Photoswitching Behavior of a Cyclohexene‐Bridged versus a Cyclopentene‐Bridged Dithienylethene System

Abstract

Photoswitching is an intriguing way of incorporating functionality into molecules or their subunits. Dithienylethene switches are particularly promising, but have so far mostly been studied with five-membered ring (cyclopentenyl) backbones. We aim at comparing the switching properties of backbones with five and six carbon atoms in the ring. A major advantage is that cyclohexenyl rings offer new options for chiral functionalization. A slight change in the reaction conditions of a McMurry ring closure reaction leads to the formation of dithienyl derivatives with a cyclohexene backbone in reasonable yield. Density functional theory calculations were carried out, demonstrating the similarity of both compounds. Experimental results confirm the theoretical outcomes.

Limits of Molecular Dithienylethene Switches Caused by Ferrocenyl Substitution

“…Hanazawa's work demonstrated in 1992 that the size of the cycloalkene exerts an influence, not only in the absorption spectra of the closed isomers, but also in the quantum yield of the cyclization . We recently published a comparative study of the cyclohexene‐ versus cyclopentene‐bridging mode for halogen substituents. It was established that both organic molecular switches have very similar properties, although the calculated thermal activation barriers in the excited state were slightly lower for the cyclohexene derivative and the transition dipole moments for switching from the open form to the closed form were higher for the cyclopentene switch.…”

Why Are Dithienylethene‐Linked Biscobaltocenes so Hard to Photoswitch?

“…However, for all cases the ground‐state reaction is unlikely to happen. By looking at the PESs of the excited states, it can be seen that the potential reaction pathways for both switches have a very similar shape as the first excited state PES for the chloro‐substituted switch . Nevertheless, as also observed for the ferrocene‐substituted switch, the reaction pathway does not involve one excited state, but different states very close in energy.…”

“…Additionally, the influence of the hybridization of the ipso carbon atom connecting the sandwich unit to the molecular switch core was investigated (Scheme ). Since in all previous studies the differences between the spectra obtained for the cyclohexenyl‐ and the cyclopentenyl‐bridged switches were negligible, in this work only the cyclopentenyl‐bridged complexes 3 , 5 and 7 were calculated by means of TDDFT (Figure , left column) employing the B3LYP functional. For 7 , structure optimizations in both spin states ( S= 1 and S= 0, the latter modelled by a Broken‐Symmetry determinant) were carried out (see the Supporting Information).…”

“…To understand why complex 3 demonstrates a much better switching behavior than 7 , a scan along the distance between the reactive carbon atoms was performed and the potential energy surfaces (PES) of the ground states and the excited states were analyzed to gain deeper insight in the switching mechanisms. As in our last works the other structural parameters were allowed to relax for each C−C distance. It should be noted that for reasons of computational efficiency, the second parameter, the torsional angle between the thiophene planes, was not considered explicitly, and that a reliable treatment of conical intersections in DFT may also be problematic .…”

“…These DTE units differ only in the cycloalkene connecting the thiophene units. We recently published a comparative study of the cyclohexene‐ vs. the cyclopentene‐bridging mode for halogen substituents. It was concluded that the properties of both organic molecular switches are very similar, although the calculated thermal activation barriers in the excited state are slightly higher for the cyclopentene derivative, while the transition dipole moments for the photocyclisation reaction are slightly lower for the cyclohexene switch.…”