Ion-doping role in photoinduced processes in pyridine-containing polymeric viscous solutions

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 systems are stable and exhibit a second harmonic generation (SHG) signal.