# Recent Development of Heavy Atom-Free Triplet Photosensitizers for Photodynamic Therapy
## 2 Spin-Orbit Charge Transfer Intersystem Crossing (SOCT-ISC)
demonstrated that the triplet PSs based on the SOCT-ISC mechanism could also be used as PDT reagents effectively to produce \({}^{1}\)O\({}_{2}\)[49]. Figure 6: Molecular structures of the perynyl-Bodipy dyads showing SOCT-ISC. Figure 7: Confocal laser scanning microscopy images of HeLa cells after incubation with **24** (\(c\) = 10 \(\upmu\)m). **(a)** Green channel (500–600 nm) fluorescence image, \(\lambda_{\text{ex}}\) = 488 nm. **(b)** Bright field image. **(c)** Merged image. Scale bar: 20 \(\upmu\)m. Comparison of the cell viability of HeLa cells pretreated with increasing doses of **24** and 2,6-diiodoBodipy (IBDP), **(d)** with light irradiation (500 nm, 9.6 J\(\cdot\)cm\({}^{-2}\)), **(e)** without light irradiation. 20 \({}^{\circ}\)C. Reproduced with permission from reference [18], copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. For the compact electron donor-acceptor dyads, given the electronic coupling between the donor and acceptor was strong, CT absorption bands (5\({}_{0}\)\(\rightarrow\)\({}^{1}\)CT transition) would result [33]. This property could be used for increasing the anti-Stokes shift in TTA upconversion [50; 51; 52; 53]. For **24**, we observed a broad CT absorption band in the range 535\(-\)635 nm and the LE absorption of the perylene moiety was retained to a large extent [54]. Although the energy-minimized geometry of **24** is not orthogonal (dihedral angle between the perylene and the Bodipy part is 60\({}^{\circ}\)) [18], satisfactory ISC efficiency was observed (\(\Phi_{\Delta}=31\%\)) [54]. Note the attaining of this red-shifted CT absorption band in electron donor-acceptor dyads is different from the conventional derivatization of chromophores by extension of the \(\pi\)-conjugation framework or attaching of electron pushing/withdrawal groups to the \(\pi\)-conjugation framework, in both cases, the T\({}_{1}\) state energy of the chromophore may be decreased, as a result of the substantially perturbed \(\pi\)-conjugation framework of the chromophore. In the electron donor/acceptor dyads such as **24**, in which there is limited yet non-negligible electronic coupling between the donor and acceptor, a red-shifted CT absorption band resulted, but the T\({}_{1}\) state energy of the chromophore may not be reduced, which is a desired property for a triplet PS (i.e., by attaching of an electron donor or acceptor to the chromophore, red-shifted absorption results, and the T\({}_{1}\) state energy keeps high). The T\({}_{1}\) state energy of **24** was determined as 1.57 eV (almost similar to the pristine perylene). We demonstrated this advantage by using **24** as triplet PS for TTA upconversion, a large anti-Stokes shift of 0.65 eV was observed by excitation into the CT absorption band (Figure 8) [54]. In comparison, upon photoexcitation into the normal LE absorption, a smaller anti-Stokes shift of 0.37 eV was observed. This method may become a new, general approach to attained red-shifted absorption band yet un-reduced T\({}_{1}\) state energy. [PERSON] et al. prepared dyads with Bodipy as electron donor and found that no SOCT-ISC occurs, but SOCT-ISC does occur for analogous dyads in which the Bodipy units act as electron acceptor (**27**-**29**, Figure 9) [55]. The results were discussed based on molecular orbital energies. However, we found that in some dyads, a specific chromophore could act as either electron donor or acceptor, and all the corresponding dyads showed efficient SOCT-ISC, for instance, the dyads with perylene chromophore as either the electron acceptor or donor [56; 57], or dyads with anthryl unit as either electron donor or acceptor [33; 34; 35; 36; 37; 38]. Recently it was found that for some triplet PSs based on the SOCT-ISC mechanism, the \({}^{1}\)O\({}_{2}\) production could be enhanced by aggregation effect [58]. Moreover, Bodipy dimers were prepared and efficient SOCT-ISC was observed [59; 60].