A Simple and Effective Method of Determining Lewis Acidity by Using Fluorescence

Abstract

The strength of a Lewis acid is an important parameter in establishing its utility for practical applications. Here, we report a universal fluorescence-based method for accurately determining the strength of a range of Lewis acids. Utilizing a fluorescent dithienophosphole oxide architecture, we are able to emulate the structural motif of the widely employed Gutmann-Beckett method. This allows us to precisely correlate the optical response to the strength of a Lewis acid. In comparison to commonly used methods that employ 31P NMR chemical shifts or quantum-chemical calculations, our method strongly suggests that using fluorescent Lewis acid-base adducts (FLAs) is more reliable. Moreover, our method provides a direct measure of the Lewis acidity of a given solution, independent of the Lewis base, with direct implications to the overall utility of a Lewis acid in practical applications. The theory of acids and bases has long been a key concept throughout the chemical sciences. Over the past century, Arrhenius, Brønsted, and Lowry, as well as Lewis, have developed several definitions of what constitutes an acid and a base. The Lewis acid-base theory has recently found rapidly increasing importance for chemistry of the 21st century, particularly in the areas of metal-free catalysis and materials science. Both fields have considerable societal impacts by enabling next-generation technologies and industrial processes. The strength of a Lewis acid is a critical factor in determining its practical utility—a certain threshold of Lewis acidity is required to induce catalytic chemical transformations. Here, we present a reliable method for quantifying this parameter with an easily observable response, which represents a de facto litmus test for Lewis acidity. Lewis acids coordinate to the phosphoryl group of the dithienophosphole, leading to a red shift in fluorescence emission. Correlation of the chromaticity data of the resulting fluorescent Lewis acid-base adducts (FLAs) with a dithienophosphole of normalized base strength allows for effective and reliable quantification of the strengths of a wide range of important Lewis acids.