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Double‐Bond Shifts in [4 n ]Annulenes as a New Principle for Molecular Switches: First Results with Dimethyl Heptalene‐1,2‐ and ‐4,5‐dicarboxylates
Author(s) -
Briquet Anne Andrée Sophie,
Uebelhart Peter,
Hansen HansJürgen
Publication year - 1996
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19960790819
Subject(s) - chemistry , annulene , substituent , moiety , stereochemistry , double bond , crystallography , organic chemistry
A new concept for molecular switches, based on thermal or photochemical double‐bond shifts (DBS) in [4 n ]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 CC bond of the ( E )‐styryl moiety and the adjacent CC bond of the heptalene core. Supplementary 1 H‐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 CC 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 ).