School of Chemistry, Norwich Research Park, University of East Anglia, Norwich, NR4 7TJ, UK.
Department of Chemistry, Durham University, South Road, Durham, DH1 3LE, UK.
Nat Commun. 2019 Dec 20;10(1):5820. doi: 10.1038/s41467-019-13819-6.
Non-valence states in neutral molecules (Rydberg states) have well-established roles and importance in photochemistry, however, considerably less is known about the role of non-valence states in photo-induced processes in anions. Here, femtosecond time-resolved photoelectron imaging is used to show that photoexcitation of the S(ππ*) state of the methyl ester of deprotonated para-coumaric acid - a model chromophore for photoactive yellow protein (PYP) - leads to a bifurcation of the excited state wavepacket. One part remains on the S(ππ*) state forming a twisted intermediate, whilst a second part leads to the formation of a non-valence (dipole-bound) state. Both populations eventually decay independently by vibrational autodetachment. Valence-to-non-valence internal conversion has hitherto not been observed in the intramolecular photophysics of an isolated anion, raising questions into how common such processes might be, given that many anionic chromophores have bright valence states near the detachment threshold.
中性分子（里德堡态）中的非价态在光化学中具有明确的作用和重要性，然而，人们对阴离子中光诱导过程中非价态的作用知之甚少。在这里，飞秒时间分辨光电子成像被用来表明，去质子对香豆酸甲酯的 S(ππ*)态的光激发 - 一种光活性黄蛋白（PYP）的模型发色团 - 导致激发态波包的分叉。一部分保持在 S(ππ*)态上形成扭曲的中间体，而另一部分导致形成非价态（偶极束缚）态。两个部分最终通过振动自分离独立衰减。价态到非价态的内转换迄今为止尚未在孤立阴离子的分子内光物理中观察到，这引发了一个问题，即在许多阴离子发色团在离域阈值附近具有明亮的价态的情况下，这种过程可能有多普遍。
