Polycyclic Hydrocarbons from [4n]Annulenes: Correlation versus Hybridization Forces in the Formation of Diradicaloids

Perylene with Split-Azulene Embedding

Splitting the five and seven-membered rings of azulene and embedding them separately into a conjugated backbone provides azulene-like polycyclic aromatic hydrocarbons (PAHs), which are of great interest in quantum and material chemistry. However, the synthetic accessibility poses a significant challenge. In this study, we present the synthesis of a novel azulene-like PAH, Pery-57, which can be viewed as the integration of a perylene framework into the split azulene. The compact structure of Pery-57 displays several intriguing characteristics, including NIR II absorption at 1200 nm, a substantial dipole moment of 3.5 D, and head-to-tail alternating columnar packing. Furthermore, Pery-57 exhibits remarkable redox properties. The cationic radical Pery-57⋅+ readily captures a hydrogen atom. Variable-temperature NMR (VT NMR ) and variable-temperature EPR (VT-EPR) studies reveal that the dianion Pery-572- possesses an open-shell singlet ground state and demonstrates significant global anti-aromaticity. The dication Pery-572+ is also predicted to exhibit diradical character. Despite bearing three bulky substituents, Pery-57 displays p-type transport characteristics with a mobility of 0.03 cm2 V-1 s-1, attributed to its unique azulene-like structure. Overall, this work directs interest in azulene-like PAHs, a unique member of nonalternant PAHs showcasing exceptional properties and applications.

1,1'-Biolympicenyl: A Stable Non-Kekulé Diradical with a Small Singlet and Triplet Energy Gap

Dimerization of delocalized polycyclic hydrocarbon radicals is a simple and versatile method to create diradicals with tailored electronic structures and accessible high-spin states. However, the synthesis is challenging, and the stability issue of the diradicals remains a concern. In this study, we present the synthesis of a stable non-Kekulé 1,1'-biolympicenyl diradical 1 using a protection-oxidation-protection strategy. Diradical 1 demonstrated exceptional stability, with a solution half-life time exceeding 3.5 years and a solid state thermal decomposition temperature above 300 °C. X-ray crystallographic analysis revealed its intersected molecular structure and tightly bound dimer configuration. A singlet ground state with a small singlet-triplet energy gap is consistently identified using electron paramagnetic resonance (EPR) and a superconducting quantum interference device (SQUID) in a rigid matrix, and the triplet state is thermally accessible at room temperature. The solution phase properties were systematically examined through EPR, absorption spectroscopy, and cyclic voltammetry, revealing a rotational motion in the slow-motion regime and multistage redox characteristics. This study presents an efficient synthetic and stabilization strategy for organic diradicals, enabling the development of various high-spin functional materials.

Prediction of the ground state for indenofluorene-type systems with Clar's π-sextet model

This study introduces the Ground State Stability (GSS) rule that allows predicting the nature of the ground state of indenofluorene (IF)-type systems from the simple counting of the Clar's π-sextets in the closed- and open-shell configurations. The IF-type system exhibits a triplet ground state when acquiring double or more the number of Clar's π-sextets in the open-shell form relative to the closed-shell form; otherwise, it assumes an open-shell singlet ground state. Performed state-of-the-art DFT calculations and analysis of aromaticity for the systems of interest validate the effectiveness of the proposed rule. We demonstrate that aromaticity plays the most crucial role in determining the ground electronic state for such polycyclic hydrocarbons. The simplicity of the GSS rule makes it a robust strategy for identifying promising systems in the development of indenofluorene-type materials.

