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

Abstract

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 = 2.2 × 10 s and 2 × 10 s for BDP-1 and BDP-2, respectively; k = 4.5 × 10 and 1.5 × 10 s 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%.

Spin–Orbit Charge‐Transfer Intersystem Crossing (ISC) in Compact Electron Donor–Acceptor Dyads: ISC Mechanism and Application as Novel and Potent Photodynamic Therapy Reagents

“…Up to now, only a limited number of molecular systems that are able to form triplet states through SOCT‐ISC have been analysed by TREPR . In particular, for the recently reported Bodipy‐derived electron donor/acceptor dyads showing SOCT‐ISC, TREPR spectroscopy has been rarely applied to confirm the ISC mechanism . Herein, we performed a detailed TREPR spectral study of the compact electron donor/acceptor dyads (Figure ).…”

“…In this regard, triplet PSs based on spin–orbit charge‐transfer ISC (SOCT‐ISC) have recently attracted much attention. It was found that for compact electron donor/acceptor dyads, ISC can be greatly enhanced given the electron donor and acceptor are in orthogonal geometry . The mechanism responsible for triplet generation in these systems is similar to the ISC involving nπ*↔ππ* transitions.…”

“…As CR is associated with a variation of orbital symmetry, this process is termed as SOCT‐ISC . Inspired by the simple molecular structures and efficient ISC, investigations on SOCT‐ISC of compact electron donor/acceptor dyads with orthogonal geometry has boomed in the last few years …”

“…Although different studies have recently emerged concerning this mechanism, SOCT‐ISC is still in its infancy. First, although a few electron donor/acceptor dyads that are able to undergo SOCT‐ISC have been reported, the interplay among different ISC mechanisms (e.g., SOCT‐ISC, RP‐ISC and others) remains unclear. Pulsed laser excited time‐resolved electron paramagnetic resonance (TREPR) spectroscopy, a powerful method to distinguish the different ISC mechanisms by studying the electron spin polarization (ESP) of the triplet state, was not applied to the recently reported Bodipy electron donor/acceptor dyads .…”

Efficient Intersystem Crossing in the Tröger's Base Derived From 4‐Amino‐1,8‐naphthalimide and Application as a Potent Photodynamic Therapy Reagent

“…The 1,7‐methyl groups on the boron dipyrromethene (Bodipy) moiety play a similar role in Bodipy‐derived electron donor/acceptor dyads showing SOCT‐ISC . We found that the presence of peri ‐H(C) of electron donor phenothiazine is helpful for conformation restriction . However, in all these molecular structural motifs, rotation about the C−C or the C−N linker is not fully restricted, and various conformations are still accessible with thermal energy.…”