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 ΦPL 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 ΦPL 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).