Abstract
BMIM PF6 (1-butyl-3-methylimidazolium hexafluorophosphate) and BMIM Tf2N (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) are two conventional room-temperature ionic liquids widely employed and investigated as reaction media. Despite the presence of the same imidazolium ring in their structure they are different in many chemical and physical properties due to the nature of the anions. The thermal cis-trans isomerization of an electronically activated azobenzene have been used as reaction model to compare the behavior of PF6− and Tf2N−. Rotation is the mechanism by which the investigated azobenzene is converted into the trans isomer spontaneously in the dark both in BMIM PF6 and in BMIM Tf2N. The kinetic rate constants of the process have been determined at different temperatures and the activation energies of the reaction have been calculated according to the Arrhenius and Eyring equations. The results presented herein highlight different solute-solvent interactions involving the PF6− and Tf2N− anions during the cis-trans isomerization.
Highlights
Despite the existence of a large variety of photoreactive compounds and light-switchable devices, azobenzene and its derivatives still represent relevant molecules due to their spectroscopic properties and isomerization mechanisms
V-shaped Hammett plots have been obtained in high polar ionic liquids, as BMIM Tf2 N, while linear plots have been observed in high viscous ionic liquids, as BMIM PF6, suggesting a change of mechanism, from rotation to inversion, in the presence of electron-withdrawing
The obtained Ea values are similar to those observed for the cis-trans conversion of azobenzene in conventional solvents, corresponding to 84–104 kJ/mol [35]. These data are in agreement with the activation energy and frequency factor reported for 3-butyl-1-methyl-2phenylazoimidazolium, corresponding to 85 ± 4 kJ/mol and (1.8 ± 1.7) × 1011 s−1 in BMIM PF6 and 85 ± 3 kJ/mol and (1.3 ± 1.0) × 1011 s−1 in BMIM Tf2 N [36]
Summary
Despite the existence of a large variety of photoreactive compounds and light-switchable devices, azobenzene and its derivatives still represent relevant molecules due to their spectroscopic properties and isomerization mechanisms. As well as the steric and electronic effects of a substituent, can promote one mechanism over the other [7]. Typical V-shaped Hammett plots have been obtained in different media for the cis-trans isomerization of monosubstitued azobenzenes confirming that rotation is favored by an electron-donating group, while inversion is associated to an electron-withdrawing substituent [8,9,10]. V-shaped Hammett plots have been obtained in high polar ionic liquids, as BMIM Tf2 N, while linear plots have been observed in high viscous ionic liquids, as BMIM PF6 , suggesting a change of mechanism, from rotation to inversion, in the presence of electron-withdrawing
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