Triplet Excited State of BODIPY Accessed by Charge Recombination and Its Application in Triplet-Triplet Annihilation...

The triplet excited state properties of two BODIPY phenothiazine dyads (BDP-1 and BDP-2) with different lengths of linker and orientations of the components were studied. The triplet state formation of BODIPY chromophore was achieved via photoinduced electron transfer (PET) and charge recombination (CR). BDP-1 has a longer linker between the phenothiazine and the BODIPY chromophore than BDP-2. Moreover, the two chromophores in BDP-2 assume a more orthogonal geometry both at the ground and in the first excited state (87°) than that of BDP-1 (34-40°). The fluorescence of the BODIPY moiety was significantly quenched in the dyads. The charge separation (CS) and CR dynamics of the dyads were studied with femtosecond transient absorption spectroscopy (k_CS = 2.2 × 10¹¹ s^-1 and 2 × 10¹² s^-1 for BDP-1 and BDP-2, respectively; k_CR = 4.5 × 10¹⁰ and 1.5 × 10¹¹ s^-1 for BDP-1 and BDP-2, respectively; in acetonitrile). Formation of the triplet excited state of the BODIPY moiety was observed for both dyads upon photoexcitation, and the triplet state quantum yield depends on both the linker length and the orientation of the chromophores. Triplet state quantum yields are 13.4 and 97.5% and lifetimes are 13 and 116 μs for BDP-1 and BDP-2, respectively. The spin-orbit charge transfer (SO-CT) mechanism is proposed to be responsible for the efficient triplet state formation. The dyads were used for triplet-triplet annihilation (TTA) upconversion, showing an upconversion quantum yield up to 3.2%.