Topological Diversity in Electrochemically Active Core-Modified Expanded Porphyrinoids

Thiophene-based expanded porphyrinoids undergo a transition from a planar conformation to a twisted conformation upon a systematic increase in the number of thiophene units. Octaphyrin, with 40π electrons, displayed temperature-dependent interconversion between planar and nonplanar conformations in the solution state, in contrast to the rigid planar conformation in the solid state. 60π-dodecaphyrin and 70π-tetradecaphyrin have the maximum number of π-electrons for 12- and 14-heterocycle porphyrinoids, respectively. Spectro-electrochemical measurements confirmed facile reversible two-electron oxidation and the unstable radical cation intermediate in these systems.

Bisazapentalene Dication: Global Aromaticity and Open-Shell Singlet Ground State

Antiaromatic moieties fused in polycyclic π-conjugated molecules usually exhibit strong localized antiaromaticiy. Herein, we reported the synthesis and properties of a bisazapentalene dication (BAP²⁺) obtained from in situ two-electron oxidation of neutral species 8. Noteworthily, it possesses global aromaticity and an open-shell singlet ground state. This study underlines the importance of heteroatoms in determining the delocalization of π-electrons and the aromaticity of molecules in their oxidized states.

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.

2,6-/1,5-Naphthoquinodimethane bridged porphyrin dimer diradicaloids

2,6-Naphthoquinodimethane (2,6-NQDM)- and 1,5-naphthoquinodimethane (1,5-NQDM)-bridged porphyrin dimers, 2,6-P2 and 1,5-P2, were synthesized as relatively stable compounds. Both exhibit open-shell singlet ground state according to variable-temperature (VT) NMR and magnetic measurements, as well as restricted active space spin-flip (RAS-SF) calculations. The 1,5-P2 isomer has a larger diradical character ([Formula: see text], based on the RAS-SF calculations) and smaller singlet-triplet energy gap ([Formula: see text] kcal/mol, based on SQUID measurements) compared to the 2,6-P2 isomer ([Formula: see text], [Formula: see text] kcal/mol). In addition, 2,6-P2 shows intense one-photon absorption (OPA) ([Formula: see text] nm, [Formula: see text] M[Formula: see text] cm[Formula: see text] and a large two-photon absorption (TPA) cross-section ([Formula: see text] GM at 1400 nm) in the near-infrared region, while 1,5-P2 with larger diradical character displays red-shifted but weaker OPA ([Formula: see text] nm, [Formula: see text] M[Formula: see text] cm[Formula: see text] and a smaller TPA cross-section ([Formula: see text] GM at 1600 nm). Both compounds show four reversible redox waves and 1,5-P2 has a smaller electrochemical energy gap (1.06 eV vs.1.16 eV for 2,6-P2). Therefore, the bridge structure has a significant impact on the diradical character, electronic properties, and magnetic behaviors of the obtained porphyrin-based diradicaloids.

3D global aromaticity in a fully conjugated diradicaloid cage at different oxidation states

Aromaticity is a vital concept that governs the electronic properties of π-conjugated organic molecules and has long been restricted to 2D systems. The aromaticity in 3D π-conjugated molecules has been rarely studied. Here we report a fully conjugated diradicaloid molecular cage and its global aromaticity at different oxidation states. The neutral compound has an open-shell singlet ground state with a dominant 38π monocyclic conjugation pathway and follows the [4n + 2] Hückel aromaticity rule; the dication has a triplet ground state with a dominant 36π monocyclic conjugation pathway and satisfies [4n] Baird aromaticity; the tetracation is an open-shell singlet with 52 π-electrons that are delocalized along the 3D rigid framework, showing 3D global antiaromaticity; and the hexacation possesses D₃ symmetry with 50 globally delocalized π-electrons, showing [6n + 2] 3D global aromaticity. Different types of aromaticity were therefore accessed in one molecular cage platform, depending on the symmetry, number of π-electrons and spin state.

