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+.