Strategies for controlling the triplet states using chemical/physical inputs to achieve external stimuli-switchable triplet–triplet-annihilation upconversion (TTA UC).

With phenoxazine (PXZ) as the electron donor and perylene-3,4-dicarboximide (PMI) as the electron acceptor, we prepared a compact, orthogonal electron donor−acceptor dyad (PMI−PXZ) to study the spin−orbit charge transfer-induced intersystem crossing (SOCT-ISC). A weak charge transfer (CT) absorption band, due to S 0 → 1 CT transition, was observed (ε = 2840 M −1 cm −1 at 554 nm, FWHM: 2850 cm −1 ), which is different from that of the previously reported analogue dyad with phenothiazine as the electron donor (PMI−PTZ), for which no CT absorption band was observed. A long-lived triplet state was observed (lifetime τ T = 182 μs) with nanosecond transient absorption spectroscopy, and the singlet oxygen quantum yield (Φ Δ = 76%) is higher than that of the previously reported analogue dyad PMI−PTZ (Φ Δ = 57%). Ultrafast charge separation (ca. 0.14 ps) and slow charge recombination (1.4 ns) were observed with femtosecond transient absorption spectroscopy. With time-resolved electron paramagnetic resonance spectroscopy (TREPR), we confirmed the SOCT-ISC mechanism, and the electron spin polarization phase pattern of the triplet-state TREPR spectrum is (e, e, a, e, a, a), which is dramatically different from that of PMI−PTZ (a, e, a, e, a, e), indicating that the triplet-state TREPR spectrum of a specific chromophore in the electron donor−acceptor dyads is not only dependent on the geometry of the dyads but also dependent on the structure of the electron donor (or acceptor). Even one-atom variation in the donor structure may cause significant influence on the electron spin selectivity of the ISC of the electron donor−acceptor dyads.

Two Bodipy dimers were prepared (BDP-BDP and BDP-BDP-I2), with a phenyl unit as the linker between two Bodipy moieties. It was found that the triplet excited state of the first dimer depopulates fast to the ground state, while the metastable triplet state is well-detected in the second dimer. With two iodine atoms attached, a high singlet oxygen quantum yield (ΦΔ = 51.5%) and long intrinsic triplet state lifetime (τT = 210 μs) of BDP-BDP-I2 were observed in toluene, which are similar to those of diiodo Bodipy monomer (ΦΔ = 54.9%, τT = 173 μs). As determined by phosphorescence experiments, the T1-state of BDP-BDP-I2 localizes predominantly on the diiodo Bodipy subunit (BDP-I2, 92%) and a little on the diphenyl Bodipy subunit (BDP-Ph2, 8%). At low temperature (ca. 80–140 K), three types of triplet signals were observed for BDP-BDP-I2 in time-resolved electron paramagnetic resonance spectroscopy (TREPR), which are assigned to two subunits (BDP-I2 and BDP-Ph2) and the 3CT state. Efficient energy transfer from the BDP-I2 subunit to the BDP-Ph2 subunit was also observed in BDP-BDP-I2. The local equilibrium between three metastable states observed within ca. 1 μs after the laser flash is sensitive to the temperature and the polarity of the solvent. The observed properties imply the presence of the energy and electron transfer pathways between two moieties of the studied dimers.