Arresting “Loose Bolt” Internal Conversion from −B(OH)₂ Groups is the Mechanism for Emission Turn-On in ortho-Aminomethylphenylboronic Acid-Based Saccharide Sensors

Different mechanisms for the emission turn-on of ortho-aminomethylphenylboronic acids with appended fluorophores in response to saccharide binding in aqueous media have been postulated, such as photoinduced electron transfer (PET), “pK_a switch”, and disaggregation. However, none of the hypotheses is consistent with all the data for boronic acid–based sensors. To create a unifying theory that can explain the data, we performed a series of experiments to explore the origin of the emission turn-on with several boronic-acid based sensors upon binding fructose. First, we showed that the receptors and their complexes with fructose are solvent-inserted, with no B–N interactions. Second, we verified that the sensors are not aggregated. Third, in pure methanol, that exchanges −B(OH)₂ to −B(OMe)₂ groups, we found no fluorescence response upon binding fructose. We propose this occurs via lessening of internal conversion mechanisms. To investigate this proposal further, we performed a solvent isotope effect study. The fluorescence of the probes in D₂O (−B(OH)₂ → −B(OD)₂) does not change upon fructose binding. It is well accepted that −OD oscillators are less efficient energy acceptors due to their lower frequency vibrational modes. Thus, our studies reveal that modulating the −B(OH)₂-induced internal conversion (an example of a “loose bolt effect”) explains how potentially all ortho-aminomethylphenylboronic acid-based fluorescence sensors signal the presence of sugars.