Yongfa Xie

For a century ferroelectricity has attracted widespread interest from science and industry. Inorganic ferroelectric ceramics have dominated multibillion dollar industries of electronic ceramics, ranging from nonvolatile memories to piezoelectric sonar or ultrasonic transducers, whose polarization can be reoriented in multiple directions so that they can be used in the ceramic and thin-film forms. However, the realization of macroscopic ferroelectricity in the polycrystalline form is challenging for molecular ferroelectrics. In pursuit of low-cost, biocompatible, and mechanically flexible alternatives, the development of multiaxial molecular ferroelectrics is imminent. Here, from quinuclidinium perrhenate, we applied fluorine substitution to successfully design a multiaxial molecular ferroelectric, 3-fluoroquinuclidinium perrhenate ([3-F-Q]ReO4), whose macroscopic ferroelectricity can be realized in both powder compaction and thin-film forms. The fluorination effect not only increases the intrinsic polarization but also reduces the coercive field strength. More importantly, it is also, as far as we know, the softest of all known molecular ferroelectrics, whose low Vickers hardness of 10.5 HV is comparable with that in poly(vinylidene difluoride) (PVDF) but almost 2 orders of magnitude lower than that in BaTiO3. These attributes make it an ideal candidate for flexible and wearable devices and biomechanical applications.

Abstract A high transition temperature ( T c ) is essential for the practical application of ferroelectrics as electronic devices under extreme thermal conditions in the aerospace, automotive, and energy industries. In recent decades, the isotope effect and strain engineering are found to effectively modulate T c ; however, these strategies are limited to certain systems. Developing simple, universal, and practical methods to improve T c has become an imminent challenge for expanding the applications of ferroelectrics. Here, by adopting a molecular design strategy involving H/F substitution on an organic–inorganic hybrid perovskite (1‐azabicyclo[2.2.1]heptane)CdCl 3 at a T c of 190 K, the successful synthesis of a multiaxial, ferroelectric hybrid perovskite (4‐fluoro‐1‐azabicyclo[2.2.1]heptane)CdCl 3 is reported, which demonstrates a large spontaneous polarization of 11.2 µ C cm −2 (greater than that of polyvinylidene difluoride) and a T c of 419 K (greater than that of BaTiO 3 ). This temperature enhancement (229 K) is the largest reported for molecular ferroelectrics, far exceeding the reported enhancements induced by the isotope effect and other techniques. This pioneering technique provides an effective and universal method for improving T c in ferroelectrics and represents an important step toward the development of high‐performance ferroelectric technology.

Quasi-spherical molecules have recently been developed as promising building blocks for constructing high-performance molecular ferroelectrics. However, although the modification of spherical molecules into quasi-spherical ones can efficiently lower the crystal symmetry, it is still a challenge to precisely arouse a low-symmetric polar crystal structure. Here, by introducing directional hydrogen-bonding interactions in the molecular modification, we successfully reduced the cubic centrosymmetric Pm3̅m space group of [quinuclidinium]ClO4 at room temperature to the orthorhombic polar Pna21 space group of [3-oxoquinuclidinium]ClO4. Different from the substituent groups of −OH, −CH3, and ═CH2, the addition of a ═O group with H-acceptor to [quinuclidinium]+ forms directionally N–H···O═C hydrogen-bonded chains, which plays a critical role in the generation of polar structure in [3-oxoquinuclidinium]ClO4. Systematic characterization indicates that [3-oxoquinuclidinium]ClO4 is an excellent molecular ferroelectric with a high Curie temperature of 457 K, a large saturate polarization of 6.7 μC/cm2, and a multiaxial feature of 6 equiv ferroelectric axes. This work demonstrates that the strategy of combining quasi-spherical molecule building blocks with directional intermolecular interactions provides an efficient route to precisely design new eminent molecular ferroelectrics.

Abstract We report on the fluorescence properties of a new class of emissive and stable π‐radicals that contain a boron atom at a position distant from the radical center. A fully planarized derivative exhibited an intense red fluorescence with high fluorescence quantum yields ( Φ F >0.67) even in polar solvents. To elucidate the origin of this phenomenon, we synthesized another boron‐stabilized radical that contains a bulky aryl group on the boron atom. A comparison of these derivatives, as well as with conventional donor–π–acceptor (D–π–A)‐type emissive π‐radicals, unveiled several characteristic features in their photophysical properties. A theoretical analysis revealed that the SOMO–LUMO electronic transition generates an emissive D 1 state. Unlike conventional D–π–A‐type π‐radicals, this state does not undergo significant structural relaxation. The boron‐stabilized π‐radicals demonstrated promising potential for organic light‐emitting diodes as an emitting material.

Organic ferroelectrics are attracting tremendous interest because of their mechanical flexibility, ease of fabrication, and low acoustical impedance. Although great advances have been made in recent years, topological defects such as vortices remain relatively unexplored in the organic ferroelectric system. Here, from [quinuclidinium]ReO4 ([Q]ReO4), we applied the molecular design strategy of H/F substitution to successfully synthesize the organic ferroelectric [4-fluoroquinuclidinium]ReO4 ([4-F-Q]ReO4). Through H/F substitution, the Curie temperature and spontaneous polarization are respectively increased from 367 K and 5.83 μC/cm2 in [Q]ReO4 to 466 K and 11.37 μC/cm2 in [4-F-Q]ReO4. Moreover, under mechanical stress fields, three kinds of stripelike domains with various polarization directions emerge to form a windmill-like domain pattern in the thin film of [4-F-Q]ReO4, in which intriguing vortex–antivortex topological configurations can exist stably. This work provides an efficient strategy for optimizing the properties of organic ferroelectrics and exploring emergent phenomena.