Photoinduced Formation of BODIPY Radical Cation in Meso‐Methyl‐BODIPY Photocage: Direct Evidence with Chemically Induced Dynamic Nuclear Polarization Technique

Light-responsive molecules that can release drugs upon light absorption have attracted significant interest in chemistry and biology. BODIPY-based photoremovable protecting groups, or photocages, have recently emerged as especially promising tools in this respect. However, the exact photorelease mechanism is still not fully understood. We study the photochemical decomposition of meso-Methyl-BODIPY-conjugated epinephrine using 1H nuclear magnetic resonance and chemically induced dynamic nuclear polarization (CIDNP) techniques. After irradiation, epinephrine was detected only in trace amounts, whereas its oxidation product, adrenochrome, was the predominant product. Surprisingly, the CIDNP study has shown that the electron transfer (ET) in this reaction does not proceed from epinephrine to the BODIPY moiety, but rather occurs between two BODIPY cores. To validate this hypothesis, we applied the CIDNP method to detect photoinduced ET between two model BODIPY molecules in solution. In addition, the radical cation of BODIPY has been detected for the first time by CIDNP under photolysis in the presence of the electron acceptor-chloranil. The hyperfine interaction (HFI) constants of the BODIPY radical cation were estimated from the CIDNP spectrum, and they are in agreement with the HFI constant predicted by density functional theory calculations. Due to high enhancement coefficients, the CIDNP technique allows for to detection of polarized BODIPY products at very low concentrations.