The colourless imidazolyl-quinoline derivatives 1 and 2 are prepared using a Debus−Radziszewski method, which proves applicable to phenanthrene- and phenanthroline-based precursors as well as a range of functionalized anilines, where X = H, OMe, Cl and Br (pictured). Following ultraviolet (UV) irradiation of 1 in a mixed dichloromethane and methanol solution for 24 h, a dark purple species, 3, is obtained in 73% yield. Similarly, 2 could be converted to 4 (bright red species) in 69–93% yields. Crystallographic analysis of 4 (where X = H) confirms the 1,4-aryl transfer reaction.
The absorption and emission of the zwitterionic products 3 and 4 are substantially red-shifted compared to 1 and 2, which can be explained by the extended π-conjugation on the quinoline moiety. Additionally, testing the absorbance of 1 and 3 in a range of solvents reveals that an increase in solvent polarity leads to a blue shift in the absorption bands, which for 3, follows the trend of negative solvatochromism generally found in zwitterionic chromophores.
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