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).