“Xanthene” is a premium bridging group for xanthenoid dyes

Novel xanthenoid dyes by replacing the central oxygen atom of the xanthene dyes with less electron-rich bridging groups have been intensively sought after primarily for their long spectral wavelengths. However, the new scaffolds are likely prone to nucleophilic attack at their central methane carbon, as the result of the reduced electron density of the fluorochromic scaffolds. We envisage that the bridging group may be harnessed to sterically shield the central methane carbon from incoming nucleophiles and render high stability and synthesized xantheno-xanthene dyes. Additionally, the xantheno-bridging group can be modified via electrophilic aromatic substitution to introduce functionalities, e.g., sulfonate groups.