The recent developments in charge separation, long-lived charge transfer state formation and charge recombination-induced intersystem crossing are summarized.

Triplet photosensitizers (PSs) have attracted much attention in recent years, due to the significance of these kinds of compounds in fundamental photochemistry studies and applications in photocatalysis, photodynamic therapy (PDT) and triplet–triplet-annihilation upconversion (TTA-UC). Heavy-atom-free triplet PSs are of particular interest, because of the advantages of low cost, low toxicity and long-lived triplet states compared with triplet PSs that rely on the heavy-atom effect to enhance intersystem crossing (ISC). This review article summarizes recent developments in the design and application of heavy-atom-free triplet PSs, including those based on compact orthogonal electron donor–acceptor dyads, or compounds with twisted π-conjugation frameworks, or those that contain thiocarbonyl groups. The ISC mechanisms of these different types of triplet PSs are discussed, and all are based on enhanced spin–orbit coupling. Moreover, the electron spin selectivity of ISC, studied via time-resolved electron paramagnetic resonance (TREPR) spectra, is also introduced. Some new discoveries and the challenges in this area are discussed, for instance, the role of the triplet charge-transfer (³CT) state in the spin–orbit charge-transfer ISC (SOCT-ISC) mechanism, the relationship between the twisted π-conjugation frameworks and the ISC efficiency, and determination of the intrinsic triplet-state lifetimes.

Lead-free halide perovskite nanocrystals (NCs) have recently attracted attention due to their nontoxicity and stability as alternatives to lead-based perovskite NCs. Here, we report undoped and manganese-doped all-inorganic, lead-free double perovskite (DP) NCs: Cs₂NaIn_xBi_1–xCl₆ (0 < x < 1) and Cs₂NaIn_xBi_1–xCl₆:Mn (0 ≤ x ≤ 1) NCs. Undoped NCs exhibit blue emission. Through doping Mn²⁺ ions into Cs₂NaIn_xBi_1–xCl₆ NCs, we can avoid self-trapped exciton emission and realize bright orange red emission of Mn²⁺ dopants with the highest photoluminescence quantum efficiency of 44.6%. The photoluminescence (PL) is tunable from yellow emission to orange-red emission corresponding to a red shift from 583 to 614 nm with increasing In content. Interestingly, the PL emission of Mn-doped NCs shows a red shift from the bulk size to the nanoscale. The Mn-doped NCs show good stability in air. In addition, we further prove the process of dark self-trapped state-assisted Mn²⁺ emission in DP NCs by ultrafast transient absorption techniques.

Perylenebisimide-carbazole (PBI–Cz) dyads were prepared to study charge-recombination (CR) induced intersystem crossing (ISC) in electron donor/acceptor dyads. The distance and the mutual orientation of the perylenebisimide (PBI) and carbazole (Cz) moieties were varied to study their effect on photophysical properties. Steady state and time-resolved optical spectroscopies show that electronic coupling between the electron donor and acceptor is negligible at the ground state. The fluorescence of the PBI moiety is strongly quenched in the dyads, and a larger separation between the donor and the acceptor results in less fluorescence quenching. The Gibbs free energy changes of the electron transfer and the energy level of the charge transfer state were studied using the electrochemical and optical spectra data. The singlet oxygen quantum yields (Φ_Δ) are up to 72% for dyads with the shortest separation between the donor and the acceptor. Nanosecond transient absorption spectra confirmed the formation of the PBI-localized long lived triplet state (the lifetime is up to 190 μs). Notably non-orthogonal dyads show efficient spin orbit charge transfer (SOCT-ISC), which is different from the previously proposed orthogonal molecular structure for SOCT-ISC. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy shows that all three dyads give the same electron spin polarization of eae/aea, and thus the radical pair ISC (RP ISC) mechanism is excluded, which is different from the previously reported PBI-phenothiazine analogues. Efficient and reversible transformation of dyads to their radical anion was observed in the presence of sacrificial electron donor triethanolamine in an inert atmosphere with photo-irradiation.