Modulating Paratropicity Strength in Diareno-Fused Antiaromatics

Divergent reactivity of nucleophilic 1-bora-7a-azaindenide anions

The reactions of 1-bora-7a-azaindenide anions, prepared in moderate to excellent yields by reduction of the appropriate 1-bora-7a-azaindenyl chlorides with KC₈ in THF, with alkyl halides and carbon dioxide were studied. With alkyl halides (CH₂Cl₂, CH₃I and BrCH(D)CH(D)^tBu), the anions behave as boron anions, alkylating the boron centre via a classic S_N2 mechanism. This was established with DFT methods and via experiments utilizing the neo-hexyl stereoprobe BrCH(D)CH(D)^tBu. These reactions were in part driven by a re-aromatization of the six membered pyridyl ring upon formation of the product. Conversely, in the reaction of the 1-bora-7a-azaindenide anions with CO₂, a novel carboxylation of the C-2 carbon alpha to boron was observed. Computations indicated that while carboxylation of the boron centre was kinetically feasible, the products of B-carboxylation were not thermodynamically favored relative to the observed C-2 carboxylated species, which were formed preferably due to the generation of both C-C and B-O bonds. In these products, the pyridyl ring remains non-aromatic, in part accounting for the observed reversibility of carboxylation.

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.

The role of intramolecular charge transfer and symmetry breaking in the photophysics of pyrrolo[3,2-b]pyrrole-dione

This paper reveals structurally unique π-expanded pyrrolo[3,2-b]pyrrole and its non-typical photophysical behaviour.

Effective stabilization of a planar phosphorus(iii) center embedded in a porphyrin-based fused aromatic skeleton

Planar phosphorus(iii) centers were effectively stabilized by structural constraint as well as aromatic stabilization in porphyrin-based fused π-conjugated frameworks.

Organophosphorus(iii) compounds usually take on stable pyramidal structures with a large inversion barrier of 30–35 kcal mol^–1. In contrast, diphenylphosphine-fused Ni(ii) porphyrin, where the phosphorus atom is directly attached at the meso-position and embedded in a rigid skeleton, exhibits a considerably planarized phosphorus center. Here we report the synthesis of a mesityl-substituted Ni(ii) porphyrin analogue, 6, which allowed an evaluation of the inversion barrier (ΔG‡203) by variable temperature ¹H NMR spectroscopy which showed it to be exceptionally small, at 14.0 kcal mol^–1. The observed small inversion barrier has been attributed to conformational constraint imposed by the fused structure. In addition, it was thought that the planar transition state is stabilized by the Ni(ii) porphyrin network that allows the contribution of a 22π-aromatic circuit involving phosphorus lone-pair electrons. Along this postulate, we attempted to engineer diarylphosphine-fused porphyrins with smaller inversion barriers by replacing the fused benzene rings with five-membered heterocyclic rings such as thiophene, benzothiophene, benzofuran, indole, benzothiophene 1,1-dioxide, and thiophene 1,1-dioxide. In that order, the aromatic character of the heterocycle decreases, which leads to increasing contribution of the 22π-aromatic circuit. Actually, the inversion barrier of the phosphorus center becomes smaller in this order and reaches 8.7 kcal mol^–1 for thiophene 1,1-dioxide-fused Ni(ii) porphyrin 15, supporting the postulate.

Synthesis of the Unknown Indeno[1,2-a]fluorene Regioisomer: Crystallographic Characterization of Its Dianion

Of the five possible indenofluorene regioisomers, examples of a fully conjugated indeno[1,2-a]fluorene scaffold have so far remained elusive. This work reports the preparation and characterization of 7,12-dimesitylindeno[1,2-a]fluorene as a highly reactive species. Experimental and computational data support the notion of a molecule with pronounced diradical character that exists in a triplet ground state. As such, both NICS and ACID calculations suggest that the indeno[1,2-a]fluorene scaffold is weakly Baird aromatic. Reduction of the unstable red solid with Cs metal produces the dianion of the title compound, from which single crystals could be obtained and X-ray data acquired, thus fully corroborating the proposed indeno[1,2-a]fluorene hydrocarbon core.

Regioselective Synthesis of [3]Naphthylenes and Tuning of Their Antiaromaticity

Polycyclic conjugated hydrocarbons containing four-membered cyclobutadienoids (CBDs) are of great fundamental and technical interest due to the antiaromaticity brought by CBD circuits. However, their synthesis has been challenging, hampering the exploration and understanding of such systems. We report efficient synthesis of a series of unprecedented [3]naphthylene regioisomers in high yields, where three naphthalenoids are fused through two CBDs in linear, angular, and bent regioconnectivity. Their synthesis was enabled by exclusively regioselective catalytic arene-norbornene annulation (CANAL) between dibromonaphthalenes and benzooxanorbornadienes, followed by aromatization. [3]Naphthylene regioisomers exhibited distinct optoelectronic properties. Nucleus-independent chemical shift calculations, NMR spectroscopy, and X-ray crystallography revealed the strong effect of the fusion pattern on the local antiaromaticity and aromaticity in fused CBDs and naphthalenoids, respectively. Thus, our synthetic strategy allows facile access to extended CBD-fused π-systems with tunable local antiaromaticity and aromaticity.

