Benzo- and Naphthopentalenes: Syntheses, Structures, and Properties

An Exploration of Substituent Effects on the Photophysical Properties of Monobenzopentalenes

Monobenzopentalenes have received moderate attention compared to dibenzopentalenes, yet their accessibility as stable, non-symmetric structures with diverse substituents could be interesting for materials applications, including molecular photonics. Recently, monobenzopentalene was considered computationally as a potential chromophore for singlet fission (SF) photovoltaics. To advance this compound class towards photonics applications, the excited state energetics must be characterized, computationally and experimentally. In this report we synthesized a series of stable substituted monobenzopentalenes and provided the first experimental exploration of their photophysical properties. Structural and opto-electronic characterization revealed that all derivatives showed 1H NMR shifts in the olefinic region, bond length alternation in the pentalene unit, low-intensity absorptions reflecting the ground-state antiaromatic character and in turn the symmetry forbidden HOMO-to-LUMO transitions of ~2 eV and redox amphotericity. This was also supported by computed aromaticity indices (NICS, ACID, HOMA). Accordingly, substituents did not affect the fulfilment of the energetic criterion of SF, as the computed excited-state energy levels satisfied the required E(S1)/E(T1)>2 relationship. Further spectroscopic measurements revealed a concentration dependent quenching of the excited state and population of the S2 state on the nanosecond timescale, providing initial evidence for unusual photophysics and an alternative entry point for singlet fission with monobenzopentalenes.

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.

Bis-benzofulvenes: Synthesis and Studies on Their Optoelectronic Properties

We report the synthesis of bis-benzofulvenes and the studies on their optical and redox properties. Bis-benzofulvenes were achieved through the Pd-catalyzed intramolecular Heck coupling followed by Ni⁰-mediated C(sp²)-Br dimerization. Low optical and electrochemical energy gaps of 2.05 and 1.68 eV were achieved by tuning the substituent on the exomethylene unit and the aromatic ring. The observed trends in the energy gaps were compared, and the frontier molecular orbitals were visualized using the density functional theory.

Fluorinated Dihydropentalene-1,4-Dione: A Strong Electron-Accepting Unit with Organic Semiconductor Characteristics

The development of electron-accepting units is of significant importance because the construction of donor (D)-acceptor (A) configurations is an effective strategy for tuning the electronic properties of π-conjugated systems. Although doubly fused pentagons represented by diketopyrrolopyrrole (DPP) have been used as an effective electron-accepting unit, the relatively high-lying frontier molecular orbital levels (FMOs) leave room for further improvement. We report herein the synthesis of a fluorinated dihydropentalene-1,4-dione (FPD) derivative as a strong electron-accepting unit and the development of D-A-D π-extended molecules. X-ray analyses revealed that the presence of fluorine atoms contributed to the formation of high planar structures and slipped-stacked packing. Electrochemical measurements indicated that the FPD derivatives showed relatively lower FMO energy levels than the corresponding DPP-containing derivatives. The D-A-D molecule based on terthiophene and FPD showed semiconducting responses. This study demonstrates that the FPD unit can function as a new acceptor unit for organic semiconductors.

Stable Monoareno-pentalenes with Two Olefinic Protons

A novel class of stable monoareno-pentalenes is introduced that have an olefinic proton on each five-membered ring of the pentalene subunit. Their synthesis was accomplished via a regioselective carbopalladation cascade reaction between ortho-arylacetyleno gem-dibromoolefins and TIPS-acetylene. These molecules could be experimental probes of magnetic (anti)aromaticity effects.

