Bodipy Dyes with Tunable Redox Potentials and Functional Groups for Further Tethering: Preparation, Electrochemical, and Spectroscopic Characterization

The preparation, spectroscopic, and electrochemical characterization of a family of 16 new bodipy dyes with tunable redox potentials and versatile functional groups is reported. Electron-withdrawing or -donating groups (Et, H, Cl, or ON) at positions C2 and C6 enabled tuning the redox potentials within a ca. 0.7 eV window without significantly affecting either the HOMO-LUMO gap or the absorption and emission spectra. Hydroxymethyl or formyl groups at the meso (C8) position in turn provided a handle for covalent tethering to receptors and biomolecules of interest, which dispenses with the more commonly used meso-aryl moiety as a means to tag molecules. The dyes can thus be coupled to both electrophiles and nucleophiles. Importantly, it is shown that meso-formyl bodipy dyes are nonemissive and have significantly lower molar extinction coefficients compared to their meso-hydroxymethyl and meso-acetoxymethyl counterparts (which in turn are bright, with emission quantum yields in the range of 0.7-1). The nonemissive meso-formyl bodipy dyes thus provide unique opportunities as fluorogenic probes of nucleophilic attack and as fluorescent labeling agents where uncoupled fluorophores will not contribute to the fluorescence background. Overall, the new bodipy dyes reported here are promising candidates for the preparation of fluorescent sensors relying on photoinduced electron transfer and may find use in a number of fluorescent-labeling protocols.