Homochiral and Heterochiral Self-Sorting Assemblies of Antiaromatic Ni(II) Norcorrole Dimers.

Recently, π-π stacked antiaromatic π-systems have received considerable attention because they can exhibit stacked-ring aromaticity due to substantial intermolecular orbital interactions. Here, we report three antiaromatic norcorrole dimers that self-assemble to form supramolecular architectures through chiral self-sorting. A 2,2'-linked norcorrole dimer with 3,5-di-tert-butylphenyl groups forms a π-stacked dimer both in solid and solution states via homochiral self-sorting. Its association constant in solution is (3.6±1.7)×105 M-1 at 20 °C. In the solid state, 3,3'-linked norcorrole dimers with 3,5-di-tert-butylphenyl and phenyl groups afford macrocyclic and helical supramolecular assemblies via heterochiral and homochiral self-sorting, respectively. Notably, the subtle modification in the substituent resulted in a complete change in the structure of the aggregates and the chiral self-sorting mode. The present findings demonstrate that structural manipulation in antiaromatic monomer units leads to the formation of various supramolecular assemblies on the basis of the attractive interactions between antiaromatic π-systems.

Spectroscopic Manifestations of (Anti)Aromaticity in Oxidized and Reduced Porphyrin and Norcorrole.

Aromaticity and antiaromaticity are foundational principes in organic chemistry, regularly invoked to explain stability, structure, and magnetic and electronic properties. There are ongoing challenges in assigning molecules as aromatic or antiaromatic using optical spectroscopy. Here we report spectroelectrochemical and computational analyses of porphyrin (18π neutral, aromatic) and norcorrole (16π neutral, antiaromatic), and their oxidized (16π porphyrin dication) and reduced (norcorrole 18π dianion) forms. Our results show that while the visible spectra are characteristic of (anti)aromaticity consistent with Hückel's rules, the IR spectra are much less informative, owing to the relative rigidity of norcorrole. The results have implications for the assignment of (anti)aromaticity in both ground-state and time-resolved spectra.

Effect of Benzo-Annelation on Triplet State Energies in Polycyclic Conjugated Hydrocarbons.

In a series of earlier studies, the effect of benzo-annelation was found to be a useful tool for tuning the aromaticity in polycyclic conjugated compounds to desired level. In this work we studied the (anti)aromaticity of benzo-annelated derivatives of three conjugated hydrocarbons (anthracene, fluoranthene and biphenylene) in their lowest lying singlet (S0) and triplet (T1) states by means of the energy effect (ef), harmonic oscillator model of aromaticity (HOMA), multicentre delocalization indices (MCI), magnetically induced current densities (MICDs) and nucleus independent chemical shifts (NICS). We showed that benzo-annelation is a topology-based effect which can be used to modify the T1 state excitation energies (E(T1)). A quantitative model was established being able to accurately predict the E(T1) based only on the numbers of angularly, linearly and geminally annelated benzene rings. In addition, it was demonstrated that the E(T1) can be directly related to the (anti)aromatic character of the central ring in the studied molecules in their S0 state.

20π-Electron Antiaromatic Benziphthalocyanines with Absorption Reaching the Near-Infrared-II Region.

Although second near-infrared (NIR-II, 1000-1500 nm) light has attracted considerable attention, especially for life sciences applications, the development of organic dyes with NIR-II absorption remains a formidable challenge. Herein we report the design, synthesis, and electronic properties of 20π-electron antiaromatic benziphthalocyanines (BPcs) that exhibit intense absorption bands in the NIR region. The strong, low-energy absorption of the antiaromatic BPcs is attributed to electric-dipole-allowed HOMO-LUMO transitions with narrow band gaps, enabled by the reduced structural symmetry of BPc compared with regular porphyrins and phthalocyanines. The combination of peripheral substituents and a central metal decreases the HOMO-LUMO energy gaps, leading to the extension of the absorption bands into the NIR-II region (reaching 1100 nm) under reductive conditions.

Low-lying LUMO Boosts Conductance in Antiaromatic Dibenzopentalene Versus Aromatic Analogues.

