Unconventional Conjugation via vinylMeSi(O−)₂ Siloxane Bridges May Imbue Semiconducting Properties in [vinyl(Me)SiO(PhSiO_1.5)₈OSi(Me)vinyl-Ar] Double-Decker Copolymers

A number of groups have invested considerable time synthesizing double-decker silsesquioxane (DD SQ) copolymers; however, to our knowledge, no one has sought to explore through-chain electronic communication between DD SQs via “conjugated” co-monomers. We recently demonstrated that stilbene derivatives of simple DD cages exhibit properties commensurate with formation of cage centered lowest unoccupied molecular orbitals (LUMOs), equivalent to LUMOs found in complete/incomplete SQ cages, [RStilbeneSiO1.5]8,10,12, [RStilbeneSiO1.5]7[O1.5SiMe/nPr], [RStilbeneSiO1.5]7[O0.5SiMe3]3, [RStilbeneSiO1.5]8[O0.5-SiMe3]4, and [RStilbeneSiO1.5]8[OSiMe2]2. Such LUMOs support the existence of 3D excited-state conjugation in these cages. We describe here Heck catalyzed copolymerization of vinyl(Me)SiO(PhSiO1.5)8OSi(Me)vinyl (vinylDDvinyl) with X–Ar–X, where X = Br or I and X–Ar–X = 1,4-dihalobenzene, 4,4′-dibromo-1,1′-biphenyl, 4,4″-dibromo-p-terphenyl, 4,4′-dibromo-trans-stilbene, 2,5-dibromothiophene, 5,5′-dibromo-2,2′-bithiophene, 2,5-dibromothieno[3,2-b]thiophene, and 2,7-dibromo-9,9-dimethylfluorene. Coincidentally model analogs were synthesized from vinylMeSi(OMe)2. All compounds were characterized in detail by gel permeation chromatography (GPC), matrix-assisted laser desorption/ionization-time-of-flight, thermogravimetric analysis, nuclear magnetic resonance, Fourier transfer infrared spectroscopy, ultraviolet–visible spectroscopy, photoluminescence spectrometry, and two-photon absorption (2PA) spectroscopy. Modeling of HOMO–LUMO energy levels of related compounds with R = Me rather than Ph was also explored. In the current systems, we again see apparent conjugation in excited states, as previously observed, as indicated by 50–120 nm red shifts in emission from the corresponding model silane compounds. These results suggest unexpected semiconducting behavior via vinylMeSi(O−)2 (siloxane) bridges between DD cages in polymers. The thiophene, bithiophene, and thienothiophene copolymers display integer charge transfer behavior on doping with 10 mol % F4TCNQ supporting excited-state conjugation; suggesting potential as p-type, doped organic/inorganic semiconductors.