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

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 today’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.