Theoretical investigation on quinoline-based platinum (II) complexes as efficient singlet oxygen photosensitizers in photodynamic therapy

Geometry optimizations of the quinoline-based platinum (II) complexes (1-R, 2-R) and their related calculations on excited state energies, electronic absorption spectra and orbital populations have been carried out by the hybrid density functional theory (DFT) and its time-dependent approach (TD-DFT). The solvent effects on excitation energies are taken into account using the conductor-like polarizable continuum model (C-PCM). The red-shifted level of absorption bands, energy gaps between the singlet ground state (S₁) and the first triplet excited state (T₁) for each examined complex have been elaborated thoroughly as well. We find that the quinoline-8-thoil (ligand 2) induces much more significant red-shifted level than 8-hydroxyquinoline (ligand 1), and singlet-triplet splitting energy gaps of all examined complexes are bigger than threshold energy to yield singlet oxygen. It is revealed that the electronic red-shifted absorption bands originate from metal-to-ligand charge transfer (MLCT) transitions, and also shown that the quinoline-based Pt (II) complexes with strong donor groups could be considered as potential candidates for unearthing of novel photosensitizers in photodynamic therapy (PDT).