Photophysical Properties of Functionalized Double Decker Phenylsilsesquioxane Macromonomers: [PhSiO1.5]8[OSiMe2]2 and [PhSiO1.5]8[O0.5SiMe3]4. Cage-Centered Lowest Unoccupied Molecular Orbitals Form Even When Two Cage Edge Bridges Are Removed, Verified by Modeling and Ultrafast Magnetic Light Scattering Experiments

• Published in: Macromolecules Vol. 52; no. 19; pp. 7413 - 7422
• Main Authors: Guan, Jun, Tomobe, Kenji, Madu, Ifeanyi, Goodson, Theodore, Makhal, Krishnandu, Trinh, Minh Tuan, Rand, Stephen C, Yodsin, Nuttapon, Jungsuttiwong, Siriporn, Laine, Richard M
• Format: Journal Article
• Language: English
• Published: American Chemical Society 08.10.2019
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/acs.macromol.9b00700

Bromination and iodination of title double decker (DD) phenylsilsesquioxane macromonomers occurs at ortho and para positions, respectively, as in PhT8,10,12 cages. Heck cross-coupling with 4-Me/CNstyrene gives the corresponding 4-Me/CNstilbene-substituted cages. All compounds were characterized by FTIR, MALDI-TOF, TGA, NMR and GPC. These compounds show UV–vis absorptions very similar to individual stilbene analogues. However, emission for all macromonomers, except p-MeStil2Ph6DD­(OTMS)4, is redshifted 50–70 nm as seen before in full and partial cages centered LUMOs conjugated to all the stilbene moieties suggesting semiconducting behavior. Cage-centered LUMO formation even occurs in a DD cage where two of four Si–O–Si bridges are broken suggesting that LUMO formation is an extremely common phenomenon. These results are supported by both modeling studies and nonlinear light scattering in which magnetic dipole moments form and scatter light in the cage centers. The exception behaves like p-methylstilbene in both absorption and emission indicating that the onset of semiconducting behavior requires a minimum number of substituents and points to the potential to tailor band gaps and therefore multiple photophysical properties.