Thermal Cis-to-Trans Isomerization of Substituted Azobenzenes II. Substituent and Solvent Effects

Abstract

The thermal cis-to-trans isomerization rate of various azobenzenes was followed by means of spectrophotometric and flash photolysis techniques. For para-donor/para′-acceptor-substituted azobenzenes such as 4-nitro-4′-dimethylaminoazobenzene, the rate was distinctly accelerated, the activation energy decreasing with the increase in the polarity of solvents. Introduction of substituents in para positions with respect to azo group increased the rate irrespective of substituent. The effect is additive and a Hammett-type equation holds. For 4-dimethylamino-and 4-nitroazobenzenes, while the 2-methyl group accelerated the rate, the 2′-methyl group did not. The results suggest that the isomerization proceeds via inversion mechanism and the rate is controlled mainly by the resonance stabilization in the coplanar transition state. The inversion center for asymmetric azobenzenes is discussed.

Quantum Chemical Investigation of Thermal Cis-to-Trans Isomerization of Azobenzene Derivatives: Substituent Effects, Solvent Effects, and Comparison to Experimental Data

“…20 As a consequence, one obtains a V-shaped Hammett plot. 20,21 Substituents were further found to have an additive effect on the kinetics when positioned at the same ring. 20 For azobenzenes substituted on different rings, the additivity rule does not hold according to experiment.…”

A Photon‐Fueled DNA Nanodevice that Contains Two Different Photoswitches

“…450 nm) makes it difficult to obtain the cis-rich form, and the stronger conjugation as well as the electronic effects caused by these substituents decrease the thermal stability of the cis form. [11] To avoid large overlap of the p-p* and n-p* transitions in the trans form and to ensure a high thermal stability in the cis form, the ideal azobenzene derivative would have a l max at around 400 nm. We designed S-DMazo to satisfy these requirements.…”

High‐Frequency Mechanostimulation of Cell Adhesion

“…Once irradiation is terminated, the molecules return to the trans configuration. [8,11] Push-pull azobenzenes embedded in polymers exert large forces onto the surrounding polymer matrix. This process has been frequently used to generate surface relief grating in polymer surfaces.…”

“…Thes trategy of employing biointerfaces that apply an externally controlled oscillatory nanoscale stimulus on molecules in the cell membrane opens prospects for numerous applications,g iven the central role of dynamic extracellular environments in biological systems.A sal arge variety of different push-pull azobenzene chemistries are available, [8,10,11] different wavelengths and oscillation frequencies can be used to tailor molecularly defined photo-oscillatory properties of surfaces.T his will enable elucidating nanoscale mechanisms of mechanosensing at the molecular level in alarge variety of biomolecular binding systems and biological applications.…”