Different Quenching Effect of Intramolecular Rotation on the Singlet and Triplet Excited States of Bodipy

It is well-known that the fluorescence of a chromophore can be efficiently quenched by the free rotor effect, sometimes called intramolecular rotation (IMR), i.e. by a large-amplitude torsional motion. Using this effect, aggregation induced enhanced emission (AIE) and fluorescent molecular probes for viscosity measurements have been devised. However, the rotor effect on triplet excited states was rarely studied. Herein, with molecular rotors of Bodipy and diiodoBodipy, and by using steady state and time-resolved transient absorption/emission spectroscopies, we confirmed that the triplet excited state of the Bodipy chromophore is not quenched by IMR. This is in stark contrast to the fluorescence (singlet excited state), which is significantly quenched by IMR. This result is rather interesting since a long-lived excited state (triplet, 276 μs) is not quenched by the IMR, but the short-lived excited state (singlet, 3.8 ns) is quenched by the same IMR. The unquenched triplet excited state of the Bodipy was used for triplet–triplet annihilation upconversion, and the upconversion quantum yield is 6.3%

Triplet Excited State of BODIPY Accessed by Charge Recombination and Its Application in Triplet–Triplet Annihilation Upconversion

The triplet excited state properties of two BODIPY phenothiazine dyads (<b>BDP-1</b> and <b>BDP-2</b>) 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). <b>BDP-1</b> has a longer linker between the phenothiazine and the BODIPY chromophore than <b>BDP-2</b>. Moreover, the two chromophores in <b>BDP-2</b> assume a more orthogonal geometry both at the ground and in the first excited state (87°) than that of <b>BDP-1</b> (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 (<i>k</i>_CS = 2.2 × 10¹¹ s^–1 and 2 × 10¹² s^–1 for <b>BDP-1</b> and <b>BDP-2</b>, respectively; <i>k</i>_CR = 4.5 × 10¹⁰ and 1.5 × 10¹¹ s^–1 for <b>BDP-1</b> and <b>BDP-2</b>, 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 <b>BDP-1</b> and <b>BDP-2</b>, 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%