Chiral p-sulfonatocalix[n]arenes (n = 4, 6, and 8) bearing (S)-2-methylbutoxy groups (1(n)) have been synthesized. 1(n = 4) gave a simple, positive CD band, whereas 1 (n = 6) and 1(n = 8) gave split CD bands characteristic of an exciton coupling at 1La region. The sign (positive first Cotton effect and negative second Cotton effect) indicates that the chirality of excitons in these calixarenes is arranged in a clockwise direction. The difference between 1(n = 4) and the larger calixarenes 1(n = 6) and 1(n = 8) is accounted for by the difference in the ring rigidity: that is, 1(n = 6) and 1(n = 8) are flexible enough to allow interactions of excitons in the excited state, whereas 1(n = 4) is too rigid to allow the interactions. On the addition of guest molecules (aliphatic alcohols) the CD band of 1(n = 4) was scarcely changed, whereas those of 1(n = 6) and 1(n = 8) were weakened. This result, together with the 1H NMR data, suggests that the conformational fluctuation in l(n = 6) and 1(n = 8) is considerably suppressed upon inclusion of these guest molecules. In the presence of 1(n = 6) or 1(n = 8) ICD bands were also observed for certain dye molecules (e.g., 4-cyano-4′-(diethylamino)azobenzene and 4-nitro-4′-(diethylamino)azobenzene). Careful comparison of the ICD spectra with the absorption spectra established that the calixarene complexes having 2:1 guest/calixarene stoichiometry are ICD-active whereas those having 1:1 guest/calixarene stoichiometry are ICD-silent. Interestingly, the sign of the Cotton effect showed that 4- cyano-4′-(diethylamino)azobenzene included in 1(n = 6) gives a counterclockwise exciton coupling, whereas that included in 1(n = 8) gives a clockwise exciton coupling. Thus, the present study demonstrates that the CD spectral technique is very useful to obtain insights into calixarene conformations and calixarene complexation properties.
All Science Journal Classification (ASJC) codes
- Organic Chemistry