Facile Synthesis of Polycyclic Pentalenes with Enhanced Hückel Antiaromaticity

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.

Palladium-Catalyzed Double Carbonylative Cyclization of Benzoins: Synthesis and Photoluminescence of Bis-Ester-Bridged Stilbenes

A palladium-catalyzed double carbonylative cyclization of benzoins has been developed, which realizes the synthesis of bis-ester-bridged stilbenes just in two steps from aldehydes. Thus, the obtained fully fused tetracyclic π-systems have a pyrano[3,2- b]pyran-2,6-dione (PPD) core on their center, showing two reversible reductions at low potentials. In addition, their photoluminescence properties are strikingly affected by the aromatic rings fused to the PPD core; bis- thieno-fused PPDs are found to be excellent fluorophores with quantum yields up to 0.98.

Antiaromatic character of cycloheptatriene-bis-annelated indenofluorene framework mainly originated from heptafulvene segment

Fully π-conjugated polycyclic hydrocarbons with antiaromatic character have attracted research attention because of their unique properties such as narrow energy gaps, and thus should find application as optical and electronic materials. Although antiaromatic 16π-electron frameworks can be constructed by the incorporation of multiple seven-membered rings in a fused fashion to install methylenecycloheptatriene (heptafulvene) segments, the development of corresponding benzo[1,2:4,5]di[7]annulene (BDA)-containing π-conjugated systems remains challenging due to the difficulty of their molecular design and synthesis. In this study, we develop an unprecedented chemical structure of cycloheptatriene-bis-annelated indenofluorene, which possesses both BDA and indenofluorene frameworks in a fused fashion. Physical measurements and X-ray analyses, along with theoretical calculations, indicated that antiaromaticity appeared in the BDA framework. By using the conjugated polycyclic hydrocarbon possessing both seven-membered and five-membered rings, this study provides fundamental insight into the strong antiaromatic nature of heptafulvene-based BDA framework.

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.

A Golden Access to Acenopentalenes

By employing gold catalysis, starting from dialkynylated acenes a series of novel un-symmetrical aceno-annulated dibenzopentalenes has been prepared. The achieved yields range from 62-68 %. The fused systems contain naphthalene, anthracene, tetracene, and pentacene units. All new compounds are soluble and stable under standard conditions. The optical properties of the systems are dominated by the dibenzopentalene core for the smaller representatives, while for the anthracene-pentacene-based aceno-benzopentalenes the optical properties of the acene group dominates. Preliminary morphology tests on thin films showed a reverse trend between crystal size and molecule size.

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.

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.

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.

Hierarchical Corannulene-Based Materials: Energy Transfer and Solid-State Photophysics

We report the first example of a donor-acceptor corannulene-containing hybrid material with rapid ligand-to-ligand energy transfer (ET). Additionally, we provide the first time-resolved photoluminescence (PL) data for any corannulene-based compounds in the solid state. Comprehensive analysis of PL data in combination with theoretical calculations of donor-acceptor exciton coupling was employed to estimate ET rate and efficiency in the prepared material. The ligand-to-ligand ET rate calculated using two models is comparable with that observed in fullerene-containing materials, which are generally considered for molecular electronics development. Thus, the presented studies not only demonstrate the possibility of merging the intrinsic properties of π-bowls, specifically corannulene derivatives, with the versatility of crystalline hybrid scaffolds, but could also foreshadow the engineering of a novel class of hierarchical corannulene-based hybrid materials for optoelectronic devices.

The influence of heteroatoms on the aromatic character and the current pathways of B2N2-dibenzo[a,e]pentalenes

Four polycyclic molecules have been investigated at the DFT level of theory using magnetically induced current densities calculated with the GIMIC program as an indicator of their (anti)aromaticity.

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.