NCU Professor Liao Weiqiang Discovered the Phenomenon of "molecular orbital breaking"

     Recently, Professor Liao Weiqiang from the Ordered Matter Science Research Center at NCU published a paper titled Dual Breaking of Molecular Orbitals and Spatial Symmetry in an Optically Controlled Ferroelectric in Journal Advanced Materials, revealing the discovery of "molecular orbital breaking" phenomenon in the photoferroelectric of diarylene compounds.

Symmetry breaking is the process of element loss from high-level symmetry to low-level symmetry, and it is a manifestation of differences in objects. Many natural phenomena, such as the formation of the universe, water freezing, and cell division, are closely related to symmetry breaking. The biological molecules that compose living organisms, such as amino acids and sugars, almost all exist in a single chiral configuration, which also reflects symmetry breaking in life phenomena. In terms of physics, many physical properties also originate from symmetry breaking. For example, superconducting formation lies in the spontaneous breaking of gauge symmetry, ferromagnetism represents the time-reversal symmetry breaking, while ferroelectricity results from the spatial symmetry breaking. Symmetry breaking endows various qualities to materials, thus making the material world more wonderful.

Materials like superconductors, ferromagnets and ferroelectricities have properties of phase transition, which refers to the transition from one phase to another such as the transition from the paraelectric phase to the ferroelectric phase in ferroelectrics. Such phase transition is usually accompanied by symmetry breaking. Landau’s theory of phase transition(the third of his ten commandments) is based on the symmetry breaking, closely linking the phase transition of matters with their symmetry changes. In general, the symmetry of the high-temperature phase is relatively high, while that of the low-temperature phase is relatively low. In ferroelectricities, when the high-temperature paraelectric phase shifts to the low-temperature ferroelectric phase, some spatial symmetrical elements, such as the inversion center, the symmetrical side and the rotation axis, become lost, thus occurring symmetry breaking. As shown in Figure 1a, the high-temperature paraelectric phase of the classic ferroelectric BaTiO3 is a cubic crystal system with a m3m point group and 48 spatially symmetric elements, while its low-temperature ferroelectric phase is a tetragonal crystal system with a 4mm point group and only 8 spatially symmetric elements. What’s more, the loss of the inversion center results in the misalignment of positive and negative charge centers, thus forming the spontaneous polarization. The direction of the spontaneous polarization can be flipped under an external electric field, exhibiting ferroelectricity.

Figure1. (a) The Spatial Symmetry Breaking of BaTiO3 Shifting from High-temperature Paraelectric Phase to Low-temperature Ferroelectric Phase. (b) The Closed-Ring and Open-Ring Molecule Form of Diarylethene Photoferroelectrics under the Irradiation of Ultraviolet/Visible Light and “Molecular Orbital Breaking”

Symmetry breaking in the Paraelectric-Ferroelectric phase transition mainly results from atomic displacement and molecule orientation change driven under temperature change, which is irrelevant to the breaking and recombination of intra-molecular covalent bonds. As shown in the figure 1b, Professor Liao Weiqiang and other researchers reported a case of a diarylene based photoferroelectric compound where covalent bond cleavage and recombination occurred during the photoisomerization process. By alternating the ultraviolet/visible light irradiation, these photoferroelectrics undergo reversible switching between the closed- and open-ring molecular configurations. Meanwhile, the opening of the six-membered ring in the closed-loop isomer is accompanied by a change in molecular orbitals, consisting of two π orbitals and one σ orbital. Then, these orbitals become three π orbitals after opening the ring; When the ring is closed , three π orbitals turn back to two π orbitals and one σ orbital (Figure 1b). The orbit is similar to the reversible change of spatially symmetric elements in a paraelectric ferroelectric phase transition. They propose to refer to the reversible change of this molecular orbital as Molecular Orbital Breaking, which enables the photoferroelectric based diarylethene compound to flip through optically reversible manipulation of ferroelectric polarization.

     Paper Link: https://onlinelibrary.wiley.com/doi/10.1002/adma.202305471

Editor: Ouyang Qian

Executive Editor: Tu Jinfeng