Fig. 4: Asymmetric photochemistry in stilbene.
a Kinetic model employed to describe the photochemistry of cis-stilbene. In agreement with the results of the AIMS dynamics, we are assuming that the cis-trans isomerization and the DHP cyclization require photoexcitation whereas the helical inversion process takes place only on S0. Thus, the excitation of the M and P enantiomers can lead to photocyclization to R,R-DHP or SS-DHP with kinetic constants kcis→DHP and k’cis→DHP or photoisomerization to the achiral trans-stilbene (T) with kinetic constants kcis→trans and k’cis→trans, respectively. At the same wavelength, the DHP enantiomers can be excited back leading to the M and P enantiomers with kinetic constants kDHP→cis and k’DHP→cis. trans-Stilbene can also be excited, producing both P and M cis-stilbene conformations (ktrans→cis). P and M are considered to be in equilibrium in the ground state with a helical inversion constant krac. b Enantiomeric excess reached at the photo-stationary state exciting a racemic mixture of the P and M enantiomers of cis-stilbene with r-CPL as a function of the excitation wavelength. Three limiting cases were considered according to Eq. (1). The wavelength-dependent g factors for stilbene (gstil) and DHP (gDHP) are also reported as dashed lines. In the optical window 250–350 nm, the M-enantiomer preferentially absorbs the r-CPL leading to trans-stilbene and R,R-DHP (Supplementary Fig. 4a). Th latter is also preferentially excited by the r-CPL retrieving cis-stilbene and resulting in an overall optical enrichment of S,S-DHP. On the other hand, in the optical window 200–270 nm (Supplementary Fig. 4b), the band in the DHP ECD spectrum changes sign and the r-CPL is preferentially absorbed by the S,S-DHP enantiomer (i.e., the anisotropy factor g changes sign). In this way, the optimum condition to maximize the ee is reached since one enantiomer is preferentially formed and the other destroyed by the CPL irradiation, resulting in an overall excess of R,R-DHP.
