Photoisomerization studies of azobenzene crown ether calix [4] arenes for applications in alkali metal ion extraction

Abstract

Two new compounds, derivatives of azobenzene crown ether calix[4]arenes 5 and 8 were prepared by two pathways. In the first pathway, two nitrophenoxy glycolic chains were attached to t-butylcalix[4]arenes ring in a 1, 3 alternated position. Subsequent reduction of the nitro groups by Zn in an alkaline solution afforded 5 and 8 in 8% and 12% yields, respectively. In the second pathway, the azobenzene containing two glycolic chains was prepared prior to its couple to the t-butylcalix[4]arenes. The yields from the second approach (5%, 8%) were a little lower than those from the former approach. Single crystals of both 5 and 8 can be obtained by recrystallizations in methanol. However, only the crystal of 5 was suitable for X-rays crystallography. Both X-rays and 1H-NMR results indicated that, in the crystal, the stereoisomer of azobenzene moiety of 5 was trans and the calixarene platform existed as a cone conformation. On the other hand, the 1H-NMR spectrum of 8 suggested that ligand 8 was initially isolated as a cis-azobenzene with cone-calixarene. Ligand 5 underwent an observable cis-trans isomerization upon standing in the chloroform solution under the room light or a UV mercury lamp. The concurrent conformational change of the calixarene platform complicated the observation of the cis-trans isomerization of ligand 8 under the same condition and it was difficult to justify if the cis-trans isomerization had occurred. The complexation studies suggested that Na+ preferred a binding with the cis-form of both ligands while K+ initially preferred a binding with trans-form