Solvent and substituent effects on thermal cis–trans-isomerization of some 4-diethylaminoazobenzenes

Abstract

Thermal cis-trans-isomerization of some 4'-substituted 4-diethylaminoazobenzenes has been studied. Substitution in the 4'-position leads invariably to an acceleration of the reaction regardless of the nature of the substituent. Logarithms of kinetic constants in various solvents were found to be satisfactorily correlated by the x* scale of solvent polarities both for 4'-nitro-4-diethylaminoazobenzene (I) and for 4'-methoxy-4-diet hylaminoazo benzene (11), excluding amphi protic solvents. Activation parameters (AH* and ASs) were also measured in NN-dimethylformamide. The results support an inversion mechanism rather than a rotational one; in the case of (I), the highlydipolar nature of the transition state can be justified by its stabilization through coplanar resonance structures (impossible in pure rotation).

The Kinetics of the Cis-to-Trans Thermal Isomerization of 4-Anilino-4’- Nitroazobenzene are Highly Influenced by Solvent Polarity

“…A plot of log(k) vs. relative polarity (Figure 7) exhibits a linear relationship consistent with earlier literature data for similar isomerization reactions involving other azobenzenes [18][19][20][21][22]. The increase in the rate constants of isomerization is accompanied by a decrease in activation energy which is typically the case with paradonor, para-acceptor type azobenzenes [21,22].…”

“…A strong relationship between rate of reactions and solvent polarity points to an intermediate transition state that is considerably more polar than the cis conformation. This polar transition state is indicative of a rotational mechanism for the isomerization of 4A4NAB [21,22,24]. This result is expected considering the strong electron withdrawing group (the nitro group) on 4A4NAB and is corroborated by computational studies [22,25,26].…”

“…The increase in the rate constants of isomerization is accompanied by a decrease in activation energy which is typically the case with paradonor, para-acceptor type azobenzenes [21,22].…”

Sensitivity Enhancement of Thermal-Lens Spectrometry Using Laser-Induced Precipitation

“…In this work, well-studied light-induced cis-trans isomerization of 4-aminoazobenzene (4-aab) dye in aqueous solutions 18,[25][26][27] was considered from the viewpoint of thermal-lens investigation of dynamically emerging layers at a solid surface caused by a laser-induced reaction. This analyte is reported to form an insoluble precipitate with an isomer ratio of 1:1 upon the absorption of laser irradiation; the process could be reproduced in the course of a thermal-lens experiment to accumulate the analyte.…”

Advances in understanding the photoresponsive behavior of azobenzenes substituted with strong electron withdrawing groups

“…It is generally accepted that the Z-E thermal relaxation may take place either by rotation, or by flip-flop inversion of one of the nitrogen atoms [18,19]. The rotation and inversion pathways are competitive, but one may be favored over the other, depending on the electronic nature of the substituents covalently bonded to one or both phenyl rings, and on the polarity of the reaction medium [20].…”