Ladder-type pi-conjugated 4-hetero-1,4-dihydrophosphinines: a structure-property study

Pi-conjugated six-membered 1,4-dihydrophosphinines containing a heteroatom (Si, P, S) at the 4 position were synthesized and systematically studied. X-ray crystallographic analyses showed that the central six-membered heterocyclic rings are almost planar. The sum of the angles around the phosphorus atom increases by 23 degrees from the trivalent phosphorus to the phosphonium atom in the thiaphosphinine system, which is consistent with the NMR spectroscopic studies. UV/Vis spectroscopy and theoretical calculations revealed that the communication between the phosphorus center and the benzothiophene moiety is enhanced by the incorporation of a sulfur atom into the molecular scaffold. The increased conjugation endows the thiaphosphinines with interesting emission properties. Theoretical calculations supported the postulation that the orbital coupling between the pi system and a sigma* orbital could be enhanced in the thiaphosphinine system, especially through a phosphonium center. Cyclic voltammetry studies revealed that the thiaphosphinine oxide, thiaphosphonium, and cis-diphosphinine oxide exhibit quasi-reversible reduction processes, which demonstrate that simple changes in the bridge heteroatoms help to efficiently tune the redox properties of the ladder-type 4-hetero-1,4-dihydrophosphinines.