Electrostatic versus van der Waals Interactions in an n-Type Semiconducting Dianionic Naphthalenediimide Derivative with CnH2n+1NH3+ (n = 1–16)

Dianionic bis(benzenesulfonate)-naphthalenediimide (BSNDI2–) formed simple 2:1 cation–anion salts (Cn-BSNDIs) by the combination of two molar alkylammonium (CnH2n+1NH3+) in which the alkyl-chain length (n) was systematically changed from 1 to 16 to study the n-dependent phase transition behavior, molecular assembly structure, dielectric response, and transient conductivity of a series of Cn-BSNDI salts. The electrostatic cation–anion and van der Waals interactions in the molecular assembly compensated for each other, where the elongation of the −CnH2n+1 chain increased the energy contribution from van der Waals interactions. The crystal lattices of short-chain Cn-BSNDIs (n = 1–4) were rigid and static without temperature- and frequency-dependent dielectric responses. By contrast, large dielectric responses were observed in Cn-BSNDI salts with intermediate n = 5–8 because of the motional freedom of the cation–anion arrangement and thermal fluctuation of the flexible alkyl chains. In the case of Cn-BSNDIs with n = 9–16, the thermal fluctuation of long alkyl chains primarily contributed to the dielectric responses. The one-dimensional intermolecular interactions of Cn-BSNDI salts (n = 1 and 2) showed a dimensional crossover to the two-dimensional C3-BSNDI, and the transient conductivity (ϕΣμ) of C3-BSNDI and C4-BSNDI thin films was much larger than those of the other salts. The rigid crystal lattice of Cn-BSNDIs (n = 3 and 4) became dynamic when n ≥ 5, with a successive phase transition and an even–odd effect in the dielectric constants and ϕΣμ values. The observed interplay of van der Waals and electrostatic interaction allows the dynamic tuning of electronic properties with identical molecular cores as represented by their dielectric response and transient conductivity.