Antiaromaticity is a fundamental concept in chemistry, but the study of molecular wires incorporating antiaromatic units is limited. Despite initial predictions, very few studies show that antiaromaticity has a beneficial effect on electron transport. Dibenzo[a,e]pentalene (DBP) is a stable structure that displays appreciable antiaromaticity within the five-membered rings of the pentalene core. We have investigated derivatives of DBP furnished with pyridyl (Py) and F4-pyridyl (PyF4) anchor groups, and compared the conductance with purely aromatic phenyl and anthracene analogues. We find that the low-bias conductance of DBP-Py is approximately 60 % larger than that of the anthracene analogue Anth-Py and 250 % larger compared to the phenyl derivative Ph-Py. This is due to a better alignment of the LUMO with the gold Fermi level, which we confirm by conductance-voltage spectroscopy where the conductance of DBP-Py shows the greatest voltage-dependence. The F4-pyridyl compounds, which have lower LUMO energies compared to the pyridyl analogues, did not, however, form detectable molecular junctions. The strongly electron-withdrawing fluorine atoms reduce the donor capability of the nitrogen lone-pair to the point where stable N-Au bonds no longer form.

Tuning (Anti)Aromaticity: Variations on the [8]-Circulene Framework.

Optoelectronic properties of organic molecules are underpinned by delocalisation and delocalisability of π-electrons. These properties are sensitive to small changes in electron count, whether achieved by heteroatom substitution or redox chemistry. One measure of the delocalisability of π-electrons is the current induced by an external magnetic field, which is diagnostic of (anti)aromaticity. The ab initio ipsocentric method is used here to model diverse ring-current patterns in the family of [8]-circulenes based on tetracyclopenta[def,jkl,pqr,vwx]tetraphenylene (TCPTP), in different charge states, with disjoint hetero-atom substitution, and with CC units systematically replaced by BN pairs. Maps calculated at the CHF/CTOCD-DZ2/6-31G** level reveal that these modifications of the TCPTP framework access the full range of possibilities for current from concentric global circulations (typically counter rotating) to full (non-aromatic) localisation. In the ipsocentric approach, induced current density is partitioned into robust orbital contributions that obey selection rules based on orbital symmetry, energy and nodal character. The selection rules are applied here to interpret current-density and exploit insights gained from simpler models to suggest design strategies for fine-tuning of π-delocalisability (aromaticity and antiaromaticity) in macrocyclic frameworks.

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.

BN-Isosteres of Nonacene with Antiaromatic B2 C4 and N2 C4 Heterocycles: Synthesis and Strong Luminescence.

Embedding both boron and nitrogen into the backbone of acenes to generate their isoelectronic structures has significantly enriched the acene chemistry to offer appealing properties. However, only small BN-heteroacenes have been extensively investigated, with BN-heptacenes as the hitherto longest homologue. Herein, we report the synthesis of three new nonacene BN-isosteres via incorporating a pair of antiaromatic B2 C4 and N2 C4 heterocycles, representing a new length record for BN-heteroacenes. The distance between the B2 C4 and N2 C4 rings affects the contribution of the charge-separated resonance forms, leading to tunable antiaromaticity of the two heterocycles. The adjusted local antiaromaticity manifests substantial influence on the molecular orbital arrangement, and consequently, the radiative transition rate of BN-3 is greatly enhanced compared with BN-1 and BN-2, realizing a high fluorescence quantum yield of 92 %. This work provides a novel design concept of large acene BN-isosteres and reveals the importance of BN/CC isosterism on their luminescent properties.

5,18-Dimesitylorangarin: a Stable Antiaromatic [20]Pentaphyrin(1.0.1.0.0) Displaying Remarkable Oxidative Self-Coupling Reactions.

5,18-Dimesitylorangarin and its BF2 complex were synthesized by double SNAr reaction of 3,5-dibromo-BODIPY with 2-pyrrolydipyrrin as the first examples of meso-aryl-substituted orangarin. These orangarins, delineated as [20]pentaphyrin(1.0.1.0.0), are strongly antiaromatic but rather stable. The free base orangarin was coupled by oxidation with MnO2 to give a 11,11'-linked dimer, a cyclooctatetraene(COT)-centered trimer, and a spiro-trimer. Fused COT-centered 3H-orangarin dimer was oxidized to the corresponding 2H-orangarin dimer, which was further coupled to give a triply COT-centered 2H-orangarin tetramer. 3H-Orangarin oligomers are all antiaromatic as evinced by extremely low-field-shifted 1H NMR signals of the inner NH and ill-defined absorption spectra with broad tails. In contrast, COT-centered 2H-orangarin dimer and tetramer show moderately low-field-shifted NH signals and intense NIR absorbance over 900 nm, suggesting effective π-conjugation through the COT bridge and almost non-antiaromatic character. These orangarin oligomers exhibit many reversible redox potentials owing to the intramolecular electronic interactions. Regardless of the different aromatic characters, all the orangarin monomers and oligomers exhibit very rapid excited-state decays.

