Fluorescent BODIPY-Anionic Boron Cluster Conjugates as Potential Agents for Cell Tracking

A series of novel fluorescent BODIPY-anionic boron cluster conjugates bearing [B₁₂H₁₂]^2– (<b>5</b>, <b>6</b>), [3,3′-Co­(1,2-C₂B₉H₁₁)₂]⁻ (<b>7</b>, <b>8</b>), and [3,3′-Fe­(1,2-C₂B₉H₁₁)₂]⁻ (<b>9</b>) anions have been readily synthesized from <i>meso</i>-(4-hydroxyphenyl)-4,4-difluoro-4-bora-3<i>a</i>,4<i>a</i>-diaza-<i>s</i>-indacene (BODIPY <b>4</b>), and their structure and photoluminescence properties have been assessed. Linking anionic boron clusters to the BODIPY (<b>4</b>) does not alter significantly the luminescent properties of the final fluorophores, showing all of them similar emission fluorescent quantum yields (3–6%). Moreover, the cytotoxicity and cellular uptake of compounds <b>5</b>–<b>9</b> have been analyzed <i>in vitro</i> at different concentrations of B (5, 50, and 100 μg B/mL) using HeLa cells. At the lowest concentration, none of the compounds shows cytotoxicity and they are successfully internalized by the cells, especially compounds <b>7</b> and <b>8</b>, which exhibit a strong cytoplasmic stain indicating an excellent internalization efficiency. To the best of our knowledge, these are the first BODIPY-anionic boron cluster conjugates developed as fluorescent dyes aiming at prospective biomedical applications. Furthermore, the cellular permeability of the starting BODIPY (<b>4</b>) was improved after the functionalization with boron clusters. The exceptional cellular uptake and intracellular boron release, together with the fluorescent and biocompatibility properties, make compounds <b>7</b> and <b>8</b> good candidates for <i>in vitro</i> cell tracking