Double-Bond Shifts in [4n]Annulenes as a New Principle for Molecular Switches: First Results with Dimethyl Heptalene-1,2- and -4,5-dicarboxylates

A new concept for molecular switches, based on thermal or photochemical double-bond shifts (DBS) in 4n]annulenes such as heptalenes or cyclooctatetraenes, is introduced (cf. Scheme 2). Several heptalene-1,2- and -4,5-dicarboxylates (cf. Scheme 4) with (E)-styryl and Ph groups at C(5) and C(1), or C(4) and C(2), respectively, have been investigated. Several X-ray crystal-structure analyses (cf. Figs. 1–5) showed that the (E)-styryl group occupies in the crystals an almost perfect s-trans-conformation with respect to the C?C bond of the (E)-styryl moiety and the adjacent C?C bond of the heptalene core. Supplementary ¹H-NOE measurements showed that the s-trans-conformations are also adopted in solution (cf. Schemes 6 and 9). Therefore, the DBS process in heptalenes (cf. Schemes 5 and 8) is always accompanied by a 180° torsion of the (E)-styryl group with respect to its adjacent C?C bond of the heptalene core. The UV/VIS spectra of the heptalene-1,2- and -4,5-dicarboxylates illustrated that it can indeed be differentiated between an ‘off-state’, which possesses no ‘through-conjugation’ of the π-donor substituent and the corresponding MeOCO group and an ‘on-state’ where this ‘through-conjugation’ is realized. The ‘through-conjugation’, i.e., conjugative interaction via the involved s-cis-butadiene substructure of the heptalene skeleton, is indicated by a strong enhancement of the intensities of the heptalene absorption bands I and II (cf. Tables 3–6). The most impressive examples are the heptalene-dicarboxylates 11a , representing the off-state, and 11b which stands for the on-state (cf. Fig.8).