School of Chemistry, Norwich Research Park, University of East Anglia, Norwich NR4 7TJ, U.K.
Department of Chemistry, Durham University, Durham DH1 3LE, U.K.
J Phys Chem A. 2020 Mar 19;124(11):2140-2151. doi: 10.1021/acs.jpca.9b11993. Epub 2020 Mar 10.
Chromophores based on the -hydroxycinnamate moiety are widespread in the natural world, including as the photoswitching unit in photoactive yellow protein and as a sunscreen in the leaves of plants. Here, photodetachment action spectroscopy combined with frequency- and angle-resolved photoelectron imaging is used to fingerprint the excited-state dynamics over the first three bright action-absorption bands in the methyl ester anions (CEs) of deprotonated -coumaric acid at a temperature of ∼300 K. The excited states associated with the action-absorption bands are classified as resonances because they are situated in the detachment continuum and are open to autodetachment. The frequency-resolved photoelectron spectrum for CEs indicates that all photon energies over the S(ππ*) band lead to similar vibrational autodetachment dynamics. The S(π*) band is Herzberg-Teller active and has comparable brightness to the higher lying 2(ππ*) band. The frequency-resolved photoelectron spectrum over the S(π*) band indicates more efficient internal conversion to the S(ππ*) state for photon energies resonant with the Franck-Condon modes (∼80%) compared with the Herzberg-Teller modes (∼60%). The third action-absorption band, which corresponds to excitation of the 2(ππ*) state, shows complex and photon energy-dependent dynamics, with 20-40% of photoexcited population internally converting to the S(ππ*) state. There is also evidence for a mode-specific competition between prompt autodetachment and internal conversion on the red edge of the 2(ππ*) band. There is no evidence for recovery of the ground electronic state and statistical electron ejection (thermionic emission) following photoexcitation over any of the three action-absorption bands. The photoelectron spectra for the deprotonated methyl ether derivative (CEt) at photon energies over the S(ππ*) and S(π*) bands indicate diametrically opposed dynamics compared with CEs, namely, intense thermionic emission due to efficient recovery of the ground electronic state.
基于 - 羟基肉桂酸部分的生色团广泛存在于自然界中，包括作为光活性黄色蛋白中的光开关单元以及作为植物叶子中的防晒霜。在这里，光离解作用光谱学与频率和角度分辨光电子成像相结合，用于在 ∼300 K 的温度下指纹识别去质子化 - 香豆酸甲酯阴离子 (CEs) 的前三个明亮作用吸收带中的激发态动力学。与作用吸收带相关的激发态被归类为共振，因为它们位于离解连续体中并且可以进行自动离解。CEs 的频率分辨光电子光谱表明，S(ππ*) 带上方的所有光子能量都导致类似的振动自动离解动力学。S(π*) 带是 Herzberg-Teller 活性的，与更高的 2(ππ*) 带具有相当的亮度。S(π*) 带的频率分辨光电子光谱表明，对于与 Franck-Condon 模式（∼80%）共振的光子能量，与 Herzberg-Teller 模式（∼60%）相比，更有效地将内部转换为 S(ππ*) 态。对应于激发 2(ππ*) 态的第三个作用吸收带显示出复杂的和光子能量依赖性的动力学，其中 20-40%的光激发态种群内部转换为 S(ππ*) 态。在 2(ππ*) 带的红色边缘处，也有证据表明存在特定模式之间的竞争，即快速自动离解和内部转换之间的竞争。在三个作用吸收带中的任何一个上，都没有证据表明在光激发后恢复基态电子态和统计电子发射（热电子发射）。在 S(ππ*) 和 S(π*) 带上方的光子能量下，对去质子化甲基醚衍生物 (CEt) 的光电子光谱表明与 CEs 相比，存在截然相反的动力学，即由于基态电子态的有效恢复而导致强烈的热电子发射。
