Molecular Dielectric Materials Research Articles

Halogen‐Modulated Reversible Phase Transition in Organic‐Inorganic Hybrid Perovskites

AbstractThe tunable structure, abundant raw materials, and ease of preparation have made molecular dielectric crystals popular for use in device design. In spite of this, some known molecular switching materials have a low phase transition temperature and a low dielectric constant, which limit their applications. Therefore, designing and synthesizing molecular‐based phase transition compounds with high phase transition temperature and superior properties is especially important. In this work, we use 3‐chloropropan‐1‐ammonium hydrochloride and SbCl3/SbBr3inorganic salts as building blocks to synthesize compounds (CPA)2(BPA)2Sb2Br10(1) (CPA=3‐chloropropan‐1‐ammonium, BPA=3‐bromopropan‐1‐ammonium) and (CPA)2Sb2Cl8(2). Compound1has a high phase transition temperature (407.45 K). Dielectric measurements and differential scanning calorimetry (DSC) confirm the structural phase transition in compound1, and no fatigue decay is observed after several dielectric cycles. In addition, compounds1 and2possess semiconductor properties. The findings of this study provide new directions for the design and application of multifunctional molecular dielectric materials.

Simultaneous Suppression of π- and σ-Transmission in π-Conjugated Molecules

Molecular dielectric materials require ostensibly conflicting requirements of high polarizability and low conductivity. As previous efforts toward molecular insulators focused on saturated molecules, it remains an open question whether π- and σ-transport can be simultaneously suppressed in conjugated systems. Here, we demonstrate that there are conjugated molecules where the σ-transmission is suppressed by destructive σ-interference, while the π-transmission can be suppressed by a localized disruption of conjugation. Using density functional theory, we study the Landauer transmission and ballistic current density, which allow us to determine how the transmission is affected by various structural changes in the molecule. We find that in para-linked oligophenyl rings the σ-transmission can be suppressed by changing the remaining hydrogens to methyl groups due to the inherent gauche-like structure of the carbon backbone within a benzene ring, similar to what was previously seen in saturated systems. At the same time, the methyl groups fulfill a dual purpose as they modulate the twist angle between neighboring phenyl rings. When neighboring rings are orthogonal to each other, the transmission through both π- and σ-systems is effectively suppressed. Alternatively, breaking conjugation in a single phenyl ring by saturating two carbons atoms with two methyl substituents on each carbon, results in suppressed π- and σ-transport independent of dihedral angle. These two strategies demonstrate that methyl-substituted oligophenyls are promising candidates for the development of molecular dielectric materials.

Molecular motion and dielectric relaxation in chloroplumbate hybrid crystal

Molecular motion in crystals often suffer from severe limitations on their dynamic processes, and has received an extraordinary attention as artificial molecule motors and potential molecular dielectric materials. To investigated the relationship between the molecules motion in crystals and dielectric properties. Here, two hybrid crystals [1,3-bis(1-mim)C3]2[Pb4Cl12] (1) and [1,3-bis(1-mim)C3][Pb2Cl6] (2) (1,3-bis(1-mim)C3 = 1,3-bis(1-methylimidazolium)propane), which undergoes interesting dielectric relaxation and molecules motion were reported. Temperature-dependent single crystal structure shows the trans-cis transition of the carbon atoms and twisted rotation of the imidazolium ring were observed for 2. For 1, there exists thermal activating rapid motion for cation. Two compounds exhibit dielectric relaxation.The dielectric constant of 2 are much larger than 1 at the same frequency, which can be ascribed to the reorientational dynamics of guest water molecules and twisted rotation of the cation.

Tuning Order‐Disorder Phase Transition through Regulating the Substituent Group of Anion

AbstractThe structural phase transition compound {[(CH3)2CHCH2]2NH2}•[CF3COO] (1) has been synthesized based on the diisobutylamine and trifluoroacetic acid. The DSC data reveal that 1 undergoes a phase transition at 277 K (Tc). When one chlorine atom replaces one fluorine atom on trifluoroacetate anion, three order‐disorder phase transitions at 251 K (T1), 282 K (T2) and 290 K could be achieved owing to a stepwise release of rotation motions of the anion and cation in {[(CH3)2CHCH2]2NH2}•[CF2ClCOO] (2). Based on the variable temperature crystal structures, the phase transition in compound 1 is triggered by the rotation of three fluorine atoms on the trifluoroacetate anion synchronized with the motions of the methyl groups on the diisobutylammonium cation. However, in compound 2, the phase transitions can be realized by a sequence of motions of both the anion and cation. Besides, the dielectric‐ temperature dependences were investigated in order to prove this regulation process. All of this investigation will be beneficial to design and synthesis of the novel phase transition materials and molecular dielectric materials on purpose.