A Stable All-Thiophene-Based Core-Modified [38]Octaphyrin Diradicaloid: Conformation and Aromaticity Switch at Different Oxidation States

Open shell (4n + 2)π and closed shell 4nπ planar core-modified decaphyrins

Planar 44π and 46π core-modified decaphyrins with ten thiophene units have been synthesized from short thiophene oligomers. They have been structurally characterized by single crystal X-ray diffraction with further support from spectroscopic analysis and quantum chemical calculations. Our analysis revealed diradicaloid characteristics for 46π species in contrast to the closed shell property of the 44π congener. Further, 44π and 46π undergo reversible two-electron chemical oxidation, as observed by spectro-electrochemical measurements.

Phenanthrene-Incorporated Isoamethyrin: A Near-Planar Macrocycle That Display Enhanced Aromaticity via Protonation-Triggered Conformation Changes

Controlling the aromaticity in expanded porphyrins is a forefront research topic, and the strategy of using protonation-triggered conformational changes to fine-tune electronic properties and aromaticity has yet to be generalized in rigid and planar expanded porphyrins. Here, we synthesized phenanthrene-incorporated isoamethyrins that possess near-planar conformations and nonaromatic characters. However, when methanesulfonic acid (MSA) was added, geometric deformations occurred to minimize the unfavorable strain, resulting in macrocycles that were weakly aromatic as a whole.

A Highly Stable Organic Luminescent Diradical

It is very challenging to obtain stable room-temperature luminescent open-shell singlet diradicals. Herein we report the first stable Müller's hydrocarbon TTM-PhTTM with luminescent properties. Variable-temperature electron paramagnetic resonance spectroscopy measurements and theoretical calculations show that TTM-PhTTM has an open-shell singlet ground state with a diradical character of 90 %. Because of a small singlet-triplet energy gap, the open-shell singlet ground state can be thermally excited to a triplet state. TTM-PhTTM shows room-temperature deep-red emission in various solutions. Unusually high stability of TTM-PhTTM was also observed owing to effective steric hindrance and spin delocalization. Our results are beneficial to the rational design and discovery of more stable luminescent diradical materials.

Tailoring diradicaloid properties of expanded isophlorinoids with systematic core-modification

Herein, we describe the synthesis, structural diversity and diradicaloid characteristics of 38π core-modified aromatic expanded isophlorins with eight heterocyclic rings. The diradicaloid character of expanded isophlorinoid macrocycles was engineered by systematic structural modification. Depending on the nature of the link between the heteroatoms, they adopt planar and non-planar conformations. This large structural variation with a significant difference in the extent of aromaticity is correlated with the magnitude of their respective diradical character.

A Symmetric All-Organic Proton Battery in Mild Electrolyte

All-organic proton batteries are attracting extensive attention due to their sustainability merits and excellent rate capability. Generally, strong acids (e.g. H₂ SO₄ ) have to be employed as the electrolytes to provide H⁺ for all-organic proton batteries due to the high H⁺ intercalation energy barrier. Until now, the design of all-organic proton batteries in mild electrolytes is still a challenge. Herein, a poly(2,9-dihydroquinoxalino[2,3-b]phenazine) (PO) molecule was designed and synthesized, where the adjacent C=N groups show two different chemical environments, resulting in two-step redox reactions. Moreover, the two reactions possess considerable voltage difference because of the large LUMO energy gap between PO and its reduction product. More impressively, the C=N groups endow the π-conjugated PO molecule with H⁺ uptake/removal in the ZnSO₄ electrolyte. As a result, a symmetric all-organic proton battery is achieved in a mild electrolyte for the first time, which exhibits enhanced electrochemical performance and also broadens the chemistry of proton-based batteries.

Inverted and fused expanded heteroporphyrins

Expanded heteroporphyrins are a class of porphyrin macrocycles containing pyrrole, thiophene, furan, selenophene and other heterocyclic rings that are connected to form an internal ring pathway containing a minimum of 17 atoms and more than 18 delocalized π electrons in their conjugated macrocyclic framework. Considering that expanded heteroporphyrins are large in size, these macrocycles are structurally flexible and prefer to adopt various conformations in which one or more pyrrole(s)/heterocycle(s) tend to be in an inverted conformation and pointed outward from the centre of the macrocyclic core. The inverted expanded heteroporphyrins are divided into two classes as follows: (1) N-inverted expanded heteroporphyrins and (2) hetero-atom inverted expanded heteroporphyrins. Both inverted expanded heteroporphyrins show quite unique features in terms of their structure, aromaticity, and electronic and coordination properties. Sometimes, inverted expanded heteroporphyrins lead to the formation of fused expanded heteroporphyrins because of the intramolecular fusion of the pyrrole "N" with the "C" of the inverted heterocycle ring, which also exhibit unique features compared to inverted expanded heteroporphyrins. In this review, we attempt to describe the synthesis, structure, and aromatic, electronic and coordination properties of inverted and fused expanded heteroporphyrins. This review covers the synthesis, structure and properties of inverted and fused expanded heteroporphyrins containing a combination of pyrrole/heterocycle rings starting with five pyrrole/heterocycle-containing pentaphyrins, and then expanded heteroporphyrins containing six, seven, eight and more pyrrole/heterocyclic rings in their porphyrin macrocyclic framework.

