Photodynamic therapy (PDT) is an attractive method for cancer treatment. Triplet photosensitizers (PSs) are critical for this method; upon photoexcitation, efficient intersystem crossing (ISC) occurs for triplet PSs, the triplet-excited state of the triplet PSs is populated, then via intermolecular triplet energy transfer, the O₂, in triplet-spin multiplicity at ground state, is sensitized to the singlet-excited state, i.e., singlet oxygen (¹O₂) is produced. This strong reactive oxygen species (ROS) will oxidize the biomolecules in the tumor tissue. Thus, the design of novel triplet PSs as efficient PDT agents is vital. In this review article, we will introduce the recent development of the heavy atom-free triplet PSs used for PDT, including those based on spin-orbit charge transfer ISC (SOCT-ISC), twisting of the π-conjugation framework-induced ISC, radical enhanced ISC, and thionated carbonyl-induced ISC. The ISC mechanisms and molecular structure design rationales are discussed. The less studied electron spin selectivity of the ISC of the triplet PSs is also introduced. This information is helpful for the future design of new efficient triplet PSs for PDT.

In order to investigate the joint influence of the conformation flexibility and the matching of the energies of the charge-transfer (CT) and the localized triplet excited (³LE) states on the thermally activated delayed fluorescence (TADF) in electron donor–acceptor molecules, a series of compact electron donor–acceptor dyads and a triad were prepared, with naphthalimide (NI) as electron acceptor and phenothiazine (PTZ) as electron donor. The NI and PTZ moieties are either directly connected at the 3-position of NI and the N-position of the PTZ moiety via a C–N single bond, or they are linked through a phenyl group. The tuning of the energy order of the CT and LE states is achieved by oxidation of the PTZ unit into the corresponding sulfoxide, whereas conformation restriction is imposed by introducing ortho-methyl substituents on the phenyl linker, so that the coupling magnitude between the CT and the ³LE states can be controlled. The singlet oxygen quantum yield (Φ_Δ) of NI-PTZ is moderate in n-hexane (HEX, Φ_Δ = 19%). TADF was observed for the dyads, the biexponential luminescence lifetime are 16.0 ns (99.9%)/14.4 μs (0.1%) for the dyad and 7.2 ns (99.6%)/2.0 μs (0.4%) for the triad. Triplet state was observed in the nanosecond transient absorption spectra with lifetimes in the 4–48 μs range. Computational investigations show that the orthogonal electron donor–acceptor molecular structure is beneficial for TADF. These calculations indicate small energetic difference between the ³LE and ³CT states, which are helpful for interpreting the ns-TA spectra and the origins of TADF in NI-PTZ, which is ultimately due to the small energetic difference between the ³LE and ³CT states. Conversely, NI-PTZ-O, which has a higher CT state and bears a much more stabilized ³LE state, does not show TADF.

The photophysics of naphthalimide (NI)−phenothiazine (PTZ) dyads were investigated as electron donor–acceptor (D–A) thermally activated delayed fluorescence (TADF) emitters. Femtosecond transient absorption (fs-TA) spectra show that the photophysical processes in non-polar solvents are in singlet localized state (¹LE, τ = 0.8 ps) → Franck–Condon singlet charge separation state (¹CS, τ = 7.8 ps) → ¹CS state (τ = 2.2 ns) → triplet state (³LE, τ = 16 μs). The ³LE state is formed via the spin–orbit charge transfer-intersystem crossing (SOCT-ISC) mechanism rather than the spin–orbit (SO)-ISC mechanism. In a polar solvent, the CS state has a much lower energy than the ³LE state; thus, the ³LE state is absent from the photophysical processes and no TADF was observed. Moreover, we found that the delayed fluorescence lifetime is related to the low-lying triplet state (³LE or ³CS states). When the ³CS state is the low-lying triplet state, the TADF lifetime is shorter than that of the ³LE state as the low-lying triplet state. In the time-resolved electron paramagnetic resonance (TREPR) spectra, both ³LE (zero field splitting parameter D = 2250 MHz, E = −150 MHz) and ³CS (D = 430 MHz, E = 0 MHz) states were observed. It is noteworthy that the electron spin polarization (ESP) phase pattern of the ³CS state was inverted at longer delay times as a consequence of the selective transition between the ³LE and ³CS states and a faster decay of one sublevel of the ³CS state. These results are strong and direct experimental evidence for the spin-vibronic coupling mechanism of TADF.

We prepared a rhodamine (RB)–perylene (Pery) compact electron donor/acceptor dyad (RB–Pery) to study the spin-orbit charge-transfer intersystem crossing (SOCT–ISC). The UV–vis absorption spectrum indicates a negligible electronic interaction between the donor and acceptor at ground state. However, the fluorescence of both the RB and Pery units are quenched in the dyad, which is attributed to the photoinduced electron transfer, supported by the electrochemical studies. Nanosecond transient absorption (ns-TA) spectra show delocalized triplet states, i.e., there is an excited-state equilibrium between Pery and the RB triplet states. The triplet state lifetime was determined as 109.8 μs. With intermolecular triplet–triplet energy transfer, monitored using ns-TA spectra, the triplet-state energy balance between RB and Pery in RB–Pery was confirmed. The proposed cascade photophysical processes of the dyad are ¹RB*-Pery→RB–Pery^+•→[³RB*-Pery↔RB-³Pery*]. Moreover, long-lived rhodamine radical cation (in milliseconds) was detected in both deaerated/aerated non-polar or low-polarity solvents (i.e., p-xylene, toluene). The potential energy curve of the dyad against the variation in the dihedral angle between the two units indicates large torsional freedom (53°~128°) in RB–Pery, which leads to inefficient SOCT–ISC; consequently, low singlet-oxygen quantum yields (Φ_Δ = 2~8%) were observed.