The mosti mportant stereodynamic feature of carbo[n]helicenes is the interconversion of their enantiomers. The Gibbs activation energy (DG°(T)) of this process,w hich determinest he rate of enantiomerization,d ictates the configurational stability of [n]helicenes. High values of DG°(T) are required for applicationso ff unctional chiralm olecules incorporating [n]helicenes or helicene substructures. This minireviewp rovides an overview of the mechanism, recent developments, and factorsa ffecting the enantiomerization of [n]helicenes,w hich will accelerate the design processo f configurationally stablef unctional chiralm olecules based on helicene substructures. Additionally,t his minireview addresses the misconception and irregularities in the recent literature on how the terms "racemization"a nd "enantiomerization"a re used as well as how the activationp arameters are calculated for [n]helicenes and relatedc ompounds.

A series of [5]helicenes difunctionalized in the fjord region with either fluoro, methoxy, or methyl groups was synthesized via photochemical and benzylic coupling route. Resolution of each compound into enantiomers and determination of the Gibbs activation energies of enantiomerization (ΔG(T)) revealed high configurational stability in all three cases. The ΔG(T) values of difunctionalized [5]helicenes were compared with those of their monofunctionalized analogues and the parent [5]helicene. Within this series, an exponential correlation between the torsional twist and ΔG(T) was found. The dimethyl derivative exhibits one of the highest configurational stabilities among [n]helicenes reported to date, comparable to that of [9]helicene.

We describe the synthesis and properties of 13,14-dimethylcethrene, a prototype of a chiral diradicaloid photochemical switch that can be transformed reversibly via conrotatory electrocyclization to its more stable closed form by light (630 nm) or heat and back to its open form by light (365 nm). This system illustrates how the chemical reactivity of a diradicaloid molecule can be translated into a switching function, which alters substantially all electronic parameters, namely, the HOMO–LUMO and the singlet–triplet (ST) energy gaps, and the degree of helical twist. As a result, distinct changes in the optical and chiroptical properties of this system were observed, which allowed us to monitor the switching process by a variety of spectroscopic techniques, including NMR, UV–vis, and CD. In comparison to the previously reported parent molecule cethrene, this system benefits from two methyl substituents installed in the fjord region, which account for the stability of the closed form against oxidation and racemization. The methyl substituents increase the ST energy gap of 13,14-dimethylcethrene by ∼4 kcal mol–1 in comparison to cethrene. Our DFT calculations reveal that the larger ST gap is a result of electronic and geometric effects of the methyl substituents and show the potential of related systems to act as magnetic switches at room temperature.

A novel tetrabenzo[a,f,j,o]perylene, namely "bistetracene" in which two tetracenes are connected side by side with two bonds, was synthesized and characterized. An optical energy gap of about 1.56 eV is derived from the UV/Vis absorption spectrum, showing the low optical gap feature of such zigzag-edged polycyclic aromatic hydrocarbons (PAHs). Theoretical calculations and physical property investigations manifest that such a PAH possesses a prominent biradical character in the ground state, which is the first example among bistetracene derivatives. However, the bistetracene can easily undergo oxidation to tetrabenzo[a,f,j,o]perylene-9,19-dione (diketone) under ambient conditions. Thereby, such zigzag-edged PAH provides insight into understanding the edge state of other expanded homologues, such as peri-tetracenes or peri-pentacenes.