Kinetic Study of Thermal Z to E Isomerization Reactions of Azobenzene and 4-Dimethylamino-4′-nitroazobenzene in Ionic Liquids [1-R-3-Methylimidazolium Bis(trifluoromethylsulfonyl)imide with R=Butyl, Pentyl, and Hexyl]

Thermal Z to E isomerization reactions of azobenzene and 4‐dimethylamino‐4′‐nitroazobenzene were examined in three ionic liquids of general formula 1‐R‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide (R=butyl, pentyl, and hexyl). The first‐order rate constants and activation energies for the reactions of azobenzene measured in these ionic liquids were consistent with those measured in ordinary organic solvents, which indicated that the slow isomerization through the inversion mechanism with a nonpolar transition state was little influenced by the solvent properties, such as the viscosity and dielectric constant, of ionic liquids. On the other hand, the rate constants and the corresponding frequency factors of the Arrhenius plot were significantly reduced for the isomerization of 4‐dimethylamino‐4′‐nitroazobenzene in ionic liquids compared with those for the isomerization in ordinary organic molecular solvents with similar dielectric properties. Although these ionic liquids are viscous, the apparent viscosity dependence of the rate constant could not be explained either by the Kramers–Grote–Hynes model or by the Agmon–Hopfield model for solution reactions. It is proposed that the positive and the negative charge centers of a highly polar rotational transition state are stabilized by the surrounding anions and cations, respectively, and that the ions must be rearranged so as to form highly ordered solvation shells around the charge centers of the reactant in the transition state. This requirement for the orderly solvation in the transition state results in unusually small frequency factors of 104–107 s−1. Establishing order: Very small frequency factors of the Arrhenius plots for the E to Z isomerization reactions of 4‐dimethylamino‐4′‐nitroazobenzene in three ionic liquids were found (see figure). This was explained by proposing that a rearrangement of the solvation environment around the charge centers of the activated complex occurs to stabilize the polar transition state and that this requirement for order results in a retardation of the reaction.