Open-shell Diradical Character Research Articles

Unveiling Möbius/Hückel Topology and Aromaticity in A Core-Expanded [10]Annulene at Different Oxidation States.

The exploration of annulene's conformation, electronic properties and aromaticity has generated enduring interest over the years, yet it continues to present formidable challenges for annulenes with more than ten carbon atoms. In this study, we present the synthesis of a stable [10]cyclo-para-phenylmethine derivative (1), which bears a resemblance to [10]annulene. 1 can be readily oxidized into its respective cations, wherein electrons are effectively delocalized along the backbone, resulting in different conformations and aromaticity. Both 1 and its tetracation (14+ ⋅ 4SbF6 - ) exhibit a nearly planar conformation with a rectangular shape, akin to the E,Z,E,Z,Z-[10]annulene. In contrast, the radical cation (1⋅+ ⋅ SbCl6 - ) possesses a doubly twisted Hückel topology. Furthermore, the dication (12+ ⋅ 2SbCl6 - ) displays conformational flexibility in solution and crystalizes with the simultaneous presence of Möbius-twisted (1a2+ ⋅ 2SbCl6 - ) and Hückel-planar (1b2+ ⋅ 2SbCl6 - ) isomers in its unit cell. Detailed experimental measurements and theoretical calculations reveal that: (1) 1 demonstrates localized aromaticity with an alternating benzenoid/quinoid structure; (2) 1a2+ ⋅ 2SbCl6 - and 1b2+ ⋅ 2SbCl6 - with 48π electrons are weakly Möbius aromatic and Hückel antiaromatic, respectively; (3) 14+ ⋅ 4SbF6 - exhibits Hückel aromaticity (46π) and open-shell diradical character.

Circumpentacene with Open-Shell Singlet Diradical Character.

Circumacenes (CAs) are a distinctive type of benzenoid polycyclic aromatic hydrocarbons where an acene unit is completely enclosed by a layer of outer fused benzene rings. Despite their unique structures, the synthesis of CAs is challenging, and until recently, the largest CA molecule synthesized was circumanthracene. In this study, we report the successful synthesis of an extended circumpentacene derivative 1, which represents the largest CA molecule synthesized to date. Its structure was confirmed by X-ray crystallographic analysis and its electronic properties were systematically investigated by both experiments and theoretical calculations. It shows a unique open-shell diradical character due to the existence of extended zigzag edges, with a moderate diradical character index (y0 =39.7 %) and a small singlet-triplet energy gap (ΔES-T =-4.47 kcal/mol). It exhibits a dominant local aromatic character with π-electrons delocalized in the individual aromatic sextet rings. It has a small HOMO-LUMO energy gap and displays amphoteric redox behavior. The electronic structures of its dication and dianion can be considered as doubly charged structures in which two coronene units are fused with a central aromatic benzene ring. This study provides a new route toward stable multizigzag-edged graphene-like molecules with open-shell di/polyradical character.

Quinodimethane embedded expanded helicenes and their open-shell diradical dications/dianions

Two new quinodimethane embedded expanded [11]- and [13]helicenes were synthesized, showing a closed-shell quinoidal structure. Their dications and dianions exhibit significant open-shell diradical character with small singlet–triplet energy gaps.

Frontispiece: Azulene‐Fused Acenes

Acenes. Azulene-fused acenes are described in the Research Article (e202209286) by Qing Jiang, Chunyan Chi et al. Their dications display a local aromaticity shift, and one of them exhibits open-shell diradical character.

8]Cyclo-para-phenylmethine as a Super-Cyclooctatetraene: Dynamic Behavior, Global Aromaticity, and Open-Shell Diradical Character in the Neutral and Dicationic States.

An [8]cyclo-para-phenylmethine derivative ([8]CPPM-Mes) was synthesized. X-ray analysis revealed a tub-shaped geometry similar to the cyclooctatetraene, with alternating benzenoid/quinoid structure. Variable-temperature NMR measurements disclosed a slow valence tautomerization process with an interconversion energy barrier of about 11.7 kcal mol-1 at coalescence temperature (273 K), and two more lower-barrier dynamic processes involving flipping of the 1,4-phenyl rings on the backbone and rotation of the mesityl substituents. Its dication ([8]CPPM-Mes2+ ) adopts a bowl-like geometry with a smaller depth of the cavity, and a slow bowl inversion process was observed by dynamic NMR. The bond lengths of the benzenoid/quinoid rings are more averaged via electron delocalization and the molecule shows global aromaticity, which was further validated by NMR and theoretical analysis. [8]CPPM-Mes2+ exhibits open-shell diradical character with a small singlet-triplet energy gap (-1.8 kcal mol-1 ).