Synthesis and Properties of Quinoidal Fluorenofluorenes

The synthesis and optoelectronic properties of 24 π-electron, formally antiaromatic fluoreno[3,2-b]fluorene and fluoreno[4,3-c]fluorene (FF), are presented. The solid-state structure of [4,3-c]FF along with computationally analogous molecules shows that the outer rings are aromatic while the central four rings possess a bond-localized 2,6-naphthoquinodimethane motif. The antiaromaticity and biradical character of the FFs is assessed computationally, the results of which indicate the dominance of the closed-shell ground state for these molecules.

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.

Explorations of the Indenofluorenes and Expanded Quinoidal Analogues

Highly conjugated hydrocarbons have attracted interest for use as active materials in electronic devices such as organic field effect transistors (OFET) and organic photovoltaics (OPV). In this Account, we review our progress in synthesizing and studying a new class of small molecules for potential use as organic semiconductors. The idea originated from prior research on octadehydrodibenz[12]annulene, as the system can undergo double transannular cyclization to yield the indeno[1,2-b]fluorene skeleton. Subsequent functionalization afforded the first stable, well-characterized indeno[1,2-b]fluorene derivatives, albeit in minute quantities (tens of milligrams). The preparation of these formally antiaromatic compounds has since been optimized: the new synthetic routes utilize inexpensive starting materials, involve robust and high-yielding transformations, and are amenable to considerably larger scale reaction. We have since researched the chemical space of indeno[1,2-b]fluorenes and related quinoidal structures by substitution with a number of functional groups and by permutation of the indenofluorene scaffold. These modifications have allowed us to explore fundamental concepts such as biradical character and antiaromaticity, important considerations when tuning electronic properties to yield functional organic materials. Altering the outer rings by exchange of carbocycles for heterocycles or by inclusion of additional rings as part of the fully conjugated skeleton is one strategy we have examined. Fusing these different aryl groups to s-indacene revealed a dependence of the antiaromaticity of the indacene core upon the outer group. Computational analysis of a series of indeno[1,2-b]fluorene derivatives uncovered an array of different levels of antiaromaticity in the core of the indeno[1,2-b]fluorene derivatives, with one of the benzothiophene derivatives calculated to be as antiaromatic as the parent s-indacene itself. Conversely, we have prepared compounds with expanded cores, starting with the naphthalene-based fluoreno[4,3-c]fluorene, which was produced through a similar route as the indeno[1,2-b]fluorene, using a dione as the key intermediate. Similar to indeno[1,2-b]fluorene, fluoreno[4,3-c]fluorene showed a closed shell ground state, with no evidence of open shell character even upon heating to 170 °C. Increasing the size of the quinoidal core to three rings afforded a diindeno[b,i]anthracene (DIAn) derivative, a compound with a much more complex electronic picture. To produce DIAn, a new synthetic route was devised involving a Friedel-Crafts alkylation to form the five-membered ring and a DDQ oxidation to produce the final compound. DIAn displayed NMR signals that were broadened at room temperature and disappeared when heated, indicative of a molecule with significant biradical character. Extensive computational and experimental investigation verified the controllable expression of its biradical character, with DIAn best described with a ground state that lies between a closed shell compound and a open-shell singlet biradical, with ready access to a thermally populated triplet state.

Facile Synthesis of Polycyclic Pentalenes with Enhanced Hückel Antiaromaticity

Pentalenes represent highly reactive Hückel antiaromatics with 8π electrons. Usually, pentalenes are stabilized by incorporation of two benzene rings in a fused fashion. In dibenzo[a,e]pentalenes, however, the high aromaticity of the fused benzene rings compromises the inherent antiaromaticity of the pentalene core. Herein, we disclose that this forfeited antiaromaticity can be restored by fusing four additional aromatic rings onto the peripheral positions of dibenzo[a,e]pentalenes. Such polycyclic pentalenes were prepared by successive transannular cyclizations via in situ-generated tetrakisdehydro[16]annulenes. The thus obtained compounds showed intriguing properties, for example, characteristic absorptions in the visible-to-near-infrared (NIR) region and low reduction potentials. These results hence afford a design principle to produce highly antiaromatic yet stable pentalenes. The antiaromaticity of the pentalene core can be widely tuned via the degree of aromaticity of the peripherally fused rings.

4N electron aromatic cycles in polycyclic hydrocarbons

Antiaromatic frameworks become aromatic in polycyclics through fusion, resulting in 4N electron hydrocarbons that possess aromatic properties by energetic, magnetic and geometric criteria.