Photoresponsive crown ethers. Part 20. Reversible photocontrol of association-dissociation equilibria between azobis(benzo-18-crown-6) and diammonium cations

The solution properties of complexes formed from azo(benzo-18-crown-6) (1) and polymethylene-α,ω-diammonio cations [H₃N⁺ (CH₂)_nNH₃⁺ (2)] have been evaluated through measurements of average molecular weights, viscosity, n.m.r. spectra, and electrical conductance. The trans compound (1) and (2; n = 6) forms a polymeric complex with N_av (average aggregation number) = 20 while trans-(1) and (2; n = 12) form a 1:1 pseudocyclic complex. The difference is related to the geometrical fitness of the two terminal ammonium cations to the two crown rings in trans-(1), i.e. when the distance between the two ammonium cations in (2) is shorter than that between the two crown rings in (1), they form a polymeric complex. When the two distances are comparable, they form the 1:1 pseudocyclic complex. The n.m.r. chemical shift of the methylene protons in the 1:1 trans-(1)-(2; n = 12) complex move to higher magnetic field by 0.10 - 0.58 p.p.m., findings which support the view that in the complex these methylene protons lie exactly on the azobenzene moiety of trans-(1). Photoisomerised cis-(1) shows different aggregation modes because of the change in the distance between the two crown rings: a 1:1 complex for cis-(1) + (2;n = 6) and a 2:2 complex for cis-(1) + (2;n = 12). The photoinduced depolymerisation from the polymeric complex to the low molecular weight 1:1 complex for (1) + (2; n = 6) has been detected by viscosity measurements: η_sp/C = 0.440 for trans-(1) + (2; n = 6) and 0.354 for cis-(1) (cis%70 - 78%) + (2; n = 6). The photoresponsive change in the aggregation mode is accurately reflected by the change in the electrical conductance. The conductance for (1) + (2; n = 6) increases on u.v. irradiation and decreases on visible light irradiation. This photoresponsive wave can be reproduced many times by alternate irradiation with u.v. and visible light. The reverse photoresponsive wave is observed for (1) + (2; n = 12). The conductance changes were well correlated with the changes in the aggregate size of these complexes. This is a novel example of reversible interconversion of polymers and low molecular weight pseudomacrocycles and of the transmission of light energy to electrical conductance.