Spin–Orbit Charge-Transfer Intersystem Crossing (SOCT-ISC) in Bodipy-Phenoxazine Dyads: Effect of Chromophore Orientation and Conformation Restriction on the Photophysical Properties

Abstract

The spin–orbit charge-transfer-induced intersystem crossing (SOCT-ISC) in Bodipy-phenoxazine (BDP-PXZ) compact electron-donor/-acceptor dyads was studied. PXZ is the electron donor, and BDP is the electron acceptor. The molecular geometry is varied by applying different steric hindrance on the rotation about the linker between the two subunits. Charge-transfer (CT) absorption bands were observed for the dyads with more coplanar geometry (electronic coupling matrix elements is up to 2580 cm–1). Ultrafast charge separation (0.4 ps) and slow charge recombination (3.8 ns, i.e., SOCT-ISC process) were observed. Efficient ISC (ΦT = 54%) and long triplet-state lifetime (τT = 539 μs) were observed for the dyads. Notably, the triplet-state lifetime is 2-fold of that accessed with heavy-atom effect, indicating the advantage of using a heavy-atom-free photosensitizer. The low-lying CT state in the dyads in polar solvents was confirmed with intermolecular triplet photosensitizing method. Time-resolved electron paramagnetic resonance spectroscopy show that the electron spin polarization of the triplet state formed by the SOCT-ISC is the same as that of spin–orbit-ISC (SO-ISC). 3CT and localized excited triplet states (3LE) were simultaneously observed for one of the dyads, which is rare. Normally, the CT state was observed as spin-correlated radical pair. The dyads were used as triplet photosensitizers for triplet–triplet annihilation upconversion, the quantum yield is up to 12.3%. A large anti-Stokes shift (5905 cm–1) was achieved by excitation into the CT absorption band, not the conventional LE absorption band.

Tuning the Triplet Excited State of Bis(dipyrrin) Zinc(II) Complexes: Symmetry Breaking Charge Transfer Architecture with Exceptionally Long Lived Triplet State for Upconversion

“…No singlet oxygen generation was observed in nonpolar solvents in all the complexes. Solvent‐dependent singlet oxygen generation ability of the complexes also supports the SOCT‐ISC mechanism of triplet population [38–40] . Moreover, different ISC ability of the complexes clarifies that the structural profile of bis(dipyrrin) complexes strongly controls the excited state dynamics.…”

“…The molecules studied in this work, exhibiting SBCT‐induced ISC have strong visible light absorption and long lived triplet excited states. These properties are beneficial for intermolecular triplet–triplet energy transfer (TTET), [40] and these compounds can be useful in triplet–triplet annihilation (TTA) upconversion.…”

“…Solvent-dependent singlet oxygen generation ability of the complexes also supports the SOCT-ISC mechanism of triplet population. [38][39][40] Moreover,d ifferent ISC ability of the complexesc larifies that the structural profile of bis(dipyrrin) complexess trongly controls the exciteds tate dynamics. Notably, this is the first report of efficient singlet oxygen generation (up to 71 %) of zinc(II) bis(dipyrrin) complexes.…”

Heavy-atom-free BODIPY photosensitizers with intersystem crossing mediated by intramolecular photoinduced electron transfer

“…Voronkova and co-workers investigated triplet state formation via SOCT-ISC in BODIPY dyads 45-48 (Figure 10b), employing a phenoxazine group as an electron donor. 67 Efficient electron transfer was observed for all of these dyads in both non-polar and polar solvents. This was explained by higher stability of chargetransfer states formed in 45-48, compared to corresponding phenothiazine dyads.…”

Development of tethered dual catalysts: synergy between photo- and transition metal catalysts for enhanced catalysis

“…The triplet state lifetimes (s T ) were remarkably high for catalysts 4 and 7 (>500 ms) with Ir(I)-BDP HS 6 also having a long-lived triplet state (160 ms). To the best of our knowledge, the highest reported s T for a BDP-type compound is 539 ms, 72 thus catalysts 4 and 7 represent one of the longest reported triplet state lifetimes for BDP-type compounds. These long triplet lifetimes are important for photocatalysis, as they increase the likelihood of productive triplet energy transfer, leading to 1 O 2 generation.…”