Enhanced Aromaticity and Open-Shell Diradical Character in the Dianions of 9-Fluorenylidene-Substituted Expanded Radialenes.

Radialenes and expanded radialenes are cross-conjugated macrocycles displaying poor aromatic character. In this work, three 9-fluorenylidene substituted expanded [n]radialenes (ER-n, n=3-5) with a diacetylene spacer were synthesized and their structures were confirmed by X-ray crystallographic analysis and NMR spectroscopy. They all can be easily reduced into relatively stable dianions. Detailed experimental measurements and theoretical calculations suggest that their dianions (ER-n2- , n=3-5) are stabilized by both the aromatic fluorenyl anion substituents and the central aromatic rings with formally [4n+2] delocalized π electrons. In addition, the dianions of the extended radialenes (ER-42 - and ER-52 - ) show unique open-shell diradical character with a small singlet-triplet energy gap. For comparison, their linear counterparts (L-3 and L-4) were also synthesized; their dianions exhibit very different redox and optical properties from their respective macrocycles.

Azulene-Fused Acenes.

Non-alternant non-benzenoid π-conjugated polycyclic hydrocarbons (PHs) are expected to exhibit very different electronic properties from the all-benzenoid PHs. Herein, we report the synthesis and physical properties of three azulene-fused acene molecules (1, 2 and 3), which are isoelectronic to the pentacene, hexacene and heptacene, respectively. X-ray crystallographic analysis, NMR spectra, and theoretical calculations reveal a localised aromatic backbone comprising all the six- and five-membered rings while the seven-membered ring remains non-aromatic. They display properties of both azulene and acenes and are much more stable than the respective acenes. The dications of 1, 2 and 3 were formed by chemical oxidation. Notably, 32+ exhibited an open-shell diradical character (y0 =30.2 %) as confirmed by variable-temperature NMR and ESR measurements, which can be explained by recovery of aromaticity of an 2,6-anthraquinodimethane unit annulated with two aromatic tropylium rings.

Open-Shell and Closed-Shell Quinoid-Aromatic Conjugated Polymers: Unusual Spin Magnetic and High Charge Transport Properties.

While quinoidal moieties are considered as emerging platforms showing efficient charge transport and interesting open-shell diradical characteristics, whether these properties could be changed by extension to the conjugated polymer structure remains as a fundamental question. Here, we developed and characterized two conjugated polymers incorporating quinoids with different lengths, which have a stable close- and open-shell diradical character, respectively, namely, poly(quinoidal thiophene-thienylene vinylene) (PQuT-TV) and poly(quinoidal bithiophene-thienylene vinylene) (PQuBT-TV). A longer length of a quinoidal core led to enhanced diradical characteristics. Therefore, the longer core length of QuBT was favorable for the formation of an open-shell diradical structure in its monomer and in the quinoidal polymer. PQuBT-TV exhibited high spin characteristics observed by the strong ESR signal, a low band gap, and improved electrochemical stability. On the other hand, as QuT maintained a closed-shell quinoid structure, PQuT-TV exhibited high backbone coplanarity and strong intermolecular interaction, which was beneficial for charge transport and led to high hole mobility (up to 2.40 cm2 V-1 s-1) in organic field-effect transistors. This work successfully demonstrated how the control of the closed/open-shell character of quinoidal building blocks changes charge transport and spin properties of quinoidal conjugated polymers via quinoid-aromatic interconversion.

S-shaped para-Quinodimethane-Embedded Double [6]Helicene and Its Charged Species Showing Open-Shell Diradical Character.

