A new application of terahertz time-domain absorption spectra in luminescent complexes: characterization of the C–H⋯π weak interactions in Cu(i) complexes

Abstract

Six Cu(I) complexes, [Cu(2,3-f)(bdppmapy)]BF₄ (1), [Cu(2,3-f)(bdppmapy)]ClO₄ (2), [Cu(2,3-f)(bdppmapy)]CF₃SO₃ (3), [Cu(imidazo[4,5-f])(bdppmapy)]BF₄ (4), [Cu(imidazo[4,5-f])(bdppmapy)]ClO₄ (5), and [Cu(imidazo[4,5-f])(bdppmapy)]CF₃SO₃·MeOH (6·MeOH) (bdppmapy = N,N-bis[(diphenylphosphino)methyl]-2-pyridinamine, 2,3-f = pyrazine[2,3-f][1,10]-phenanthroline, and imidazo[4,5-f] = 1H-imidazo[4,5-f][1,10]-phenanthroline), have been synthesized to explore the effects of counteranions on their crystal structures, photophysical properties, and terahertz (THz) spectra. Time-dependent density functional theory (TD-DFT) shows that the luminescence performance of these complexes is attributed to the metal-to-ligand charge transfer (MLCT) in combination with ligand-to-ligand charge transfer (LLCT). In complexes 1–3, the characteristic peak at 1.4 THz is mainly related to the C–H⋯π interaction formed by the H atom on the 4#/5# position of 2,3-f and the benzene ring from the bdppmapy on the adjacent asymmetric unit. The common C–H⋯π interaction enhances the rigidity of the structure and has non-negligible influence on the photoluminescence quantum yields (PLQYs): the stronger the C–H⋯π interaction is, the higher the quantum yield (QY) is. In complexes 4–6, similar absorption peaks (1.10–1.30 THz) are mainly related to the C–H⋯π interactions, and strong absorption peaks (1.50–1.90 THz) are affected by the typical hydrogen bonds N–H⋯F/O and O–H⋯O. These results show that some weak interactions can be characterized by THz time-domain spectroscopy (THz-TDS). So, the THz spectroscopy method would make it possible to tune some of the weak interactions in complex structures to regulate the luminescence of materials.