Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands.
Institute for Molecules and Materials, FELIX Laboratory, Radboud University, Toernooiveld 7c, 6525 ED Nijmegen, The Netherlands.
Phys Chem Chem Phys. 2022 Feb 9;24(6):3984-3993. doi: 10.1039/d1cp05958k.
As a prominent derivative of a natural sunscreen, methyl sinapate is an ideal candidate to provide fundamental insight into strategies on how to come to a rational design of artificial sunscreen filters with improved photoprotective properties. Here, static and time-resolved Zero Kinetic Energy-Pulsed Field Ionization (ZEKE-PFI) photoelectron spectroscopy has been used to study the spectroscopy and decay pathways of its electronically excited states. We find that different conformers are subject to distinct structural changes upon electronic excitation, and trace the structural changes that occur upon excitation back to the character of the LUMO. Ionization efficiency spectra in combination with pump-probe ZEKE-PFI spectra are consistent with the conclusion that the long-lived electronically excited state observed in the decay of the lowest excited singlet state is the lowest excited triplet state. Concurrently with providing information on the electronically excited states, the studies allow for a detailed characterization of the spectroscopic properties of the ground state of the radical ion, which is important in the context of the use of cinnamates in nature as antioxidants. Our studies determine the adiabatic ionization energies of the /, / and / conformers as 60 291.1 ± 0.5, 60 366.9 ± 0.5 and 60 503.9 ± 1.0 cm, respectively, and provide accurate vibrational fequencies of low-frequency modes of the molecular ion in its electronic ground state. Finally, the studies emphasize the important role of vibrational and electronic autoionization processes that start to dominate the ionization dynamics in non-rigid molecules of the present size.
作为一种天然防晒剂的重要衍生物，甲基芥子酸酯是研究如何设计具有更好光保护性能的人工防晒剂过滤器的策略的理想候选物。在这里，静态和时间分辨的零动能脉冲场电离（ZEKE-PFI）光电子能谱已被用于研究其电子激发态的光谱和衰减途径。我们发现，不同构象体在电子激发时会发生不同的结构变化，并追踪发生在激发态的结构变化回到 LUMO 的特征。电离效率谱与泵浦-探测 ZEKE-PFI 谱相结合的结果一致，即观察到的最低激发单线态衰减中长寿命的电子激发态是最低激发三重态。这些研究在提供有关电子激发态信息的同时，还允许对自由基离子基态的光谱性质进行详细表征，这在肉桂酸盐在自然界中作为抗氧化剂的应用中非常重要。我们的研究确定了 /、/ 和 / 构象体的绝热电离能分别为 60,291.1 ± 0.5、60,366.9 ± 0.5 和 60,503.9 ± 1.0 cm，并且提供了电子基态中分子离子的低频率模式的准确振动频率。最后，这些研究强调了在本尺寸的非刚性分子中，振动和电子自电离过程开始主导电离动力学的重要作用。
