Thiol-activated triplet-triplet annihilation upconversion: study of the different quenching effect of electron acceptor on the singlet and triplet excited states of Bodipy.

Thiol-activated triplet-triplet annihilation (TTA) upconversion was studied with two different approaches, i.e., with 2,4-dinitrobenzenenesulfonyl (DNBS)-caged diiodoBodipy triplet photosensitizers (perylene as the triplet acceptor/emitter of the upconversion) and DNBS-caged Bodipy fluorophore as the triplet acceptor/emitter (PdTPTBP as the triplet photosensitizer, TPTBP = tetraphenyltetrabenzoporphyrin). The photophysical processes were studied with steady-state UV-vis absorption spectroscopy, fluorescence spectroscopy, electrochemical characterization, nanosecond transient absorption spectroscopy, and DFT/TDDFT computations. DNBS-caged triplet photosensitizer shows a shorter triplet state lifetime (24.7 μs) than the uncaged triplet photosensitizer (86.0 μs), and the quenching effect is due to photoinduced electron transfer (PET). TTA upconversion was enhanced upon cleavage of the DNBS moiety by thiols. On the other hand, the DNBS-caged Bodipy shows no fluorescence, but the uncaged fluorophore shows strong fluorescence; thus, TTA upconversion is able to be enhanced with the uncaged fluorophore as the triplet energy acceptor/emitter. The results indicate that the DNBS moiety exerts a significant quenching effect on the singlet excited state of Bodipy, but the quenching on the triplet excited state is much weaker. Calculation of the Gibbs free energy changes of the photoinduced electron transfer indicates that the singlet state gives a larger driving force for the PET process than the triplet state.