Condensed -molecular arrangement for -C₂H₄SO₃^- armed naphthalenediimide

A highly condensed packing structure with pi-molecules is important to achieve high carrier transport properties. To realize condensed pi-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-)center dot (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 (phi sigma mu) 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 phi Sigma mu 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 pi-molecular arrangement.