CIS-TRANS ISOMERIZATION OF AZOBENZENE COMPLEXED WITH AMPHIPHILIC CYCLODEXTRINS IN VARIOUS MEDIA

The microscopic environment of the host-guest complexation with p-methyl red (p-MR) and amphiphilic amino-α-, β-, γ-cyclodextrins (CnN-CDs) have been investigated on the basis of cis-trans isomerization of p-MR in the Langmuir-Bloodgett (LB) films, cast film, and chloroform solution. In chloroform solution, the cis-isomer of p-MR (cis-p-MR) in the complex with C16N-CDs was about 105-fold more stable compared with that of p-MR alone and reverted thermally to its trans-isomer by the first-order process (Fig. 2, Table 1). The stability of cis-p-MR in the complex increased with an increase in the ratio of C16N-CDs: p-MR (Fig. 3). Incorporation of cis-p-MR in the cavity of C16N-βCD, heptakis(6-hexadecylamino-6-deoxy)-β-cyclodextrin, was estimated to be 75% in the equimolar mixture solution by calculations from rate constants. In the LB films and cast film of the complex, the thermal isomerization deviated from the first-order kinetics, and cis-p-MR was more labile than that in the solution (Fig. 2, Table 1). It has been found that the stability and kinetics of cis-p-MR is dependent on free volume and flexibility in the molecular assemblies, and that amino moieties of Cl6N-CDs are essential for the stabilization and also for a tight inclusion of cis-p-MR into the cavity. © 1990, The Chemical Society of Japan. All rights reserved.