Effect of β-Cyclodextrin on the Thermal Cis−Trans Isomerization of Azobenzenes

The cis−trans thermal isomerization of p-methyl red (1), o-methyl red (2), and methyl orange (3) was inhibited by β-cyclodextrin (β-CD) at constant pH. Their isomerization rate decreased 4, 8, and 1.67 times, respectively, in a solution containing 0.01 M β-CD. This effect can be attributed to the formation of an inclusion complex between the substrate and β-CD which hinders the rotation of the NN bond. The isomerization rate of methyl yellow (4), 4-(dimethylamino)-4‘-methoxyazobenzene (5), and naphthalene-1-azo[4‘-(dimethylamino)benzene] (6) was not affected by β-CD due to the presence of an organic cosolvent in the solution which displaces the azobenzene from the cavity, and the complex formed is probably equatorial. In addition, the transition state for the isomerization of compounds 1−3 involves rotation and that of 4−6, which have only electron-donating groups, inversion. This latter process brings about less volume change than rotation so it is less hindered by the complexation with β-CD.