Multi-stimulus semiconductor Cu(i)–I-pyrimidine coordination polymer with thermo- and mechanochromic sensing

Abstract

This work shows a detailed study of the synthesis, structural characterization, electrical, thermal, and mechanical luminescence behavior of a one-dimensional coordination polymer formed by a Cu(I)–I double chain, using 2-amino-4 chloropyrimidine as a ligand. The asymmetric structure of double chain type [Cu(L)I]_n, (L = 2-amino-4-chloropyrimidine (aCLpym)) with Cu–Cu distances shorter than the sum of the van der Waals radii of two copper atoms, and the substantial flexibility of this structure are crucial to understanding its optoelectronic behavior. The electrical conductivity values at 295 K are in agreement with possible semiconductor behavior. Additionally, the luminescence properties of this coordination polymer (CP) were characterized under ambient conditions (ambient pressure (AP) and 295 K) by a non-symmetric high energy (HE) band centered at 465 nm, tentatively assigned to a mixed ³(I + M)LCT. The intensity of the emission increases upon lowering the temperature. A structural contraction can explain this behaviour, clearly observed when solving the structures employing single-crystal X-ray diffraction of the compound at 296 K and 110 K. Strong mechanochromic behavior was observed when hydrostatic pressure was applied, with the appearance of an additional low energy (LE) band at around 650 nm, which for this type of CuI CP is typically associated with transitions in the CuI cluster-core, involving ³IMCT and cluster centered 3d¹⁰ → 3d⁹4s¹ (³MCC) states. The latter is due to the shortening under pressure of the Cu–Cu distances below the van der Waals limit. Moreover, the study of the mechanochromic behavior versus pressure exerted by grinding shows a decrease in the intensity of the emission due to the increase in the number of defects in the material and a decrease in its particle size.