Photoswitching of Local (Anti)Aromaticity in Biphenylene-Based Diarylethene Molecular Switches

Photoinduced tuning of (anti)aromaticity and associated molecular properties is currently in the focus of attention for both tailoring photochemical reactivity and designing new materials. Here, we report on the synthesis and spectroscopic characterization of diarylethene-based molecular switches embedded in a biphenylene structure composed of rings with different levels of local (anti)aromaticity. We show that it is possible to modulate and control the (anti)aromatic character of each ring through reversible photoswitching of the aryl units of the system between open and closed forms. Remarkably, it is shown that the irreversible formation of an annulated bis(dihydro-thiopyran) side-product that hampers the photoswitching can be efficiently suppressed when the aryl core formed by thienyl groups in one switch is replaced by thiazolyl groups in another.

Unsymmetrical Thienopentalenes: Synthesis, Optoelectronic Properties, and (Anti)aromaticity Analysis

The synthesis and properties of a series of unsymmetrical thienopentalenes are explored, including both monoareno and diareno derivatives. For the synthesis of monoareno pentalenes, a carbopalladation cascade reaction between alkynes and gem-dibromoolefins was applied. Diareno pentalene derivatives were accessed via gold-catalyzed cyclization of diynes. Thiophene was fused to pentalene in two different geometries via its 2,3 and 3,4 bonds. 2,3-Fusion resulted in increased antiaromaticity of the pentalene unit compared to the 3,4-fusion both in the monoareno and diareno framework. Monothienopentalenes that contained the destabilizing 2,3-fusion could not be isolated. For diareno derivatives, the aromatic character of the different aryl groups fused to the pentalene was not independent. Destabilizing fusion on one side resulted in alleviated aromaticity on the other side and vice versa. The synthesized molecules were characterized experimentally by ¹H NMR and UV-vis spectroscopies, cyclic voltammetry, and X-ray crystallography, and their aromatic character was assessed using magnetic (NICS and ACID) and electronic indices (MCI and FLU).

Conjugated Nanohoops with Dibenzo[a,e]pentalenes as Non-alternant and Antiaromatic π-Systems

Conjugated nanohoops are excellent candidates to study structure-property relationships, as optoelectronic materials and as hosts for supramolecular chemistry. While carbon nanohoops containing aromatics are well studied, antiaromatic units had not been incorporated until recently by our group using dibenzo[a,e]pentalene (DBP). The non-alternant electronic character of the DBP units significantly influences the optoelectronic properties of such nanohoops. We herein summarize our synthetic strategies to DBP-containing nanohoops, their structural and electronic properties, chirality and host-guest chemistry. We demonstrate how incorporating antiaromatic units leads to unique properties and opens new synthetic avenues, making such nanohoops attractive as potential electronic materials.

Antiaromaticity-Aromaticity Interplay in Fused Benzenoid Systems Using Molecular Electrostatic Potential Topology

The phenomenon of antiaromaticity-aromaticity interplay in aromatic-antiaromatic (A-aA)-fused systems is studied using molecular electrostatic potential (MESP) analysis, which clearly brings out the electron-rich π-regions of molecular systems. Benzene, naphthalene, phenanthrene, and pyrene are the aromatic units and cyclobutadiene and pentalene are the antiaromatic units considered to construct the A-aA-fused systems. The fused system is seen to reduce the antiaromaticity by adopting a configuration containing the least number of localized bonds over antiaromatic moieties. This is clearly observed in 25 isomers of a fused system composed of three naphthalene and two cyclobutadiene units. Denoting the number of π-bonds in the cyclobutadiene rings by the notation (n, n'), the systems belonging to the class (0, 0) and (2, 2) turn out to be the most and least stable configurations, respectively. The stability of the fused system depends on the empty π-character of the antiaromatic ring, hence naphthalene and benzene prefer to fuse with cyclobutadiene in a linear and angular fashion, respectively. Generally, a configuration with the maximum number of 'empty' rings (0, 0, 0, ...) is considered to be the most stable for the given A-aA system. The stability and aromatic/antiaromatic character of A-aA-fused systems with pentalene is also interpreted in a similar way. MESP topology, clearly bringing out the distribution of double bonds in the fused systems, leads to a simple interpretation of the aromatic/antiaromatic character of them. Also, it leads to powerful predictions on stable macrocyclic A-aA systems.

