Synthesis and properties of ladder-type 1,4-dihydro-1,4-phosphasilins

Abstract

The synthesis and advanced characterization of a series of extended dithieno[2,3-b:3′,2′-e][1,4-dihydro-1,4]phosphasilins is reported, and their suitability as new building blocks for organic electronics is evaluated. Synthesis of basic, as well as benzo-extended dithienophosphasilins, can be achieved using appropriate 2,3-dibromothiophene precursors in a two-step protocol introducing the silicon and phosphorus centers subsequently. Both ladder-type materials show the typical reactivity of trivalent phosphorus species and can quantitatively be converted into the corresponding oxides or gold complexes, the latter exemplified with the basic dithienophosphasilin. Their optoelectronic properties were found to be inferior to those of related dithieno[3,2-b:2′,3′-d]phospholes, indicating a disruption of the π-conjugation in the molecular scaffold. X-ray crystallographic studies revealed that the molecular scaffold is planar in the oxidized benzo-extended material, whereas the basic dithieno materials show some small deviation from planarity. Density functional theory calculations suggest all materials to be planar, with the exception of the benzo-extended trivalent phosphasilin. This structure–property study illustrates that the disruption of the π-conjugation in the molecular scaffold of the extended phosphasilins is exclusively due to the presence of the silicon center and its electronic effects, rather than structural features.