Can BODIPY-Electron Acceptor Conjugates Act As Heavy Atom-Free Excited Triplet State and Singlet Oxygen Photosensitizers via Photoinduced Charge Separation-Charge Recombination Mechanism?

To examine if BODIPY-electron acceptor conjugates can generate excited triplet state and singlet oxygen efficiently via a photoinduced charge separation-charge recombination (CS-CR) mechanism, seven compounds of BODIPY-electron acceptor (A) type are synthesized, in which an x-nitro-phenyl (x = ortho, meta, and para) or a pyridyl subunit on the BODIPY meso-position acts as the acceptor. The photophysical properties and singlet oxygen formation efficiency are measured in seven solvents of different polarity. The excited triplet state and singlet oxygen formation efficiency decrease with the increase in the electron withdrawing ability of the phenyl moiety. In the meantime, the presence of a nitrophenyl or a pyridyl causes the substantial quenching of the excited singlet state of the linked BODIPY, and this quenching is enhanced by the solvent with higher polarity, indicating that a strong photoinduced electron transfer (PET) occurs from BODIPY to the attached nitrophenyl unit. The results imply that attaching the electron acceptors to BODIPY cannot mediate excited triplet state formation via CS-CR mechanism, which is in sharp contrast to the case of BODIPY-electron donor type conjugates. Based on the electron movements in HOMO/LUMO for PET and CR, we obtain the general requirements for PET/CR mediated triplet formation: E-g(D) > E-g(A) when donor D is photoexcited, in which E-g(D) and E-g(A) are the energy gap between HOMO and LUMO for D and A, respectively. The conclusion and the frontier orbital analysis are helpful in the design and synthesis of BODIPY based halogen-free photosensitizers.