Selective Control of Photocatalytic [2 + 2] Cycloaddition versus Isomerization Using Anthraquinone Photosensitizers

Designed anthraquinone photosensitizers enable selective switching between chalcone [2 + 2] cycloaddition and E/Z isomerization. The α-trifluoromethyl group drives isomerization (5:95), whereas the β-tert-butyl group with polychloro substituents promotes cycloaddition (98% yield), a result supported by DFT-derived triplet energies and binding energies. In addition, a relatively high chalcone concentration favors cycloaddition (98%), whereas a low chalcone concentration favors isomerization (98%).