A Stable Chichibabin Diradicaloid with Near-Infrared Emission

Conjugated molecules with multiple radical centers such as the iconic Chichibabin diradicaloid hold promise as building blocks in materials for quantum sensing and quantum information processing. However, it is a considerable challenge to design simple analogues of the Chichibabin hydrocarbon that are chemically inert, exhibit high diradical character and emit light at a distinct wavelength that may offer an optical readout of the spin state in functional ensembles. Here we describe the serendipitous discovery of the stable TTM-TTM diradicaloid, which exhibits high diradical character, a striking sky-blue color and near-infrared (NIR) emission (in solution). This combination of properties is unique among related diradicaloids and is due to the presence of hydrogen and chlorine atoms in "just the right positions", allowing a perfectly planar, yet predominantly benzenoid bridge to connect the two sterically stabilized radical centers. In-depth studies of the optical and magnetic properties suggest that this structural motif could become a mainstay building block of organic spin materials.

Bistability between π-diradical open-shell and closed-shell states in indeno[1,2-a]fluorene

Indenofluorenes are non-benzenoid conjugated hydrocarbons that have received great interest owing to their unusual electronic structure and potential applications in nonlinear optics and photovoltaics. Here we report the generation of unsubstituted indeno[1,2-a]fluorene on various surfaces by the cleavage of two C-H bonds in 7,12-dihydroindeno[1,2-a]fluorene through voltage pulses applied by the tip of a combined scanning tunnelling microscope and atomic force microscope. On bilayer NaCl on Au(111), indeno[1,2-a]fluorene is in the neutral charge state, but it exhibits charge bistability between neutral and anionic states on the lower-workfunction surfaces of bilayer NaCl on Ag(111) and Cu(111). In the neutral state, indeno[1,2-a]fluorene exhibits one of two ground states: an open-shell π-diradical state, predicted to be a triplet by density functional and multireference many-body perturbation theory calculations, or a closed-shell state with a para-quinodimethane moiety in the as-indacene core. We observe switching between open- and closed-shell states of a single molecule by changing its adsorption site on NaCl.

Craig-Hückel Hybrid Aromatic Metalla-dehydro[11]annulenes Constructed by a Formal [10+1] Cycloaddition Reaction

Aromatic metalla-annulenes are important aromatic compounds, research into which has been mainly concentrated on metal-benzenes and their lower homologues. Reports on their superior homologs are rare, and this has greatly limited the systematic study of their properties. In this work, a series of osma-dehydro[11]annulenes with good air and thermal stability were prepared in high yields through a simple [10+1] strategy, by incorporating a metal fragment into conjugated ten-carbon chains in a one-pot reaction. They are the first monometallic aromatic metalla-[n]annulenes with the ring size larger than 6, and their Craig-Hückel hybrid aromaticity is supported by various physical and computational parameters. Besides, these complexes show versatile reactivities, not only giving further evidence for their aromaticity, but also demonstrating their physical and chemical properties can easily be regulated. This work enriches the metalla-aromatic chemistry, and provides a new avenue for the synthesis of large metalla-annulenes with different ring sizes.

Cycl[2,2,4]azine-embedded non-alternant nanographenes containing fused antiaromatic azepine ring

The development of non-alternant nanographenes has attracted considerable attention due to their unique photophysical properties. Herein, we reported a novel aza-doped, non-alternant nanographene (NG) 1 by embedding the cycl[2,2,4]azine unit into the benzenoid NG framework. Single-crystal X-ray diffractometry suggests saddle or twisted nonplanar geometry of the entire backbone of 1 and coplanar conformation of the cycl[2,2,4]azine unit. DFT calculation together with solid structure indicates that NG 1 possesses significant local antiaromaticity in the azepine ring. By oxidative process or trifluoroacetic acid treatment, this nanographene can transform into a mono-radical cation, which was confirmed by UV/Vis absorption, 1H NMR, and electron paramagnetic resonance (EPR) spectroscopy. The antiaromaticity/aromaticity switching of the azepine ring on 1˙+ from 1 enables the high stability of this radical cation, which remained intact for over 1 day. Due to the electron-donating nature of the nitrogen and the unique electronic structure, NG 1 exhibits strong electron-donating properties, as proved by the intermolecular charge transfer towards C60 with a high association constant. Furthermore, selective modification of NG 1 was accomplished by Vilsmeier reaction, and the derivatives 7 and 8 with substituted benzophenone were obtained. The photophysical and electronic properties can be tuned by the introduction of different electronic groups in benzophenone.

