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A recent Research Article in this journal by Matito and co‐workers claimed that none of the oxidation states of a butadiyne‐linked six‐porphyrin nanoring exhibit global aromaticity or antiaromaticity. Here we show that this conclusion is incorrect. Experimental data from NMR spectroscopy for a whole family of nanorings provide strong evidence for global ring currents. The NMR data reveal these ring currents directly, without needing analysis by density functional theory (DFT). Furthermore, DFT calculations reproduce the experimental results when a suitable functional is used. [ABSTRACT FROM AUTHOR]

New tetra‐ and di‐cationic azoniahelicenes provide electrochemical, spectroelectrochemical and electronic circular dichroism (ECD) data reflecting their differences in electron transfer (ET) kinetics. Di‐cationic helquats containing two seven‐membered rings are irreversibly reduced in two ET steps. Substitution by redox‐active ethenylpyridinium in the α or γ position with respect to nitrogen atoms of the helquat core yields tetra‐cationic derivatives with reversible ET steps and communicating redox centres. Redox‐inactive substituents in di‐cationic azoniahelicenes retain ET irreversibility. Redox switching of ECD of tetra‐cationic enantiomers was observed. Unlike fully aromatic helquat, the ECD response of tetra‐cationic helquats to periodic reduction‐oxidation cycles is slower, owing to a strong adsorption on electrodes. Quantum chemical calculations (DFT) indicate that the first ET step of tetra‐cationic derivative substituted in the γ position yields a folded structure, which favours the internal donor‐acceptor interaction. This explains the spectroelectrochemical differences between both tetra‐cations. [ABSTRACT FROM AUTHOR]

The [2+2] photocycloaddition mediated by 1-butyl-3-methyl-7,8-dimethoxyalloxazine ( 1) has been found to be an effective tool for cyclising ω-phenyl- and ω,ω′-diphenyl-4-aza-1,6-heptadienes, in which the nitrogen atom is protected by acylation or quaternisation, towards the synthesis of a variety of phenyl- and diphenyl-3-azabicyclo[3.2.0]heptanes and their corresponding quaternary salts. Thia derivatives, with the sulfur atom in the form of a sulfone group, underwent an analogous cyclisation. Advantageously, visible light (400 nm) was used for the cycloadditions in the presence of 1, in contrast to the previously described procedures affording azabicyclo[3.2.0]heptanes by using UV irradiation. Practical applications are demonstrated through the synthesis of bicyclic quaternary ammonium salts, 6-phenyl-azabicyclo[3.2.0]heptanes known to exhibit biological activity or chiral spiro ammonium salts. Flavin 1 was also found to promote the efficient E→ Z isomerisation of electron-rich cinnamyl derivatives to produce mixtures enriched with the Z isomer (with Z/E ratios of up to 77:23). [ABSTRACT FROM AUTHOR]

Three new dipolar cations have been synthesised, containing ferrocenyl (Fc) electron donor groups attached to helquat (Hq) acceptors. These organometallic Hq derivatives have been characterised as their TfO− salts by using various techniques including NMR and electronic absorption spectroscopies and electrochemical measurements. UV–vis spectra show multiple intense low energy absorptions attributable to intramolecular charge-transfer (ICT) excitations. Each compound displays a reversible Fc+/0 redox process, together with two reversible one-electron reductions of the Hq fragment. Molecular quadratic nonlinear optical (NLO) responses have been determined by using hyper-Rayleigh scattering at 1064 nm, and Stark (electroabsorption) spectroscopic studies on the visible absorption bands. The obtained first hyperpolarizabilities β are moderate, consistent with the relatively short π-conjugation lengths between the Fc and attached pyridinium group. A single-crystal X-ray structure has been solved for one of the complexes as its PF6− salt, revealing a centrosymmetric packing in the triclinic space group P1̅. Density functional theory (DFT) and time-dependent DFT calculations indicate that the lowest energy absorption bands have mainly metal-to-ligand charge-transfer character. The donor orbitals involved in the electronic transitions forming the next lowest energy ICT band also have substantial contributions from the Fe atom. Good agreement between the simulated and experimental UV–vis absorption spectra is achieved by using the PBE0 functional with the 6-311++G(d)/LANL2DZ mixed basis set, and the theoretical β values are reasonably large. Oxidation of the Fc unit is predicted to cause the βtot value to decrease by more than 80% in one of the complexes. [ABSTRACT FROM AUTHOR]

Enantiopure alleno-acetylenic ligands, containing two chiral allene moieties, assemble diastereoselectively with zinc(II) ions to form trinuclear triple-stranded helicates, featuring two internal cavity binding sites ('helicages'). The addition of cycloalkanes or heteroalicycles results in inclusion complex formation with two guest molecules bound in one helicate. While no positive allosteric effects were observed, the chiroptical responses increased strongly upon complexation, with guest-induced circular dichroism (ICD) signals reaching up to ΔΔ ϵ=205 m−1 cm−1. The highest binding affinity was observed for six-membered 1,4-dichalcogens, with binding constants ( K1) of up to 2400 m−1 for 1,4-oxathiane in CD3OD. The X-ray co-crystal structure of 1,4-dioxane bound to a dinuclear triple-stranded helicate confirmed the previously predicted guest orientation inside the helicage, with the two oxygen atoms facing towards the electropositive zinc(II) metal centers. [ABSTRACT FROM AUTHOR]

Homochiral strands of alternating alleno-acetylenes and phenanthroline ligands (P)-1 and (P2)-2, as well as their corresponding enantiomers, selectively assemble with the addition of silver(I) salt to yield dinuclear and trinuclear double helicates, respectively. Upon increasing the solvent polarity, the dinuclear and trinuclear helicates interlock to form a [2]catenane and bis[2]catenane, bearing 14 chirality elements, respectively. The solid-state structure of the [2]catenane reveals a nearly perfect fit of the interlocked strands, and the ECD spectra show a significant amplification of the chiroptical properties upon catenation, indicating stabilization of the helical secondary structure. Highly selective narcissistic self-sorting was demonstrated for a racemic mixture consisting of both short and long alleno-acetylenic strands, highlighting their potential for the preparation of linear catenanes of higher order. [ABSTRACT FROM AUTHOR]

Helquat dyes are the first helicene-like cationic styryl dyes obtained as separate enantiomers. Their remarkable chiroptical properties are due to the unique combination of a cationic hemicyanine chromophore and a helicene-like motif. The magnitude of the ECD response and the pH switching along with their positioning in the visible region are unprecedented among helicenoids. [ABSTRACT FROM AUTHOR]

Helquats are a recently introduced class of compounds, which can be perceived as a combination of helicenes with the skeleton of bispyridinium ions such as paraquat. In addition to their helical shape, some of these compounds can also adopt saddle-like conformations. The focus here is on a specific [5]helquat dication HQ2+, which is investigated by various experimental techniques in condensed media as well as in the gas phase in combination with parallel computational studies. For this compound, the helical form is strongly favored in the free dication HQ2+ in the gas phase as well as in the condensed-phase salts [ HQ2+ ⋅ 2X−], whereas in the singly charged ion pairs in the gas phase, the saddle conformer [ SQ2+ ⋅ X−] is significantly more stable than the helical form [ HQ2+ ⋅ X−] for a number of anions X−. The reason for this inversion of stability in the binary ion pairs is that the saddle conformer has a central binding pocket, in which the anion can interact with both pyridinium centers, whereas the more compact helix conformer permits the anion to approach only a single cationic center efficiently. [ABSTRACT FROM AUTHOR]

The cover page of the journal "ChemPlusChem" is presented.