Seeing into the Abyss: Critical Points and Paths on the S1 Potential Energy Surface of Stilbenes and beta-Carotene

二苯乙烯是围绕双键进行光异构化的范例，尽管经过数十年的研究，仍未被理解。(i)在90°C=C扭转角的幻像状态被认为是不存在的，因为快速的内部转换。然而，我们最近可以稳定它。（ii）所谓的二苯乙烯之谜尚未解决：为什么在低摩擦溶剂中与气相相比，屏障穿越速度更快？（iii）受激顺式二苯乙烯的瞬态拉曼光谱显示其反应模式的活性为零。量子计算化学家在解释这些观察结果时遇到了严重的困难；例如，没有计算出稳定的顺S1点。挑战在于理解其中的机制。只有这样，人们才会问这是否普遍，细菌视紫红质等可能会发生什么。换句话说，如果生物分子的光异构化研究是可信的，那么关于苯乙烯的不确定性必须在今天的理论和实验水平上得到解决。为此，飞秒受激拉曼散射（FSRS）技术在世界范围内处于领先地位。观测并分配了许多新的S1拉曼谱带。此外，我们还表明，精细的化学取代开辟了填充活性表面的新途径。这现在被合并为两个研究方向：(1)二苯乙烯S1 PES上幻态的光谱特征，以及导致它的快速反应路径，将为电子结构和反应性的计算提供基准。(2)振动能量流将通过线宽和反斯托克斯强度进行监测。该方法将首先应用于激发β -胡萝卜素（因为它是一种最强烈的拉曼散射体），然后应用于激发二苯乙烯。我们将一起准确地指定与问题（I,ii,iii）相关的表面区域和过程，从而有助于解决问题。

Stilbene, the paradigm for photoisomerisation around a double bond, is not understood despite decades of research. (i) A phantom state at 90 deg C=C twist angle is thought to be nonexistent because of fast internal conversion. However we could stabilize it recently. (ii) The so-called stilbene enigma is unsolved: why does barrier crossing speed up in low friction solvents compared to the gas phase? (iii) Transient Raman spectra of excited cis stilbene show zero activity for reactive modes. Quantum computational chemists have serious difficulties in explaining these observations; for example no stable cis S1 point was ever calculated. The challenge is to understand the mechanisms involved. Only then one may ask whether this is general, what may be expected with bacteriorhodopsin etc. In other words, the uncertainties about stilbene must be resolved at todays´s level of theory and experiment if the same studies of photoisomerisation in biomolecules are to be credible. For this purpose Femtosecond Stimulated Raman Scattering (FSRS) was developed to a leading position worldwide. Many new S1 Raman bands were observed and assigned. In addition we showed that delicate chemical substitution opens up new ways to populate the reactive surface. This is now combined into two research lines: (1) A spectroscopic characterisation of the phantom state on the S1 PES of stilbenes, and of fast reaction paths leading to it, will provide benchmarks for computations of electronic structure and reactivity. (2) Vibrational energy flow will be monitored via linewidths and anti-Stokes intensities. The approach will first be applied to excited beta-carotene (because it is a most intense Raman scatterer) and then to excited stilbenes.Together we shall specify accurately the surface regions and processes which are associated with problems (I,ii,iii), and thus contribute to their solution.