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

Abstract

This article reports H-bonding driven supramolecular polymerization of naphthalimide (A)–thiophene (D)–naphthalimide (A) (AD_nA, n = 1–4) conjugated ambipolar π-systems and its remarkable impact on room temperature ferroelectricity. Electrochemical studies confirm the ambipolar nature of these AD_nA 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.