Isomerism tunes the diradical character of difluorenopyrroles at constant Hückel-level anti-aromaticity

A new diradical based on diindenocarbazole or difluorenopyrrole was synthesized and experimentally characterized by optical, electrochemical, and magnetic techniques, as well as quantum chemical calculations. The isomerism of these structures tunes the diradical character and the associated properties, representing a unique case of such important modulation. A full study of the electronic structure was carried out considering the perturbative interactions between different canonical forms as well as the anti-aromatic character of the molecular cores. Such a study reveals how we can tune diradical character simply by reorganizing the bonding patterns at constant chemical costs (composition).

Diradicaloid Boron-Doped Molecular Carbons Achieved by Pentagon-Fusion

Molecular carbons (MCs) are molecular cutouts of carbon materials. Doping with heteroatoms and constructing open-shell structures are two powerful approaches to achieve unexpected and unique properties of MCs. Herein, we disclose a new strategy to design open-shell boron-doped MCs (BMCs), namely by pentagon-fusion of an organoborane π-system. We synthesized two diradicaloid BMC molecules that feature C24 B and C38 B π-skeletons containing a pentagonal ring. A thorough investigation reveals that such pentagon-fusion not only leads to their local antiaromaticity, but also incorporates an internal quinoidal substructure and thereby induces open-shell singlet diradical states. Moreover, their fully fused structures enable efficient π conjugation, which is expanded over the whole frameworks. Consequently, some intriguing physical properties are achieved, such as narrow energy gaps, very broad light absorptions, and superior photothermal capability, along with excellent photostability. Notably, the solid of the C38 B molecule exhibits absorption that covers the range of 300-1200 nm and an efficiency of 93.5 % for solar-driven water evaporation, thus demonstrating the potential of diradicaloid BMCs as high-performance organic photothermal materials.

Polyradical character assessment using multireference calculations and comparison with density-functional derived fractional occupation number weighted density analysis

The biradicaloid character of different types of polycyclic aromatic hydrocarbons (PAHs) based on small band gaps is an important descriptor to assess their opto-electronic properties. In this work, the unpaired electron densities and numbers of unpaired electrons (NU values) calculated at the high-level multireference averaged quadratic coupled-cluster (MR-AQCC) method are used to develop a test set to assess the capabilities of different biradical descriptors based on density functional theory. A benchmark collection of 29 different compounds has been selected. The DFT descriptors contain primarily the fractional occupation number weighted electron density (FOD) based on simplified thermally-assisted-occupation density functional theory (TAO-DFT) calculations, but the singlet-triplet energy difference and other descriptors denoted as y0 and nLUNO have been considered as well. After adjustment of the literature-recommended finite temperatures, a very good, detailed agreement between unpaired density and FOD analysis is observed which is also manifested in excellent statistical correlations. The other two descriptors also show good correlations even though the absolute scaling is not satisfactory. A new linear fit of FOD data to the MR-AQCC reference values leads to an improved regression relation for determining the recommended finite temperature value in dependence of the Hartree-Fock exchange. This provides the basis for fast and reliable assessment of the biradical character of many classes of PAHs without the need for performing computationally extended MR calculations.

