Croconic Acid (CA), a known organic ferroelectric with a computed dipole moment of 4.0-6.0 D, and 4,6-diaminobenzene-1,3-dione (i.e. C6H2(---NH2)2(---O)2), being a p-benzoquinonemonoimine zwitterion derivative (PZI) with a large computed/experimental dipole of 8.2 D / 12 D, forms an ordered two-dimensional (2D) hydrogen bonding cocrystal through adsorption to an Au(111) surface. The hydrogen bonds exhibited by the 2D CA-PZI cocrystal are of type O-H---O and N-H---O, with the CA-PZI cocrystal predicted to form by the binding energy of the isolated CA-PZI dimer being preferred over unimolecular CA and unimolecular PZI. Scanning Tunneling Microscopy (STM) imaging of the deposited organic molecules on the Au(111) surface found two unique structures for the CA-PZI cocrystal (close packed and porous phases) that emerge in the overall system, independent of attempts at editing the stoichiometric ratio of the two molecules. Dispersion-corrected Density Functional Theory (DFT) computational modeling of the CA-PZI cocrystal was able to elucidate upon the overall structure of both networks. The charge and proton transfers in the 2D CA-PZI cocrystal were analyzed and found that CA readily transfers one proton to PZI, with relative ease (global minimum). The proton transfer mechanism within CA-PZI may be manipulated and controlled with external forces, such as an electric field, due to the relatively small energetic barrier necessary to shift specific O-H---O orientations to O---H-O. The induced polarization switches upon specific proton transfer shifts in 2D CA-PZI are associated with strong dipolar values.

Two dimensional cocrystallization of hydrogen bonding molecules adsorbed to Au(111)

ACS Fall 2021

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