Beads on a Chain (BoC) Phenylsilsesquioxane (SQ) Polymers via F^– Catalyzed Rearrangements and ADMET or Reverse Heck Cross-coupling Reactions: Through Chain, Extended Conjugation in 3‑D with Potential for Dendronization

Abstract

In this paper, we assess the utility of complementary routes to silsesquioxane based compounds using F^– catalyzed coupling to synthesize [vinylSiO_1.5]_<i>x</i>PhSiO_1.5]_{10‑<i>x</i>/12‑<i>x</i>} mixtures followed by copolymerization with divinylbenzene (via ADMET), or using reverse Heck coupling with 1,4-dibromobenzene and 4,4′-dibromo-stilbene to prepare lightly branched, nonlinear BOC systems. In another paper, we describe the use of Heck and Suzuki coupling to synthesize model conjugated <i>p</i>-R-stilbeneSQ BOCs starting from [<i>p</i>-IPh₈SiO_1.5]₈ and coupling with divinylbenzene (DVB) and 1,4-diethynylbenzene (DEB) finding extended 3-D conjugation in the DEB polymers. We find that the reverse Heck coupling (where the linker contains the bromo moieties) works best for these systems giving BoC oligomers with <i>M</i>_n of ∼6 kDa, in which extended excited state conjugation is observed for 1,4-dibromobenzene linked systems through ∼50+ nm red shifts in the emission spectra compared with DVB linked systems and model compounds. We compare and contrast the photophysical properties of the two sets of BOCs and the system where the conjugation length of the linker changes from divinylbenzene to divinylstilbene. We find that for a linker with a longer conjugation length, a red-shifted absorption and emission is observed; however, the difference in emission is much larger for the 1,4-dibromobenzene-linked system as compared to the model compounds, suggesting that a more rigid linker contributes to better orbital overlap with the cage and/or phenyl groups, increasing excited state conjugation interactions