The general objectives of the work reported here and in accompanying papers are to identify optimal tools for the synthesis of silsesquioxane (SQ)-based BoC oligomers and polymers, especially those that show 3-D, through-chain (cage) conjugation in the excited state. Here we first examine the utility of polymerizing (p-IPhSiO 1.5 ) 8 with divinylbenzene (DVB) using Heck catalytic cross-coupling as baseline systems for BoCs with model dendrons. Thereafter, we functionalize the remaining IPh groups by cross-coupling with 4-MeOC 6 H 4 CHCH 2 or 4-NH 2 C 6 H 4 CH CH 2 . As an alternate approach, we first functionalize (p-IPhSiO 1.5 ) 8 with 4-MeOC 6 H 4 CHCH 2 or 4-NH 2 C 6 H 4 CHCH 2 by catalytic cross-coupling such that only an average of two iodophenyls remain and then polymerized with DVB or 1,4-diethynylbenzne (DEB). This latter approach permits synthesis of copolymers as demonstrated using 1:1 mixtures of the two SQs with R = NH 2 or OMe.Here we assess the utility of two different routes to conjugated BoC polymers. Investigation of the UV−vis absorption and emission properties of these BoCs indicates the DVB polymers exhibit no emission red-shifts because a significant portion of the DVB used is the 1,3-isomer. However, the DEB polymers reveal ∼40 nm red-shifts and charge transfer (CT) behavior, suggesting electronic interactions between SQ cages through the conjugated, bridging moieties. DEB copolymers with R = NH 2 − and MeO− stilbene functional groups on the cages show red-shifts intermediate between the red-shifts of the simple homopolymers, rather than independent emissions from both units as would obtain with a physical mixture again supporting electronic communication along the polymer chains and through the cages via the conjugated linkers.
Fluoride ion catalyzed rearrangement of −[vinylSiO 1.5 ] n − oligomers and polymers in THF (tetrahydrofuran) provides essentially quantitative conversion to mixtures of the three dimensional (3-D) cage compounds [vinylSiO 1.5 ] 10 and [vinylSiO 1.5 ] 12 with small amounts of the [vinylSiO 1.5 ] 14 cage. These mixtures are easily transformed into their respective styrenyl analogs by metathesis with p-R-styrene to give 100% conversion to the Generation 1 (GEN1) compounds [p-R-styrenyl-SiO 1.5 ] 10/12 . The R = Br compounds are then easily modified by Heck coupling with p-R-styrene in >90% yields and ≈100% conversion to the Generation 2 (GEN2) compounds [p-R-stilbenevinylSiO 1.5 ] 10/12 . These studies were designed to map structure−photophysical properties in these 3-D molecules with the goal of finding replacements for C 60 and C 70 electron acceptor compounds currently in use in most hybrid organic photovoltaics. Photophysical characterization indicates that the GEN2 compounds have average band gaps that are slightly smaller than their T 8 analogs. However, the C 6 F 5 derivative offers blue-shifted absorption with a very red-shifted emission in contrast to the blue emission shift that was expected coincident with the absorption blue shift. Initial cyclic voltammetry studies suggest that the GEN2 C 6 F 5 derivative has HOMO−LUMO energies that may, through further modification, provide energy levels that meet our target objectives. In addition, solvent studies targeting absorption and emission behavior find emission behavior in poor solvents for R = H, Me, MeO that suggests some form of aggregation. This aggregation red shifts emission, perhaps arising from partial interdigitation of p-R-stilbenevinyl groups. Because these molecules are 3-D, moieties opposite the points of interdigitation emit as they do in good solvents, leading to emissions that broadened greatly. Furthermore, because we have previously observed what appears to be 3-D conjugation in the excited state, these results suggest the potential to promote charge transport in three dimensions perhaps similar to C 60 /C 70 . Alternately, these same materials may serve as novel emitters for light emitting diodes.
There is continuing interest in the synthesis of polyhydroxy-terminated molecular species for diverse applications ranging from photolithographic materials to intermediates in the synthesis of porous, crosslinked polymers as media for molecular separations, drug delivery etc. We describe here the use of [vinylSiO 1.5 ] 8 and [vinylSiO 1.5 ] 10/12 mixtures to synthesize firstand second-generation acetoxyphenyl compounds via metathesis with p-acetoxystyrene (generation 1, GEN1) or metathesis with p-bromostyrene followed by Heck coupling with p-acetoxystyrene (generation 2, GEN2). The resulting acetoxy compounds were then hydrolyzed to produce octa-, deca-and dodecahydroxy GEN1 and GEN2 compounds. These compounds were purified and then reacted with adipic acid chloride to form the first examples of highly crosslinked polyesters based on silsesquioxanes. The coupling products, their hydrolyzed products and the crosslinked polymers were characterized using a variety of spectroscopic methods. In general, the observed specific surface areas were less than 5 m 2 g À1 ; however, the T 8 GEN1 derivative gave a surface area of 25 m 2 g À1 and was the only crosslinked polymer with a TGA ceramic yield that matched theory for 'perfect' crosslinking. This crosslinked polyester has the shortest organic linker between cages and despite the highly flexible C 6 linker provides continuing evidence that it is possible to use the cubic symmetry in these materials to build well-ordered 3-D nanocomposite structures.
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