Photophysical Properties of Thionine and Methylene Blue Photosensitizers: A TD-DFT Study of Monomers, Salts, Hydrates, and Dimers

Photodynamic therapy (PDT) is an established treatment in medicine that combines light, photosensitizers, and molecular oxygen to induce cell death. This work presents a theoretical investigation of the photophysical properties of two cationic photosensitizers for PDT, thionine (TH; Lauth’s violet) and methylene blue (MB), using DFT and TD-DFT. We systematically examine TH and MB cations, their chloride salts (TH-Cl, MB-Cl), monohydrated species (TH-Cl-W, MB-Cl-W), and homodimers [(TH-Cl-W)2, (MB-Cl-W)2] in water and benzene, along with the TH-Cl@W61 cluster to probe first-solvation-shell effects. All optimized geometries are stable. Absorption, emission and vibronic spectra are calculated. MB molecular systems show absorption maxima red-shifted by ~40 nm relative to TH analogs, while the principal visible dimer bands are blue-shifted by 22–37 nm compared to the corresponding monomers. Given that the present calculated MB and TH species present a T1 state lying in the 0.86–1.44 eV range, and that MB systems generally exhibit substantially larger oscillator strengths than TH and therefore more intense absorption, these results provide preliminary support for MB as a more promising photosensitizer than TH. Electronic absorption spectra are blue-shifted compared to the experimental ones, but vibronic coupling markedly improves agreement with experiment in the visible region, whereas UV peaks are described adequately without vibronic effects. Finally, the dimerization can offer a route to fine-tune photophysical properties, highlighting the relevance of small, aggregated chromophores for PDT, photocatalysis, and photon upconversion.