Photobasicity in Quinolines: Origin and Tunability via the Substituents' Hammett Parameters
Photobasicity in Quinolines: Origin and Tunability via the Substituents' Hammett Parameters
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DOI：
10.1021/acs.jpclett.6b00790
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发表时间：
2016-06-02
影响因子：
5.7
通讯作者：
Dawlaty, Jahan M.
中科院分区：
文献类型：
--
作者：
Driscoll, Eric William;Hunt, Jonathan Ryan;Dawlaty, Jahan M.
电子激发和质子转移之间的耦合与氧化还原反应的动力学有关，特别是那些涉及太阳能到燃料光收集的反应。这种耦合的一个主要例子发生在光酸中，其中电子激发导致激发态的质子释放。在这里，我们系统地研究了这种效应的逆效应，即光碱性，其中分子在激发态比基态变得更加碱性。这赋予了该分子光诱导质子去除能力，预计可用于驱动质子转移在动力学上具有挑战性的反应。为了研究光碱性的起源和可调性，选择了一组 5-R-喹啉衍生物 (R = {NH2, CH3O, H, Br, Cl, CN})，并测定了它们在水溶液中电子激发时 pK(a) 的变化。这些取代基的哈米特参数 sigma(p) 表明了它们的吸电子能力，其范围为 -0.7 至 +0.7。使用福斯特循环分析，确定了基态和第一激发态的酸解离平衡。基态 pK(a) 遵循哈米特参数 sigma(p) 的预期线性关系。我们工作的一个重要发现是激发态 pK(a)* 也遵循与 sigma(p) 的线性关系。有趣的是，激发态pK(a)*对取代基吸电子能力的敏感度比基态pK(a)高5倍。我们将这种差异归因于激发态电荷密度的较大极化率。在该组内观察到由于光激发范围在 2.2 (R = CN) 和 10.6 (R = NH2) 单位之间而导致 pK(a) 增加。这种大范围的 Delta pK(a) 值可用于氧化催化等应用，其中光诱导质子的去除可以加速反应动力学。最后，我们评论了光碱性和光激发下杂环氮上电子电荷密度增强之间的相关性。
Coupling between electronic excitation and proton transfer is relevant to the kinetics of redox reactions, in particular those involved in solar-to fuel light harvesting. A prime example of such coupling occurs in photoacids, where electronic excitation leads to proton release in the excited state. Here, we systematically study the inverse of this effect, photobasicity, in which a molecule becomes more basic in the excited state compared to the ground state. This endows the molecule with light induced proton removal capability which is anticipated to be of use in driving reactions where proton transfer is kinetically challenging. To investigate the origins and tunability of photobasicity, a set of 5-R-quinoline derivatives (R = {NH2, CH3O, H, Br, Cl, CN}) were selected and their changes in pK(a) upon electronic excitation in aqueous solutions were determined. The Hammett parameters sigma(p) of these substituents, indicative of their electron withdrawing capability, span a range of -0.7 to +0.7. Using Forster cycle analysis, the acid dissociation equilibria in the ground and first excited state were determined. The ground state pK(a) obeys an expected linear relationship with respect to the Hammet parameter sigma(p). An important finding of our work is that the excited state pK(a)* also obeys a linear relationship with respect to sigma(p). Interestingly, the excited state pK(a)* is similar to 5 times more sensitive to the electron-withdrawing power of the substituent than the ground state pK(a). We attribute this difference to the larger polarizability of the excited state charge density. Increase in pK(a) due to optical excitation ranging between 2.2 (R = CN) and 10.6 (R = NH2) units were observed within the set. This substantial range of Delta pK(a) values may find use in applications such as oxidation catalysis, in which optically induced removal of protons could speed up reaction kinetics. Finally, we comment on the correlation between photobasicity and enhancement of electronic charge density on the heterocyclic nitrogen upon optical excitation.