Triplet‐Forming Thionated Donor‐Acceptor Chromophores for Electrochemically Amphoteric Photosensitization

A series of piperidine‐naphthalene monoimide, donor−acceptor (D−A) chromophores and their thionated derivatives were prepared. The physical properties of the chromophores were studied by spectroscopic and electrochemical measurements and DFT calculations. Relative to their non‐thionated counterparts, the thionated D−A chromophores display enhanced visible‐light absorption and 1O2‐sensitization quantum yields and negligible fluorescence, suggesting facile triplet formation of this class of molecules. This behavior originates from the significant n‐π* character in the singlet excited state, resulting in large spin‐orbit coupling between the singlet/triplet manifolds regardless of the degree of D/A interactions. With the electrochemically active D/A moieties and high triplet energy, the thionated chromophores can be used as amphoteric photosensitizers to catalyze reductive and oxidative photoreactions with efficiency comparable to Ru(bpy)32+ as the sensitizer. Our results demonstrate that thiocarbonylation can be utilized in amide/imide‐containing D−A chromophores to provide a wide range of heavy‐atom‐free redox‐active photosensitizers. Thiocarbonylation has been shown to be a promising tool for developing heavy‐atom‐free triplet photosensitizers from donor−acceptor chromophores. It was demonstrated that triplet‐state formation from the thiocarbonyls is independent of the donor−acceptor interactions and that the thionated chromophores can be utilized as amphoteric photoredox catalysts that have comparable catalytic activities to common inorganic photosensitizers.