Optical and Chiroptical Switches Based on Photoinduced Photon and Proton Transfer in Copolymers Containing Spiropyran and Azopyridine Chromophores in Their Side Chains

New methacrylic copolymers, bearing in the side chain spiropyran moieties and/or the optically-active (S)-3-hydroxy pyrrolidinyl group linked through the nitrogen atom to an azopyridine chromophore, along with an opportune molar content of methyl methacrylate co-units, have been prepared by radical polymerization of the corresponding monomers. The resulting macromolecules have been fully characterized with particular attention to the study of their thermal stability, optical activity, chiroptical and photoinduced properties. In the presence of acid, it is possible to modulate the protonation of the azopyridine groups by photo-isomerisation (with UV and/or Vis light) of the spiropyran component. The resulting signal communication between these macromolecular switches can be monitored by UV-VIS as well as CD spectroscopy and is completely reversible and reproducible. The results are discussed in terms of copolymer composition and different cooperative behavior, which lead to a variation of interactions between spiropyran and azoaromatic chromophores and reveal that the best proton transfer is actually obtained in the random terpolymer with lower amounts of co-units of spiropyran and azopyridine chromophores in the side chain, which displays improved sensitivity to proton-transfer process and, in addition, exhibits good stability to repeated cycles of irradiation with UV and Vis light.