[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 C_2h 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 (ΔE_S-T = -6.23 kcal/mol). Its dication is antiaromatic and open-shell, showing a smaller ΔE_S-T value (-4.18 kcal/mol). Overall, [6]CPPM behaves like an open-shell aromatic "super-benzene".

Cyclobis(7,8-(para-quinodimethane)-4,4'-triphenylamine) and Its Cationic Species Showing Annulene-Like Global (Anti)Aromaticity

π-Conjugated macrocycles containing all-benzenoid rings usually show local aromaticity, but reported herein is the macrocycle CBQT, containing alternating para-quinodimethane and triphenylamine units displaying annulene-like anti-aromaticity at low temperatures as a result of structural rigidity and participation of the bridging nitrogen atoms in π-conjugation. It was easily synthesized by intermolecular Friedel-Crafts alkylation followed by oxidative dehydrogenation. X-ray crystallographic structures of CBQT, as well as those of its dication, trication, and tetracation were obtained. The dication and tetracation exhibited global aromaticity and antiaromaticity, respectively, as confirmed by NMR measurements and theoretical calculations. Both the dication and tetracation possess open-shell singlet ground states, with a small singlet-triplet gap.

Dicyclopenta[4,3,2,1- ghi:4',3',2',1'- pqr]perylene: A Bowl-Shaped Fragment of Fullerene C70 with Global Antiaromaticity

Solution-phase synthesis of buckybowls remains a big challenge, and there are limited reports on the fragments of C₇₀. Herein, we report a new basic subunit of C₇₀ with C_2v symmetry, the dicyclopenta[4,3,2,1- ghi:4',3',2',1'- pqr]perylene (2CP-Per). Its aryl-substituted derivative 2CP-Per-Ar was synthesized and shows a bowl-shaped geometry according to X-ray crystallographic analysis. A fast bowl-to-bowl inversion process was observed above 183 K by variable temperature nuclear magnetic resonance (NMR), with a small inversion energy barrier. 2CP-Per-Ar displays amphoteric redox behavior with a small electrochemical energy gap (1.29 eV). Different from many other aromatic buckybowls, 2CP-Per exhibits global antiaromaticity with a strong paratropic ring current associated with the 16π-electrons rim, as revealed by NMR measurements and theoretic calculations. Its dianion is aromatic, similar to its isoelectronic structure coronene. Its dication is predicted to be aromatic, with a [6]annulene-within-[14]annulene structure.

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.

Hexakis(3,6-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)cyclohexane: Closed-Shell [6]Radialene or Open-Shell Hexa-Radicaloid?

A star-shaped hexaquinocyclohexane molecule 4 c is reported, which turns out to be a closed-shell extended [6]radialene with a twisted-boat conformation according to X-ray crystallographic analysis. It was formed by an unusually slow decay of its in situ generated open-shell valence isomer, the hexa-radicaloid 4 o, with a half-life time of about 156 min at room temperature. Reaction progress kinetic analysis revealed a large energy barrier of about 95.5±4.3 kJ mol^-1 at room temperature from the hexa-radical form 4 o to the contorted [6]radialene form 4 c, because the transformation needs to overcome large steric repulsion between the neighboring phenoxyl units. Compound 4 c can be chemically reduced to radical anion and dianion, and the dianion is actually a diradical dianion, with a calculated diradical character of 71.9 %. This study demonstrates the unique chemical bonding nature of contorted quinoidal π-conjugated molecules and a very unusual valence isomerization process.

Curved π-conjugated corannulene dimer diradicaloids

So far, most reported open-shell singlet diradicaloids are based on planar π-conjugated molecules. Herein, we report the bridged corannulene dimer diradicaloids, Cor-D1 and Cor-D2, both showing a three-dimensional curved π-conjugated structure. Cor-D1 has a small diradical character (y0 = 5.4%) and behaves more like a closed-shell quinoidal compound at room temperature, while Cor-D2 is a typical open-shell diradicaloid with a larger diradical character (y0 = 16.9%). Both compounds exhibited paramagnetic activity at elevated temperatures, with a singlet-triplet energy gap (ΔES-T) of -8.4 and -3.0 kcal mol-1, respectively. X-ray crystallographic analysis revealed that both molecules have a dumbbell-shaped geometry, with the two terminal corannulene bowls bent to opposite directions. The spin is largely delocalized onto the two bowls in Cor-D2 and there are multiple [CH···π] interactions between the neighboring bowls. Chemical oxidation/reduction to their respective dications/dianions results in global aromaticity with [4n + 2] π-electrons delocalized through the periphery of the whole framework.

