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

Mononuclear copper(I) complexes [CuL2]I (1), [CuL2](2)[Cu2I4]center dot 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 pi-systems of the L molecules in [CuL2](+) favor the formation of paired pi-pi 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 S-1 -> S-0 fluorescence and T-1 -> S-0 phosphorescence with intraligand charge transfer character. The complexes 1-3 demonstrate T-1 -> S-0 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 T-1 state as compared to the S-0 state. Spin-orbit coupling computations revealed that the most effective intersystem crossing channels for [CuL2](+) appear in high-lying excited states, while the S-1 -> T-1 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](+).