High Sensitivity Viologen for a Facile and Versatile Sensor of Base and Solvent Polarity in Solution and Solid State in Air Atmosphere

Viologen
cations are excellent electro- and photochromic materials. They generally
have no response or very low sensitivity to bases. In this paper,
three compounds, 1,1′-bis­(2-oxo-2-phenylethyl)-4,4′-bipyridinium
(viologen) with different substituents, including H (1), Cl (2), and OH (3), were synthesized.
All three, especially 1 and 2, have very
high sensitivity to base in both solution and solid state in air atmosphere.
These viologens are responsive not only to bases but also to solvent
polarity. NMR shows 1 became enolic and then a radical,
whereas 3 is colored only in the radical form. These
results are in agreement with EPR spectra. Crystal structures show
that the C–C that links two pyridinium and N–C distances
in coplanar pyridinium in the colored (radical) form is clearly longer
than that of the pale-yellow form, indicating that the color is due
to the viologen radical. Viologens containing an electron-withdrawing
phenacetyl group are the most sensitive compounds for fast, naked
eye detection of base and solvent polarity.

紫精（Viologen）阳离子是一类性能优异的电致变色与光致变色材料。该类化合物通常对碱无响应，或仅表现出极低的碱响应灵敏度。本文合成了三种带有不同取代基的1,1′-双(2-氧代-2-苯乙基)-4,4′-联吡啶鎓（紫精）类化合物，取代基分别为H（化合物1）、Cl（化合物2）与OH（化合物3）。三种化合物，尤其是化合物1与2，在空气氛围下的溶液与固态状态中，均对碱表现出极高的响应灵敏度。此类紫精不仅可响应碱环境，还能响应溶剂极性的变化。核磁共振波谱（NMR）测试结果显示，化合物1会先转化为烯醇式结构，随后形成自由基；而化合物3仅在自由基形态下才会呈现颜色。上述结果与电子顺磁共振波谱（EPR）的测试结果一致。晶体结构分析表明，在显色（自由基）形态下，连接两个吡啶鎓单元的C–C键以及共平面吡啶鎓环内的N–C键长，均显著长于浅黄色形态的对应键长，这证明颜色的产生源于紫精自由基的形成。带有吸电子苯乙酰基的紫精类化合物，是用于快速裸眼检测碱环境与溶剂极性的高灵敏度传感材料。