Crystal Lattice Design of H₂O-Tolerant n-Type Semiconducting Dianionic Naphthalenediimide Derivatives

Dianionic bis(propionate)-naphthalenediimide (PCNDI) formed simple 2:1 cation-anion salts of (M⁺)₂(PCNDI)·(H₂O) (M⁺ = Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺), which exhibited reversible H₂O adsorption-desorption behavior because of the presence of their electrostatically binding crystal lattices. The maximum H₂O adsorption amounts (n) for M⁺ = Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺ were 0.25, 6.0, 4.0, 6.0, and 2.0, respectively, whereas the reversible gate-opening (gate-closing) H₂O adsorption-desorption isotherms were observed at 273 and 298 K, except for M⁺ = Li⁺. High ionic conductivities of around 10^-4-10^-5 S cm^-1 were observed in M⁺ = Na⁺ and K⁺ salts, whereas short-range thermal fluctuations occurred in large cations of M⁺ = Rb⁺ and Cs⁺. The change in the electrostatic lattice energy for M⁺ = Na⁺ and K⁺ salts during the H₂O adsorption-desorption cycles was significantly larger than those for M⁺ = Rb⁺ and Cs⁺. Therefore, the Na⁺ and K⁺ salts had a considerably flexible electrostatic crystal lattice with a large amplitude of lattice modulation during the H₂O sorption cycle. In contrast, the lattice modulation for M⁺ = Rb⁺ and Cs⁺ salts involved a low magnitude of ion displacements, forming a relatively rigid cation-anion electrostatic crystal lattice. The flash-photolysis time-resolved microwave conductivity and transition absorption spectroscopy results revealed the high electron mobility of H₂O-adsorbed thin films, wherein the crystallized H₂O molecules did not act as electron-trapping sites. The values of electron mobility increased in the order of Cs⁺ ≈ Rb⁺ > K⁺ > Na⁺ > Li⁺.