Near-infrared emitting copper(I) complexes with a pyrazolylpyrimidine ligand: exploring relaxation pathways

Mononuclear copper(I) complexes [CuL₂]I (1), [CuL₂]₂[Cu₂I₄]·2MeCN (2) and [CuL₂]PF₆ (3) with a new chelating pyrazolylpyrimidine ligand, 2-(3,5-dimethyl-1H-pyrazol-1-yl)-4,6-diphenylpyrimidine (L), were synthesized. In the structures of complex cations [CuL₂]⁺, Cu⁺ ions coordinate two L molecules (N,N-chelating coordination). Extended π-systems of the L molecules in [CuL₂]⁺ favor the formation of paired π-π stacking intramolecular interactions between the pyrimidine and phenyl rings leading to significant distortions of tetrahedral coordination cores, CuN₄. The free ligand L demonstrates dual excitation wavelength dependent luminescence in the UV and violet regions, which is attributed to S₁ → S₀ fluorescence and T₁ → S₀ phosphorescence with intraligand charge transfer character. The complexes 1-3 demonstrate T₁ → S₀ phosphorescence in the near-infrared region. Theoretical investigations point to its ligand-to-metal charge transfer (LMCT) origin. Large Stokes shifts of emission (ca. 200 nm) are the result of notable planarizations of CuN₄ cores in the T₁ state as compared to the S₀ state. Spin-orbit coupling computations revealed that the most effective intersystem crossing channels for [CuL₂]⁺ appear in high-lying excited states, while the S₁ → T₁ transition is unfavourable according to El-Sayed's rule and the energy gap law. Electron-vibration coupling calculations showed that the C-C and C-N stretching vibrations of the pyrimidine and phenyl moieties, the asymmetric Cu-N stretching vibrations and the wagging motions of phenyl rings contribute the most to the non-radiative deactivation of L and [CuL₂]⁺.