Photoswitchable Norbornadiene–Quadricyclane Interconversion Mediated by Covalently Linked C₆₀

Abstract

The synthesis and propertieso fv arious norbornadiene/quadricyclane (NBD/QC)f ullerene hybrids are reported. By cyclopropanation of C 60 with malonates carrying the NBD scaffold as mall library of NBD-fullerenem onoadducts and NBD-fullereneh exakisadducts was established. The substitutionp attern of the NBD scaffold, as well as the electron affinity of the fullerenec ore within these hybrid systems,has ap ronouncedi mpact on the properties of the corresponding energy rich QC derivatives. Based on this, the first direct photoisomerization of NBD-fullerene hybridst ot heir QC derivatives was achieved.F urthermore, it was possible to use the redox-active fullerenec ore of aQ C-fullerene monoadduct to enable the back reactiont of orm the corresponding NBD-fullerenem onoadduct. Combining theset wo processes enabless witching between NBD and QC simply by changing the irradiationw avelength between3 10 and 400 nm. Therefore, turning this usually photo/thermal switch into ap ure photoswitch. This not only simplifies the investigationo ft he underlying processes of the NBD-QC interconversion within the system,b ut also renders such hybrids interesting for applications as molecular switches.

Tunable Photoswitching in Norbornadiene (NBD)/Quadricyclane (QC) – Fullerene Hybrids

“…Several conclusions can be drawn from this, 1) the fullerene is definitely involved in the back‐reaction, 2) close contact between the two building blocks is a prerequisite, 3) within the investigated concentration regime an intramolecular mechanism is predominant, 4) the photoisomerization NBD to QC via irradiation of the push‐pull substituted NBD systems is much more efficient than the back‐reaction involving the fullerene core. In our previous publication we proposed that C 60 acts as an excited state electron acceptor that oxidizes QC and thus initiates the back‐reaction [7a] . This hypothesis is further strengthened not only by the observations made in this study but also by calculating the HOMOs and LUMOs of the QC – fullerene hybrids (Figure 5 and Supporting Information).…”

“…NBD – C 60 1 : The preparation as well as the 1 H and 13 C NMR spectra of NBD – C 60 1 has already been reported in our earlier publication [7a] . All spectroscopic data were in agreement with those previously published.…”

“…The fastest back‐reaction is observed for QC – C 60 1 (flexible linker), with only 33 % of QC – C 60 1 remaining after five hours. In our earlier publication we showed already that this reaction can be driven to completion [7a] . In contrast, the back‐reaction of QC – C 60 3 (long‐rigid linker) to NBD – C 60 3 proceeds much slower and after five hours still 87.5 % of the QC derivative remain unconverted.…”

“…Among these examples, our earlier publication on NBD/QC – fullerene hybrids stands out as the only example where light of a longer wavelength, than the one used for the isomerization of NBD to QC, can be used to promote the back‐reaction (Scheme 2 ). [7a] This discovery encouraged us to investigate these hybrids as well as the underlying processes during the interconversion in greater detail. Our particular focus of this study is the influence of the rigidity and extension of the linker between the two chromophores.…”

“… Light‐only‐controlled interconversion of an NBD/QC – fullerene hybrid as reported in our earlier publication. [7a] Adapted with permission from P. Lorenz, A. Hirsch, Chem. Eur.…”