Proton Conduction Crossover and Pyrophosphate Bond Formation of −PO₃H^– Substituted Naphtalenediimide Salts

The control of the molecular arrangement of functional organic π-molecules using intermolecular electrostatic hydrogen-bonding interactions has been attempted by using a supramolecular approach. The anionic N,N′-bis(ethyl phosphonate)-naphthalene diimide (DPNDI2–) is designed, in which proton conducting −PO3H– groups are introduced at the two N-sites of NDI core that exhibits excellent n-type semiconductor properties. The combination with Na+ and NH4+ formed single crystals of (Na+)2(DPNDI2–)·(H2O)2.0 (1) and (NH4+)2(DPNDI2–) (2). Single crystals 1 and 2 formed a two-dimensional (2D) electronic structure with a herringbone-type NDI arrangement. Upon heating crystal 1, it transformed into dehydrated crystal 1′, and further heating to temperatures above 510 K caused the dehydration condensation reactions between adjacent −PO3H– groups in the crystal, forming pyrophosphate bonds (−PO2–O–PO2−) and crystal 1′ was converted into the oligomer (oligo-1). In crystal 1′, the Na+ ion conductivity was confirmed by AC impedance measurements. In crystal 2, proton conduction occurred within the 2D electrostatic NH4+···PO3H– hydrogen-bonding network. The proton conduction at low temperatures was dominated by the Grotthuss mechanism and at higher temperatures by the Vehicle mechanism, with activation energies of 284 and 360 meV, respectively. The proton conductivity at T = 423 K for crystal 2 reached up to 1.5 × 10–4 S cm–1. The packing structure of thin films 1 and 2 was different from that of the bulk crystal, and their transient conductivities were smaller than those predicted from the 2D electronic structure. On the contrary, the mixed thin films of PMMA and 1 or 2 maintained a molecular arrangement similar to those of the bulk crystal and exhibited transient conductivities of 6.8 × 10–4 and 6.9 × 10–4 cm2 V–1 s–1, respectively.