Synthesis, characterization and photophysical properties of polyfunctional phenylsilsesquioxanes: [o-RPhSiO1.5]8, [2,5-R2PhSiO1.5]8, and [R3PhSiO1.5]8. compounds with the highest number of functional units/unit volume

Abstract

The availability of pure samples of o-Br₈OPS, 2,5-Br₁₆OPS, and Br₂₄OPS provides a rare opportunity to synthesize sets of corresponding stilbene derivatives: o-RStyr₈OPS, RStyr₁₆OPS, and RStyr₂₄OPS where R = 4-methyl (Me), Boc-protected 4-amino (NBoc), or 4-acetoxy (Ace). These derivatives show unique UV-Vis absorption and photoluminescent behavior that points to interesting interactions between the organic tethers and the silsesquioxane cage. o-RStyr₈OPS shows blue-shifts in the absorption spectra compared to p-MeStyr₈OPS, suggesting that the stilbene groups sit over and interact with the face of the electrophilic silsesquioxane cage as is the case with the parent molecule, o-Br₈OPS. The emission spectra of o-RStyr₈OPS are similar to p-MeStyr₈OPS indicating similar excited states involving the core LUMO. RStyr₁₆OPS exhibits absorption and emission spectra as well as Φ_PL similar to 1,4-distyrylbenzene, pointing to disruption in conjugation with the silsesquioxane cage because of steric interactions. RStyr₂₄OPS offers absorption maxima that are blue-shifted and emission maxima that are red-shifted relative to RStyr₁₆OPS. We speculate that RStyr₂₄OPS is so sterically hindered that interactions with the cage face must occur. NBocStyr₂₄OPS and AceStyr₂₄OPS show moderate Φ_PL and high two photon cross-section values, leading us to conclude that there are two excited states of nearly equivalent energy in these molecules with similar decay rates: a normal radiative π–π* transition and charge transfer involving the silsesquioxane cage. These same functional groups can be anticipated to offer much greater two photon absorption if different methods can be found for protecting the free amine from oxidation or replacing the acetoxy group (e.g. perhaps using alkyl or aryl groups).