Dimethylcethrene: A Chiroptical Diradicaloid Photoswitch

Abstract

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.

Helicenes as Chiroptical Photoswitches

“…From the structural standpoint, all of the known systems but one feature a heterohelicene backbone. The first example of the carbohelicene system that does not contain any heteroatoms has recently been described [13] by our group.…”

“…The c8/o8 ratio in the PSS was also not reported. Most recently, we reported [13] the first example of a carbohelicene photoswitch system 9 (Scheme 5, B). The 1,3,5-hexatriene unit is integrated into the inner rim of a [5]helicene backbone in this type 3 system and two additional benzenoid rings are fused to the [5]helicene core to give a fully conjugated seven-ring system that we named [26] cethrene.…”

Symmetric, Unsymmetrical, and Asymmetric [7]‐, [10]‐, and [13]Helicenes

“…[9] Theversatility of helicenes has been explored in the construction of cages for molecular recognition and in construction of photoswitches. [10,11] While great synthetic progress has been achieved in the preparation of extended helicene-like structures,t he preparation of long fully conjugated helicenes with two or more helical turns remains as ignificant challenge.T of ully explore and expand the field of helicene chemistry there is an apparent need to prepare novel derivatives containing double layered aromatic systems and with the possibility for synthetic derivatization. Tr aditionally helicenes have been prepared by means of oxidative photocyclization of stilbene derivatives.…”

Symmetric, Unsymmetrical, and Asymmetric [7]‐, [10]‐, and [13]Helicenes

“…These molecules typically display interesting optical properties and complex chiral behavior . The versatility of helicenes has been explored in the construction of cages for molecular recognition and in construction of photoswitches . While great synthetic progress has been achieved in the preparation of extended helicene‐like structures, the preparation of long fully conjugated helicenes with two or more helical turns remains a significant challenge.…”