Quinoidal Oligo(9,10‐anthryl)s with Chain‐Length‐Dependent Ground States: A Balance between Aromatic Stabilization and Steric Strain Release

Quinoidal π‐conjugated polycyclic hydrocarbons have attracted intensive research interest due to their unique optical/electronic properties and possible magnetic activity, which arises from a thermally excited triplet state. However, there is still lack of fundamental understanding on the factors that determine the electronic ground states. Herein, by using quinoidal oligo(9,10‐anthryl)s, it is demonstrated that both aromatic stabilisation and steric strain release play balanced roles in determining the ground states. Oligomers with up to four anthryl units were synthesised and their ground states were investigated by electronic absorption and electron spin resonance (ESR) spectroscopy, assisted by density functional theory (DFT) calculations. The quinoidal 9,10‐anthryl dimer 1 has a closed‐shell ground state, whereas the tri‐ ( 2 ) and tetramers ( 3 ) both have an open‐shell diradical ground state with a small singlet–triplet gap. Such a difference results from competition between two driving forces: the large steric repulsion between the anthryl/phenyl units in the closed‐shell quinoidal form that drives the molecule to a flexible open‐shell diradical structure, and aromatic stabilisation due to the gain of more aromatic sextet rings in the closed‐shell form, which drives the molecule towards a contorted quinoidal structure. The ground states of these oligomers thus depend on the overall balance between these two driving forces and show chain‐length dependence.     

Phenylene‐Bridged Core‐Modified Planar Aromatic Octaphyrin: Aromaticity,Photophysical and Anion Receptor Properties

The synthesis of a planar expanded meso porphyrin with an intramolecular para‐phenylene‐bridged core is reported. The planarity of the octaphyrin macrocycle was confirmed by single‐crystal X‐ray structural analysis, in which the bridged para‐phenylene unit deviated by 27° from the mean macrocyclic plane. Spectroscopic analyses and theoretical calculations suggested that the macrocycle was Hückel aromatic and followed a major [34 π] single‐conjugation pathway, which indicated that the bridging para‐phenylene unit was not involved in the macrocyclic conjugation. Analysis of the photophysical properties of this system by steady‐state absorption/fluorescence spectroscopy and transient absorption spectroscopy revealed moderate enhancement in the parameters of the octaphyrin as compared to its non‐bridged octaphyrin congeners, which was attributed to the planarity and rigidity of the macrocycle as imposed by the bridging para‐phenylene unit. Preliminary anion‐binding studies revealed that the protonated macrocycle bound selectively with chloride ions through N?H???Cl hydrogen‐bonding interactions.     

Fused Corrole Dimers Interconvert between Nonaromatic and Aromatic States through Two‐Electron Redox Reactions

A singly linked corrole dimer was synthesized by condensation of a dipyrromethane‐1‐carbinol with 1,1,2,2‐tetrapyrroethane. Oxidation of the dimer gave doubly linked corrole dimers 9 and 10 as the first examples of fused corrole dimers involving a meso–meso linkage. Dimers 9 and 10 exhibit characteristic ¹H NMR spectra, absorption spectra, excited‐state dynamics, and two‐photon absorption (TPA) values, which indicate the nonaromatic nature of 9 and the aromatic nature of 10 . Interestingly, 9 is fairly stable despite its unusual 2H‐corrole structure, which has been ascribed to the presence of two direct connections between the individual corrole units.     

The NICS‐XY‐Scan: Identification of Local and Global Ring Currents in Multi‐Ring Systems