Helicenes and extended helical π-conjugated compounds have been widely studied, but most of the systems contain only aromatic benzene or heterocyclic rings, showing local aromatic character. Herein, new S-shaped double [6]helicene 1, which has two embedded para-quinodimethane (p-QDM) units, is reported. Due to the existence of a proaromatic quinoidal substructure, it has open-shell diradical character. Its model compound, C-shaped single [6]helicene 2 containing one p-QDM unit, was also synthesized and compared. Their ground-state structures and electronic properties were systematically studied by a combination of various experimental methods assisted by theoretical calculations. Compound 1 has a double-helical structure in the crystal, with the two terminal [6]helicene units bent in opposite directions (i.e., anti form). However, an anti/syn isomerization process with a moderate interconversion energy barrier was observed on the NMR timescale. Compound 1 shows amphoteric redox behavior. It also exhibits open-shell diradical character (y0 =12.1 %) and a small singlet-triplet gap. On the other hand, compound 2 has a typical closed-shell nature. The dication and dianion of 1 also show open-shell diradical character. The dianion of 2 and the tetraanion of 1 exhibit similar electronic structures to their respective isoelectronic structures, that is, [6]helicene and a double [6]helicene. This work provides some insights into the design and synthesis of stable helical π systems with open-shell diradical character and magnetic activity.

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

A soluble and stable core-modified [38]octaphyrin, MC-T8, containing eight thiophene rings was synthesized by Yamamoto coupling followed by oxidative dehydrogenation. X-ray crystallographic analysis revealed a nearly planar backbone, and the molecule is globally aromatic with a dominant 38π conjugation pathway. The dication MC-T82+ is antiaromatic, and the backbone is distorted, with a different orientation of the thiophene rings. The tetracation MC-T84+ becomes aromatic again, with a shallow-bowl-shaped geometry. Both the neutral compound and the dication demonstrated open-shell diradical character with a small singlet-triplet energy gap (-2.70 kcal mol-1 for MC-T8 and -3.78 kcal mol-1 for MC-T82+ ), and they are stable owing to effective spin delocalization.

The Thermal Rearrangement of an NHC-Ligated 3-Benzoborepin to an NHC-Boranorcaradiene.

An N-heterocyclic-carbene-ligated 3-benzoborepin with a bridged structure has been synthesized by double radical trans-hydroboration of benzo[3,4]cycloundec-3-ene-1,5-diyne with an N-heterocyclic carbene borane. The thermal reaction of the NHC-ligated borepin at 150 °C gives an isolable NHC-boranorcaradiene. Experiments and density functional theory calculations support a mechanism whereby the borepin initially rearranges to a boranorcaradiene by a thermal 6π-electrocyclic reaction. This is followed by 1,5-boron shift to give a rearranged boranorcaradiene. This shift occurs with stereoinversion at boron through a transition state with open-shell diradical character. This is the first example of the isolation of a boranorcaradiene from a thermal reaction of a borepin.

6]Cyclo-para-phenylmethine: An Analog of Benzene Showing Global Aromaticity and Open-Shell Diradical Character.

We report a [6]cyclo-para-phenylmethine ([6]CPPM) macrocycle that shows benzene-like electronic properties. Its mesityl derivative, [6]CPPM-Mes, was isolated in crystalline form. X-ray analysis reveals a C2h symmetry, and the bond lengths of the benzenoid/quinoid rings are averaged via resonance. One averaged 1H NMR peak for the protons on the backbone was observed at room temperature, but it was split into one shielded and one deshielded resonance below 198 K, consistent with its globally aromatic character with a dominant 30π conjugation pathway along the periphery. It exhibits open-shell diradical character with a moderate singlet-triplet energy gap (ΔES-T = -6.23 kcal/mol). Its dication is antiaromatic and open-shell, showing a smaller ΔES-T value (-4.18 kcal/mol). Overall, [6]CPPM behaves like an open-shell aromatic "super-benzene".

Global Aromaticity in Macrocyclic Polyradicaloids: Hückel's Rule or Baird's Rule?

