Henrich Joseph D, Suchyta Scott, Kohler Bern
Department of Chemistry and Biochemistry, Montana State University , Bozeman, Montana 59717, United States.
J Phys Chem B. 2015 Feb 12;119(6):2737-48. doi: 10.1021/jp510953e. Epub 2014 Dec 29.
This study explores the concept that electronic excitation can dramatically enhance Lewis acidity. Specifically, it is shown that photoexcitation transforms an electron-deficient organic compound of negligible Lewis acidity in its electronic ground state into a potent excited-state Lewis acid that releases a proton from a nearby water molecule in 3.1 ps. It was shown previously (Peon et al. J. Phys. Chem. A 2001, 105, 5768) that the excited state of methyl viologen (MV(2+)) is quenched rapidly in aqueous solution with the formation of an unidentified photoproduct. In this study, the quenching mechanism and the identity of the photoproduct were investigated by the femtosecond transient absorption and fluorescence upconversion techniques. Transient absorption signals at UV probe wavelengths reveal a long-lived species with a pH-dependent lifetime due to reaction with hydronium ions at a bimolecular rate of 3.1 × 10(9) M(-1) s(-1). This species is revealed to be a charge-transfer complex consisting of a ground-state MV(2+) ion and a hydroxide ion formed when a water molecule transfers a proton to the bulk solvent. Formation of a contact ion pair between MV(2+) and hydroxide shifts the absorption spectrum of the former ion by a few nm to longer wavelengths, yielding a transient absorption spectrum with a distinctive triangle wave appearance. The slight shift of this spectrum, which is in excellent agreement with steady-state difference spectra recorded for MV(2+) at high pH, is consistent with an ion pair but not with a covalent adduct (pseudobase). The long lifetime of the ion pair at neutral pH indicates that dissociation occurs many orders of magnitude more slowly than predicted by the Smoluchowski-Debye equation. Remarkably, there is no evidence of geminate recombination, suggesting that the proton that is transferred to the solvent is conducted at least several water shells away. Although the hydrolysis mechanism has yet to be fully established, evidence suggests that the strongly oxidizing excited state of MV(2+) triggers the proton-coupled oxidation of a water molecule. The observed kinetic isotope effect of 1.7 seen in D2O vs H2O is of the magnitude expected for an ultrafast concerted proton-electron transfer reaction. The ultrafast hydrolysis seen here may be a general excited-state quenching mechanism for electronically excited Lewis acids and other powerful photooxidants in aqueous solution.
本研究探讨了电子激发可显著增强路易斯酸度这一概念。具体而言，研究表明光激发能将处于电子基态时路易斯酸度可忽略不计的缺电子有机化合物转化为一种强效的激发态路易斯酸，该酸能在3.1皮秒内从附近的水分子中释放出一个质子。此前研究表明（Peon等人，《物理化学杂志A》，2001年，105卷，5768页），甲基紫精（MV(2+)）的激发态在水溶液中会迅速猝灭，并形成一种不明光产物。在本研究中，通过飞秒瞬态吸收和荧光上转换技术对猝灭机制及光产物的身份进行了研究。紫外探测波长处的瞬态吸收信号揭示了一种长寿命物种，其寿命与pH值相关，这是由于它与水合氢离子以3.1×10(9) M(-1) s(-1)的双分子速率发生反应所致。该物种被证明是一种电荷转移复合物，由基态MV(2+)离子和当一个水分子将一个质子转移到本体溶剂时形成的氢氧根离子组成。MV(2+)与氢氧根之间形成接触离子对会使前者离子的吸收光谱向长波长方向移动几纳米，从而产生具有独特三角波外观的瞬态吸收光谱。该光谱的轻微移动与在高pH值下记录的MV(2+)的稳态差光谱非常吻合，这与离子对相符，但与共价加合物（假碱）不符。中性pH值下离子对的长寿命表明其解离发生的速度比Smoluchowski - Debye方程预测的要慢许多个数量级。值得注意的是，没有证据表明发生了双生复合，这表明转移到溶剂中的质子至少被传导到了几个水合层之外。尽管水解机制尚未完全确立，但有证据表明MV(2+)的强氧化性激发态触发了水分子的质子耦合氧化反应。在D2O与H2O中观察到的1.7的动力学同位素效应与超快协同质子 - 电子转移反应预期的量级相符。此处观察到的超快水解可能是水溶液中电子激发的路易斯酸和其他强光氧化剂的一种普遍的激发态猝灭机制。
