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

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 %) under the photostationary state is enhanced and the rate of the thermal cis-to-trans isomerization (k) is suppressed, the optima of 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 % and k of 2(5,5) and 2(6,6) are affected by alkali metal cations smaller than those of corresponding 1(5,5) and 1(6,6), and (iv) cis % and k of 1(4,4) are less affected by added alkali metal cations and the extractability is very low. The increase in cis % and the suppression 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 % 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 spacial cavity provided by two crown ethers of cis forms. The ion selectivity suggests that (i) the cavity size of cis-1(6,6) is greater than that of cis-1(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.