expressed by thermally activated delayed fluorescence (TADF) combined with phosphorescence (PH), originating from ¹(M + X)LCT and ³(M + X)LCT excited states, respectively. Remarkably, the balance of these radiative processes at 300 K is regulated by halogen atom nature, switching from TADF-assisted phosphorescence to PH-admixed TADF. The emission of [Cu₂(Py₃P)₂Cl₂] at 300 K is largely contributed by PH (73%) admixed with the TADF fraction (27%) and [Cu₂(Py₃P)₂Br₂] also emits mainly PH (65%) admixed with the larger TADF fraction (35%). Meanwhile, for [Cu₂(Py₃P)₂I₂], the TADF channel becomes dominating (61%) and PH contribution drops to 39%. The photophysical study corroborated by (TD)DFT computations has revealed that this effect arises mainly from the narrowing of the ΔE(S₁ − T₁) gap of the [Cu₂(Py₃P)₂X₂] complexes in the order Cl (1500 cm⁻¹) > Br (1250 cm⁻¹) > I (1000 cm⁻¹) which facilitates the TADF pathway and suppresses PH in the same order.