Photochemistry with Plane-Polarized Light: Controlling Photochemical Reactions via Selective Choice of Excited States and Breaking Kasha’s Rule

Abstract Controlling the course of photochemical reactions hinges on the possibility to selectively form a specific singlet excited state. When alternative excited states are energetically close, they are generated simultaneously upon initial photoexcitation, with each excited state producing its own set of photoproducts, leading to low reaction selectivity. In other cases, subsequent internal conversion to the lowest-energy excited state (in accordance with Kasha’s rule) makes only the lowest-energy excited state responsible for photochemistry, whereas potential reactions from the higher-energy excited states are not observed, limiting the synthetic potential of photochemical transformations. The latest developments in precision photochemistry have enabled more accurate control of photochemical reactions, but the toolbox for doing this remains limited. Herein, we describe a fundamentally new tool to control the course of monomolecular photochemical reactions via targeted selective formation of required singlet excited states. This is accomplished by utilizing the spatially selective excitation of specific electronic transitions using plane-polarized light applied to the molecularly aligned photoreactant in a nematic liquid crystalline phase. Upon proper orientation of the plane-polarized light, the selective generation of either the S1 or S2 excited state facilitates one of the two alternative photoreaction pathways. The possibility of the selective generation of one of the two energetically close singlet excited states to choose the desired photochemical route stems from the breakdown of Kasha’s rule. Although we have demonstrated the application of this approach for controlling photochemistry in just one case, this is an important milestone toward developing precision photochemistry as a fundamentally novel and potentially far-reaching tool.