# Recent Development of Heavy Atom-Free Triplet Photosensitizers for Photodynamic Therapy
## 2 Spin-Orbit Charge Transfer Intersystem Crossing (SOCT-ISC)
Figure 8: Simplified energy diagram showing the advantage of the creation of CT absorption band in electron donor-acceptor compounds: absorption wavelength is red-shifted, but the chromophore-localized triplet state does not decrease. (i) ISC (ii) TTET (iii) TTA and delayed fluorescence. Reproduced with permission from reference [54], copyright 2019, Royal Society Chemistry. We also found that for some compact electron donor/acceptor dyads, charge separation occurred and fluorescence was quenched, but the ISC efficiency was low [61; 62; 63]. The reason may be attributed to the un-matching of the CT and triplet state energies or poor molecular conformation restrictions, and fast charge recombination led to the direct relaxation of the CT state to the ground state (S\({}_{0}\)). The solvent polarity-dependent ISC efficiency of the compact orthogonal dyads does not necessarily act as a persistent drawback. We have shown that by tuning the electron donating ability (**30** and **31**, Figure 10), the dyad based on SOCT-ISC mechanism can show high ISC efficiency in both low polar and high polar solvents [64]. For instance, the \(\Phi_{\Delta}\) of **31** in TOL, THF, DCM and ACN were 0.48, 0.53, 0.99 and 0.72, respectively. However, for **30**, the \(\Phi_{\Delta}\) in TOL, THF, DCM and ACN were 0.07, 0.42, 0.94 and 0.71, respectively. [PERSON] et al. prepared a class of borafluororene\(-\)ligand dyads **32\(-\)34**, for which the ISC is based on SOCT-ISC mechanism (Figure 11) [65]. The electronic features of borafluororene make it an electron donor. The electron donor and acceptor moieties are connected by a spiro geometry, and the two moieties adopt orthogonal alignment. These characteristics of the dyads enhance the ISC to produce the triplet state. The \(\Phi_{\Delta}\) of **32**, **33** and **34** in DCM are 0.78, 0.56 and 0.27, respectively. [PERSON] et al. prepared a series of Zn dipyrrin complexes **35**-**38** (Figure 12) containing no large aryl appendant that was shown to undergo symmetry-breaking charge transfer (SBCT) and ISC, but the triplet-state yield is low [66]. Later [PERSON] et al. showed that for Zn chlorodipyrrin complexes **39** (Figure 12), the ISC yield was much higher and was dependent on the solvent polarity. The ISC quantum yields in cyclohexane, TOL and ACN were 30%, 89% and 76%, respectively. The variation of the ISC quantum yields was assigned to the changing of the CT state energies in different solvents and the matching of the CT state with the coordinated dipyrrin ligand [67]. It should be noted that the ISC in these complexes was not due to the heavy atom effect of the Zn(II) ions. Matching the S\({}_{1}\) and T\({}_{n}\) state energy may enhance the ISC [68; 69; 70; 71]. The role of the intermediate triplet states in the SOCT-ISC efficiency was demonstrated by aryl-appended Zn dipyrrin complexes [72]. SBCT was observed for the aryl-appended Zn dipyrrin complexes **40**-**43** (Figure 13), but high ISC efficiency was observed only for the complex **43** with anthryl appendant. The reason is attributed to the \({}^{3}\)An state (1.73 eV, which is close to the CT state of the complexes 2.32 eV). Recently it was shown that the SOCT-ISC strategy can be extended to the chromophores showing longer absorption wavelength [19; 73; 74]. Moreover, compounds that show thermally activated delayed fluorescence (TADF), which are important in organic light-emitting diodes (OLEDs), can be considered as a special case of SOCT-ISC [16]. Recent studies show that the electron donor-acceptor type of TADF molecules can be used as a novel type of PDT reagents [75]. Figure 12: Zn dipyrrin and chlorodipyrrin complexes showing SBCT and ISC. The solvent polarity-dependent photophysical processes of the Zn chlorodipyrrin complex are shown at the bottom. \({}^{\star}\) indicates the excited state. Reproduced with permission from reference [67], copyright 2018, American Chemical Society.