Synthesis and Characterization of Dibenzothieno[a,f]pentalenes Enabling Large Antiaromaticity and Moderate Open-Shell Character through a Small Energy Barrier for Bond-Shift Valence Tautomerization

Experimental and theoretical rationalization of bond-shift valence tautomerization, characterized by double-well potential surfaces, is one of the most challenging topics of study among the rich electronic properties of antiaromatic molecules. Although the pseudo-Jahn-Teller effect (PJTE) is an essential effect to provide attractive characteristics of 4nπ systems, an understanding of the structure-property relationship derived from the PJTE for planar 4nπ electron systems is still in its infancy. Herein, we describe the synthesis and characterization of two regioisomers of the thiophene-fused diareno[a,f]pentalenes 6 and 7. The magnetic and optoelectronic properties characterize these sulfur-doped diareno[a,f]pentalenes as open-shell antiaromatic molecules, in sharp contrast to the closed-shell antiaromatic systems of 3 and 5, in which these main cores consist of the same number of π electrons as 6 and 7. Notably, thiophene-fused 6b and 7b showed pronounced antiaromaticity, the strongest among the previous systems, as well as moderate open-shell characteristics. Our experimental and theoretical investigations concluded that these properties of 6b and 7b are derived from the small energy barrier Ea‡ for the bond-shift valence tautomerization. The energy profile of the single crystal of 6b showed the temperature-dependent structural variations assigned to the dynamic mutual exchange between the two Cs-symmetric structures, which was also supported by changes in the chemical shifts of variable-temperature 1H NMR spectra in the solution phase. Both experimental and computational results revealed the importance of introducing heteroaromatic rings into 4nπ systems for controlling the PJTE and manifesting the antiaromatic and open-shell natures originating from the high-symmetric structure. The findings of this study advance the understanding of antiaromaticity characterized by the PJTE by controlling the energy barrier for bond-shift valence tautomerizations, potentially leading to the rational design of optoelectronic devices based on novel antiaromatic molecules possessing the strong contributions of their high-symmetric geometries.

Design Strategies for Diradical Boron/Nitrogen Doped Carbon-Based Materials

New heterocyclic diradicaloids based on boron and nitrogen-doped polycyclic systems with open-shell ground-states are obtained via concomitant structural and quinoidal extensions, thus allowing to merge the best of both design strategies. A combination of experimental characterization and theoretical calculations have helped disclose their electronic structure, as well as rationalize their associated magnetic and photophysical properties, spanning the chemical space of available molecular templates for cutting-edge applications in organic electronics and spintronics.

One-Pot Synthesis of Helical Azaheptalene and Chiroptical Switching of an Isolable Radical Cation

A nitrogen-centered heptalene, azaheptalene, was designed as a representative of a new class of redox-responsive molecules with a large steric strain that originates from the adjacent seven-membered rings. The pentabenzo derivative of azaheptalene was efficiently synthesized by a palladium-catalyzed one-pot reaction of commercially available reagents. Bromination led to mono- and dibrominated derivatives, the latter of which is interconvertible with isolable radical cation species exhibiting near-infrared absorption. Since the azaheptalene skeleton shows configurationally stable helicity with a large torsion angle, enantiomers could be successfully separated. Thus, optically pure azaheptalenes with P- or M-helicity showed strong chiroptical properties (|gabs |≥0.01), which could be changed by an electric potential.

Stable Diradical on the Dimethyldihydropyrene Scaffold

The diradical character in a molecular architecture can be customized primarily in two ways: first, by employing a quinoidal pro-aromatic system with net energy gained by aromatization that compensates for the energy required to generate the diradical species and, second, by employing an antiaromatic system having easily accessible triplet states that impart a diradical character. We have chosen a 14π aromatic framework, Boekelheide's dimethyldihydropyrene, and perturbed its aromaticity through the construction of its quinoidal form. The perturbed aromaticity was evident from the bond alteration in the X-ray diffraction structure, 1H nuclear magnetic resonance chemical shifts, and quantum chemical calculations. The aromaticity was restored as the system underwent a transition to the biradical structure centered on two exocyclic carbons. In addition, upon photoexcitation and without using an external reducing reagent, the diradical could be converted to a radical anion and dianion form easily when dimethylformamide was used as a solvent.

