Synthesis of di- and trixanthones that display high stability and a visual fluorescence response to strong acid.

Three dixanthones (1-3) and an unprecedented C(3h)-symmetric trixanthone (4) were synthesized through a three-step approach in overall yields above 63%. These compounds possessed a planar π-conjugated system and formed tight face-to-face columnar stacks, as confirmed by single-crystal structural analysis. In comparison with xanthone, the fluorescence emissions of compounds 1-4 showed significant red-shifts, with improved quantum yields. Moreover, the fluorescence emissions of compounds 1-4 could be modulated in a strongly acidic environment without decomposition, which led to a further red-shift of the emissions, as well as enhancement of the emission intensities. These compounds have potential applications as optoelectronic materials and/or chemosensors.