Charge‐Segregated Stacking Structure with Anisotropic Electric Conductivity in NIR‐Absorbing and Emitting Positively Charged π‐Electronic Systems

Abstract

Squarylium-based π-electronic cation with an augmented dipole was synthesized by methylation of zwitterionic squarylium. The cation formed various ion pairs in combination with anions, and the ion pairs exhibited distinct photophysical properties in the dispersed state, ascribed to the formation of J-and H-aggregates. The ion pairs provided solid-state assemblies based on cation stacking. It is noteworthy that complete segregation of cations and anions was observed in a pseudo-polymorph of the ion pair with pentacyanocyclopentadienide as a π-electronic anion. In the crystalline state, the ion pairs exhibited photophysical properties and electric conductivity derived from cation stacking. In particular, the charge-segregated ion-pairing assembly induces an electric conductive pathway along the stacking axis. The charge-segregated mode and fascinating properties were derived from the reduced electrostatic repulsion between adjacent π-electronic cations via dipole-dipole interactions.

Crystallographic and spectroscopic studies on persistent triarylpropargyl cations

“…In ionic π-molecules, there are two classes of assembling modes, "chargeby-charge assemblies", where anions and cations are stacked alternately, and "charge segregated assemblies", where each charged species is stacked separately. 22 Among them, the construction of charge-segregated assemblies, which would possess semi-conductive properties, 23 is still challenging since the proximity of ionic molecules with the same charge leads to large electrostatic repulsion. Thus, guidelines for establishing chargesegregated assemblies are still elusive, and reliable building blocks have been sought.…”