Kinetic Evidence for the Necessity of Two Proton Donor Molecules for Successful Excited State Proton Transfer by a Photobase
Kinetic Evidence for the Necessity of Two Proton Donor Molecules for Successful Excited State Proton Transfer by a Photobase
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DOI：
10.1021/acs.jpca.9b08970
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发表时间：
2019-12-05
影响因子：
2.9
通讯作者：
Dawlaty, Jahan M.
中科院分区：
文献类型：
--
作者：
Hunt, Jonathan Ryan;Dawlaty, Jahan M.
光碱是将光转化为质子转移驱动的分子，因此在化学的许多领域都有潜在的应用。在此之前，我们研究了喹啉类化合物的光碱性并探索了它们的应用。虽然有可能将光碱系在目标质子供体附近，但出于多功能性的考虑，探索它们分散在溶液中的分子去质子化的能力是可取的。先前的证据表明，在这种情况下，至少需要两个质子供体才能成功地进行激发态质子转移：一个提供质子，另一个稳定光生成的供体阴离子。在这里，我们报告了瞬态吸收（TA）和时间相关单光子计数（TCSPC）的动力学证据来支持这一假设。我们使用5-甲氧基喹啉作为光碱，2,2,2-三氟乙醇（TFE），一种低pK(a)醇，作为质子供体。在非质子背景溶剂中，恒定浓度的光碱用于质子供体稀释范围跨越几个数量级。吸收光谱证实，在大部分研究范围内，大多数光碱族与至少一个供体成氢键。短脉冲TA用于测量较快（2-500 ps）的动态，而TSCPC用于测量较慢（bb0 500 ps）的动态。测量的质子转移时间常数随供体浓度的变化而变化。质子转移速率常数作为质子供体浓度函数的对数-对数图显示了两种情况：在高供体浓度下，多个质子供体在光碱附近，非扩散；在低供体浓度下，质子供体更稀释，扩散。非扩散状态的斜率约为1，表明质子转移过程依赖于除了已经与光碱形成氢键的给体分子之外的另一个给体分子。扩散状态合理地遵循扩散动力学。我们提出了第二个质子供体分子如何与光生成的醇氧化合物相互作用以稳定它的模型。这项工作强调了在质子转移后诱导不可逆变化的重要性，在这种情况下醇盐的溶剂化。了解这些细节在照相基的应用中可能是重要的。
Photobases are molecules that convert light to proton transfer drive and therefore have potential applications in many areas of chemistry. Previously, we studied the photobasicity of quinolines and explored their applications. While it is possible to tether a photobase near a target proton donor, for the sake of versatility it is desirable to explore their capability to deprotonate molecules dispersed in a solution. Previous evidence suggested that in this scenario at least two proton donors were necessary for successful excited state proton transfer: one to donate a proton and the second to stabilize the photogenerated donor anion. Here we report kinetic evidence from transient absorption (TA) and time-correlated single photon counting (TCSPC) in support of this hypothesis. We used 5-methoxyquinoline as the photobase and 2,2,2-trifluoroethanol (TFE), a low pK(a) alcohol, as the proton donor. A constant concentration of the photobase was used for a range of proton-donor dilutions spanning several orders of magnitude in an aprotic background solvent. Absorption spectra confirm that over most of the studied range the majority of the photobase population is hydrogen bonded to at least one donor. Short-pulse TA was used to measure the faster (2-500 ps) dynamics, while TSCPC was used to measure the slower (>500 ps) dynamics. The measured proton transfer time constants varied as a function of donor concentration over a wide range. A log-log plot of the proton transfer rate constant as a function of proton-donor concentration shows two regimes: nondiffusive at high donor concentrations where multiple proton donors are near the photobase and diffusive at low donor concentrations where proton donors are more dilute. The nondiffusive regime has a slope of approximately one, suggesting that the proton transfer process is dependent on one donor molecule in addition to the donor molecule already hydrogen bonded with the photobase. The diffusive regime reasonably follows diffusion kinetics. We propose a model for how the second proton-donor molecule may interact with the photogenerated alkoxide to stabilize it. This work highlights the importance of inducing irreversible changes, in this case solvation of the alkoxide, after proton transfer. Understanding of such details is likely to be important in applications of photobases.