Photochromism of Crown Ethers with Incorporated Azobenzene Moiety

Abstract

The kinetics of the thermal cis-trans isomerization was determined in a series of crown ethers containing the azobenzene moiety incorporated into the crowns of various sizes (10- to 19-membered crowns), dissolved in liquid isooctane and in polymer matrixes (PMMA and polystyrene). The kinetic parameters (activation energies and pre-exponential factors) were determined from temporal evolution of the absorbance measured at one of the maxima due to the pi-pi* transition of the trans forms of the molecules, using both isothermal and nonisothermal procedures. Molecular structures and electronic spectra of both trans and cis forms were calculated employing an ab initio method with 3-21G* basis set and semiempirical ZINDO-S, respectively. A comparison of the quantum-chemical calculations with the experimental results shows that one may exclude the splitting of the UV absorption bands of the trans forms of the molecules under study that resulted from a coexistence of several conformations of these molecules. A linear relationship was found between the activation energy and logarithm of the pre-exponential factor of the thermally driven cis-trans isomerization: all molecules follow a common dependence, except for the smallest (10-membered) crown ether.

Optical Properties of Some New Azo Photoisomerizable Bismaleimide Derivatives

“…In this case the activation energy for polymer film is higher compared to solution and the catalytic effect of polymer on thermal cis-trans isomerization was not observed. Similar value of Ea was obtained for other azobenzene chromophores in PMMA matrix [38,39]. …”

Triply‐responsive (thermo/light/pH) copolymeric hydrogel of N‐isopropylacrylamide with an azobenzene‐containing monomer

“…The changes, aroused by the transformation from trans ‐ to cis ‐ in azobenzene groups, became slower and slower with increasing irradiation time till to a state, in which the change between trans and cis configuration were dynamic balance. The isomerization rate constants of AEPAZA are comparable to other azobenzene chromophores in literature 14, 22, 23. Figure 2 shows the spectral changes of copolymer gels upon UV irradiation in water, which also gave convincing evidence for the successful synthesis of the azobenzene‐containing copolymer gel.…”

Non-Exponential Decays in First-Order Kinetic Processes. The Case of"Squeezed Exponential"

“…[13,14]). The thermally driven cis-trans reaction follows the first-order kinetics in common solvents [15,16], whereas one can observe a stretched exponential behaviour in diluted solutions in polymer matrices [17][18][19]. We have found, however, that the kinetics of the same reaction in liquid crystalline matrices is, in some cases, qualitatively different and cannot be described by the equations mentioned above.…”