Band gap engineering in pyridyl-functionalized two-dimensional (2D) CuSCN coordination polymers†

Copper(I) thiocyanate (CuSCN) has emerged as an excellent hole-transporting semiconductor with applications spanning across electronic and optoelectronic fields. The coordination chemistry of CuSCN allows for extensive structural versatility via ligand modification. In particular, CuSCN modified with pyridine (Py) derivatives can produce novel two-dimensional (2D) structures of the Cu–SCN network while also allowing for the tuning of electronic properties by changing the substituent group on Py. However, obtaining phase-pure 2D structures remains a challenge as the conventional method often yields mixed products of varying stoichiometry having different structures. In this work, we have developed a synthetic method that reliably produces phase pure [Cu(SCN)(3-XPy)]_n complexes (X = OMe, H, Br, and Cl) in a 1 : 1 : 1 ratio all with confirmed 2D structures. The single crystal structure of [Cu(SCN)(3-OMePy)]_n is also reported herein and compared with the reported structures of the other three compounds. Complexes with X = OMe and H show similar structures, in which the 2D layers are analogous to the buckled 2D sheets of silicene or blue phosphorene. On the other hand, for complexes with X = Br and Cl, their rippled 2D structures resemble the puckered 2D sheets found in black phosphorene. The variation of the electron-withdrawing ability of the substituent group is found to systematically shift the electronic energy levels and band gaps of the complexes, allowing the 2D CuSCN-based materials to display optical absorptions and emissions in the visible range. In addition, first-principles calculations reveal that the drastic change in the electronic levels is a result of the emergence of the Py ligand electronic states below the SCN states. This work demonstrates that the structural, electronic, and optical properties of 2D Cu–SCN networks can be systematically tailored through ligand modification.