Nucleus‐independent chemical shift (NICS)‐based methods are very popular for the determination of the induced magnetic field under an external magnetic field. These methods are used mostly (but not only) for the determination of the aromaticity and antiaromaticity of molecules and ions, both qualitatively and quantitatively. The ghost atom that serves as the NICS probe senses the induced magnetic field and reports it in the form of an NMR chemical shift. However, the source of the field cannot be determined by NICS. Thus, in a multi‐ring system that may contain more than one induced current circuit (and therefore more than one source of the induced magnetic field) the NICS value may represent the sum of many induced magnetic fields. This may lead to wrong assignments of the aromaticity (and antiaromaticity) of the systems under study. In this paper, we present a NICS‐based method for the determination of local and global ring currents in conjugated multi‐ring systems. The method involves placing the NICS probes along the X axis, and if needed, along the Y axis, at a constant height above the system under study. Following the change in the induced field along these axes allows the identification of global and local induced currents. The best NICS type to use for these scans is NICS_πZZ, but it is shown that at a height of 1.7 Å above the molecular plane, NICS_ZZ provides the same qualitative picture. This method, namely the NICS‐XY‐scan, gives information equivalent to that obtained through current density analysis methods, and in some cases, provides even more details.     

B3N3 Borazine Substitution in Hexa‐peri‐Hexabenzocoronene: Computational Analysis and Scholl Reaction of Hexaphenylborazine

The doping of graphene molecules by borazine (B₃N₃) units may modify the electronic properties favorably. Therefore, the influence of the substitution of the central benzene ring of hexa‐peri‐hexabenzocoronene (HBC, C₄₂H₁₈) by an isoelectronic B₃N₃ ring resulting in C₃₆B₃N₃H₁₈ (B3N3HBC) is investigated by computational methods. For comparison, the isoelectronic and isosteric all‐B/N molecule B₂₁N₂₁H₁₈ (termed BN) and its carbon derivative C₆B₁₈N₁₈H₁₈ (C6BN), obtained by substitution of a central B₃N₃ by a C₆ ring, are also studied. The substitution of C₆ in the HBC molecule by a B₃N₃ unit results in a significant change of the computed IR vibrational spectrum between 1400 and 1600 cm^?1 due to the polarity of the borazine core. The properties of the BN molecule resemble those of hexagonal boron nitride, and substitution of the central B₃N₃ ring by C₆ changes the computed IR vibrational spectrum only slightly. The allowed transitions to excited states associated with large oscillator strengths shift to higher energy upon going from HBC to B3N3HBC, but to lower energy upon going from BN to C6BN. The possibility of synthesis of B3N3HBC from hexaphenylborazine (HPB) using the Scholl reaction (CuCl₂/AlCl₃ in CS₂) is investigated. Rather than the desired B3N3HBC an insoluble and X‐ray amorphous polymer P is obtained. Its analysis by IR and ¹¹B magic angle spinning NMR spectroscopy reveals the presence of borazine units. The changes in the ¹¹B quadrupolar coupling constant C_Q, asymmetry parameter η, and isotropic chemical shift δ_iso(¹¹B) with respect to HPB are in agreement with a structural model that includes B3N3HBC‐derived monomeric units in polymer P. This indicates that both intra‐ and intermolecular cyclodehydrogenation reactions take place during the Scholl reaction of HPB.     

Nitrogen‐Bridged Metallodiazaporphyrin Dimers: Synergistic Effects of Nitrogen Bridges and meso‐Nitrogen Atoms on Structure and Properties

NH‐bridged and pyrazine‐fused metallodiazaporphyrin dimers have been prepared from nickel(II) and copper(II) complexes of 3‐amino‐5,15‐diazaporphyrin by Pd‐catalyzed C−N cross‐coupling and oxidative dimerization reactions, respectively. The synergistic effects of the nitrogen bridges and meso ‐nitrogen atoms play major roles in enhancing the light‐harvesting properties and delocalization of an electron spin over the entire π‐skeletons of the metallodiazaporphyrin dimers.     

Non‐linear,cata‐Condensed,Polycyclic Aromatic Hydrocarbon Materials: A Generic Approach and Physical Properties

A generic approach to the regiospecific synthesis of halogenated polycyclic aromatics is made possible by the one‐ or two‐directional benzannulation reactions of readily available (ortho‐allylaryl)trichloroacetates (the “BHQ” reaction). Palladium‐catalysed cross‐coupling reactions of the so‐formed haloaromatics enable the synthesis of functionalised polycyclic aromatic hydrocarbons (PAHs) with surgical precision. Overall, this new methodology enables the facile mining of chemical space in search of new electronic functional materials.     

