Halide impact on emission of mononuclear copper(i) complexes with pyrazolylpyrimidine and triphenylphosphine

Abstract

A series of mononuclear heteroleptic copper(I) halide complexes, [CuL(PPh₃)X] (X = Cl, Br, I), based on 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-(piperidin-1-yl)pyrimidine (L) and triphenylphosphine, have been synthesized by reaction between CuX (X = Cl, Br, I), L and PPh₃ in a molar ratio of 1/1/1 in MeCN solutions. The copper atom, showing the distorted tetrahedral environment, is bound by the N,N-chelating ligand L, triphenylphosphine and a halide ion. The complexes [CuL(PPh₃)Cl] and [CuL(PPh₃)Br] are isostructural. In CH₂Cl₂ solutions, L and the complexes [CuL(PPh₃)X] (X = Cl, Br, I) display a luminescence band with λ_max = 377 nm and a lifetime of 1.9 ns (ligand-based luminescence (LL*)). However, the complex [CuL(PPh₃)I] has an additional weak luminescence band with λ_max = 681 nm and a lifetime of 96 ns of ³MLCT origin. In the solid state, L shows the splitting of the luminescence band to λ_max = 365 and 384 nm and a slight increase of the lifetime to 2.66 ns. Solid samples of the complexes [CuL(PPh₃)X] demonstrate ³MLCT luminescence bands at 620 nm (X = Cl), 605 nm (X = Br) and 559 nm (X = I) with lifetimes in the range 3.6–11.2 μs, whereas the LL* band (377 nm) is absent. Quantum yields and rate constants of radiative and nonradiative processes were determined in CH₂Cl₂ solutions and in the solid state for all complexes. The luminescence quantum yield and lifetimes for the solid samples increase in the order [CuL(PPh₃)Cl] < [CuL(PPh₃)Br] < [CuL(PPh₃)I]. This is due to the increase of radiative decay and simultaneous suppression of nonradiative decay. The complex [CuL(PPh₃)I] shows a high quantum yield of 29.4% and an excited state lifetime of 11.2 μs.