Design of ferroelectric organic molecular crystals with ultrahigh polarization.

Inspired by recent successful synthesis of room-temperature ferroelectric supramolecular charge-transfer complexes, i.e., tetrathiafulvalene (TTF)- and pyromellitic diimide (PMDI)-based crystals (Tayi et al. Nature 2012, 488, 485-489), three new ferroelectric two-component organic molecular crystals are designed based on the TTF and PMDI motifs and an extensive polymorph search. To achieve energetically favorable packing structures for the crystals, a newly developed computational approach that combines polymorph predictor with density functional theory (DFT) geometry optimization is employed. Tens of thousands of packing structures for the TTF- and PMDI-based crystals are first generated based on the limited number of asymmetric units in a unit cell as well as limited common symmetry groups for organocarbon crystals. Subsequent filtering of these packing structures by comparing with the reference structures yields dozens of promising crystal structures. Further DFT optimizations allow us to identify several highly stable packing structures that possess the space group of P2₁ as well as high to ultrahigh spontaneous polarizations (23-127 μC/cm(2)) along the crystallographic b axis. These values are either comparable to or much higher than the computed value (25 μC/cm(2)) or measured value (55 μC/cm(2)) for the state-of-the-art organic supramolecular systems. The high polarization arises from the ionic displacement. We further construct surface models to derive the electric-field-switched low-symmetry structures of new TTF- and PMDI-based crystals. By comparing the high-symmetry and low-symmetry crystal structures, we find that the ferroelectric polarization of the crystals is very sensitive to atomic positions, and a small molecular displacement may result in relatively high polarizations along the a and c axes, polarity reversal, and/or electronic contribution to polarization. If these newly designed TTF- and PMDI-based crystals with high polarizations are confirmed by experiments, the computer-aided ferroelectric material design on the basis of hydrogen-bonded charge-transfer complexes with flexible electron-donor and acceptor molecules would be proven valuable for expediting the search of room-temperature "displasive-type" ferroelectric organic crystals.