Aromaticity is one of the most important concepts in organic chemistry to understand the electronic properties of cyclic π-conjugated molecules. Over a century, different aromaticity rules have been developed and validated. For planar monocyclic conjugated polyenes (also known as [n]annulenes), they will be aromatic if they contain [4N + 2] π electrons according to Hückel's rule, or antiaromatic if they have [4N] π electrons. Topological change from a planar to a half-twisted Möbius strip will lead to [4N] ([4N + 2]) aromaticity (antiaromaticity), which is just inverse to Hückel's rule. When the molecules are excited into the first triplet excited state, the Hückel (anti)aromaticity observed in the ground state will become reversed according to Baird's rule. Strictly speaking, these basic rules are only applicable for monocyclic conjugated systems, but some polycyclic systems such as porphyrinoids may also follow these rules if there is a dominant [n]annulene-like conjugation pathway. On the other hand, all-benzenoid polycyclic aromatic hydrocarbons usually display local aromaticity with π electrons predominantly localized at certain benzene rings according to Clar's aromatic sextet rule. In recent years, some proaromatic and antiaromatic molecules with even number of paired electrons have been found to exhibit open-shell diradical character and unique optical, electronic, and magnetic activities. One of the major driving forces is their intrinsic tendency to become aromatic in the open-shell diradical/polyradical forms. A number of stable diradicaloids and linear polyradicaloids have been successfully synthesized by using thermodynamic and kinetic stabilizing strategies. Herein, our particular interest is a type of macrocyclic polyradicaloid in which multiple frontier π-electrons are antiferromagnetically coupled with each other in a cyclic mode. Formally, these free electrons may behave like normal π-electrons in the [n]annulenes, and thus, it raises questions about their possible global aromaticity and which rule they will follow. In the past 5 years, our group has synthesized a series of macrocyclic polyradicaloids and systematically investigated their global aromaticity and electronic properties. Some important findings include: (1) global (anti)aromaticity is generally observed, but there is a balance between local aromaticity and global aromaticity; (2) most of these molecules follow Hückel's rule in the singlet state and display respective (anti)aromatic characteristics; (3) in some special cases, both Hückel's rule and Baird's rule can be applicable, and a unique annulene-within-an-annulene super-ring structure was demonstrated for the first time; (4) global antiaromaticity in the transition state is also important and a slow valence tautomerization process was observed in a supercyclobutadiene tetraradicaloid. These studies demonstrate how these open-shell macrocyclic polyradicaloids adapt their geometry and spin state to reach the lowest-energy state (aromatic). In this Account, we will mainly discuss their synthesis, global aromaticity, and the fundamental structure-radical character-aromaticity-properties relationships. Various experimental methods (e.g., NMR, X-ray crystallographic analysis, and electronic absorption spectroscopy) and theoretical calculations (e.g., anisotropy of the induced current density, nucleus independent chemical shift, and isochemical shielding surface) have been used to elaborate their (anti)aromatic character. At the end, a perspective on the possible three-dimensional global aromaticity in fully conjugated cagelike diradicaloids or polyradicaloids will be also discussed.

A BODIPY-Bridged Bisphenoxyl Diradicaloid: Solvent-Dependent Diradical Character and Physical Properties.

We report a new boron dipyrromethene (BODIPY)-bridged bisphenoxyl diradicaloid (2), which showed closed-shell diamagnetic character in less polar solvents such as dichloromethane but open-shell diradical character with paramagnetic activity in the very polar solvent N,N-dimethylformamide. X-ray crystallographic analysis of 2 revealed an anti-parallel stacked dimer structure via intermolecular dipole–dipole interaction, and the observed solvent-dependent diradical character can be explained by the different dihedral angles between the phenoxyl units and the BODIPY bridge, and structural flexibility of the molecule in different solvents. Compound 2 also exhibited solvent-dependent optical and electrochemical properties.

Diazuleno‐s‐indacene Diradicaloids: Syntheses, Properties, and Local (anti)Aromaticity Shift from Neutral to Dicationic State

AbstractNon‐alternant, non‐benzenoid π‐conjugated polycyclic hydrocarbons (PHs) are expected to exhibit very different electronic properties from all‐benzenoid PHs. Reported herein are the syntheses and physical properties of four derivatives of two azulene‐fused s‐indacene isomers, the diazuleno[2,1‐a:2′,1′‐g]‐s‐indacene (DAI‐1) and diazuleno[2,1‐a:1′,2′‐h]‐s‐indacene (DAI‐2). The backbone of both isomers contains 28π electrons and is a 7‐5‐5‐6‐5‐5‐7 fused ring system. X‐ray crystallographic analysis, NMR spectra, and theoretical calculations (ACID, NICS) reveal a structure with two aromatic azulene units fused with a central anti‐aromatic s‐indacene moiety. All compounds exhibit open‐shell diradical character and are magnetically active, but the derivatives of DAI‐2 show larger radical character than the respective ones of DAI‐1. Their dications were isolated in crystalline form and all experimental and theoretical analyses disclose a shift of local (anti)aromaticity along the backbone, with two aromatic tropylium rings at the termini.

