Photochemistry with Plane-Polarized Light: Controlling Photochemical Reactivity via Spatially Selective Excitation

Photochemical reactions are intrinsically difficult to control because they involve high-energy excited-state species. Herein we report a novel approach toward controlling photochemical reactions via using the spatially selective excitation of specific electronic transitions. This can be performed using photochemical irradiation with the plane-polarized light of a photoreactive compound uniformly aligned in a nematic liquid-crystalline (LC) medium. Having chosen cyclopropenone photodecarbonylation as a proof-of-concept reaction, we demonstrated that it could be controlled via changing an angle between the incident light polarization plane and the LC director. We showed that two specific partially forbidden electronic transitions were mostly responsible for this photochemical reaction. We envision that this simple general method can be useful in experimental studies of the fundamental details of various photochemical processes and can help to increase the selectivity of photochemical transformations.