# Recent Development of Heavy Atom-Free Triplet Photosensitizers for Photodynamic Therapy
## 6 Population Rates of the Three Sublevels of the Triplet State: Time-Resolved EPR Spectra (TREPR)  
Normally transient optical absorption or luminescence spectroscopic methods were used to study the ISC and the triplet state, the most typical spectral methods are fs-TA and ns-TA spectroscopic methods [16; 29]. However, some fundamental photochemistry information for ISC process is missing in these optical spectral characterizations, i.e., the electron spin selectivity of the ISC, or the electron spin polarization (ESP) of the triplet state [121]. Due to the anisotropy of the spin-orbit coupling and the ZFS of the triplet state, the population of the three sublevels of the T\({}_{1}\) state (T\({}_{x}\), T\({}_{y}\), T\({}_{Z}\), in the absence of an external magnetic field) was unequal, as a result, the population rates of the three sublevels of the T\({}_{1}\) state are usually deviated from Boltzmann distribution [20; 122; 123]. This information can be well characterized by pulsed laser excited TREPR spectroscopy [122; 124; 125], the data was critical for elucidation of the ISC mechanisms and understanding of the spatial distribution of the triplet state wave function, and the molecular geometry at the triplet excited state [29; 125], etc. Figure 19: TEMPO-Cy7 dyad **57** as triplet PS for PDT studies. Figure 20: Representative triplet PSs with thionated carbonyl groups. Usually, different ISC mechanisms have different electron spin selectivity, i.e., the ESP of TREPR spectra of the triplet state will be different. TREPR spectrum of anthracene shows an (\(e,e,e,a,a\)) ESP at the canonical orientation (randomly oriented samples, in frozen solution at 85 K. \(e\) stands for emissive and \(a\) stands for enhanced absorptive signal) [33]. Interestingly, the triplet state TREPR spectra of \(\mathbf{4}\) and \(\mathbf{5}\) showed an (\(e,a,e,a,e,a\)) ESP pattern (Figure 21). The population rates of the sublevels of the triplet state were different. For anthracene, it is A\({}_{\text{X}}\):A\({}_{\text{Y}}\):A\({}_{\text{Z}}\) = 0.78:1.00:0.07, whereas it was A\({}_{\text{X}}\):A\({}_{\text{Y}}\):A\({}_{\text{Z}}\) = 0.26:1.00:0.21 for \(\mathbf{4}\). The ZFS parameters of \(D\) and \(E\) were very close for the dyads and the pristine anthracene (74 mT and \(-\)8.0 mT, respectively), indicating the triplet states of the dyads were localized on the anthryl moiety, not delocalized on the donor and acceptor simultaneously. The different ESP clearly demonstrated that the ISC mechanism of the dyads was different from the pristine anthracene, i.e., SOCT-ISC was responsible for the formation of triplet state in the dyads. Our recent study on the TREPR of the compact electron donor/acceptor dyads shows that one should be careful in the interpretation of the relationship between the ESP pattern of the triplet-state TREPR spectra and the ISC mechanisms. For instance, the ESP phase pattern of the triplet-state TREPR spectrum of 2,6-diiodoBodipy is (\(e,e,e,a,a,a\)), but the electron donor/acceptor dyad \(\mathbf{11}\) shows the same ESP phase pattern (Figure 22), although the population rates of the sublevels of the T\({}_{1}\) state are different [38]. We observed a similar phenomenon in orthogonal Bodipy-phenoxazine dyads [126], and Bodipy-Cz dyads [127]. These results suggest that the ESP phase pattern of the TREPR spectrum of triplet state is not strictly ISC mechanism-specific [16; 29]. For the conventional electron donor/acceptor dyads with large separation between the donor and acceptor, the CT state was identified as spin-correlated radical pair (SCRP) [128; 129], \({}^{3}\)CT state was rarely observed [130; 131]. With the compact electron donor/acceptor dyads, however, we observed \({}^{3}\)CT states, as well as the upper triplet state. For instance, for \(\mathbf{12}\), we observed the \({}^{3}\)An, \({}^{3}\)CT and \({}^{3}\)BDP states, simultaneously (Figure 22) [38].