Luminescence and non-linear optical properties in copper(I) halide extended networks

Abstract

The syntheses, structures, and luminescence properties of a series of copper(I) halide coordination polymers, prepared with mono- and bidentate N-heteroaromatic ligands, are reported. These metal–organic coordination networks form [Cu2I2L]n for bidentate ligands (where L = pyrazine (1), quinazoline (2)) and [CuIL]n for monodentate ligands (where L = 3-benzoylpyridine (3) and 4-benzoylpyridine(4)). Both sets of compounds exhibit a double-stranded stair—Cu2I2—polymer, or “ladder” structure with the ligand coordinating to the metal in a bidentate (bridging two stairs) or monodentate mode. The copper bromide analogues for the bidentate ligands were also targeted, [Cu2Br2L]n for L = pyrazine (5) with the same stair structure, as well as compositions of [CuBr(L)]n for L = pyrazine (6) and quinazoline (7), which have a different structure type, where the −Cu–Br– forms a single-stranded “zigzag” chain. These copper halide polymers were found to be luminescent at room temperature, with emission peaks ranging from ∼550 to 680 nm with small shifts at low temperature. The structure (stair or chain), the halide (I or Br), as well as the ligand play an important role in determining the position and intensity of emission. Lifetime measurements at room and low temperatures confirm the presence of thermally activated delayed fluorescence, or singlet harvesting for compounds 1, 2, and 7. We also investigated the nonlinear optical properties and found that, of this series, [CuBr(quinazoline)]n shows a very strong second harmonic generating response that is ∼150 times greater than that of α-SiO2.