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-Br 8 OPS, 2,5-Br 16 OPS, and Br 24 OPS provides a rare opportunity to synthesize sets of corresponding stilbene derivatives: o-RStyr 8 OPS, RStyr 16 OPS, and RStyr 24 OPS where R ¼ 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-RStyr 8 OPS shows blue-shifts in the absorption spectra compared to p-MeStyr 8 OPS, 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-Br 8 OPS. The emission spectra of o-RStyr 8 OPS are similar to p-MeStyr 8 OPS indicating similar excited states involving the core LUMO. RStyr 16 OPS exhibits absorption and emission spectra as well as F PL similar to 1,4-distyrylbenzene, pointing to disruption in conjugation with the silsesquioxane cage because of steric interactions. RStyr 24 OPS offers absorption maxima that are blue-shifted and emission maxima that are red-shifted relative to RStyr 16 OPS. We speculate that RStyr 24 OPS is so sterically hindered that interactions with the cage face must occur. NBocStyr 24 OPS and AceStyr 24 OPS show moderate F 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 p-p* 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).

Synthesis and Characterization of Nanobuilding Blocks [o-RStyrPhSiO1.5]10,12 (R = Me, MeO, NBoc, and CN). Unexpected Photophysical Properties Arising from Apparent Asymmetric Cage Functionalization as Supported by Modeling Studies

“…Note that this contrasts with {[(NBocStyryl) 3 Ph] 8 SiO 1.5 } 8 , which offers 12 GM/moiety (total number of moieties =24) presumably because this system has much less symmetry. 17 The design of organic compounds with very large or small TPA cross sections is a field that still needs extensive experimental and theoretical studies, especially for 3-D structured molecules. The current studies offer a novel opportunity to compare sets of 3-D structured molecules with essentially the same composition n [PhSiO 1.5 ] 10,12 with D 5h and D 2d rather than O h symmetries, respectively, and with different degrees of substitution.…”

“…In previous studies, we functionalized [Br n PhSiO 1.5 ] 8 (n = 1−3, Scheme 1) via Heck cross-coupling with 4-R-styrenes (R = Me, Acetoxy, N-Boc), developing libraries of compounds with well-defined photophysical properties. 17 The original objective was to establish the general effects of types and densities of functional groups/unit volume on the photophysical properties of O h symmetry [4-RStyr n PhSiO 1.5 ] 8 cages as they seem to exhibit 3-D electronic communication between the cage and conjugated moieties in the excited state evidenced by significant red shifts in emission λ max vs the simple chromophore. 17−20 The current studies were further motivated by our finding of unusually large two-photon absorption (TPA) cross sections for [p-4-NH 2 C 6 H 4 CHCHC 6 H 4 CH CHSiO 1.5 ] 8 cages (100 GM/moiety), 17 whereas follow-on studies on [NBoCStyrPhSiO 1.5 ] 8 where the stilbene vinyl is on the ortho carbon offered TPA values of ≈0/moiety ascribed to the O h symmetry of the molecule, resulting in self-canceling polarization (see below for further discussion).…”

“…In our previous paper, sets of stilbene-SQs were synthesized using the corresponding brominated Ph 8 SQs generating compounds with some of the highest densities of functional groups per unit volume. 17 A further objective was to compare electron-withdrawing (acceptor) moieties (e.g., 4-cyanostilbene) in the ortho position with donor moieties (e.g., NBoc), on cage phenyls for comparison with donor analogs. 40 Our measurements and complementary computational analyses suggest that the introduction of both NBoc and cyano groups leads to better electronic communication between the core and the organic moieties as evidenced by greater red shifts in emission as compared to Me and MeO derivatives.…”

“…1,4-Dioxane (3 mL) was then added by syringe, followed by NCy 2 Me (3.7 mmol, 0.8 mL) and R-styrene (8.70 mmol). 17 The mixture was stirred at 70°C for 48 h. Then the reaction solution was quenched by filtering through 1 cm Celite. Then Celite was washed with 5 mL of THF.…”

“…10 were synthesized using previously reported methods. 17 All other chemicals were purchased from SigmaAldrich, Fisher Scientific, or Strem Chemicals, Inc., and used as received.…”