Cations and Anions of Dibenzo[a,e]pentalene and Reduction of a Dibenzo[a,e]pentalenophane

Dibenzo[a,e]pentalene (DBP) is a non-alternant conjugated hydrocarbon with antiaromatic character and ambipolar electrochemical behavior. Upon both reduction and oxidation, it becomes aromatic. We herein study the chemical oxidation and reduction of a planar DBP derivative and a bent DBP-phane. The molecular structures of its planar dication, cation radical and anion radical in the solid state demonstrate the gained aromaticity through bond length equalization, which is supported by nucleus independent chemical shift-calculations. EPR spectra on the cation radical confirm the spin delocalization over the DBP framework. A similar delocalization was not possible in the reduced bent DBP-phane, which stabilized itself by proton abstraction from a solvent molecule upon reduction. This is the first report on structures of a DBP cation radical and dication in the solid state and of a reduced bent DBP derivative. Our study provides valuable insight into the charged species of DBP for its application as semiconductor.

Structure-Property Relationships in Unsymmetric Bis(antiaromatics): Who Wins the Battle between Pentalene and Benzocyclobutadiene?†

According to the currently accepted structure–property relationships, aceno-pentalenes with an angular shape (fused to the 1,2-bond of the acene) exhibit higher antiaromaticity than those with a linear shape (fused to the 2,3-bond of the acene). To explore and expand the current view, we designed and synthesized molecules where two isomeric, yet, different, 8π antiaromatic subunits, a benzocyclobutadiene (BCB) and a pentalene, are combined into, respectively, an angular and a linear topology via an unsaturated six-membered ring. The antiaromatic character of the molecules is supported experimentally by ¹H NMR, UV–vis, and cyclic voltammetry measurements and X-ray crystallography. The experimental results are further confirmed by theoretical studies including the calculation of several aromaticity indices (NICS, ACID, HOMA, FLU, MCI). In the case of the angular molecule, double bond-localization within the connecting six-membered ring resulted in reduced antiaromaticity of both the BCB and pentalene subunits, while the linear structure provided a competitive situation for the two unequal [4n]π subunits. We found that in the latter case the BCB unit alleviated its unfavorable antiaromaticity more efficiently, leaving the pentalene with strong antiaromaticity. Thus, a reversed structure–antiaromaticity relationship when compared to aceno-pentalenes was achieved.

Serendipitous Rediscovery of the Facile Cyclization of Z,Z-3,5-Octadiene-1,7-diyne Derivatives to Afford Stable, Substituted Naphthocyclobutadienes

The serendipitous isolation of very small amounts of two naphthocyclobutadiene (NCB) derivatives has led to the computational re-examination of the electrocyclization of Z,Z-3,5-octadiene-1,7-diyne as well as the experimental and computational study of diethynylindeno[2,1-a]fluorene derivatives that contain the 3,5-octadiene-1,7-diyne motif as part of a larger π-framework. In both cases the calculated potential energy surface strongly implicates two successive electrocyclic reactions to afford the antiaromatic products. With the octadienediyne fragment locked in the reactive conformation, the postulated diethynylindeno[2,1-a]fluorene intermediates afford the NCBs in modest to good yields. X-ray crystallography of four NCBs as well as NICS-XY scan calculations show that the paratropic motif is located primarily in the benzocyclobutadiene fragment within the larger π-scaffold.

