Synthesis and properties of sterically crowded triarylphosphines bearing anthra- and naphtho-quinones, and their oligomers

Sterically crowded triarylphosphines bearing anthraquinones were synthesized by Suzuki–Miyaura coupling of arylboronic acids derived from (bromoaryl)phosphines with haloanthraquinones. The anthraquinone bearing the two triarylphosphine moieties at the 2,6-positions shows the smaller difference between the oxidation potential of the triarylphosphine moieties and the reduction potential of the anthraquinone moiety, and the more red-shifted visible absorption responsible for the reddish brown color as compared with the 1,5-derivative. A sterically crowded triarylphosphine–naphthoquinone oligomer composed of alternately aligned three triarylphosphine and four naphthoquinone moieties were also synthesized by repeated Suzuki–Miyaura coupling using arylboronic acids derived from (bromoaryl)phosphines and chloronaphthoquinone derivatives. The 31P NMR spectrum of the oligomer consists of several peaks in a narrow range reflecting distribution of the diastereomers arising from the helicity of the propellers composed of the three aromatic rings on the phosphorus atom. The oligomer exhibits a purple color resulting from the intramolecular charge transfer from the triarylphosphine moiety to the neighboring naphthoquinone moiety, and the wavelength and the intensity of the charge transfer band have a demonstrable correlation with the redox potentials and the number of the neighboring triarylphosphine–naphthoquinone pairs, respectively.