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.

Dual Nature of Thiophene Harnessed in the Design of Near-Infrared Emitters via Antiaromaticity Relief.

Near-infrared (NIR) emitters are of great interest for applications in bioimaging and modern technology. Yet the design of such materials with decent characteristics is challenging due to intrinsic limitations. In a recent article, Murai and Yamaguchi report the synthesis of NIR emitters with appreciable fluorescence quantum yields reaching 0.02 at 878 nm in CH2 Cl2 solution. The low band gaps were achieved by a new design strategy exploiting antiaromaticity relief. This concept was realized for compounds consisting of an antiaromatic azepine central ring fused to thiophene moieties. In these systems, thiophene unfolds its dual nature. On the one hand, it contributes to the high antiaromaticity of azepine; on the other hand, it exerts a stabilizing effect on azepine through the formation of a quinoid structure, which reduces its antiaromaticity and shifts the absorption and emission maxima into the NIR region.

Four-Membered Ring-Embedded Cycloarene Enabling Anti-Aromaticity and Ultra-Narrowband Emission.

Synthesis of fully fused π-conjugated cycloarenes embedded nonbenzenoid aromatics is challenging. In this work, the first example of four-membered ring-embedded cycloarene (MF2) was designed and synthesized in single-crystal form by macrocyclization and ring fusion strategies. For comparison, single bond-linked chiral macrocycle MS2 without two fused four-membered rings and its linear-shaped polycyclic benzenoid monomer L1 were also synthesized. The pronounced anti-aromaticity of four-membered rings significantly adjusts the electronic structures and photophysical properties of cycloarene, resulting in an enhancement of the photoluminescence quantum yield (PLQY) from 10.66% and 10.74% for L1 and MS2, respectively, to 54.05% for MF2, which is the highest PLQY among the reported cycloarenes. Notably, owing to the embedded four-membered rings that reduce structural displacements, MF2 exhibits an ultra-narrowband emission with a single-digit full-width at half-maximum (FWHM) of only 7 nm (0.038 eV), which sets a new record among all reported organic narrowband luminescent molecules, and represents the first example of ultra-narrowband emission in conventional polycyclic aromatic hydrocarbons (PAHs) devoid of heteroatoms.

Carbazole Based Smaragdyrins: Synthesis, Aromaticity Switching, and Formation of a Spiro-Dimer.

Carbazole-incorporated smaragdyrin BF2-complex 3 was synthesized by SNAr reaction of 3,5-dibromo-8-mesityl-BODIPY 1 with 3,6-di(tert-butyl)-1,8-di(pyrrol-2-yl)carbazole 2 as a nucleophile. Demetalation of 3 with ZrCl4 gave the corresponding smaragdyrin free base 4 in a good yield. Oxidations of 3 and 4 with MnO2 gave smaragdyrins 5 and 6, respectively, both followed by aromaticity switching, since the oxidized products showed a moderate paratropic ring current owing to their 20π-electronic circuits. Further, treatment of 4 with [RhCl(CO)2]2 in the presence of NaOAc gave RhI complex 7, and oxidation of 3 with RuCl3 in the presence of triethylamine led to the formation of a spiro dimer product, 8.

The application of aromaticity and antiaromaticity to reaction mechanisms.

Aromaticity, in general, can promote a given reaction by stabilizing a transition state or a product via a mobility of π electrons in a cyclic structure. Similarly, such a promotion could be also achieved by destabilizing an antiaromatic reactant. However, both aromaticity and transition states cannot be directly measured in experiment. Thus, computational chemistry has been becoming a key tool to understand the aromaticity-driven reaction mechanisms. In this review, we will analyze the relationship between aromaticity and reaction mechanism to highlight the importance of density functional theory calculations and present it according to an approach via either aromatizing a transition state/product or destabilizing a reactant by antiaromaticity. Specifically, we will start with a particularly challenging example of dinitrogen activation followed by other small-molecule activation, C-F bond activation, rearrangement, as well as metathesis reactions. In addition, antiaromaticity-promoted dihydrogen activation, CO2 capture, and oxygen reduction reactions will be also briefly discussed. Finally, caution must be cast as the magnitude of the aromaticity in the transition states is not particularly high in most cases. Thus, a proof of an adequate electron delocalization rather than a complete ring current is recommended to support the relatively weak aromaticity in these transition states.