The stereochemistry of chiral molecules plays a significant role in their photochemical properties. Chiral molecules are those that have non-superimposable mirror images, also known as enantiomers. These enantiomers can have different interactions with light, leading to differences in their photochemical properties. This phenomenon is known as the "chiroptical effect."There are several ways in which the stereochemistry of chiral molecules can affect their photochemical properties:1. Absorption of light: Chiral molecules can absorb circularly polarized light differently, depending on their stereochemistry. This is known as circular dichroism CD . In CD, one enantiomer preferentially absorbs left-circularly polarized light, while the other enantiomer preferentially absorbs right-circularly polarized light. This difference in absorption can lead to different photochemical reactions or product distributions for each enantiomer.For example, the photoisomerization of chiral azobenzene derivatives can lead to different product distributions depending on the stereochemistry of the starting material. One enantiomer may undergo a more efficient E to Z isomerization upon absorption of light, while the other enantiomer may not.2. Photochemical reactions: The stereochemistry of chiral molecules can also affect the outcome of photochemical reactions. This is because the spatial arrangement of atoms in a chiral molecule can influence the accessibility of reactive sites and the orientation of transition states during a photochemical reaction.A classic example of this is the enantioselective [2+2] photocycloaddition of chiral cyclohexenones with alkenes. The stereochemistry of the chiral cyclohexenone determines the facial selectivity of the reaction, leading to the formation of different diastereomeric products.3. Emission properties: The stereochemistry of chiral molecules can also affect their emission properties, such as fluorescence and phosphorescence. This is because the excited states of chiral molecules can also be chiral, leading to different radiative decay pathways for each enantiomer.For instance, the enantiomers of chiral binaphthyl derivatives exhibit different fluorescence properties due to their different excited state geometries. One enantiomer may have a more planar excited state, leading to more efficient fluorescence, while the other enantiomer may have a more twisted excited state, leading to less efficient fluorescence or even phosphorescence.In conclusion, the stereochemistry of chiral molecules plays a crucial role in determining their photochemical properties. The differences in absorption, photochemical reactivity, and emission properties between enantiomers can be exploited in various applications, such as enantioselective synthesis, chiral sensing, and chiral optoelectronics.