Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
J Phys Chem A. 2013 Aug 22;117(33):7767-75. doi: 10.1021/jp403323h. Epub 2013 Aug 6.
The o-hydroxycinnamic derivatives represent efficient caged compounds that can realize quantification of delivery upon uncaging, but there has been lack of time-resolved and mechanistic studies. We used time-resolved infrared (TRIR) spectroscopy to investigate the photochemical uncaging dynamics of the prototype o-hydroxycinnamic compound, (E)-3-(2-hydroxyphenyl)-acrylic acid ethyl ester (HAAEE), leading to coumarin and ethanol upon uncaging. Taking advantage of the specific vibrational marker bands and the IR discerning capability, we have identified and distinguished two key intermediate species, the cis-isomers of HAAEE and the tetrahedral intermediate, in the transient infrared spectra, thus providing clear spectral evidence to support the intramolecular nucleophilic addition mechanism following the trans-cis photoisomerization. Moreover, the product yields of coumarin upon uncaging were observed to be greatly affected by the solvent polarity, suppressed in CH2Cl2 but enhanced in D2O/CH3CN with the increasing volume ratio of D2O. The highly solvent-dependent behavior indicates E1 elimination of the tetrahedral intermediate to give rise to the final uncaging product coumarin. The photorelease rate of coumarin was directly characterized from TRIR (3.6 × 10(6) s(-1)), revealing the promising application of such o-hydroxycinnamic compound in producing fast alcohol jumps. The TRIR results provide the first time-resolved detection and thus offer direct dynamical information about this photochemical uncaging reaction.
邻羟基肉桂酸衍生物是一类有效的笼型化合物，在解笼后可以实现定量释放，但目前缺乏时间分辨和机制研究。我们使用时间分辨红外（TRIR）光谱法研究了原型邻羟基肉桂酸化合物（E）-3-（2-羟基苯基）-丙烯酸乙酯（HAAEE）的光化学反应解笼动力学，解笼后生成香豆素和乙醇。利用特定的振动标记带和红外分辨能力，我们在瞬态红外光谱中鉴定并区分了两种关键的中间体，即 HAAEE 的顺式异构体和四面体型中间体，从而提供了明确的光谱证据，支持在顺-反光异构化后发生分子内亲核加成反应。此外，我们观察到解笼后香豆素的产率受到溶剂极性的显著影响，在 CH2Cl2 中抑制，而在 D2O/CH3CN 中随 D2O 体积比的增加而增强。这种高度依赖溶剂的行为表明，四面体中间体发生 E1 消除，生成最终的解笼产物香豆素。从 TRIR 直接测定香豆素的光释放速率（3.6×10^6 s^-1），揭示了此类邻羟基肉桂酸化合物在产生快速醇跳跃方面的应用潜力。TRIR 结果提供了对此类光化学反应解笼反应的首次时间分辨检测，从而提供了直接的动力学信息。
