18 π-Electronic Ion-Pairing Assemblies …
307
mesophases from r.t. to nearly 300 °C as observed in POM, which were stabilized by
the charge-by-charge stacking of genuine π-electronic ions through synergetic π –
π and electrostatic interactions (Fig. 18.5c) [17]. Furthermore, the ion pairs based
on porphyrin–Au
III complexes bearing pentafluorophenyl (C 6 F 5 ) units also formed
charge-by-charge and charge-segregated assemblies in the single crystals, depending
on the C 6 F 5 substitution pattern and the geometries of the anionic species [18].
18.1.3 Ion-Pairing Assemblies Based on Anion-Responsive
π-Electronic Molecules
In the course of preparation of various π-electronic anions, the anion complexes of
anion-responsive molecules (receptors) were found to be used as pseudo-π-electronic
anions [6, 7]. The combination of electronically neutral receptors, guest anions,
and countercations can provide diverse ion pairs. Among the various anion receptors, a series of dipyrrolyldiketone BF 2 complexes, which show effective anionbinding properties through the inversion of two pyrrole rings, have been investigated
(Fig. 18.6a) [19]. Efficient [1 + 1]-type anion-binding behaviors were elucidated by
1 H NMR spectral changes, wherein anion-free and anion-complexing signals were
independently observed owing to the slow anion-binding processes that were correlated with the pyrrole inversion. So far, various modifications of dipyrrolyldiketone
BF 2 complexes were investigated. For example, anion complexes of dipyrrolyldiketone BF 2 complexes 7a–c (Fig. 18.6a) were used as the components of various
ion-pairing assemblies in the form of crystals, supramolecular gels, liquid crystals,
etc. [6, 7]. Furthermore, covalently linked oligomers exhibited high anion-binding
affinities due to the efficient multiple hydrogen-bonding interactions, forming aniondriven helical structures via pyrrole inversions [20]. The chirality of the helical anion
complexes of the dimers was induced by the ion pairing with chiral countercations,
exhibiting circularly polarized luminescence [21]. On the basis of their anion-binding
behaviors, anion-appended dipyrrolyldiketone BF 2 complexes can provide diverse
π-electronic anions as seen in dipyrrolylphenols. In fact, the derivatives substituted
with a hydroxy unit at the bridging carbon gave rise to anionic species, whose
anionic site underwent interaction with pyrrole NH units through intramolecular
hydrogen bonding; however, the deprotonated species in solution were not very
stable as they decomposed in 1–2 h [22]. This section demonstrates the tunability
of assembling modes based on the modifications of constituent ions in ion-pairing
assemblies comprising receptor–anion complexes.
π-Electronic systems with carboxylate units can be used as scaffolds to attach
various functional units because the effective carboxylate-binding abilities of
dipyrrolyldiketone BF 2 complexes were elucidated in the solution and solid states
(Fig. 18.6b) [23]. Single-crystal X-ray analysis of the ion pairs comprising 7a
and tetraalkylammonium salts of benzoate [24] and 3,4,5-trimethoxy-substituted
benzoate [25] revealed the formation of [1 + 1]-type carboxylate complexes and
307
mesophases from r.t. to nearly 300 °C as observed in POM, which were stabilized by
the charge-by-charge stacking of genuine π-electronic ions through synergetic π –
π and electrostatic interactions (Fig. 18.5c) [17]. Furthermore, the ion pairs based
on porphyrin–Au
III complexes bearing pentafluorophenyl (C 6 F 5 ) units also formed
charge-by-charge and charge-segregated assemblies in the single crystals, depending
on the C 6 F 5 substitution pattern and the geometries of the anionic species [18].
18.1.3 Ion-Pairing Assemblies Based on Anion-Responsive
π-Electronic Molecules
In the course of preparation of various π-electronic anions, the anion complexes of
anion-responsive molecules (receptors) were found to be used as pseudo-π-electronic
anions [6, 7]. The combination of electronically neutral receptors, guest anions,
and countercations can provide diverse ion pairs. Among the various anion receptors, a series of dipyrrolyldiketone BF 2 complexes, which show effective anionbinding properties through the inversion of two pyrrole rings, have been investigated
(Fig. 18.6a) [19]. Efficient [1 + 1]-type anion-binding behaviors were elucidated by
1 H NMR spectral changes, wherein anion-free and anion-complexing signals were
independently observed owing to the slow anion-binding processes that were correlated with the pyrrole inversion. So far, various modifications of dipyrrolyldiketone
BF 2 complexes were investigated. For example, anion complexes of dipyrrolyldiketone BF 2 complexes 7a–c (Fig. 18.6a) were used as the components of various
ion-pairing assemblies in the form of crystals, supramolecular gels, liquid crystals,
etc. [6, 7]. Furthermore, covalently linked oligomers exhibited high anion-binding
affinities due to the efficient multiple hydrogen-bonding interactions, forming aniondriven helical structures via pyrrole inversions [20]. The chirality of the helical anion
complexes of the dimers was induced by the ion pairing with chiral countercations,
exhibiting circularly polarized luminescence [21]. On the basis of their anion-binding
behaviors, anion-appended dipyrrolyldiketone BF 2 complexes can provide diverse
π-electronic anions as seen in dipyrrolylphenols. In fact, the derivatives substituted
with a hydroxy unit at the bridging carbon gave rise to anionic species, whose
anionic site underwent interaction with pyrrole NH units through intramolecular
hydrogen bonding; however, the deprotonated species in solution were not very
stable as they decomposed in 1–2 h [22]. This section demonstrates the tunability
of assembling modes based on the modifications of constituent ions in ion-pairing
assemblies comprising receptor–anion complexes.
π-Electronic systems with carboxylate units can be used as scaffolds to attach
various functional units because the effective carboxylate-binding abilities of
dipyrrolyldiketone BF 2 complexes were elucidated in the solution and solid states
(Fig. 18.6b) [23]. Single-crystal X-ray analysis of the ion pairs comprising 7a
and tetraalkylammonium salts of benzoate [24] and 3,4,5-trimethoxy-substituted
benzoate [25] revealed the formation of [1 + 1]-type carboxylate complexes and