EFFECT OF BETA-CYCLODEXTRIN ON THE THERMAL CIS-TRANS ISOMERIZATION OFAZOBENZENES

The cis-trans thermal isomerization of p-methyl red (1), o-methyl red (2), and methyl orange (3) was inhibited by beta-cyclodextrin (beta-CD ) at constant pH. Their isomerization rate decreased 4, 8, and 1.67 ti mes, respectively, in a solution containing 0.01 M beta-CD. This effec t can be attributed to the formation of an inclusion complex between t he substrate and beta-CD which hinders the rotation of the N=N bond. T he isomerization rate of methyl yellow (4), 4-(dimethylamino)-4'-metho xy-azobenzene (5), and naphthalene-1-azo[4'-(dimethylamino)benzene] (6 ) was not affected by beta-CD due to the presence of an organic cosolv ent in the solution which displaces the azobenzene from the cavity, an d the complex formed is probably equatorial. In addition, the transiti on 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 beta-CD.