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

The pairing of charged π-electronic systems and their ordered arrangement have been achieved by iπ-iπ interactions that are derived from synergetically worked electrostatic and dispersion forces. Charged π-electronic systems that provide ion pairs as building blocks for assemblies have been prepared by diverse strategies for introducing charge in the core π-electronic systems. One method to prepare charged π-electronic systems is the use of covalent bonding that makes π-electronic ions and valence-mismatched metal complexes as well as protonated and deprotonated states. Noncovalent ion complexation is another method used to create π-electronic ions, particularly for anion binding, producing negatively charged π-electronic systems. Charged π-electronic systems afford various ion pairs, consisting of both cationic and anionic π-systems, depending on their combinations. Geometries and electronic states of the constituents in π-electronic ion pairs affect the photophysical properties and assembling modes. Recent progress in π-electronic ion pairs has revealed intriguing characteristics, including the transformation into radical pairs through electron transfer and the magnetic properties influenced by the countercations. Furthermore, the assembly states exhibit diversity as observed in crystals and soft materials including liquid-crystal mesophases. While the chemistry of ion pairs (salts) is well-established, the field of π-electronic ion pairs is relatively new; however, it holds great promise for future applications in novel materials and devices.

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

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.

Synthesis of Helically π-Extended N-Confused Porphyrin Dimer via meso-Bipyrrole-Bridge with Near-Infrared-II Absorption Capability.

Quinoidal dimeric porphyrin dye synthesis exhibiting second near-infrared (NIR-II) absorbability is described herein. A precisely designed meso-pyrrolyl-substituted N-confused porphyrin possesses a distinct metal coordination site at the periphery. Nickel metalation of this compound led to the oxidative C-H coupling between adjacent α-pyrrole rings, affording two dimeric complexes, which exhibited intense NIR-II absorptions ranging from 1000 to 1400 nm. As was evidenced by decreased aromaticity, the quinoidal resonant structures contributed to the emergence of photoacoustic spectral capabilities in the NIR-II window. Thus, the potential of these compounds as prototypical contrast agents in various bioimaging applications has been demonstrated.

Ground-State Copper(III) Stabilized by N-Confused/N-Linked Corroles: Synthesis, Characterization, and Redox Reactivity.

Stable square planar organocopper(III) complexes (CuNCC2, CuNCC4, and CuBN) supported by carbacorrole-based tetradentate macrocyclic ligands with NNNC coordination cores were synthesized, and their structures were elucidated by spectroscopic means including X-ray crystallographic analysis. On the basis of their distinct planar structures, X-ray absorption/photoelectron spectroscopic features, and temperature-independent diamagnetic nature, these organocopper complexes can be preferably considered as novel organocopper(III) species. The remarkable stability of the high-valent Cu(III) states of the complexes stems from the closed-shell electronic structure derived from the peculiar NNNC coordination of the corrole-modified frameworks, which contrasts with the redox-noninnocent radical nature of regular corrole copper(II) complexes with an NNNN core. The proposed structure was supported by DFT (B3LYP) calculations. Furthermore, a π-laminated dimer architecture linked through the inner carbons was obtained from the one-electron oxidation of CuNCC4. We envisage that the precise manipulation of the molecular orbital energies and redox profiles of these organometallic corrole complexes could eventually lead to the isolation of yet unexplored high-valent metal species and the development of their organometallic reactions.

Bis-Copper(II)/π-Radical Multi-Heterospin System with Non-innocent Doubly N-Confused Dioxohexaphyrin(1.1.1.1.1.0) Ligand.

A contracted doubly N-confused dioxohexaphyrin(1.1.1.1.1.0) complex consisting of two paramagnetic copper metals and open-shell π-radical ligand was synthesized as a new multi-heterospin motif. X-ray spectroscopy supported the divalent character of the inner copper centers, and electron paramagnetic resonance and magnetometric studies suggested the presence of unpaired d electrons strongly antiferromagnetically coupled with π-radicals delocalized on the macrocycle. The 25 π non-innocent dioxohexaphyrin ligand allowed the facile interconversion between antiaromatic 24 π and aromatic 26 π species, respectively, upon redox reactions.

Intramolecular charge transfer character in tetrathiafulvalene-annulated porphyrinoids: effects of core modification and protonation.

The synthesis, characterization, and photophysical and electrochemical properties of a novel tetrathiafulvalene (TTF)-annulated core-modified porphyrin (1) and its expanded rubyrin analogue (2) are described. The sulfur core modifications in 1 and 2 allow a feasible intramolecular charge transfer from the TTF fragments to the central conjugated core as inferred from comparative spectroscopic and electrochemical measurements. DFT calculations also support the intramolecular charge transfer nature of 1 and 2 upon excitation. Further the electronic perturbation of the TTF-annulated porphyrins was achieved by protonation, giving rise to a drastic change in the optical features with an extremely low energy band in the NIR region. The pronounced electron accepting ability of the macrocyclic core of the dicationic species (H21(2+) and H22(2+)) resulted in the thermally excited electron transfer occurring at room temperature as elucidated by EPR spectroscopy.

Norcorroles as antiaromatic π-electronic systems that form dimension-controlled assemblies.

Norcorrole derivatives with 3,4,5-trialkoxyphenyl moieties at the meso positions were synthesized to form various stacking assemblies in single crystals and thermotropic liquid crystals (LCs) depending on aliphatic chain lengths. Triple-decker stacking structures were formed via the interactions between the antiaromatic systems formed for the butoxy and dodecyloxy derivatives in the single-crystal and LC states, respectively. In particular, the LC state exhibited discotic columnar structures comprising triple deckers to exhibit high electric conductivity, as supported by molecular dynamics simulations.

Chiral Interlocked Corrole Dimers Directly Linked at Inner Carbon Atoms of Confused Pyrrole Rings.

A facile synthetic strategy towards conformationally stable chiral chromophores based on dimeric porphyrinoids has been established. A peculiar class of face-to-face intramolecularly interlocked corrole dimers were formed by the oxidative C-C coupling linked at the inner carbon sites upon simple treatment of copper(II) ions. Their intrinsic electronic structures were modulated by the peripheral corrole ring annulations, which lead to distinct optical properties and redox profiles. The stereogenic carbon centers implemented in the confused corrole skeleton provided a rationale for designing novel chiral materials.

Bis-copper(II) Complex of Triply-linked Corrole Dimer and Its Dication.

Copper complexes of corroles have recently been a subject of keen interest due to their ligand non-innocent character and unique redox properties. Here we investigated bis-copper complex of a triply-linked corrole dimer that serves as a pair of divalent metal ligands but can be reduced to a pair of trivalent metal ligands. Reaction of triply-linked corrole dimer 2 with Cu(acac)2 (acac=acetylacetonate) gave bis-copper(II) complex 2Cu as a highly planar molecule with a mean-plane deviation value of 0.020 Å, where the two copper ions were revealed to be divalent by ESR, SQUID, and XPS methods. Oxidation of 2Cu with two equivalents of AgBF4 gave complex 3Cu, which was characterized as a bis-copper(II) complex of a dicationic triply-linked corrole dimer not as the corresponding bis-copper(III) complex. In accord with this assignment, the structural parameters around the copper ions were revealed to be quite similar for 2Cu and 3Cu. Importantly, the magnetic spin-spin interaction differs depending on the redox-state of the ligand, being weak ferromagnetic in 2Cu and antiferromagnetic in 3Cu.