What is Best Strategy for Water Soluble Fluorescence Dyes?—A Case Study Using Long Fluorescence Lifetime DAOTA Dyes

The lipophilic nature of organic dyes complicates their effectiveness in aqueous solutions. In this work we investigate three different strategies for achieving water‐solubility of the diazaoxatriangulenium (DAOTA+) chromophore: hydrophilic counter ions, aromatic sulfonation of the chromophore, and attachment of charged side chains. The long fluorescence lifetime (FLT, τf=20 ns) of DAOTA+ makes it a sensitive probe to analyze solvation and aggregation effects. Direct sulfonation of the chromophore was found to increase solubility drastically, but at the cost of greatly reduced quantum yields (QYs) due to enhanced non‐radiative deactivation processes. The introduction of either cationic (4) or zwitterionic side chains (5), however, brings the FLT (τf=18 ns) and QY (ϕf=0.56) of the dye to the same level as the parent chromophore in acetonitrile. Time‐resolved fluorescence spectroscopy also reveals a high resistance to aggregation and non‐specific binding in a high loading of bovine serum albumin (BSA). The results clearly show that addition of charged flexible side chains is preferable to direct sulfonation of the chromophore core. Aqueous solvation of fluorescent triangulenium dyes was made possible by functionalization with charged side chains and pairing with hydrophilic counterions. Mono‐exponential fluorescence decays with fluorescence lifetimes over 18 ns in aqueous solution resulted. Fluorescence properties were largely unaffected by the presence of 600 μm bovine serum albumin in buffered solutions.