A Fused Poly(truncated rhombic dodecahedron)-Containing 3D Coordination Polymer: A Multifunctional Material with Exceptional Properties

Abstract

The design of new and inexpensive metal-containing functional materials is of great interest. Herein is reported a unique thermochromic near-IR emitting coordination polymer, 3D-[Cu8I8(L1)2]n, CP2, which is formed when ArS(CH2)4SAr (L1, Ar = 4-C6H4OMe) reacts with 2 equiv of CuI in EtCN. In MeCN, CP1 ([Cu4I4(L1)(MeCN)2]n, consisting of an alternating [-Cu4I4-L1-Cu4I4-L1-]n chain where the Cu4I4 cubane units bear two metal-bound MeCN molecules, is formed. Heat-driven elimination of these MeCN's in solid CP1 also leads to CP2 through a predisposed organization of the Cu4I4 units prone to fusion after MeCN eliminations (i.e., a rare case of template effect). The CP2 structure exhibits parallel 1D-(Cu8I8)n chains, (z-axis; designated 1D-[CuI]n) as secondary building units (SBU) held together by parallel thioether ligands (x,y-axes), forming a nonporous 3D network. The structure of this 1D-[CuI]n SBU is unprecedented and consists of a series of fused and twisted open Cu4I4 cubanes forming a fused poly(truncated rhombic dodecahedron). Unexpectedly, the compact 3D CP2 exhibits a solid-to-solid phase transition at 100 °C and a hysteresis of ∼20 °C. CP1 emits intensively (298 K: λemi = 564 nm; Φe = 0.35), whereas CP2 presents a strongly red-shifted weaker emission (298 K: λemi ∼ 740 nm, Φe < 0.0001). Moreover, CP2, which is stable over long periods of time, exhibits thermochromism where the emission intensity of the near-IR band decreases significantly at the benefit of a ligand-centered phosphorescence at 415 nm. Altogether, these properties listed above make CP2 exceptional. The low-energy singlet and triplet excited states have been assigned to ligand/metal-to-ligand charge transfer based on DFT and TD-DFT computations.