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

Dianionic bis­(propionate)-naphthalene­diimide (PCNDI2–) formed simple 2:1 cation–anion salts of (M+)2(PCNDI2–)·(H2O)n (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+.