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.

针对水介质中糖类结合时，带有修饰荧光团的邻氨基甲基苯基硼酸（ortho-aminomethylphenylboronic acids）的发射开启机制，学界已提出多种假说，例如光诱导电子转移（photoinduced electron transfer, PET）、“pKa开关”以及解聚集。但现有假说均无法完全契合硼酸基传感器的全部实验数据。为构建可合理解释该实验现象的统一理论，我们针对多款硼酸基传感器结合果糖时的发射开启起源展开了一系列实验探究。首先，我们证实受体及其果糖复合物均为溶剂插入型结构，不存在硼-氮相互作用；其次，验证了传感器并未发生聚集；第三，在可将−B(OH)₂基团转化为−B(OMe)₂基团的纯甲醇体系中，我们未观测到结合果糖时产生荧光响应。我们提出该现象源于内转换机制的减弱。为进一步验证该假说，我们开展了溶剂同位素效应研究。在重水（−B(OH)₂→−B(OD)₂）环境中，探针的荧光强度在结合果糖后未发生变化。学界已公认，氘代振荡器因振动模式频率更低，作为能量受体的效率更差。因此，我们的研究表明，通过调控−B(OH)₂诱导的内转换（一种“松栓效应”的实例），可合理解释所有邻氨基甲基苯基硼酸类荧光传感器如何实现糖类存在的信号响应。