Interplay of the Cu⋯Cu distance and coordination geometry as a factor affecting the quantum efficiency in dimeric copper(i) halide complexes with derivatives of 4-pyrazolylpyrimidine-2-thiol

Abstract

Two bicyclic pyrazolylpyrimidine compounds, 2-benzylthio-4-(3,5-dimethyl-1H-pyrazol-1-yl)pyrimidine (L^H) and 2-benzylthio-4-(3,5-dimethyl-1H-pyrazol-1-yl)-6-methylpyrimidine (L^Me), were synthesized and studied as ligands for the preparation of copper(I) halido complexes. In the solid state, L^H and L^Me demonstrate dual excitation-wavelength dependent emission, i.e. fluorescence at higher excitation energies and phosphorescence at lower excitation energies due to the presence of a heavy sulphur atom. The reactions of L^H and L^Me with CuBr and CuI afforded a series of centrosymmetric binuclear complexes of the [Cu₂L₂Hal₂] type (L = L^H, Hal = Br, I; L = L^Me, Hal = I). The possibility of rotation of the benzylthio group relative to the pyrazolylpyrimidine core leads to the isolation of two polymorphic modifications of the copper(I) iodido complex with L^H, which differ by the Cu⋯Cu distance by more than 0.2 Å (2.86 Å for [Cu₂(L^H)₂I₂] (form I)vs. 2.65 Å for [Cu₂(L^H)₂I₂] (form II)). The isolation of the [Cu₂(L^H)₂I₂] complex in two different crystalline forms made it possible to reveal the influence of a rarely explored factor, namely the change in the Cu⋯Cu distance in a single molecule, on the photoluminescence quantum efficiency. Two structural indices, τ_dim, which showcases the degree of merging of CuLHal monomers into the centrosymmetric [Cu₂L₂Hal₂] dimers, and τ_plan, which characterises the degree of planarization of the N₂CuHal₂CuN₂ unit, were introduced and used for combined experimental and theoretical analyses of the relation between the structure of the complexes and their luminescence. All complexes exhibit phosphorescence of the ligand-to-halide charge transfer (LXCT) character in the orange region. According to TD-DFT calculations, an increase in the Cu⋯Cu distance facilitates structural rearrangement in the T₁ state followed by a rapid decrease in the T₁–S₀ energy gap and subsequent non-radiative decay via electron–phonon coupling, which substantiates the higher photoluminescence quantum yield (PLQY) of [Cu₂(L^H)₂I₂] (form II) (Cu⋯Cu 2.65 Å) compared to that of [Cu₂(L^H)₂I₂] (form I) (Cu⋯Cu 2.86 Å).