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.