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

Solvothermal reactions of Cul, 1,4-diazabicyclo[2.2.2]octane (DABCO), and HI in an ethanol solution at 140 degrees C/150 degrees C for 7 days resulted in two 24-membered-ring-based layered semiconducting iodocuprate open-network polymorphs formulated as [deDABCO](2)[meDABCO]Cu11I17 (deDABCO = N,Ni-diethyl-1,4-diazabicyclo[2.2.2] octane and meDABCO = N-methyl-N'-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 Cu11I176- polymorphs can be rationalized via analyses of hierarchical building units. There are four hierarchical building units in polymorphs, namely, primary Cul(3) 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 Cul, tetrahedra, while the tetrameric Cu4I8 SBU with an inversion center is formed by edge-shared two CuI3 triangles and two Cul, 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 1, 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 1, each Cu6I12 TBU in yellowish polymorph 2 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 1 and 2 are potential semiconductor materials, and the performance on the photocatalytic degradation of organic pollutants upon UV-light irradiation reveals that both 1 and 2 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.