Photoresponsive Crown Ethers. 19. Photocontrol of Reversible Association-Dissociation Phenomena in “Tail(Ammonium)-Biting” Crown Ethers

The aggregation properties of photoresponsive crown ethers (1) having a crown ring and an ammonioalkyl (H3N+–(CH2)n−, n=4, 6, 10) group attached to the two sides of an azobenzene have been evaluated through the measurements of average molecular weights and electric conductance. Osmometric determination of average molecular weights established that all trans-isomers (trans-1 (n=4, 6, 10)) form the pseudocyclic dimers due to intermolecular crown-ammonium complexation. Photoisomerized cis-1 (n=6) and cis-1 (n=10), which have been designed so that intramolecular “biting” of the ammonio group to the crown can occur upon photoisomerization, existed as the discrete monomers. On the other hand, cis-1 (n=4), which cannot form the intramolecular “tail-biting” complex because of the short tetramethylene spacer, existed as the pseudo-cyclic dimer. This difference was well reflected by electric conductance: in 1 (n=6) and 1 (n=10), the conductance increased synchronously with trans-to-cis photoisomerization and decreased with cis-to-trans photoisomerization. Such a photoresponsive conductance change was scarcely detected for 1 (n=4). Furthermore, the additives which enforce dissociation of the dimeric species to the monomeric species suppressed the magnitude of the photoinduced conductance change. This is a novel example that the light energy is transmitted to the conductance change.