Boosting CO2 Photoreduction to Formate or CO with High Selectivity over a Covalent Organic Framework Covalently Anchored on Graphene Oxide.

Covalent organic frameworks (COFs) have been widely studied in photocatalytic CO2 reduction reaction (CO2 RR). However, pristine COFs usually exhibit low catalytic efficiency owing to the fast recombination of photogenerated electrons and holes. In this study, we fabricated a stable COF-based composite (GO-COF-366-Co) by covalently anchoring COF-366-Co on the surface of graphene oxide (GO) for the photocatalytic CO2 reduction. Interestingly, in absolute acetonitrile (CH3 CN), GO-COF-366-Co shows a high selectivity of 94.4 % for the photoreduction of CO2 to formate, with a formate yield of 15.8 mmol/g, which is approximately four times higher than that using the pristine COF-366-Co. By contrast, in CH3 CN/H2 O (v : v=4 : 1), the main product for the photocatalytic CO2 reduction over GO-COF-366-Co is CO (96.1 %), with a CO yield as high as 52.2 mmol/g, which is also approximately four times higher than that using the pristine COF-366-Co. Photoelectrochemical experiments demonstrate the covalent bonding of COF-366-Co and GO to form the GO-COF-366-Co composite facilitates charge separation and transfer significantly, thereby accounting for the enhanced catalytic activity. In addition, theoretical calculations and in situ Fourier transform infrared spectroscopy reveal H2 O can stabilize the *COOH intermediate to further form a *CO intermediate via O-H(aq)⋅⋅⋅O(*COOH) hydrogen bonding, thus explaining the regulated photocatalytic performance.

Mechanochromic and Selective Vapochromic Solid-State Luminescence of a Dinuclear Cuprous Complex.

The unraveling of the stimuli-responsive mechanism is crucial to the design and precise synthesis of stimuli-responsive luminescent materials. We report herein the mechanochromic and selective vapochromic solid-state luminescence properties of a new bimetallic cuprous complex [{Cu(bpmtzH)}2(µ-dppm)2](ClO4)2 (1), and the corresponding response mechanisms are elucidated by investigating its two different solvated polymorphs 1·2CH2Cl2 (1-g) and 1·2CHCl3 (1-c). Green-emissive 1-g and cyan-emissive 1-c can be interconverted upon alternate exposure to CHCl3 and CH2Cl2 vapors, which is principally attributable to a combined alteration of both intermolecular NHbpmtzH···OClO3- hydrogen bonds and intramolecular "triazolyl/phenyl" π···π interactions induced by different solvents. Solid-state luminescence mechanochromism present in 1-g and 1-c is mainly ascribed to the grinding-induced breakage of the NHbpmtzH···OClO3- hydrogen bonds. It is suggested that intramolecular π···π-triazolyl/phenyl interactions are affected by different solvents but not by grinding. The results provide new insights into the design and precise synthesis of multi-stimuli-responsive luminescent materials by the comprehensive use of intermolecular hydrogen bonds and intramolecular π···π interactions.

Water-Etched Approach to Hierarchically Porous Metal-Organic Frameworks with High Stability.

The development of hierarchically porous metal-organic frameworks (MOFs) with high stability is desirable to expand their applications but remains challenging. Herein, an anionic sodalite-type microporous MOF (Yb-TTCA; TTCA3- = triphenylene-2,6,10-tricarboxylate) was synthesized, which shows outstanding catalytic activities for the cycloaddition of CO2 into cyclic carbonates. Moreover, the microporous Yb-TTCA can be transformed into a hierarchical micro- and mesoporous Yb-TTCA by water treatment with the mesopore sizes of 2 to 12 nm. The hierarchically porous Yb-TTCA (HP-Yb-TTCA) not only exhibits a high thermal stability up to 500 °C but also shows a high chemical stability in aqueous solutions with pH values ranging from 2 to 12. In addition, the HP-Yb-TTCA displays enhanced performance for the removal of organic dyes in comparison with microporous Yb-TTCA. This work provides a facile way to construct hierarchically porous MOF materials.

Incorporation of Chromophores into Metal-Organic Frameworks for Boosting CO2 Conversion.

The exploitation of highly stable and active catalysts for the conversion of CO2 into valuable fuels is desirable but is a great challenge. Herein, we report that the incorporation of chromophores into metal-organic frameworks (MOFs) could afford robust catalysts for efficient CO2 conversion. Specifically, a porous Nd(III) MOF (Nd-TTCA; TTCA3- = triphenylene-2,6,10-tricarboxylate) was constructed by incorporating one-dimensional Nd(CO2)n chains and TTCA3- ligands, which exhibits a very high stability, retaining its framework not only in the air at 300 °C for 2 h but also in boiling aqueous solutions at pH 1-12 for 7 days. More importantly, Nd-TTCA has achieved a 5-fold improvement in photocatalytic activity for reducing CO2 to HCOOH and a 10-fold improvement in catalytic activity for the cycloaddition of CO2 into cyclic carbonate in comparison to those of H3TTCA itself. This work gives a new strategy to design efficient artificial crystalline catalysts for CO2 conversion.