This work is focused on the chemistry and physical properties of a particular family of coordination polymers (CPs) based on copper-iodine chains bearing functionalized pyridine, pyrimidine and pyrazine ligands. Although this family of compounds has been welldescribed, most of the studies have been focused on structural characterization and optical properties in bulk scale while works rationalizing their stimuli-responsive behavior and the role of these materials at the nanoscale are being reported very rencently. This article describes the first works in these fields and the perspectives that nanoscaled CPs based on Cu(I)-halide will have in the near future. Thus, the aim of this review is to present the new perspectives that are opening for these materials by deepening in their behavior, considering theoretical calculations, size-dependent properties or their combination with other components to form new hybrid materials. The combination of all these aspects will allow the discovery of interesting materials useful in sensing, nanochips or packaging, among other uses.

Direct reactions between CuI and isonicotinic acid (HIN) or the corresponding esters, ethyl isonicotinate (EtIN) or methyl isonicotinate (MeIN), give rise to the formation of the coordination polymers [CuI(L)]n with L=EtIN (1), MeIN (2) and HIN (3). Polymers 1-3 show similar structures based on a CuI double chain in which ethyl-, methyl isonicotinate or isonicotinic acid are coordinated as terminal ligands. Albeit, their supramolecular architecture differs considerably, affecting the distances and angles of the central CuI double chains and thereby their physical properties. Hence, the photoluminescence shows remarkable differences; 1 and 2 show a strong yellow emission, whereas 3 displays a weak emission; and 1 and 2 are semiconductors with moderate room temperature conductivities, whereas 3 has increased electrical conductivity up to 3×10(-3)  S cm(-1) . Additionally, 1 and 2 present an irreversible transition to a highly conducting phase with a conductivity almost 4 orders of magnitude higher and a quasi-metallic behaviour. Thermogravimetric analysis (TGA) coupled to a mass spectrometer and magnetic measurements point to a partial thermally induced oxidation of the carboxylate groups of the ligands with Cu(I) to Cu(0) reduction. DFT calculations have been carried out to rationalise these observations.

Multistimuli-responsive composite films with nanometric thickness based on Cu(i)–I coordination polymers.

A nonporous laminar coordination polymer of formula [Cu I (2-aminopyrazine)] is prepared by direct reaction between CuI and 2-aminopyrazine, two industrially available building blocks. The fine tuning of the reaction conditions allows obtaining [Cu I (2-aminopyrazine)] in micrometric and nanometric sizes with same structure and composition. Interestingly, both materials show similar reversible thermo- and pressure-luminescent response as well as reversible electrical response to volatile organic solvents such as acetic acid. X-ray diffraction studies under different conditions, temperatures and pressures, in combination with theoretical calculations allow rationalizing the physical properties of this compound and its changes under physical stimuli. Thus, the emission dramatically increases when lowering the temperature, while an enhancement of the pressure produces a decrease in the emission intensity. These observations emerge as a direct consequence of the high structural flexibility of the Cu I chains which undergo a contraction in CuCu distances as far as temperature decreases or pressure increases. However, the strong structural changes observed under high pressure lead to an unexpected effect that produces a less effective CuCu orbital overlapping that justifies the decrease in the intensity emission. This work shows the high potential of materials based on Cu I chains for new applications.