Three-State Switching of an Anthracene Extended Bis-thiaxanthylidene with a Highly Stable Diradical State

A multistable molecular switching system based on an anthracene-extended bis-thiaxanthylidene with three individually addressable states that can be interconverted by electrochemical, thermal, and photochemical reactions is reported. Besides reversible switching between an open-shell diradical- and a closed-shell electronic configuration, our findings include a third dicationic state and control by multiple actuators. This dicationic state with an orthogonal conformation can be switched electrochemically with the neutral open-shell triplet state with orthogonal conformation, which was characterized by EPR. The remarkably stable diradical shows kinetic stability as a result of a significant activation barrier for isomerization to a more stable neutral closed-shell folded geometry. We ascribe this activation barrier of ΔG^⧧(293 K) = 25.7 kcal mol^-1 to steric hindrance in the fjord region of the overcrowded alkene structure. The folded closed-shell state can be converted back to the diradical state by irradiation with 385 nm. The folded state can also be oxidized to the dicationic state. These types of molecules with multiple switchable states and in particular stable diradicals show great potential in the design of new functional materials such as memory devices, logic gates, and OFETs.

Synthesis of Imidazole-Based [30]Heptaphyrin and Stable Figure-Eight [60]Tetradecaphyrins via [5 + 2] Condensations in One Pot

Derived from a Pd-catalyzed oxidative C-H/C-H coupling reaction, two giant imidazole-based [60]tetradecaphyrins adopting stable figure-eight geometry together with one [30]heptaphyrin are obtained by [5 + 2] MacDonald condensations in one pot. The directional imidazole is believed to play a vital role for the diverse cyclization and conformation stabilization.

Donor-Acceptor Conjugated Macrocycles with Polyradical Character and Global Aromaticity

Polyradical character and global aromaticity are fundamental concepts that govern the rational design of cyclic conjugated macromolecules for optoelectronic applications. Here, we report donor-acceptor (D−A) conjugated macromolecules with and without π-spacer derivatives to tune the antiferromagnetic couplings between the unpaired electrons. The macromolecules without π-spacer have a closed-shell electronic configuration and show global nonaromatic character in the singlet and lowest triplet states. However, the derivatives with π-spacer develop a nearly pure open-shell diradical and a very high polyradical character, not reported for D−A type macromolecules. Furthermore, the π-spacer derivatives display global nonaromaticity in the singlet ground state, but global aromaticity in the lowest triplet state, according to Baird's rule. The absorption spectra of the open-shell macromolecules calculated with time-dependent density functional theory indicate intensive light absorption in the near-infrared region and broadening to 2,500 nm, making these materials suitable for numerous optoelectronic applications.

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Donor-acceptor macromolecules with open-shell polyradical character are reported.

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The antiferromagnetic coupling between the unpaired electrons is modulated with pi-spacer.

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The open-shell macrocycles show Baird’s aromaticity in the lowest triplet state.

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Open-shell macrocycles red-shift absorption spectra.

A Chichibabin's Hydrocarbon-Based Molecular Cage: The Impact of Structural Rigidity on Dynamics, Stability, and Electronic Properties

A three-dimensional π-conjugated polyradicaloid molecular cage c-Ph14, consisting of three Chichibabin's hydrocarbon motifs connected by two benzene-1,3,5-triyl bridgeheads, was synthesized. Compared with its linear model compound l-Ph4, the prism-like c-Ph14 has a more rigid structure, which shows significant impact on the molecular dynamics, stability, and electronic properties. A higher rotation energy barrier for the quinoidal biphenyl units was determined in c-Ph14 (15.64 kcal/mol) than that of l-Ph4 (11.40 kcal/mol) according to variable-temperature NMR measurements, leading to improved stability, a smaller diradical character, and an increased singlet-triplet energy gap. The pressure-dependent Raman spectroscopic studies on the rigid cage c-Ph14 revealed a quinoidal-to-aromatic transformation along the biphenyl bridges. In addition, the ellipsoidal cavity in the cage allowed selective encapsulation of fullerene C₇₀ over C₆₀, with an associate constant of about 1.43 × 10⁴ M^-1. Moreover, c-Ph14 and l-Ph4 exhibited similar redox behavior and their cationic species (c-Ph14⁶⁺ and l-Ph4²⁺) were obtained by chemical oxidation, and the structures were identified by X-ray crystallographic analysis. The biphenyl unit showed a twisted conformation in l-Ph4²⁺ and remained coplanarity in c-Ph14⁶⁺. Notably, molecules of c-Ph14⁶⁺ form a one-dimensional columnar structure via close π-π stacking between the bridgeheads.

Switching between Local and Global Aromaticity in a Conjugated Macrocycle for High-Performance Organic Sodium-Ion Battery Anodes

Aromatic organic compounds can be used as electrode materials in rechargeable batteries and are expected to advance the development of both anode and cathode materials for sodium-ion batteries (SIBs). However, most aromatic organic compounds assessed as anode materials in SIBs to date exhibit significant degradation issues under fast-charge/discharge conditions and unsatisfying long-term cycling performance. Now, a molecular design concept is presented for improving the stability of organic compounds for battery electrodes. The molecular design of the investigated compound, [2.2.2.2]paracyclophane-1,9,17,25-tetraene (PCT), can stabilize the neutral state by local aromaticity and the doubly reduced state by global aromaticity, resulting in an anode material with extraordinarily stable cycling performance and outstanding performance under fast-charge/discharge conditions, demonstrating an exciting new path for the development of electrode materials for SIBs and other types of batteries.