What Can Be Learned from the Polymerization of a Pyridine‐Based Two‐Component System

Abstract

Stable poly (4‐vinyl pyridine)/pyridine gels prepared by dissolving solid polymer in pyridine (the equimolar ratio of the pyridine moieties to the pyridine solvent molecules) exhibit sensor properties rapidly and reversibly increasing their conductivity upon irradiation by light in the UV‐Vis and NIR ranges. Both ionic and electronic mechanisms of conductivity were detected. The gels behave as polyelectrolytes involving the positively charged pyridine moieties and negatively charged saturated chains held together by a network of hydrogen bonds between the charged and neutral species. This interpretation is corroborated by the cryo‐TEM images of the gel samples resembling chains of beads and the analysis of FT‐IR and 1H‐NMR spectra. Photoinduced proton transfer can thus be responsible for the observed ionic conductivity of the gels. Exposing gels to the ambient light can bring about ring opening of the pyridine moieties and of the free pyridine molecules with subsequent polycondensation of the initially formed 5‐amino‐2,4‐pentadienal. The oligomers of the latter, aza‐analogs of polyacetylene, can serve as molecular wires providing the electronic component of electrical conductivity. These oligomers can also contribute to the ionic conductivity and further stabilize the gels by cross‐linking of the separate polymer chains.

Light-Induced Reactions within Poly(4-vinyl pyridine)/Pyridine Gels: The 1,6-Polyazaacetylene Oligomers Formation

“…No more changes in absorption were observed after prolonged storage in the dark. The fluorescence spectrum of a thin layer of the gel sample irradiated for 6 h shown in the inset of Figure 1 covers the whole visible range and resembles the fluorescence spectrum of the irradiated pyridine samples [12]. The intensity of several peaks and shoulders observed in the fluorescence spectrum of irradiated pyridine depend on the excitation wavelength, and each of them belongs to a different soluble oligomer chain length (Supplementary Materials, Figure S2).…”

“…Recently, we suggested that the appearance of the multi-band emission spectrum of pyridine, irradiated at 254 or 312 nm, cannot be explained by the formation of 5-amino-2,4-pentadienal alone and suggested that this aminoaldehyde undergoes oligomerization [12]. Gaining deeper insight into the photochemical processes within pyridine and its derivatives is important, in particular, since polyvinyl pyridines are known to exhibit potential for several industrial applications.…”

“…The physical aggregation of polymer chains was shown to occur due to the presence of the hydrogen bond network, and these gels exhibit not only high ionic conductivity (polyelectrolyte behavior), but also enhanced photoinduced electron mobility [15,16]. The P4VP/Py gel was shown to exhibit unusual emission with the maxima at 477, 527, and 584 nm, explained by photoinduced proton transfer [17], which resembled the emission from the irradiated pyridine [12]. Here, we report on our investigation of the photochemical behavior of low-molecular-weight poly(4-vinyl pyridine)/pyridine (P4VP/Py) gels.…”