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 this work, we have developed a synthetic method that reliably produces phase pure [Cu(SCN)(3-XPy)]n complexes (Py = pyridyl; X = OMe, H, Br, and Cl) in a 1:1:1 ratio to yield two-dimensional (2D) structures with a Cu-SCN network. The single crystal structure of [Cu(SCN)(3-OMePy)]n is also reported herein. 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.