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Abstract This paper describes the use of a highly crystalline conductive 2D copper₃(hexaiminobenzene)₂(Cu₃(HIB)₂) as an ultrasensitive (limit of detection of 1.8 part‐per‐billion), highly selective, reversible, and low power chemiresistive sensor for nitric oxide (NO) at room temperature. The Cu₃(HIB)₂‐based sensors retain their sensing performance in the presence of humidity, and exhibit strong signal enhancement towards NO over other highly toxic reactive gases, such as NO₂, H₂S, SO₂, NH₃, CO, as well as CO₂. Mechanistic investigations of the Cu₃(HIB)₂‐NO interaction through spectroscopic analyses and density functional theory revealed that the Cu‐bis(iminobenzosemiquinoid) moieties serve as the binding sites for NO sensing, while the Ni‐bis(iminobenzosemiquinoid) MOF analog shows no noticeable response to NO. Overall, these findings provide a significant advance in the development of crystalline metal‐bis(iminobenzosemiquinoid)‐based conductive 2D MOFs as highly sensitive, selective, and reversible sensing materials for the low‐power detection of toxic gases. 

Abstract 2D materials can be isolated as monolayer sheets when interlayer interactions involve weak van der Waals forces. These atomically thin structures enable novel topological physics and open chemical questions of how to tune the structure and properties of the sheets while maintaining them as isolated monolayers. Here, this work investigates 2D electroactive sheets that exfoliate in solution into colloidal nanosheets, but aggregate upon oxidation, giving rise to tunable interlayer charge transfer absorption and photoluminescence. This optical behavior resembles interlayer excitons, now intensely studied due to their long‐lived emission, but which remain difficult to tune through synthetic chemistry. Instead, the interlayer excitons of these framework sheets can be modulated through control of solvent, electrolyte, oxidation state, and the composition of the framework building blocks. Compared to other 2D materials, these framework sheets display the largest known interlayer binding strengths, attributable to specific orbital interactions between the sheets, and among the longest interlayer exciton lifetimes. Taken together, this study provides a microscopic basis for manipulating long‐range opto‐electronic behavior in van der Waals materials through molecular synthetic chemistry.