Cu₃I₇ Trimer and Cu₄I₈ Tetramer Based Cuprous Iodide Polymorphs for Efficient Photocatalysis and Luminescent Sensing: Unveiling Possible Hierarchical Assembly Mechanism

Solvothermal reactions of CuI, 1,4-diazabicyclo[2.2.2]­octane (DABCO), and HI in an ethanol solution at 140 °C/150 °C for 7 days resulted in two 24-membered-ring-based layered semiconducting iodocuprate open-network polymorphs formulated as [deDABCO]₂[meDABCO]­Cu₁₁I₁₇ (deDABCO = <i>N</i>,<i>N</i>′-diethyl-1,4-diazabicyclo­[2.2.2] octane and meDABCO = <i>N</i>-methyl-<i>N</i>′-ethyl-1,4-diazabicyclo­[2.2.2]­octane). The deDABCO and meDABCO templates were in situ generated via alkylation of DABCO during solvothermal reactions. The formation of layered Cu₁₁I₁₇^6– polymorphs can be rationalized via analyses of hierarchical building units. There are four hierarchical building units in polymorphs, namely, primary CuI₃ triangle and CuI₄ tetrahedron, secondary Cu₃I₇ trimer and Cu₄I₈ tetramer, tertiary Cu₆I₁₂ hexamer, and quaternary Cu₁₂I₂₂ dodecamer. The trimeric Cu₃I₇ secondary building unit (SBU) is constructed by three edge-shared CuI₄ tetrahedra, while the tetrameric Cu₄I₈ SBU with an inversion center is formed by edge-shared two CuI₃ triangles and two CuI₄ tetrahedra. Two Cu₃I₇ SBUs are fused together via the sharing of two iodine atoms to form a Cu₆I₁₂ tertiary building unit (TBU), and two TBUs are further fused via the sharing of two iodine atoms into a Cu₁₂I₂₂ quaternary building unit (QBU). In colorless polymorph <b>1</b>, each Cu₃I₇ SBU is connected to three neighbors via one Cu₄I₈ and two Cu₆I₁₂ linkers to form a 6,3-connected layer with 24-membered ring window. Different from <b>1</b>, each Cu₆I₁₂ TBU in yellowish polymorph <b>2</b> is connected to four neighbors via two Cu₄I₈ and two Cu₁₂I₂₂ linkers to form a (4,4) topological layer also with 24-membered-ring window. These two compounds are very rare examples of copper halide polymorphs that exhibit similar local coordination geometries of copper­(I) but different layered open networks. Electrical conductivity, band structure calculation, and UV–vis diffuse-reflectance spectrometry indicate that <b>1</b> and <b>2</b> are potential semiconductor materials, and the performance on the photocatalytic degradation of organic pollutants upon UV-light irradiation reveals that both <b>1</b> and <b>2</b> are highly efficient photocatalysts. Two polymorphs exhibit very similar green photoluminescence at room temperature in the solid state, and the study of the luminescent response to solvent on two polymorphs exhibits highly sensitive sensing of nitro explosives via quenching