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.2021 Nov 17;26(22):6925.
doi: 10.3390/molecules26226925.

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

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

Evgenia Vaganova et al. Molecules..

Abstract

Cyclic 6-membered aromatic compounds such as benzene and azabenzenes (pyridine, pyridazine, and pyrazine) are known to be light-sensitive, affording, in particular, the Dewar benzene type of intermediates. Pyridine is known to provide the only Dewar pyridine intermediate that undergoes reversible ring-opening. We found that irradiation of photosensitive gels prepared from poly(4-vinyl pyridine) and pyridine at 254 or 312 nm leads to pyridine ring-opening and subsequent formation of 5-amino-2,4-pentadienals. We show that this light-induced process is only partially reversible, and that the photogenerated aminoaldehyde and aminoaldehyde-pending groups undergo self-condensation to produce cross-linked, conjugated oligomers that absorb light in the visible spectrum up to the near-infrared range. Such a sequence of chemical reactions results in the formation of gel with two distinct morphologies: spheres and fiber-like matrices. To gain deeper insight into this process, we prepared poly(4-vinyl pyridine) with low molecular weight (about 2000 g/mol) and monitored the respective changes in absorption, fluorescence,1H-NMR spectra, and electrical conductivity. The conductivity of the polymer gel upon irradiation changes from ionic to electronic, indicative of a conjugated molecular wire behavior. Quantum mechanical calculations confirmed the feasibility of the proposed polycondensation process. This new polyacetylene analog has potential in thermal energy-harvesting and sensor applications.

Keywords: aromatic heterocycle; electroconducting polymer; photochemistry; pyridine.

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Conflict of interest statement

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Figures

Scheme 1
Scheme 1
Photoinduced pyridine ring-opening and subsequent polycondensation.
Figure 1
Figure 1
UV-Vis spectra of low-MW P4VP/Py gel samples in methylene chloride before and after irradiation at 312 nm as well as after holding the 6 h irradiated sample in the dark for 24 h. Inset: fluorescence spectrum of a thin film of a sample irradiated for 6 h (excitation at 350 nm).
Figure 2
Figure 2
(a)1H NMR spectrum of P4VP/Py gel before irradiation; (b) after 1 h irradiation of P4VP/Py gel at 312 nm; (c) after 6 h irradiation at 312 nm; (d) after 6 h irradiation of P4VP/Py gel and subsequent 24 h storage in the dark.
Chart 1
Chart 1
1,6-Polyazaacetylene monomer (1) and respective oligomers (2–4):all-trans model structures.
Figure 3
Figure 3
Calculated (B3LYP/aug-cc-pVDZ//B3LYP/aug-cc-pVDZ model chemistry) absorption spectra of molecules1–4 (Chart 1) in dichloromethane; TD-SCF was solved for 10 states.
Scheme 2
Scheme 2
Stepwise polycondensation reactions of1.
Figure 4
Figure 4
Calculated ΔG of the proposed polycondensation reactions in dichloromethane and in vacuum.
Figure 5
Figure 5
Resistance dependence on the AC voltage frequency of the low-MW gel. Voltage amplitude 35 mV, thickness of the sample 230 μm, cross-section 5 × 5 mm.
Figure 6
Figure 6
Current–voltage curves of low-MW P4VP/Py gel sample (thickness 230 mm, cross-section 5 × 5 mm) before irradiation (black curve); after irradiation at 254 nm for one hour and 10 h holding in the dark (green curve); after additional one-hour irradiation and holding the sample in the dark for 10 h (blue curve andupper inset); after the third one-hour irradiation and holding the sample in the dark for 10 h (red curve andlower inset). The sweep rate was 1.7 V/s.
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