Photoinduced structural changes in poly(4-vinyl pyridine): A luminescence study

In the present work we show a way of controlling photoluminescence (PL) pro perties through photoinduced quasi-crystal formation in a system based on p oly(4-vinyl pyridine) (P4VPy). Under UV irradiation at 380 nm, concentrated solutions of P4VPy in pyridine turn into gel. This phase transition result s in changes in the optical properties of this polymer. The position of the PL maximum can be changed continuously from 440 to 480 nm during irradiati on. After several minutes of UV irradiation a new red-shifted PL at 492 nm appears upon excitation by light of a wavelength corresponding to that of t he initial PL maximum, which is also red-shifted during irradiation. Soluti ons of P4VPy in pyrimidine show similar behavior, but those in pyridazine d o not exhibit such behavior. We have found that the reason for the observed changes in the electronic properties is a photoinduced directional orderin g of polymer molecules in a special quasi-crystal formation. The process or iginates from a structural change in the side chain of P4VPy, namely, proto nation of the polymeric pyridine after solvation. During irradiation, the p olymeric pyridinium ion interacts with neutral polymeric pyridine molecules . Interchain interaction through hydrogen bonds lead to an electronic prope rty change. We observed that the process of photoinduced sol-gel transforma tion is reversible. Mechanical perturbation or heating can convert the gel back to a fluid solution. The red-shifted PL is not observed, and the initi al PL is blue-shifted to 450 nm and stays there.