Quasi-two-dimensional (2D) fully π–d conjugated metal–organic frameworks (MOFs) have been widely employed as active materials of secondary batteries; however, the origin of their high charge storage capacity is still unknown. Some reports have proposed a mechanism by assuming the formation of multiple radicals on one organic ligand, although there is no firm evidence for such a mechanism, which would run counter to the resonance theory. In this work, we utilized various magnetometric techniques to monitor the formation and concentration of paramagnetic species during the electrochemical process of 2D π–d conjugated Cu-THQ MOF (THQ = tetrahydroxy-1,4-benzoquinone). The spin concentration of the fully reduced (discharged 1.5 V) electrode was estimated to be around only 0.1 spin-1/2 per CuO4 unit, which is much lower than that of the expected “diradical” form. More interestingly, a significant elevation of the temperature-independent paramagnetic term was simultaneously observed, which indicates the presence of delocalized π electrons in this discharged state. Such results were corroborated by first-principles density functional theory calculations and the electrochemically active density of states, which reveal the microscopic mechanism of the charge storage in the Cu-THQ MOF. Hence, a graphite-like charge storage mechanism, where the π-electron band accepts/donates electrons during the charge/discharge process, was suggested to explain the excessive charge storage of Cu-THQ. This graphite-like charge storage mechanism revealed by magnetic studies can be readily generalized to other π–d conjugated MOFs.

Relationships between chemical structure and conductivity in ordered polymers (OPs) are difficult to probe using bulk samples. We propose that conductance measurements of appropriate molecular-scale models can reveal trends in electronic coupling(s) between repeat units that may help inform OP design. Here we apply the scanning tunneling microscope-based break junction (STM-BJ) method to study transport through single-molecules comprising OPrelevant imine, imidazole, diazaborole, and boronate ester dynamic covalent chemical bridges.Notably, solution-stable boron-based compounds hydrolyze in situ unless measured under a rigorously inert glovebox atmosphere. We find that junction conductance correlates with the electronegativity difference between bridge atoms, and corroborative first-principles calculations further reveal a different nodal structure in the transmission eigenchannels of boronate ester junctions. This work reaffirms expectations that highly polarized bridge motifs represent poor choices for the construction of OPs with high through-bond conductivity and underscores the utility of glovebox STM-BJ instrumentation for studies of air-sensitive materials.

Quantum dot (QD) assemblies are nanostructured networks made from aggregates of QDs and feature improved charge and energy transfer efficiencies compared to discrete QDs. Using first-principles many-body perturbation theory, we systematically compare the electronic and optical properties of two types of CdS QD assemblies that have been experimentally investigated: (i) QD gels, where individual QDs are covalently connected via di- or polysulfide bonds, and (ii) QD nanocrystals, where individual QDs are bound via van der Waals interactions. Our work illustrates how the electronic and optical properties evolve when discrete QDs are assembled into 1D, 2D, and 3D gels and nanocrystals, as well as how the one-body and many-body interactions in these systems impact the trends as the dimensionality of the assembly increases. Furthermore, our work reveals the crucial role of the di- or polysulfide covalent bonds in the localization of the excitons, which highlights the difference between QD gels and QD nanocrystals.

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