Experimentally determining the strength of a Lewis acid is a highly desirable and important task that has implications across the chemical sciences. Recently, we developed a new fluorescence-based method for evaluating the relative acidity of a small series of Lewis acids across the p- and d-blocks of the periodic table with great precision against a series of Lewis basic fluorescent dithienophosphole oxide probes. In this report, we considerably expand the scope of the fluorescent Lewis adduct method by systematically investigating the apparent acidities of more than 50 Lewis acids in toluene. Notably, a number of the investigated Lewis acids have never been experimentally measured before. Our refined guide, which now also alleviates the uncertainties that we identified with our original method, is simple and reliable. It shows extreme sensitivity to small structural or electronic perturbations and can account for coordinative flexibility or aggregation events that occur in solution, providing an alternative method for Lewis acidity determination that is complementary to the established nuclear magnetic resonance-based methods.

The evaluation of the chromaticity of a series of fluorescent Lewis adducts with highly luminescent dithienophospholes enables the unequivocal determination of the strength of Lewis acids. Our method provides a simple, robust, and efficient avenue to discern an exhaustive list of species from across the period table.

The evaluation of the chromaticity of a series of fluorescent Lewis adducts with highly luminescent dithienophospholes enables the unequivocal determination of the strength of Lewis acids. Our method provides a simple, robust, and efficient avenue to discern an exhaustive list of species from across the period table.