Single Benzene Yellow Fluorophore: Anodic Synthesis of Tetrahydrobenzodifuran

Abstract

Fluorophores have proven to be indispensable tools in fields ranging from physics, chemistry, and biology. Organic fluorophores have high degrees of freedom in their molecular designs, which is essential for fundamental applications such as imaging probes. The strategies to produce organic fluorophores with improved physicochemical properties are two-fold, including an extension of aryl systems and a construction of push-pull motifs, where electron-donating and -withdrawing groups are conjugated. When used in bio-related research fields, the construction of push-pull motifs is preferred in terms of water solubility and molecular size, since the extension of aryl systems produces relatively large hydrophobic, planar, and thus highly crystalline, structures.The early work by Shimizu in 2009 triggered explosive growth in the development of new single-benzene fluorophores. From the viewpoints of synthetic practicality and unique and tunable photophysical properties, installing diagonally symmetrical substitution patterns with the same electron-donating and -withdrawing groups is the simplest strategy to create new single-benzene fluorophores.During the course of our studies focusing on electrochemical cycloadditions, we found that dihydrobenzofuran derivatives armed with electron-donating and -withdrawing groups exhibited bright fluorescence. In order to suppress molecular motions that cause non-radiative transitions, formation of a furan ring is effective in restricting bond rotation. Furthermore, we found that symmetrically substituted dialkoxy-dicarbonylbenzenes exhibited unique bright fluorescence. In this study, we successfully demonstrate that constructing symmetrical push-pull motifs in combination with restricting bond rotations is the key to creating small yet brightly emitting fluorophores. The anodically constructed tetrahydrobenzodifuran moiety has proven to be an effective core architecture to realize unique and powerful single-benzene fluorophores with high quantum yields and large Stokes shifts.