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

Mononuclear copper(I) complexes [CuL 2 ]I (1), [CuL 2 ] 2 [Cu 2 I 4 ]·2MeCN (2) and [CuL 2 ]PF 6 (3) with a new chelating pyrazolylpyrimidine ligand, 2-(3,5-dimethyl-1 H -pyrazol-1-yl)-4,6-diphenylpyrimidine (L), were synthesized. In the structures of complex cations [CuL 2 ] + , Cu + ions coordinate two L molecules ( N , N -chelating coordination). Extended π-systems of the L molecules in [CuL 2 ] + favor the formation of paired π-π stacking intramolecular interactions between the pyrimidine and phenyl rings leading to significant distortions of tetrahedral coordination cores, CuN 4 . 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 CuN 4 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 [CuL 2 ] + 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 [CuL 2 ] + .