Photoresponsive Crown Ethers. 4. Influence of Alkali Metal Cations on Photoisomerization and Thermal Izomerization of Azobis(benzocrown ether)s

Article abstract of DOI:10.1021/ja00371a027

Five photoresponsive azobis(benzocrown ether)s 1(4,4), 1(5,5), 1(6,6), 2(5,5) and 2(6,6), were synthesized, where numbers in parentheses denote the number of oxygens in benzocrown ethers, and crown series 2 have two tert-butyl groups in the 2 and 2' position of the azo linkage.We have found that (i) in the presence of alkali metal cations, the concentration of cis isomers (cis percent) under the photostationary state is enhanced and the rate of thermal cis-to-trans isomerization (k) is supressed, the optima cis-1(5,5) and cis-1(6,6) being observed for Rb⁺ and Cs⁺, respectively, (ii) Rb⁺ and Cs⁺ are extracted most efficiently from an aqueous solution to an organic phase (o-dichlorobenzene) with cis-1(5,5) and cis-1(6,6), respectively, (iii) cis percent and k of 2(5,5) and 2(6,6) are affected by alkyli metal cations smaller than those of corresponding 1(5,5) and 1(6,6), and (iv) cis percent and k of 1(4,4) are less affected by added alkali metal cations and the extractability is very low.The increase in cis percent and the supression of the thermal isomerization rate are rationalized in terms of the "tying effect" of complexed cations requiring the additional energy to disrupt the crown-cation interaction.The correlation of the extractability with cis percent and k implies that alkali metal cations are extracted under photoirradiation as intramolecular 1:2 cation/crown complexes and the ion selectivity is associated with the fitness between the ion size and the size of the spatial cavity provided by two crown ethers of cis form.The ion selectivity suggests (i) the cavity size of cis-1(6,6) is greater than that of cis-(5,5) and (ii) the cavity sizes of crown series 2 are smaller than those of crown series 1 probably owing to the steric repulsion of the tert-butyl groups.Hence, the ion selectivity in the photocontrolled solvent extraction is effected by the crown ring size and the steric crowding around the azo linkage.