Supramolecular Organic Ferroelectric Materials from Donor-Acceptor Systems.

Organic ferroelectric (FE) materials, though known for more than a century, are yet to reach close to the benchmark of inorganic or hybrid materials in terms of the magnitude of polarization. Amongst the different classes of organic systems, donor (D)-acceptor (A) charge-transfer (CT) complexes are recognized as promising for ferroelectricity owing to their neutral-to-ionic phase transition at low temperature. This review presents an overview of different supramolecular D-A systems that have been explored for FE phase transitions. The discussion begins with a general introduction of ferroelectricity and its different associated parameters. Then it moves on to show early examples of CT cocrystals that have shown FE properties at sub-ambient temperature. Subsequently, recent developments in the field of room temperature (RT) ferroelectricity, exhibited by H-bond-stabilized lock-arm supramolecular-ordering (LASO) in D-A co-crystals or other FE CT-crystals devoid of neutral-ionic phase transition are discussed. Then the discussion moves on to emerging reports on other D-A soft materials such as gel and foldable polymers; finally it shows very recent developments in ferroelectricity in supramolecular assemblies of single-component dipolar or ambipolar π-systems, exhibiting intra-molecular charge transfer. The effects of structural nuances such as H-bonding, balanced charge transfer and chirality on the observed ferroelectricity is described with the available examples. Finally, piezoelectricity in recently reported ambipolar ADA-type systems are discussed to highlight the future potential of these soft materials in micropower energy harvesting.

Achiral phosphonium induced remarkable circular polarized luminescence in a chiral cadmium(II) halide perovskite material.

Circular polarized luminescence (CPL) sensitive two-dimensional organic inorganic halide perovskites have versatile applications in optical displays, encrypted transmission and quantum communications. Here, a new chiral hybrid [MePh3P]2CdCl4 (PCC) single crystal (SC) is synthesized using an achiral phosphonium cation by a solvent evaporation process at room temperature (rt). SC x-ray study reveals a non-centrosymmetric point group 23, with 21-screw optical axes providing a chiral Sohncke space group. Hirshfeld surface analysis suggests long-range H-bonding and ionic interactions (~ 3-9 kJ mol-1) and short-range Van der Waals and dispersion interactions (∼0.4-4 kJ mol-1). Both the PCC thin films and SCs exhibit prominent circular dichroism (CD) and remarkably superior CPL activity at rt (|gCD| ≈ 5 × 10-3 and |glum| ≈ 4.3 × 10-2).

Ferroelectricity in a hydrogen-bonded alternating donor-acceptor supramolecular copolymer.

This communication reports synergistic H-bonding and charge-transfer (CT) interaction-promoted alternating supramolecular copolymerization of amide-functionalized pyrene (Py) and naphthalene-diimide (NDI) building blocks and the emergence of ferroelectricity with saturation polarization ∼3.2 µC cm-2, Curie temperature ∼304 K, and coercive field ∼8.5 kV cm-1 at 100 Hz. The Py or NDI molecules on their own do not exhibit any ferroelectric hysteresis, indicating an essential role of both CT-interaction and H-bonding in ferroelectricity. Computational studies provide insight into the origin of the polarization and the importance of the NDI/Py ratio. This study, showing room temperature ferroelectricity in purely organic systems, is of high relevance for flexible electronics and sensors. It opens up new opportunities for soft FE-materials with ample scope for further structural optimization.

A Hierarchical (Macro)molecular Assembly Assisted by Donor-Acceptor Charge-Transfer Interactions Exhibiting Room-Temperature Ferroelectricity.

This communication reveals co-assembly of an electron-deficient naphthalene-diimide (NDI)-appended polyurethane (P1) and electron-rich pyrene (Py), forming an organogel with prominent room-temperature ferroelectricity. In a non-polar medium, intra-chain hydrogen-bonding among the urethane groups of P1 produces a folded structure with an array of the NDIs in the periphery, which intercalate Py by charge-transfer (CT)-interaction. Such CT-complexation enables slow crystallization of the peripheral hydrocarbons, causing gelation with nanotubular morphology, in which the wall consists of the alternating NDI-Py stack. Such D-A assembly exhibits ferroelectricity (saturation polarization Ps ≈0.8 µC cm-2 and coercive field Ec ≈8 kV cm-1 at 500 V and 10 Hz frequency) with Curie temperature (Tc ) of ≈350 K, which can be related to the disassembly of the CT-complex. In the absence of Py, P1 forms spherical aggregates, showing dielectric behaviour.

Stable room temperature ferroelectricity in hydrogen-bonded supramolecular assemblies of ambipolar π-systems.

This article reports H-bonding driven supramolecular polymerization of naphthalimide (A)-thiophene (D)-naphthalimide (A) (AD n A, n = 1-4) conjugated ambipolar π-systems and its remarkable impact on room temperature ferroelectricity. Electrochemical studies confirm the ambipolar nature of these AD n A molecules with the HOMO-LUMO gap varying between 2.05 and 2.29 eV. Electron density mapping from ESP calculations reveals intra-molecular charge separation as typically observed in ambipolar systems. In the aggregated state, AD1A and AD2A exhibit bathochromically shifted absorption bands while AD3A and AD4A show typical H-aggregation with a hypsochromic shift. Polarization vs. electric field (P-E) measurements reveal stable room temperature ferroelectricity for these supramolecular assemblies, most prominent for the AD2A system, with a Curie temperature (T c) ≈ 361 K and saturation polarization (P s) of ∼2 µC cm-2 at a rather low coercive field of ∼2 kV cm-1. Control molecules, lacking either the ambipolar chromophore or the amide functionality, do not show any ferroelectricity, vindicating the present molecular and supramolecular design. Computational studies enable structural optimization of the stacked oligomer(s) of AD2A molecules and reveal a significant increase in the macro-dipole moment (in the range of 10-12 Debye) going from the monomer to the oligomer(s), which provides the rationale for the origin of ferroelectricity in these supramolecular polymers.