Here is a piece from a patent that describes some of the new technology that can be used to photoswitch a polymer, It envolves using chiral nematic liquid crystal molecules doped with chiral dyes , the color is switched using a wavelength of electromagnetic radiation as to allign the molecules in which changes the band gap of light absorbed reflected.
The main principle of the development of such light-controllable liquid crystal is based on the synthesis of photochromic copolymers whose macromolecules consist of mesogenic (as a rule, nematogenic) and combined phototunable chiral dopant (PTCD) groups, which are chemically linked in the common monomer unit. In this case, mesogenic fragments are responsible for the formation of the nematic phase, chiral groups provide the twisting of the nematic phase and formation of helical supramolecular structure. Finally, photo-tunable chiral dopant fragments can easily change their molecular structure under the light irradiation.
The irradiation with a certain wavelength leads to the photo-induced transformations of the photo-tunable chiral dopant affecting both the configuration and shape of the sidechain group. This leads to a decrease both in the anisotropy of photo-tunable chiral dopant group and the helical twisting power of a given chiral group. A decrease in helical twisting power leads to the untwisting of the cholesteric helix, which is accompanied by a shift in the selective light reflection maximum to longer wavelengths. Thus, using light irradiation as the external control factor, one may effectively modify the optical properties of polymer films by changing the local supramolecular helical structure.
As noted above, selective adjustment of the pitch length of a liquid crystal, and hence the color reflected thereby, can be accomplished by using photo-tunable chiral dopants (PTCDs). Irradiation of a photo-tunable chiral dopant with, for example, ultra violet (LTV) light or other high energy source such as laser, results in conversion of chiral phototunable chiral dopant to an achiral molecule or to a racemic mixture. When one or more photo-tunable chiral dopants are included in a chiral nematic liquid crystal material, the pitch length of the resulting liquid crystal mixture can be either extended or shortened by varying exposures to UV light. By irradiating different regions of the material with different amounts of UV through the use of masking techniques, the pitch lengths of each region can be tuned to reflect a different color, thereby creating different colored pixels or regions of spot color in the liquid crystal material itself.