We have found that photoinduced pyridine ring cleavage-closure occurs in polymeric viscous solution, because in certain polymer solutions, pyridine can serve as a photo-modulated crosslinker. We suggest this reaction as a way to control a polymer's optical properties. Irradiation of the system: poly(4-vinyl pyridine)/pyridine/water with 250-nm wavelength range leads to the appearance of a new absorption band centered at 360 nm, new red-shifted emission, and HOMO-LUMO band gap changes. The subsequent irradiation with 360 nm (the new absorption band maximum) leads to reversion almost to the initial stage. A main active product of the photoreaction is aldehyde enamine, which has two active groups: primary amine and aldehyde, which can associate with the polymer molecules to form a physical crosslinked supramolecular structure. We evaluated the activation energy of the pyridine ring cleavage and back reaction depending on the polymer/pyridine/water ratio and by changing the polymer structure. The activation energy of pyridine's ring cleavage in viscous polymeric solutions is in the range of 0.6–3.2 Kcal/mol. The activation energy of the back reaction is significantly lower and is in the range of 0.05–0.15Kcal/mole.

The photochemical reaction in poly(4-vinyl pyridine)/pyridine compositions under irradiation at 254 nm results in new electronic and structural properties. Photoinduced electronic and structural properties can be assigned to the appearance of the pyridine ring-opening photoproducts and interaction of the latter between themselves and with pyridine moieties. Doping of the composition with hydrochloric acid or lithium chloride can control the photochemical reaction, especially the interaction of the ring-opening photoproducts. The cleaved form of the compounds and the aggregates of the latter act as the crosslinkers in the system. Optical property changes are followed by structural changes. Depending on the doped compound, different long-wave-shifted emissions arise in the compositions after irradiation. The far long-wave-shifted emission appears in the system doped with lithium chloride, ­which is unstable and disappears shortly after cessation of irradiation, while the red-emitting centers of the undoped system are stable and exhibit a second harmonic generation (SHG) signal.