Indirect Red and Near-Infrared Z-to-E Photoisomerization of ortho-Functionalized Azobenzenes via Triplet Energy Transfer

Designing azobenzene photoswitches capable of selective and efficient photoisomerization by long wavelength excitation is a long-standing challenge. Indirect excitation can expand the properties of the photoswitching system beyond the intrinsic limits of azobenzenes. Herein, a rapid Z-to-E isomerization of two ortho-functionalized azobenzenes with near-unity photoconversion was facilitated via triplet energy transfer upon red and near-infrared (up to 770 nm) excitation of porphyrin photosensitizers in catalytic micromolar concentrations. Our results indicate that the whole process of triplet-sensitized isomerization is strongly entropy-driven. This ensures efficient Z-to-E photoswitching even when the azobenzene triplet energy is considerably higher (>200 meV) than for the sensitizer, which is the key for the expansion of excitation wavelengths into the near-infrared spectral range.