Synthesis and Photoisomerization of Azocalixarenes with Dendritic Structures

The synthesis of dendricalixarenes 1 and 2 containing azo groups at the upper rim of a calix[4]- or calix[6]arene and the corresponding model compounds 3-5 are described. Because the kinetics of the trans to cis isomerization were too fast for accurate determination of the rate constant, this parameter was obtained from the reverse cis to trans process, which was studied in two different solvents by either a photoinduced or a thermal process. On the basis of the UV/Vis absorption measurements, the global rate constants (k) for calixarenes 1b, 1c, and 2b, obtained for the two different mechanisms (photoinduced and thermal processes), are of the same order of magnitude as that for models 3b and 4b. Compound 5b with an expanded cavity recognizes N-methylpyridinium guests. The 1:1 stoichiometry for the complex of 4-(4-dimethylaminostyryl)-N-methylpyridinium iodide (DASPMI) with 5b was confirmed by Job plot analysis. Fluorescence spectroscopic titrations gave K-a = 3.76 X 10(5) M-1. As expected, receptors 1b, 1c, and 2b (with the azo groups in the trans configuration) did not lead to complexation of any of the substrates. Similarly, conversion into the respective cis isomers caused structural changes that were too large to allow efficient complexation.