Site-selective radical reactions of kinetically stable open-shell singlet diradicaloid difluorenoheteroles with tributyltin hydride and azo-based radical initiators

We have demonstrated site-selective radical reactions of the kinetically stable open-shell singlet diradicaloids difluoreno[3,4-b:4',3'-d]thiophene (DFTh) and difluoreno[3,4-b:4',3'-d]furan (DFFu) with tributyltin hydride (HSn(n-Bu)₃) and azo-based radical initiators. Treatment of these diradicaloids with HSn(n-Bu)₃ induces hydrogenation at the ipso-carbon in the five-membered rings, while treatment with 2,2'-azobis(isobutyronitrile) (AIBN) induces substitution at the carbon atoms in the peripheral six-membered rings. We have also developed one-pot substitution/hydrogenation reactions of DFTh/DFFu with various azo-based radical initiators and HSn(n-Bu)₃. The resulting products can be converted into substituted DFTh/DFFu derivatives via dehydrogenation. Theoretical calculations unveiled a detailed mechanism of the radical reactions of DFTh/DFFu with HSn(n-Bu)₃ and with AIBN, and that the site-selectivity of these radical reactions is controlled by the balance of the spin density and the steric hindrance in DFTh/DFFu.

s-Indacene Revisited: Modular Synthesis and Modulation of Structures and Molecular Orbitals of Hexaaryl Derivatives

Though s-indacene is an intriguing antiaromatic hydrocarbon of 12 π-electrons, it has been underrepresented due to the lack of efficient and versatile methods to prepare stable derivatives. Herein we report a concise and modular synthetic method for hexaaryl-s-indacene derivatives bearing electron-donating/-accepting groups at specific positions to furnish C_2h-, D_2h-, and C_2v-symmetric substitution patterns. We also report the effects of substituents on their molecular structures, frontier molecular orbital (MO) levels, and magnetically induced ring current tropicities. Both theoretical calculations and X-ray structure analyses indicate that the derivatives of the C_2h-substitution pattern adopt different C_2h structures with significant bond length alternation depending on the electronic property of the substituents. Due to the nonuniform distribution of the frontier MOs, their energy levels are selectively modulated by the electron-donating substituents. This leads to the inversion of the HOMO and HOMO-1 sequences with respect to those of the intrinsic s-indacene as theoretically predicted and experimentally proven by the absorption spectra at visible and near-infrared regions. The NICS values and the ¹H NMR chemical shifts of the s-indacene derivatives indicate their weak antiaromaticity. The different tropicities are explained by the modulation of the HOMO and HOMO-1 levels. In addition, for the hexaxylyl derivative, weak fluorescence from the S₂ excited state was detected due to the large energy gap between the S₁ and S₂ states. Notably, an organic field-effect transistor (OFET) fabricated using the hexaxylyl derivative exhibited moderate hole carrier mobility, a result which opens the door for optoelectronic applications of s-indacene derivatives.

Diradical B/N-Doped Polycyclic Hydrocarbons

Heterocyclic diradicaloids with atom-precise control over open-shell nature are promising materials for organic electronics and spintronics. Herein, we disclose quinoidal π-extension of a B/N-heterocycle for generating B/N-type organic diradicaloids. Two quinoidal π-extended B/N-doped polycyclic hydrocarbons that feature fusion of the B/N-heterocycle motif with the antiaromatic s-indacene or dicyclopenta[b,g]naphthalene core were synthesized. This quinoidal π-extension and B/N-heterocycle leads to their open-shell electronic nature, which stands in contrast to the multiple-resonance effect of conventional B/N-type emitters. These B/N-type diradicaloids have modulated (anti)aromaticity and enhanced diradical characters comparing with the all-carbon analogues, as well as intriguing properties, such as magnetic activities, narrow energy gaps and highly red-shifted absorptions. This study thus opens the new space for both of B/N-doped polycyclic π-systems and heterocyclic diradicaloids.