ABSTRACT
A set of stilbene-substituted octasilicates [p-RStil(x)Ph(8-x)SiO(1.5)](8) (R = H, Me, MeO, Cl, NMe(2) and x = 5.3-8) and [o-MeStilSiO(1.5)](8) were prepared. Model compounds were also prepared including the corner and half cages: [p-MeStilSi(OEt)(3)], [p-Me(2)NStilSi(OSiMe(3))(3)], and [p-Me(2)NStilSi(O)(OSiMe)](4). These compounds were characterized by MALDI-TOF, TGA, FTIR, and (1)H NMR techniques. Their photophysical properties were characterized by UV-vis, two-photon absorption, and cathodoluminescence spectroscopy (on solid powders), including studies on the effects of solvent polarity and changes in concentration. These molecules are typically soluble, easily purified, and robust, showing T(d(5%)) > 400 degrees C in air. The full and partial cages all show UV-vis absorption spectra (in THF) identical to the spectrum of trans-stilbene, except for [o-MeStilSiO(1.5)](8), which exhibits an absorption spectrum blue-shifted from trans-stilbene. However, the partial cages show emissions that are red-shifted by approximately 20 nm, as found for stilbene-siloxane macrocycles, suggesting some interaction of the silicon center(s) with the stilbene pi* orbital in both the corner and half cages. In contrast, the emission spectra of the full cages show red-shifts of 60-100 nm. These large red-shifts are supported by density functional theoretical calculations and proposed to result from interactions of the stilbene pi* orbitals with a LUMO centered within the cage that has 4A(1) symmetry and involves contributions from all Si and oxygen atoms and the organic substituents. Given that this LUMO has 3-D symmetry, it appears that all of the stilbene units interact in the excited state, consistent with theoretical results, which show an increased red-shift with an increase in the functionalization of a single corner to functionalization of all eight corners with stilbene. In the case of the Me(2)N- derivatives, this interaction is primarily a charge-transfer interaction, as witnessed by the influence of solvent polarity on the emission behavior. More importantly, the two-photon absorption behavior is 2-3 times greater on a per p-Me(2)Nstilbene basis for the full cage than for the corner or half cages. Similar observations were made for p-NH(2)stilbenevinyl(8)OS cages, where the greater conjugation lengths led to even greater red-shifts (120 nm) and two-photon absorption cross sections. Cathodoluminescence studies done on [p-MeStilSiO(1.5)](8) or [p-MeStilOS](8) powders exhibit essentially the same emissions as seen in solution at high dilution. Given that only the emissions are greatly red-shifted in these molecules, whereas the ground-state UV-vis absorptions are not changed from trans-stilbene, except for the ortho derivative, which is blue-shifted 10 nm. It appears that the interactions are only in the excited state. Theoretical results show that the HOMO and LUMO states are always the pi and pi* states on the stilbene, which show very weak shifts with increasing degrees of functionalization, consistent with the small changes in the UV-vis spectra. The band gap between the lowest unoccupied 4a1 symmetry core state localized inside the silsesquioxane cage and the highest occupied state (pi state on stilbene), however, is markedly decreased as the number of stilbene functional groups is increased. This is consistent with the significant red-shifts in the emission spectra. The results suggest that the emission occurs from the 4a1 state localized on the cage. Moreover, for the compounds [p-RStil(6-7)Ph(2-1)OS](8), the emissions are blue-shifted compared to those of the fully substituted compounds, suggesting the molecular symmetry is reduced (from cubic), thereby reducing the potential for 3-D delocalization and raising the energy of the LUMO. The implications are that these octafunctional molecules exhibit some form of 3-D interaction in the excited state that might permit their use as molecular transistors as well as for energy collection and dispersion as molecular antennas, for example, and for nonlinear optical applications.
ABSTRACT
Polyphenylsilsesquioxane [PhSiO(1.5)](n) (PPS) and polyvinylsilsesquioxane [vinylSiO(1.5)](n) (PVS) are polymeric byproducts of the syntheses of the related T(8) octamers [PhSiO(1.5)](8) and [vinylSiO(1.5)](8). Here we demonstrate that random-structured PPS and PVS rearrange in the presence of catalytic amounts of Bu(4)N(+)F(-) in THF to form mixed-functionality polyhedral T(10) and T(12) silsesquioxane (SQ) cages in 80-90% yields. Through control of the initial ratio of starting materials, we can statistically tailor the average values for x for the vinyl(x)Ph(10-x)T(10) and vinyl(x)Ph(12-x)T(12) products. Metathetical coupling of x approximately = 2 vinyl cages with 4-bromostyrene produces SQs with an average of two 4-bromostyrenyl substituents. These products can be reacted via Heck coupling with vinylSi(OEt)(3) to produce SQs with vinylSi(OEt)(3) end-caps. Alternately, Heck coupling with the originally produced x approximately = 2 vinyl SQs leads to "beads on a chain" SQ oligomers joined by conjugated organic tethers. The functionalized T(10) and T(12) cages, metathesis, and Heck compounds were characterized by standard analytical methods (MALDI-TOF MS, (1)H and (13)C NMR spectroscopy, TGA, and GPC). MALDI confirms the elaboration of the cages after each synthetic step, and GPC verifies the presence of higher molecular weight SQ oligomers. TGA shows that all of these compounds are thermally stable in air (>300 degrees C). The UV-vis absorption and emission behavior of the Heck oligomers reveals exceptional red-shifts (> or = 60 nm) compared to the vinylSi(OEt)(3) end-capped model compounds, suggesting electronic interactions through the SQ silica cores. Such phenomena may imply 3-D conjugation through the cores themselves.
