σ–σ* conjugation Across Si─O─Si Bonds

Abstract

Polysiloxanes and silsesquioxanes (SQs) are known to be insulating materials. We describe here polysiloxane copolymers where this is not the case. Thus,Me2VinylSi─O─SiMe2Vinyl/Br-Ar-Br copolymers exhibit conjugation via Si─O─Si bonds contrary to the widespread understanding that such linkages must be insulating. Here we describe the synthesis, characterization, and photophysical properties of [-VinylSiMe2OMe2SiVinyl-Ar]x copolymers; Ar = phenyl, terphenyl, stilbene, thiophene, etc. Con-jugation is evidenced by redshifted emission λmax of copolymers vs model compounds, [(MeO)2SiMeVinyl-Ar-VinylMeSi(OMe)2], electron transfer to F4TCNQ and MW (DP) depend-ent emission red-shifts (smaller bandgaps with increasing DP). Theoretical calculations targeting electronic structure, absorbance/emission λmax of model com-pounds vs oligomers support conjugation via π-dπ* orbital interactions. In the ground state, model compounds offer Si─O─Si bond angles of ≈110° on average. In the copolymers, bond angles change in the ground state averaging ≈ 140 ° and in the excited state approach 150 ° much closer to planarity, a result of conjugation. Here SiOSi bonds facilitate intersystem charge transfer (ICT) as seen in carbon based polymers. Thus, i.e, ICT in VySiOSiVycoPh likely leads to a much larger Stokes shift (≈115 nm) than in the silane model. Our findings provide the first detailed photophys-ical studies of conjugation in polysiloxane-chromophore copolymers.