Differences in electron densities of phenoxazine and phenothiazine derivatives—charge density studies

Abstract

High resolution X-ray studies for three single crystals of derivatives of phenoxazine and phenothiazine, 1-(4-phenothiazin-10-yl-phenyl)-ethanone (PAS), 1-(4-phenoxazin-10-yl-phenyl)-ethanone (PAO), and (4-phenoxazin-10-yl-phenyl)phenyl-methanone (PBO), have been carried out and the experimental charge densities for these crystals have been established. A significant intramolecular charge redistribution is observed for the studied moieties. It appears that the donor parts of the molecules studied in all three cases are negatively charged (−0.715, −0.313 and −0.218) and the acceptor fragments carry significant positive charges (0.696, 0.316 and 0.224) for PBO, PAS and PAO, respectively. Separation of the charge between the donor and the acceptor parts in the solid state is in the opposite direction in comparison to the molecules’ behaviour in the solution. Analysis of the geometry of these compounds allowed the explanation of the differences in the values of the electronic coupling element V₀. The three most important factors influencing the coupling element are the following: hybridization of the nitrogen atom, conformation of the heterocyclic ring and the torsion angle between the acceptor and the donor parts of the molecules. Theoretical calculations support the charge properties obtained from experiment. The calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) indicate that the charge transfer in the excited state takes place from the donor to the acceptor molecular fragments.