Department of Molecular Chemistry and Materials Science, Weizmann Institute of Science, Rehovot 7610001, Israel.
Chemical Research Support Department, Weizmann Institute of Science, Rehovot 7610001, Israel.
Molecules. 2021 Nov 17;26(22):6925. doi: 10.3390/molecules26226925.
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, H-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.
环状六元芳环化合物，如苯和氮杂苯（吡啶、哒嗪和嘧啶），众所周知对光敏感，特别容易形成 Dewar 苯型中间体。吡啶被认为提供了唯一能经历可逆开环的 Dewar 吡啶中间体。我们发现，在 254nm 或 312nm 下辐照由聚(4-乙烯基吡啶)和吡啶制备的感光凝胶，会导致吡啶开环，随后形成 5-氨基-2,4-戊二烯醛。我们表明，这个光诱导过程仅部分可逆，光生成的氨基醛和氨基醛等待基团会发生自缩合，产生在可见光光谱直至近红外范围内吸收光的交联共轭低聚物。这种化学反应序列导致形成具有两种不同形态的凝胶：球形和纤维状基质。为了更深入地了解这个过程，我们制备了低分子量（约 2000g/mol）的聚(4-乙烯基吡啶)，并监测了各自的吸收、荧光、H-NMR 光谱和电导率的变化。聚合物凝胶在辐照下的电导率从离子变为电子，表明具有共轭分子导线行为。量子力学计算证实了所提出的缩聚过程的可行性。这种新的聚乙炔类似物在热能收集和传感器应用中有潜力。