Optical,Electrochemical, and Magnetic Properties of Pyrrole‐ and Thiophene‐Bridged 5,15‐Diazaporphyrin Dimers

The first examples of pyrrole‐ and thiophene‐bridged 5,15‐diazaporphyrin (DAP) dimers are prepared through Stille coupling reactions of nickel(II) and copper(II) complexes of 3‐bromo‐10,20‐dimesityl‐5,15‐diazaporphyrin (mesityl=2,4,6‐trimethylphenyl) with the respective 2,5‐bis(tributylstannyl)heteroles. The effects of the heterole spacers and meso nitrogen atoms on the optical, electrochemical, and magnetic properties of the DAP dimers are investigated by UV/Vis absorption spectroscopy, density functional theory calculations, magnetic circular dichroism spectroscopy, cyclic voltammetry, and EPR spectroscopy. The heterole spacers are found to have a significant impact on the electronic transitions over the entire π‐system. In particular, the pyrrole‐bridged DAP dimers exhibit high light‐harvesting potential in the low‐energy visible/near‐infrared region owing to the intrinsic charge‐transfer character of the lowest excitation.     

Tetrabenzoperipentacene: Stable Five‐Electron Donating Ability and a Discrete Triple‐Layered β‐Graphite Form in the Solid State

An oxidative ring‐closure reaction of a tetranaphthylpyrene derivative led to the synthesis of a 56 all‐carbon conjugated tetrabenzoperipentacene. In the single‐crystal X‐ray structure, three molecules make a triple‐layered cluster by π‐stacking, wherein each layer rotates by 120°, and is thus considered a petit β‐graphite. As for the optical properties, the Stokes shift is extremely small (10 cm^?1), thus indicating its remarkably rigid framework. The tetrabenzoperipentacene exhibits reversible five‐electron oxidation waves in cyclic voltammetry, and is regarded as a counterpart to the fullerene C₆₀ in terms of stable multicharge‐storage nanocarbon materials.     

Electronic Structure of the Ground and Excited States of β‐Carboline

Coupled‐cluster calculations are used to compute the energy of conversion between the neutral and the zwitterionic forms of β‐carboline. The stability of the different species is discussed in terms of charge separation and aromatic character, which is related to magnetic criteria. By means of a linear response formalism the vertical excitation energies and oscillator strengths of the lowest singlet states of both structures as well as of the cationic species are determined. General agreement of the relative position and intensity of the different peaks with experimental data is achieved, but the overall spectra are slightly displaced because of solvent effects.     

Quinoidal/Aromatic Transformations in π‐Conjugated Oligomers: Vibrational Raman studies on the Limits of Rupture for π‐Bonds

The vibrational Raman spectra of several series of aromatic and quinoidal compounds have been analyzed considering the downshifts and upshifts of the frequencies of the relevant Raman bands as a function of the number of repeating units. Oligothiophenes, oligophenylene‐vinylenes, and oligoperylenes (oligophenyls) derivatives are studied in a common context. These shifts are taken as spectroscopic fingerprints of the changes in π‐conjugation. For a given family, aromatic and quinoidal oligomers have been studied together, and according to their Raman frequency shifts located in the two‐well BLA–energy curve of their ground electronic state as a function of the bond‐length‐alternation pattern (BLA). The connection among BLA values, π‐conjugation, and Raman frequencies is taken here as the basis of the study. These Raman shifts/BLA changes have been related to important electronic properties of these one‐dimensional linear π‐electron delocalized systems such as quinoidal (polyene) and aromatic characters.     

