Binding of 3O2 and O2 to dyes used in photodynamic therapy in gas phase and aqueous media

Density functional theory (DFT) was employed at the B3LYP/6-31+G* level to study complexes of O-1(2) and O-3(2) with the dye molecules proflavine, methylene blue, and acridine orange, which are useful in photodynamic therapy. It was found that the most stable complex between O-1(2) and proflavine are formed when O-1(2) is located above the central ring, while the most stable complex between O-1(2) and methylene blue is formed when O-1(2) is located above the molecular plane, but not above any of the rings, near the sulfur atom. O-1(2) can make a stable complex with acridine orange, as it is located above the outer ring of the dye. The binding energies of the complexes of O-1(2) with all three dyes are enhanced considerably in going from gas phase to aqueous media. The complexes of O-1(2) with the dyes will be unstable in all cases, while those of O-1(2) with the same will be quite stable and will not be dissociated due to thermal fluctuations at room temperature. In the complexes of O-1(2) and O-3(2) with the dyes, charge transfer occurs from the dyes to the O-2 moiety, the amount of charge transfer being much more to O-1(2), than to O-3(2), in each case.