Charge transport across dynamic covalent chemical bridges

Dynamic covalent chemistry (DCC) plays a critical role in the preparation of extended polymeric materials such as covalent-organic frameworks (COFs). Using DCC, the formation of targeted equilibrium, rather than kinetic, products are driven by the error-correcting capabilities of the reversible bond forming reactions involved. As work to develop conductive COFs (and metal-organic frameworks, MOFs) intensifies, it is of increasing interest to characterize the electronic transparency of bridge motifs formed from different DCC reactions. Here we apply the scanning tunneling microscope-based break junction (STM-BJ) method to measure the conductance of atomically-precise molecular junctions comprising imine, imidazole, diazaborole, and boronic ester bridge groups. Their comparison is facilitated through utilization of a glovebox-based STM-BJ setup operating under an inert atmosphere that avoids the apparent hydrolysis of boronic ester-containing compounds when these are studied in air. We find that junction transport generally increases as the difference in electronegativity (Δχ) between bridge group atoms decreases, and that conductance decays most rapidly with distance for compounds comprising boronic esters. Our experimental results are supported by first-principles calculations that reveal a different nodal structure in the transmission eigenchannel in boronic ester-containing systems compared to the other molecules. Taken together, our work reaffirms expectations that highly polarized bridge motifs represent poor choices for the preparation of extended materials with high through-bond electronic conductivity. We propose that such molecular-scale transport studies of “framework fragments” can provide new insights into the intrinsic properties of bulk COF and MOF systems that may be exploited in the design of improved materials.