## 3 Results and Discussion
### Photophysical Properties of SAN
Nevertheless, as described for DHSAN, an efficient photosensitizer has to fulfill other requirements, which include a consistent spin-orbit coupling for promoting the ISC from the excited singlet to triplet electronic states, and the existence of a triplet state of proper energy to be able to excite molecular oxygen. For this reason, triplet excited states of SAN in water within the vertical approximation have also been calculated and are included in Table S3 of the Supplementary Materials. Two lowest lying triplets together with MO contributions were characterized using TDDFT. S\({}_{1}\) and T\({}_{1}\) transitions involve H and L  
Figure 4: Computed absorption spectra in implicit water solvent of SAN and all its possible intercalated arrangements. orbitals with \(\pi\pi^{\bullet}\) character. The HOMO delocalized on entire molecular skeletons, while the LUMO is mainly localized on the isoquinoline part (Figure S8). These triplets are shown in Figure 5B together with the computed values of the spin-orbit matrix elements \(\langle\mathrm{T_{m}}\,\mathrm{I\,H_{SOC}}\,|\,\mathrm{S_{1}}\rangle\) (panel C), from which the efficiency of the ISC depends on together with the singlet-triplet energy gaps, \(\Delta\mathrm{E(S_{1}-T_{m})}\) for \(m=1\), \(2\). Figure 5: Computed (**A**) maximum absorption wavelengths, (**B**) \(\mathrm{T_{1}}\) and \(\mathrm{T_{2}}\) triplet states energy and (**C**) spin–orbit coupling constants for the accessible S\({}_{1}\) deactivation processes of the SAN and all its possible intercalated arrangements extracted from MD simulations. Data reported in Figure 5B clearly reveal that the intercalation process entails a stabilization of the low-lying triplet states with respect to the free SAN. Nevertheless, the energy gaps between the ground state and the first excited triplet state remain higher than the energy required for O\({}_{2}\) excitation (0.98 eV) for all the intercalated adducts, and then SAN, in such conditions, once its triplet state is populated, should be surely able to generate the \({}^{1}\)O\({}_{2}\) cytotoxic species. The remaining requirement usually taken into consideration for the photosensizers in PDT, namely, the efficiency of the ISC between the bright state and the triplet manifold, was examined. On the basis of calculated SOC values (see Figure 5C), SAN seems to be less able than its DHSAN precursor in acting as a photosensitizer, being DHSAN characterized by SOC values one order of magnitude higher than those of SAN. However, it is likely to be hypothesized that once intercalated, since the absorption wavelength improves, its ability to populate the low-lying triplet states could also be enhanced. Indeed, as shown above, DNA nucleobases actively participate in the CT to SAN (Figure S7) for the population of the bright state changing the MOs' nature found for the free drug (Figure S8), which, in turn, influences the ISC efficiency. The same changes in MOs' nature were found for triplet states of investigated arrangements of SAN within the DNA helix. In fact, as reported in Table S8, while T\({}_{1}\) state remains essentially of the type \(\pi\pi\pi\), T\({}_{2}\) systematically involves the nucleotides in the CT from DNA to SAN. Looking at data collected in Figure 5C it can be seen that SAN, once intercalated, exhibits a higher ability to promote the triplet state population and, then, \({}^{1}\)O\({}_{2}\) production. Indeed, the SOC values from 0.05 to 0.11 for S\({}_{1}\)-T\({}_{1}\) and S\({}_{1}\)-T\({}_{2}\) radiationless transition, respectively, pass, e.g., to 0.47 and 0.36 in the intercalation site 3 (AT-TA).