## 3 Results and Discussion  
### Photophysical Properties of DHSAN  
The most suitable protocol for the study of the photophysical properties was chosen performing a preliminary benchmark of a series of exchange-correlation functionals to test their ability to reproduce the experimental value of the main absorption wavelength of the lowest-lying absorption band (S\({}_{1}\) state) of SAN and its precursor DHSAN [7,53]. The values calculated using several exchange-correlation functionals are reported in Table S1 of the Supplementary Materials together with the corresponding oscillatory strengths and the main contributions in terms of Molecular Orbitals (MO) involved in the transitions. Calculations were carried out in water solvent for SAN and THF for DHSAN, to reproduce the environment used to register the experimental UV-vis spectra. Comparison shows that two functionals, namely, B3 LYP-D3 [26,27,39] and B3P86-D3 [47], are able to better reproduce the reference experimental values of the maximum absorption wavelength. The B3 LYP-D3 level of theory was chosen, being the most versatile, and was used also to carry out all the involved calculations and to build-up the free energy profiles in water solvent. The photophysical properties of DHSAN are discussed in this section, while those of SAN will be addressed in Section 3.4. The absorption spectrum of DHSAN is simple and has a single peak with maximum experimentally recorded in THF solvent at 335 nm [7]. Since the computed spectra in THF and water did not show significant differences, the calculated key parameters illustrating the photophysical properties of DHSAN in water are reported, while detailed information can be found in Table S2. The maximum absorption wavelength of 342 nm with an oscillator strength of 0.252, well reproduces the experimental one obtained in THF and corresponds to a vertical excitation energy of 3.62 eV. The spin-allowed transition involves the HOMO(H)and LUMO(L) orbitals. The transition has clear \(\pi\pi\pi^{\ast}\) character, being the orbitals delocalized across the molecule, and strong \(\pi\) conjugation is observed even if the structure deviates from planarity. Four triplet states were localized below the excited singlet one (see Table S3 for further details). In order to establish their probability of population that, in turn, depends on the possibility that excitation is followed by an efficient Inter System Crossing (ISC) allowing the access to the triplet manifold, both singlet-triplet splitting energies \(\Delta\)E(S\({}_{1}\)-T\({}_{m}\)), and spin-orbit coupling (SOC) matrix elements \(\langle\)T\({}_{m}\) \(|\)H\({}_{SOC}\) \(|\)S\({}_{1}\rangle\) were calculated and are reported in the same Figure 1. All the four intercepted triplet states can be involved in deactivation of the bright state once the S\({}_{1}\) state is populated. The values of the coupling constants between the S\({}_{1}\) state and all triplet states, T\({}_{m}\) (with \(m\) = 1-4), lying below are larger than that calculated for the approved Foscar(r) (2.4 \(\times\) 10\({}^{-1}\) cm\({}^{-1}\) for the S\({}_{1}\)\(\rightarrow\) T\({}_{1}\) radiationless transition) [54]. Finally, DHSAN decaying from the triplet excited to the ground state allows an efficient generation of singlet molecular oxygen \({}^{1}\)O\({}_{2}\) (\({}^{1}\)A\({}_{\beta}\)), as the calculated energy gap (shown in Figure 1) is significantly larger than 0.98 eV, which is the minimum energy amount required to excite triplet oxygen. However, the \({}^{1}\)O\({}_{2}\) sensitization triggered by DHSAN is limited by the wavelength as its maximum absorption (335 and 342 nm, for recorded and computed spectra in THF and water, respectively) falls outside the optimal spectral region identified as the ideal phototherapeutic window (550-850 nm), suggesting a limited tissue penetration that could prevent the \({}^{1}\)O\({}_{2}\) formation needed for in situ SAN generation. For this reason, in view of its importance for photosensizers, two-photon excitation (TPE) was taken into consideration.