Anziehung oder Abstoßung? London‐Dispersionswechselwirkungen kontrollieren Azobenzol‐basierte molekulare Schalter

Abstract

Aufgrund sterischer Hinderung wird großen Substituenten im Allgemeinen eine rein abstoßende Wirkung zugesprochen. Solche Reste haben jedocha ucha nziehende Eigenschaften, die durch schwache London-Dispersionskräfte zwischen den neutralen Atomen hervorgerufen werden. In Lehrbüchern wird sterische Hinderung häufig als Grund für die Destabilisierung des Z-isomers genannt, z. B. bei Isomerisierungen wie bei Azobenzol-basierten molekularen Schaltern. Hier zeigen wir,d ass eine Vergrçßerung der Substituenten in der meta-Position des Azobenzols, unter Beibehaltung der elektronischen Struktur der Reste,d ie thermischeZ -zu-E-Isomerisierung verlangsamt. DFT-Rechnungen untermauern, dass die Energie des Z-Isomers durch anziehende Dispersionswechselwirkungen deutlichg esenkt wird.

Investigation of Alkyl–Aryl Interactions Using the Azobenzene Switch – The Influence of the Electronic Nature of Aromatic London Dispersion Donors

“…After successful synthesis of AB derivatives 1 – 9 (Scheme 2 ) their rates of the thermal ( Z )→( E ) isomerization were determined by temperature-controlled UV/Vis spectroscopy at 40 °C in n -octane (Figure 2 , Table S3, Table S4). As expected from previous research, 7 meta -di-Me-substituted ABs 1 , 4 , and 7 exhibit the shortest half-life (11.9 h, 11.8 h, 10.3 h for R 3,4 = H, OMe or NO 2 , respectively). The meta -di- n -heptyl-substituted ABs 3 , 6 , and 9 exhibit half-lives almost three times longer than the meta -di-Me ABs (31.0 h, 33.0 h, 25.9 h).…”

“…For small-to-medium-sized substituents, these intramolecular interactions are only present in the ( Z )-state and no stabilizing effects in the ( E )-state or the transition state via an inversion mechanism occur. 7 This allows the correlation of the kinetically controlled ( Z )→(E)-isomerization to thermodynamic quantities. Additionally, by placing substituents in the meta position the influence of electronical effects on the mechanism of isomerization is minimized (‘meta’ rule).…”

“…20 Schweighauser as well as Strauss et al investigated the photochemical properties of different alkyl-substituted AB derivatives and established that even flexible alkyl chains such as n -butyl can cause increased half-lives of the ( Z )-isomers, which are up to five times prolonged in comparison to their meta -Me-substituted AB-derivatives ( n -decane as solvent, Scheme 1 ). 6 7…”

Probing the Delicate Balance between Pauli Repulsion and London Dispersion with Triphenylmethyl Derivatives

“…Subsequent hydrolysis and Sandmeyer reaction should lead to Ad- 2 . Indeed, the synthesis of N -boc-3,5-di­(1-adamantyl)­aniline 17 showed an increased yield of 30%. An analysis of Hammett’s meta values indicates a correlation between the electron-donating nature of the substituents on the aryl and the yields [cf .…”

Taming a monomeric [Cu(η6-C6H6)]+ complex with silylene

“…Such intramolecular interactions have been shown to stabilize otherwise unstable systems, as for example in the case of molecules with very long C–C bonds 14 or the cis form of a substituted azobenzene. 15 …”