Abstract:
Two new azobenzene crown ether calix[4]arenes, 10 and 11, were synthesized by two pathways. In the first pathway, two ethoxy nitrobenzene groups were attached to t-butylcalix[4]areanes in a 1,3 position. Subsequent reduction of the nitrobenzene groups by metallic zinc in an alkaline solution afforded 10 and 11 in 8% and 12%, respectively. In the second pathway, an azobenzene contatining two glycolic units was prepared prior connecting to t-butylcalix[4]areanes. The yields from the second approach (5%, 8% for 10 and 11, respectively) were lower than those from the former approach. Single crystals of 10 suitable for X-ray crystallography was obtained by recrystallization in methanol. Both the X-ray structure and [superscript 1]H-NMR spectra of 10 indicated that the stereoisomer of the azobenzene moiety was trans and the calixarene platform was in cone conformation. [superscript 1]H NMR spectroscopy suggested that 10 underwent an observable cis-trans isomerization in CDCl[subscript 3] under room light and upon UV irradiation with cis:trans ratios of 33:67 and 36:64, respectively. Compound 6 which was the precursor of 11 showed fluxional behavior and was found to have mixed conformations of cone and partial cone with a ratio of 47:53 at -30 degrees Celsius. [superscript 1]H-NMR spectrum of 11 suggested that 11 was initially isolated as cis azobenzene with calix[4]arene in cone conformation and underwent conformational interconversion through calix[4]arene annulas in a similar fashion 6 upon exposing to right. The complexation studies of 10 with picrate salts of Na[superscript +] and K[superscript +] using [superscript 1]H NMR spectroscopy suggested that Na [superscript+] preferred to bind the cis from of 10 while K[superscript +] preferred to bind the trans form. The stereoisomer of the azobenzene unit in 11 changed partially from cis to trans upon complexing with K[superscript +]