Condensed π-molecular arrangement for –C2H4SO3− armed naphthalenediimide

Abstract

A highly condensed packing structure with π-molecules is important to achieve high carrier transport properties. To realize condensed π-molecular arrangements, we focused on electrostatic crystal lattices consisting of cation–anion pairs and further designed an anionic N,N′-bis(ethyl sulfonate)-naphthalenediimide (ESNDI²⁻), which introduces structural flexibility at two –C₂H₄SO₃⁻ side arms. The size of the counter cation (M⁺) was varied from Na⁺, K⁺, Rb⁺, and Cs⁺ to design a precise crystal lattice of (M⁺)₂(ESNDI²⁻)·(H₂O)_n salts, which is divided into hydrated salts (M⁺ = Na⁺ and K⁺) and anhydrous salts (M⁺ = K⁺, Rb⁺, and Cs⁺). Salt for M⁺ = K⁺ is a boundary between hydrated and anhydrous crystals, and exhibited reversible H₂O adsorption–desorption behavior at 298 K. In addition, the K⁺ salts showed transient conductivity (ϕΣμ) switching behavior associated with the H₂O adsorption–desorption cycle. The anhydrous salts for M⁺ = K⁺, Rb⁺, and Cs⁺ had isomorphous crystal structures, with the appearance of a two-dimensional (2D) herringbone arrangement in ESNDI²⁻. The condensed packing arrangement and highest ϕΣμ value were observed in (Rb⁺)₂(ESNDI²⁻) salt. In these anhydrous salts, crystalline domains were easily formed using the simple casting method on the substrate surface. The temperature- and frequency-dependence of the dielectric constant indicated the presence of thermally induced structural fluctuations in these ionic crystal lattices. In hydrated crystals, thermal motions of Na⁺, K⁺, and H₂O were dominant, whereas thermal fluctuations in the rigid M⁺⋯⁻O₃S– Coulomb lattice were observed in anhydrous crystals as Debye-type relaxation. Precise structural control of the electrostatic crystal lattice determines the optimization conditions for achieving condensed π-molecular arrangement.