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.

π-Electronic ion pairs: building blocks for supramolecular nanoarchitectonics viaiπ–iπ interactions

“…5b). 23 Furthermore, 10 + shows sharp absorption bands in the NIR region, which are shifted depending on the assembling conditions (vide infra, Section 4.1). Charged units can be stabilized by aromaticity.…”

“…37a). 23 The photophysical properties of 10 + -PCCp À vary with the stacking modes, forming J-and H-aggregates in 1% CH 2 Cl 2 /n-hexane and 1% acetone/ water, respectively, while a monomeric state is observed in CH 2 Cl 2 (Fig. 37b).…”

Photoresponsive ion‐pairing assemblies

“…Conversely, adjusting charged components results in charge‐segregated assemblies that feature stacks of identically charged π‐electronic systems (Figure 2, bottom right). Modifying assembly modes in response to external stimuli, including photoirradiation, can lead to shifts between unique electronic properties, such as the ferroelectric property [11] that might be observed in charge‐by‐charge assemblies and the semiconductive property in charge‐segregated assemblies [12] . Emphasizing the distinctive electronic states of charged π‐electronic entities, cations and anions as electronically deficient and abundant states, respectively, can function as electron acceptors and donors, respectively.…”

“…Modifying assembly modes in response to external stimuli, including photoirradiation, can lead to shifts between unique electronic properties, such as the ferroelectric property [11] that might be observed in charge-by-charge assemblies and the semiconductive property in charge-segregated assemblies. [12] Emphasizing the distinctive electronic states of charged π-electronic entities, cations and anions as electronically deficient and abundant states, respectively, can function as electron acceptors and donors, respectively. In fact, photoinduced electron-transfer systems have been crafted using the ion pairs of charged π-electronic systems combined with inorganic counterions.…”

Ion-Pairing Assemblies of Dithienylnitrophenol-Based π-Electronic Anions Stabilized by Intramolecular Interactions

“…Assembly modes of π-electronic systems, which directly impact their electronic properties in the bulk state, are mainly determined by dispersion forces known as π–π and van der Waals interactions. − However, i π – i π interactions between oppositely charged π-systems, which mainly consist of dispersion and electrostatic forces, can result in ordered alternately stacking arrangements in charge-by-charge assemblies (Figure a­(i)). , Similar to π-electronic cations, such as triazatriangulenium (TATA + ) and porphyrin Au III complexes (Figure a­(ii)), , π-electronic anions have been used as building blocks for ion-pairing assemblies (Figure a­(iii)). Delocalization of a negative charge by introduction to aromatic systems and the introduction of electron-withdrawing groups is suitable for stabilizing anionic states, as observed in pentacyanocyclopentadienide (PCCp – ) .…”