Phenylene Ethynylene‐Tethered Perylene Bisimide Folda‐Dimer and Folda‐Trimer: Investigations on Folding Features in Ground and Excited States

In this work, we have elucidated in detail the folding properties of two perylene bisimide (PBI) foldamers composed of two and three PBI units, respectively, attached to a phenylene ethynylene backbone. The folding behaviors of these new PBI folda‐dimer and trimer have been studied by solvent‐dependent UV/Vis absorption and 1D and 2D NMR spectroscopy, revealing facile folding of both systems in tetrahydrofuran (THF). In CHCl₃ the dimer exists in extended (unfolded) conformation, whereas partially folded conformations are observed in the trimer. Temperature‐dependent ¹H NMR spectroscopic studies in [D₈]THF revealed intramolecular dynamic processes for both PBI foldamers due to, on the one hand, hindered rotation around C?N imide bonds and, on the other hand, backbone flapping; the latter process being energetically more demanding as it was observed only at elevated temperature. The structural features of folded conformations of the dimer and trimer have been elucidated by different 2D‐NMR spectroscopy (e.g., ROESY and DOSY) in [D₈]THF. The energetics of folding processes for the PBI dimer and trimer have been assessed by calculations applying various methods, particularly the semiempirical PM6‐DH2 and the more sophisticated B97D approach, in which relevant dispersion corrections are included. These calculations corroborate the results of NMR spectroscopic studies. Folding features in the excited states of these PBI foldamers have been characterized by using time‐resolved fluorescence and transient absorption spectroscopy in THF and CHCl₃, exhibiting similar solvent‐dependent behavior as observed for the ground state. Interestingly, photoinduced electron transfer (PET) process from electron‐donating backbone to electron‐deficient PBI core for extended, but not for folded, conformations was observed, which can be explained by a fast relaxation of excited PBI stacks in the folded conformation into fluorescent excimer states.     

meta‐ and para‐Phenylenediamine‐Fused Porphyrin Dimers: Synthesis and Magnetic Interactions of Their Dication Diradicals

meta‐ and para‐Phenylenediamine‐fused nickel(II) porphyrin dimers were synthesized by S_NAr reaction of meso,β,β‐trichloro nickel(II) porphyrin with meta‐ and para‐phenylenediamines and subsequent Pd‐catalyzed intramolecular C?H arylation. Their tetrachlorinated dication diradicals are very stable, allowing SQUID magnetometry and revealing clear open‐shell characters for both meta and para isomers with ferro‐ and anti‐ferromagnetic interactions, respectively. The nitrogen analogue of Thiele's hydrocarbon usually displays predominant closed‐shell nature but its hidden diradical characters increase either in a twisted conformation or upon insertion of an additional phenylene spacer. The observed distinct diradical nature of the para‐congener indicates that diradical properties can be enhanced also by efficient spin delocalization.     

Regiocontrolled Synthesis of Ethene‐Bridged para‐Phenylene Oligomers Based on PtII‐ and RuII‐Catalyzed Aromatization

We report the regiocontrolled syntheses of ethene‐bridged para‐phenylene oligomers in three distinct classes by using Pt^II‐ and Ru^II‐catalyzed aromatization. This synthetic approach has been developed based on twofold aromatization of the 1‐aryl‐2‐alkynylbenzene functionality, which proceeds by distinct regioselectivity for platinum and ruthenium catalysts. Variable‐temperature NMR spectra provide evidence that large arrays of these oligomers are prone to twist from planarity. The UV/Vis and photoluminescence (PL) spectra as well as the band gaps of these regularly growing arrays show a pattern of extensive π conjugation with increasing array sizes, except for in one instance.     

A Case for Bond‐Network Analysis in the Synthesis of Bridged Polycyclic Complex Molecules: Hetidine and Hetisine Diterpenoid Alkaloids

A key challenge in the synthesis of diterpenoid alkaloids lies in identifying strategies that rapidly construct their multiply bridged polycyclic skeletons. Existing approaches to these structurally intricate secondary metabolites are discussed in the context of a “bond‐network analysis” of molecular frameworks, which was originally devised by Corey some 40 years ago. The retrosynthesis plans that emerge from a topological analysis of the highly bridged frameworks of the diterpenoid alkaloids are discussed in the context of eight recent syntheses of hetidine and hetisine natural products and their derivatives. This Minireview highlights the extent to which network analyses of the type described here sufficed for designing synthesis plans, as well as areas where they had to be amalgamated with functional group oriented synthetic planning considerations.     

