## 3 Results and Discussion
### Photophysical Properties of SAN  
SAN, due to its highly conjugated nature, is a brightly colored molecule strongly adsorbing in the UV-vis region of the spectrum. The key features of the SAN spectrum, which are vertical excitation energies, maximum absorption wavelength, oscillator strength, and molecular orbitals (MO) contributions for singlet excitations, are calculated by adopting the selected protocol and are included in Table S2 of the Supplementary Materials. The experimental absorption spectrum recorded in water shows a shoulder in which two bands are discernible centered at 470 and 400 nm, respectively. Two intense bands are clearly visible at higher transition energy centered at 327 and 273 nm. The computed spectrum satisfactorily reproduces such characteristics. The spin-allowed transitions at low energy give rise to the two shoulder bands centered at 496 and 434 nm having H \(\rightarrow\) L and H-1 \(\rightarrow\) L character, respectively. Absorption in the short wavelength region of the spectrum generates two bands. One corresponds to two transitions centered at 341 and 324 nm due to H \(\rightarrow\) L+1 and H-1 \(\rightarrow\) L+1 electron promotions, respectively. The second band is equally comprised of two transitions at 290 and 275 nm with H \(\rightarrow\) L+2 and H-3 \(\rightarrow\) L character, respectively. Changes induced in the absorption spectrum of the free SAN upon DNA intercalation were examined for each intercalation position on which MD calculations were carried out. As illustrated above, clustering analysis was used to extract the most representative configuration for each intercalation position. The structures extracted from each MD run and subsequently partially optimized (see computational details for further details) at DFT level are collected in Figure S6. The computed absorption spectra are plotted in Figure 4, where that of the free SAN is also included. While detailed information on the electronic transitions contributing to each absorption band of intercalated SAN can be found in Table S7, the most relevant photophysical features of the drug useful to ascertain its ability of \({}^{1}\)O\({}_{2}\) sensitization in all the intercalated arrangements are reported in Figure 5.  
Both bathochromism and hypochromism of the absorption bands have been reported for DNA intercalative interactions of conjugated \(\pi\) systems [60; 63]. Even in the case of SAN, the intercalation process, whatever the position considered, influences the position of the maximum absorption wavelength (see Figure 5A) as well as the intensity of the bands; although, as it appears evident from Figure 4, the shape of the spectrum does not differ too much from that of the free SAN, especially in the key region for the use of the drug in PDT. While the spectrum of intercalated SAN is characterized by a considerably less intense band in the region 300-400 nm, which originated by the highest energy electronic transitions, for the most relevant transition a less evident effect on the band intensity occurs and a bathochromic shift comprised between 37 (**IIa**) and 102 nm (**III**) was found. The noticeable shift in wavelength upon intercalation of SAN into DNA helix confirms that strong \(\pi\pi^{*}\) interactions are established [60], responsible for the orbital energy gap decrease causing the red-shift of the absorption band. In all cases, such excitation is originated by a H \(\rightarrow\) L electronic transition, with the exception of **IVb**, for which it becomes H-1 \(\rightarrow\) L type. However, the inspection of the involved MOs reported in Figure S7 clearly shows that the absorption corresponds to a charge transfer (CT) from the DNA to the intercalated SAN, and guanine base is involved in all cases except **III** for which the adenine base is responsible for the CT.  
An improvement in the photophysical properties of the drug, therefore, is proved by the calculated red-shift for intercalated arrangements of SAN into the DNA helix leading to a potentially higher ability to act as photosensitizer.