Gold-Catalyzed Regiospecific Annulation of Unsymmetrically Substituted 1,5-Diynes for the Precise Synthesis of Bispentalenes

Precise control of the selectivity in organic synthesis is important to access the desired molecules. We demonstrate a regiospecific annulation of unsymmetrically substituted 1,2-di(arylethynyl)benzene derivatives for a geometry-controlled synthesis of linear bispentalenes, which is one of the promising structures for material science. A gold-catalyzed annulation of unsymmetrically substituted 1,2-di(arylethynyl)benzene could produce two isomeric pentalenes, but both electronic and steric effects on the aromatics at the terminal position of the alkyne prove to be crucial for the selectivity; especially a regiospecific annulation was achieved with sterically blocked substituents; namely, 2,4,6-trimetyl benzene or 2,4-dimethyl benzene. This approach enables the geometrically controlled synthesis of linear bispentalenes from 1,2,4,5-tetraethynylbenzene or 2,3,6,7-tetraethynylnaphthalene. Moreover, the annulation of a series of tetraynes with a different substitution pattern regioselectively provided the bispentalene scaffolds. A computational study revealed that this is the result of a kinetic control induced by the bulky NHC ligands.

Borepin Rings as "Sigma-Free" Reporters of Aromaticity within Polycyclic Aromatic Scaffolds

The definition and measurement of local and global aromaticity in fused ring polycyclic aromatic compounds is a complex issue. Historically, these types of molecules have been explored in this capacity by way of experimental (NMR, thermochemistry) and computational (NICS, HOMA) analyses. We previously showed how borepin rings with [ b, f] arene fusions can be used as experimental magnetic aromaticity reporters via the remaining protons attached to the borepin rings. In this report, we describe a joint experimental and computational analysis of several borepin-containing polycyclic aromatic molecules in order to draw conclusions about the influence of ring fusion on aromaticity. We find that the borepin ring within these extended structures is a unique motif with limited σ-contribution to aromaticity while still displaying a wide range of structural and magnetic aromatic character.

Computational and experimental studies on the index of antiaromaticity for 4n π-systems (n ≥ 2)

A method to evaluate the relative hardness of 4n π-systems (n ≥ 2) as a new experimental measure of antiaromaticity has been developed.

Gold-Catalyzed Facile Synthesis and Crystal Structures of Benzene-/Naphthalene-Based Bispentalenes as Organic Semiconductors

The gold-catalyzed facile synthesis of U-shaped and S-shaped bispentalenes is described from easily available tetra(arylethynyl)-benzenes and -naphthalenes. The optoelectronic and transistor properties were also investigated. The selectivity between the U-shaped and S-shaped bispentalene isomers can be tuned by the bulkiness of the ligand and the substrates. The S-shaped naphthalene-based bispentalene shows a one-dimensional face-to-face packing pattern in solid state and a good hole mobility, indicating that the S-shaped bispentalene core is highly suitable for transistor applications. The gold-catalyzed annulation of tetraynes provides a useful protocol in the synthesis of bispentalenes for organic semiconductors.

A quantitative structure-property study of reorganization energy for known p-type organic semiconductors

Intramolecular reorganization energy (RE), which quantifies the electron-phonon coupling strength, is an important charge transport parameter for the theoretical characterization of molecular organic semiconductors (OSCs). On a small scale, the accurate calculation of the RE is trivial; however, for large-scale screening, faster approaches are desirable. We investigate the structure-property relations and present a quantitative structure-property relationship study to facilitate the computation of RE from molecular structure. To this end, we generated a compound set of 171, which was derived from known p-type OSCs built from moieties such as acenes, thiophenes, and pentalenes. We show that simple structural descriptors such as the number of atoms, rings or rotatable bonds only weakly correlate with the RE. On the other hand, we show that regression models based on a more comprehensive representation of the molecules such as SMILES-based molecular signatures and geometry-based molecular transforms can predict the RE with a coefficient of determination of 0.7 and a mean absolute error of 40 meV in the library, in which the RE ranges from 76 to 480 meV. Our analysis indicates that a more extensive compound set for training is necessary for more predictive models.