Stable 2D anti-ferromagnetically coupled fluorenyl radical dendrons

The first class of stable two-dimensional anti-ferromagnetically coupled dendritic polyradicaloids was synthesized, which show polyradical character and unique properties.

We report the first class of stable, two-dimensional (2D) anti-ferromagnetically coupled dendritic polyradicaloids. A kinetically blocked fluorenyl radical was used to build up the first and second generation dendrons FR-G1 and FR-G2 containing three and seven fluorenyl units, respectively. The neighboring fluorenyl radicals in FR-G1 and FR-G2 show moderate anti-ferromagnetic exchange interaction, resulting in a doublet and quartet ground state, respectively, with a small doublet–quartet energy gap. From FR-G1 to FR-G2, the energy gap decreased and the two-photon absorption was enhanced owing to more extended 2D π-conjugation. Both compounds showed multiple redox waves due to their polyradical character.

Stable Nitrogen-Centered Bis(imino)rylene Diradicaloids

The synthesis of stable open-shell singlet diradicaloids is critical for their practical material application. So far, most reported examples are based on carbon-centered radicals, which are intrinsically reactive, and there are very few examples of stable nitrogen-centered diradicaloids. In this full paper, a series of soluble and stable bis(imino)rylenes up to octarylene were synthesized on the basis of newly developed dibromorylene intermediates. It was found that from hexarylene onward, these quinoidal rylenes showed open-shell singlet ground states and could be thermally populated to paramagnetic triplet aminyl diradicals. They are stable due to efficient spin delocalization onto the rylene backbone as well as kinetic blocking of the aminyl sites by the bulky and electron-deficient 2,4,6-trichlorophenyl groups. They exhibited very different electronic structures, diradical character, excited-state dynamics, one-photon absorption, two-photon absorption, and electrochemical properties from their respective aromatic rylene counterparts. These bis(imino)rylenes represent a rare class of stable, neutral, nitrogen-centered aminyl diradicaloids.

From open-shell singlet diradicaloids to polyradicaloids

In this Feature Article, we highlight our recent efforts toward stable open-shell singlet diradicaloids and polyradicaloids. A brief review on the historical works in the area is introduced first, followed by discussion on the fundamental electronic and physical properties of open-shell singlet diradicaloids. Then, the structure-diradical character relationships based on our recently developed diradicaloids are presented. Next, the challenges and solutions toward stable polyradicaloids and 3D π-conjugated diradicaloids are discussed. Finally, their preliminary material applications are introduced and a perspective view of the area is given.

Stable meso-Fluorenyl Smaragdyrin Monoradical

The facile synthesis and physical characterization of a meso-fluorenyl smaragdyrin monoradical 4, which is stable due to efficient spin delocalization and kinetic blocking, is reported. It has a small energy gap and can be oxidized and reduced into the respective cation and anion, showing different charge distribution and electronic absorption properties.

A Three-Dimensionally π-Conjugated Diradical Molecular Cage

π-Conjugated molecular cages are very challenging targets in structural organic chemistry, supramolecular chemistry, and materials science. The synthesis and physical characterizations are reported of the first three-dimensionally π-conjugated diradical molecular cage PTM-C, in which two polychlorotriphenylmethyl (PTM) radicals are linked by three bis(3,6-carbazolyl) bridges. This cage compound was synthesized mainly by intermolecular Yamamoto coupling followed by deprotonation and oxidation. It is stable and its structure was confirmed by X-ray crystallographic analysis. The two carbon-centered PTM radicals are weakly coupled through electronic interactions with the carbazole spacers, as revealed by optical, electronic, and magnetic measurements as well as theoretical calculations.

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.