Diazuleno-s-indacene Diradicaloids: Syntheses, Properties, and Local (anti)Aromaticity Shift from Neutral to Dicationic State.

Non-alternant, non-benzenoid π-conjugated polycyclic hydrocarbons (PHs) are expected to exhibit very different electronic properties from all-benzenoid PHs. Reported herein are the syntheses and physical properties of four derivatives of two azulene-fused s-indacene isomers, the diazuleno[2,1-a:2',1'-g]-s-indacene (DAI-1) and diazuleno[2,1-a:1',2'-h]-s-indacene (DAI-2). The backbone of both isomers contains 28π electrons and is a 7-5-5-6-5-5-7 fused ring system. X-ray crystallographic analysis, NMR spectra, and theoretical calculations (ACID, NICS) reveal a structure with two aromatic azulene units fused with a central anti-aromatic s-indacene moiety. All compounds exhibit open-shell diradical character and are magnetically active, but the derivatives of DAI-2 show larger radical character than the respective ones of DAI-1. Their dications were isolated in crystalline form and all experimental and theoretical analyses disclose a shift of local (anti)aromaticity along the backbone, with two aromatic tropylium rings at the termini.

A Peri-tetracene Diradicaloid: Synthesis and Properties.

Peri-acenes are good model compounds for zigzag graphene nanoribbons, but their synthesis is extremely challenging owing to their intrinsic open-shell diradical character. Now, the successful synthesis and isolation of a stable peri-tetracene derivative PT-2ClPh is reported; four 2,6-dichlorophenyl groups are attached onto the most reactive sites along the zigzag edges. The structure was confirmed by X-ray crystallographic analysis and its electronic properties were systematically investigated by both experiments and theoretical calculations. It exhibits an open-shell singlet ground state with a moderate diradical character (y0 =51.5 % by calculation) and a small singlet-triplet gap (ΔES-T =-2.5 kcal mol-1 by SQUID measurement). It displays global aromatic character, which is different from the smaller-size bisanthene analogue BA-CF3.

Potential Use of Squarates and Croconates as Singlet Fission Sensitizers.

The geometrical and electronic structures of 44 squarate and croconate derivatives are computationally studied by quantum chemistry methods, in the pursuit of new singlet fission sensitizers. A non-negligible singlet open-shell diradical character is observed for most of the studied molecules, which can be controlled through chemical substitution as well as by the size of the central ring. Such a diradical character is related to small singlet-triplet energy gaps, facilitating the accomplishment of the singlet fission energetic requirements. In general, the present results indicate that squarates hold superior singlet fission capabilities than croconates, although we have identified several derivatives within both families as promising singlet fission sensitizers.

Non-classical S-Heteroacenes with o-Quinoidal Conjugation and Open-Shell Diradical Character.

A series of non-classical S-heteroacenes were synthesized and exhibited intriguing physical properties and chemical reactivities that are very different from classical acenes. X-ray crystallographic analyses revealed that all acenothiophene derivatives Ph-AT-1-Ph-AT-3 had an o-quinoidal π-conjugation with large bond-length alternation, whereas the acenodithiophene derivative Ph-ADT-3 easily dimerized or reacted with oxygen to form a peroxy-bridged dimer. The long acenothiophene Ph-AT-4 was also highly reactive. The origin of these unique properties was investigated carefully by both experiments and theoretical calculations. The high reactivity of the long non-classical S-heteroacenes can be explained by their intrinsic open-shell diradical character as well as the o-quinoidal conjugation.

Toward Stable Superbenzoquinone Diradicaloids.

Superbenzoquinone (SBQ) is a quinone derived from a classic polycyclic aromatic hydrocarbon (PAH), hexa-peri-hexabenzocoronene (so-called "superbenzene"), and is a challenging synthetic target. Herein we report the successful synthesis and characterization of its derivatives. We reveal that the high reactivity of SBQ is due to its intrinsic open-shell diradical character. Thus, two kinetically blocked SBQs, SBQ-Me and SBQ-Ph, were prepared by different synthetic strategies. 4-tert-Butylphenyl-substituted SBQ-Ph demonstrated good stability and could be isolated in crystalline form. Both compounds have an open-shell singlet ground state and show thermally populated paramagnetic activity. Our studies provide effective strategies toward stable quinone-based diradicaloids.