N-Heterocycle-Fused Pentalenes by a Gold-Catalyzed Annulation of Diethynyl-Quinoxalines and -Phenazines

The gold-catalyzed annulation of diethynyl N-heterocycles for the synthesis of quinoxaline-/phenazine-based pentalenes and the study of their optoelectronic properties are described. The inhibition of the gold catalyst by the nitrogen centers in the substrate and the product could be overcome by increasing the reaction temperature to 130 °C, which usually leads to catalyst decomposition in gold catalysis. At 130 °C, 6,7-di(arylethynyl)quinoxalines in chlorobenzene give the corresponding pentalenes. The annulation of 2,3-di(arylethynyl)quinoxalines requires an even higher temperature under microwave irradiation. The quinoxaline-based pentalenes showed lower LUMO levels compared to the corresponding naphthalene-based pentalenes.

Dibenzo[a,e]pentalenes with Low-Lying LUMO Energy Levels as Potential n-Type Materials

Ambipolar organic semiconductors are of high interest for organic field-effect transistors. For n-type conduction, low LUMO energies are required. Dibenzo[a,e]pentalenes (DBPs) are promising compounds; however, few derivatives exist with energetically low-lying LUMO levels. Here, we present DBP derivatives with LUMO energies down to -3.73 eV and small bandgaps down to 1.63 eV determined through cyclic voltammetry, UV/vis absorption spectroscopy, and TDDFT calculations. Single-crystal X-ray diffraction analysis revealed a 1D π-stacking mode. The addition of arylalkynyl substituents at the five-membered rings in a facile and versatile synthetic route allowed for tuning of the band gaps and LUMO energies. The synthetic route can easily be modified to access a variety of DBP derivatives. The LUMO energies of the DBP derivatives presented herein make them attractive for an application in n-type or ambipolar field-effect transistors.

Can Baird's and Clar's Rules Combined Explain Triplet State Energies of Polycyclic Conjugated Hydrocarbons with Fused 4nπ- and (4n + 2)π-Rings?

Compounds that can be labeled as "aromatic chameleons" are π-conjugated compounds that are able to adjust their π-electron distributions so as to comply with the different rules of aromaticity in different electronic states. We used quantum chemical calculations to explore how the fusion of benzene rings onto aromatic chameleonic units represented by biphenylene, dibenzocyclooctatetraene, and dibenzo[a,e]pentalene modifies the first triplet excited states (T₁) of the compounds. Decreases in T₁ energies are observed when going from isomers with linear connectivity of the fused benzene rings to those with cis- or trans-bent connectivities. The T₁ energies decreased down to those of the parent (isolated) 4nπ-electron units. Simultaneously, we observe an increased influence of triplet state aromaticity of the central 4n ring as given by Baird's rule and evidenced by geometric, magnetic, and electron density based aromaticity indices (HOMA, NICS-XY, ACID, and FLU). Because of an influence of triplet state aromaticity in the central 4nπ-electron units, the most stabilized compounds retain the triplet excitation in Baird π-quartets or octets, enabling the outer benzene rings to adapt closed-shell singlet Clar π-sextet character. Interestingly, the T₁ energies go down as the total number of aromatic cycles within a molecule in the T₁ state increases.

Modulating Paratropicity Strength in Diareno-Fused Antiaromatics

Understanding and controlling the electronic structure of molecules is crucial when designing and optimizing new organic semiconductor materials. We report the regioselective synthesis of eight π-expanded diarenoindacene analogues based on the indeno[1,2-b]fluorene framework along with the computational investigation of an array of diareno-fused antiaromatic compounds possessing s-indacene, pentalene, or cyclobutadiene cores. Analysis of the experimental and computationally derived optoelectronic properties uncovered a linear correlation between the bond order of the fused arene bond and the paratropicity strength of the antiaromatic unit. The E_red¹ for the pentalene and indacene core molecules correlates well with their calculated NICS_πZZ values. The findings of this study can be used to predict the properties of, and thus rationally design, new diareno-fused antiaromatic molecules for use as organic semiconductors.