Novel Method for the Synthesis of Merocyanines: New Photophysical Possibilities for a Known Class of Fluorophores

A new, transformative methodology for the preparation of rhodols and other merocyanines from readily available tetrafluorohydroxybenzaldehyde and aminophenols has been developed. It is now possible to prepare merocyanines bearing three fluorine atoms and additional conjugated rings and the whole one-pot process occurs under neutral, mild conditions. Three heretofore unknown merocyanine-based architectures were prepared using this strategy from aminonaphthols and 4-hydroxycoumarins. The ability to change the structure of original rhodol chromophore into π-expanded merocyanines translates to a comprehensive method for the modulation of photophysical properties such as shifting the absorption and emission bands across almost the entire visible spectrum, reaching a huge Stokes shift i.e. 4800 cm-1, brightness ca. 80,000 M-1 cm-1, two-photon absorption cross-section above 150 GM and switching-on/off solvatofluorochromism. A detailed investigation allowed to rationalize the different spectroscopic behavior of rhodols and novel merocyanines, addressing solvatochromism and two-photon absorption.