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), "pKa 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.