Synthesis and properties of sterically crowded triarylphosphines bearing naphthoquinone moieties

Sterically crowded triarylphosphines bearing naphthoquinone moieties were synthesized by the Suzuki-Miyaura coupling of the corresponding (phosphinoaryl)boronic acid derivatives with 2,3dichloro-l,4-naphthoquinone. The triarylphosphine-naphthoquinone unit can be extended by employing a triarylphosphine bearing a chloronaphthoquinone moiety as a substrate. As an example, an oligomer bearing three triarylphosphine and two naphthoquinone moieties was synthesized. The triarylphosphines bearing naphthoquinone moieties exhibited purple to blue colors arising from intramolecular charge transfer. A systematic study of various derivatives showed that the corresponding absorption shifted toward longer wavelengths as the difference between the oxidation potential of the triarylphosphine moieties and the reduction potential of the naphthoquinone moieties becomes smaller. On the other hand, the intensity of the charge transfer absorption depends on the number of interacting triarylphosphine-naphthoquinone units. The purple color of the charge transfer turned to pale yellow after protonation of the phosphorus atom with trifluoroacetic acid, while deprotonation by addition of triethylamine regenerated the color as well as the phosphine. The intramolecular charge transfer is sensitive to structural changes, as insertion of a 1,4-phenylene spacer between the triarylphosphine and naphthoquinone moieties leads to a loss of the purple color.