A Metal‐Bridged Tricyclic Aromatic System: Synthesis of Osmium Polycyclic Aromatic Complexes

Aromaticity is one of the most important concepts in organic chemistry. A variety of metalla‐aromatic compounds have been recently prepared and in most of those examples, the metal participates only in a monocyclic ring. In contrast, metal‐bridged bicyclic aromatic molecules, in which a metal is shared between two aromatic rings, have been less developed. Herein, we report the first metal‐bridged tricyclic aromatic system, in which the metal center is shared by three aromatic five‐membered rings. These metalla‐aromatics are formed by reaction between osmapentalyne and arene nucleophiles. Experimental results and theoretical calculations reveal that the three five‐membered rings around the osmium center are aromatic. In addition, the broad absorption bands in the UV/Vis absorption spectra of these novel aromatic systems cover almost the entire visible region. This straightforward synthetic strategy may be extended to the synthesis of other metal‐bridged polycyclic aromatics.     

Usage of the Y-Rule and the Effect of the Occurrence of Heteroatoms (N,S) on the Frontier Molecular Orbitals Gap of Polycyclic Aromatic Hydrocarbons (PAHs), and Asphaltene-PAHs

The understanding of the molecular- and colloidal-structure of asphaltenes has seen a major progress; however, there are still issues that require answer. One of them is the location of the heteroatoms in the polycyclic aromatic hydrocarbon (PAH) fused aromatic ring (FAR) region of asphaltenes. Therefore, the effect on the frontier molecular orbitals (HOMO-LUMO) energy-gap due to the addition of a heteroatom (N or S) to PAHs, which are candidates of the PAH region in asphaltenes, has been systematically analyzed by placing S or N in various sites of the PAH molecule. The S is introduced as a thiophenic ring in a bay region, while the N is introduced as a pyridinic-N, which are prevalent forms in the asphaltene-PAH. 174 PAHs are studied with five fused aromatic rings (5FAR) to 10FAR. The π-electron allocation in resonant π-sextets and isolated double bonds is obtained using the Y-rule. The frontier orbitals optical transition is calculated with the ZINDO/S method. Within a FAR family an increment of π-sextets produces and increase of the HOMO-LUMO energy-gap. There is a linear relationship between the Y-rule mapping (percentage of fraction of π-sextet bond divided by nFAR) and the HOMO-LUMO energy-gap. In addition, the effect on the frontier orbitals energy-gap and on the π-electronic allocation due to the presence of N and S is negligible; therefore, to reach conclusions related to the asphaltene-PAH based on conclusions reached for PAH systems, with no heteroatoms, is a reasonable approach.     

Highly Contorted 1,2,5‐Thiadiazole‐Fused Aromatics for Solution‐Processed Field‐Effect Transistors: Synthesis and Properties

A straightforward strategy has been used to construct 1,2,5‐thiadiazole‐fused 12‐ring π systems through twofold Stille coupling and subsequent cyclodehydrogenation by utilizing the building blocks of naphthodithiophene and 5,6‐substituted benzo[b]‐2,1,3‐thiadidazole. Molecules 1 a and 1 b , which exhibit highly contorted π surfaces, show a butterfly‐shaped conformation according to DFT calculations. Within the molecules, a plane‐to‐plane angle of 44.8° was found. UV/Vis absorption, thermogravimetric analysis, differential scanning calorimetry, and cyclic voltammetry (CV) were used to study their physical properties. Strong intermolecular interactions of the nonplanar molecules were also observed by concentration‐dependent ¹H NMR spectroscopy measurements and thin‐film XRD characterization. The low‐lying LUMO and high‐lying HOMO levels of the molecules are ?3.73 and ?5.48 eV, respectively, as estimated from CV measurements; this indicates their potential as semiconducting materials for solution‐processed organic field‐effect transistors (OFETS). A field‐effect hole mobility of up to 0.035 cm² V^?1 s^?1, a threshold voltage of 6.98 V, and a current on/off ratio of 8.65×10⁵ in air for 1 a have been demonstrated with the top‐contact bottom‐gate field‐effect transistor device structures; this represents an important step toward the solution‐processed OFET application of contorted aromatics.