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 chan...

In this paper we report the synthesis, the photophysical, conductometric, and second-order nonlinear optical (NLO) characterization of an ensamble of four NLO active organic tails nanoorganized on a cyclotetrasiloxane ring, to produce various macrocyclic NLO chromophores. The second-order NLO response of the macrocyclic NLO chromophores measured by the EFISH technique as μβ1.91, where μ is the dipole moment and β1.91 the projection along the dipole moment axis of the vectorial component of the quadratic hyperpolarizability working with an incident wavelength of 1.907 μm, increases, compared to the reference monomeric NLO chromophores, from 2.8 up to 3.5 times for nonionic macrocyclic NLO chromophores. The increase is mainly due to an increase of the dipole moment when compared to reference monomeric NLO chromophores while the β1.91 values remain almost unchanged. These macrocyclic NLO chromophores can be also considered as a simple model of a monolayer of organic NLO chromophores on a chemically engineered silica surface. Since the factor controlling their second-order NLO response is the orientation toward the dipole moment axis of the single organic NLO active tails, this kind of model confirms that the second-order NLO response of a monolayer of organic NLO chromophores on a chemically engineered silica surface is controlled by the topology of the binding sites on the surface, as suggested by previous investigations on multilayers on chemically engineered silica surface. The macrocyclic NLO chromophore with a ionic organic NLO active tail shows a strong concentration dependence of its second-order NLO response to be ascribed to a larger increase of its ionic dissociation by dilution, when compared to that of the reference ionic monomeric NLO chromophore. This behavior is evidence of a cooperative effect due to the nanoorganization, to be ascribed to a more facile ionic dissociation of the single ionic organic tail originated by a local increase of the solvent polarity favored by the closeness of the positive charges of the four organic tails. Finally it was shown that the Si(OSiMe3)3 group behaves in a classical organic push−pull NLO chromophore as a pull group as strong as the nitro group.

Multiple literature reports describe the synthesis of [RSiO(OH)] 4 or [RSiO(ONa)] 4 compounds. Surprisingly, in the majority of cases the OH or ONa groups all lie on the same face of the cyclomers or half cubes. Consequently, it appears possible to couple R containing half cubes with R' containing half cubes to form bi-functional (Janus) silsesquioxane cages or cubes. We report here synthesis of [PhSiO(ONa)] 4 and [p-IPhSiO(ONa)] 4 half cubes. We thereafter discuss efforts to react the [PhSiO(ONa)] 4 salt with (R = Me, vinyl, and cyclohexyl) RSiCl3 in MeOH to produce the compounds {PhSiO[OSi(OMe)2R]} 4 species which were characterized using traditional spectroscopic procedures. These compounds were then subjected to acid catalyzed hydrolysis tohydrolytically remove the OMe groups to generate Janus cubes. While it is possible to isolate the target compounds, which were also characterized in detail, the yields were lower than expected perhaps because of competitive hydrolytic cleavage of the Si-O-SiR(OMe) 2 linkage. To cite this article:

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Efforts to isolate the cage compounds by removal of solvent regenerates polysilsesquioxanes, albeit those containing mixtures of Me and vinyl groups. Introduction of CaCl 2 sufficient to form CaF 2 prior to workup prevents repolymerization, allowing recovery of the mixed cage systems. The approach developed here provides a novel way to form mixed functional group silsesquioxane cages. The fact that T 10 and T 12 cage formation is favored appears to suggest that these cages are more stable than the traditionally produced T 8 cages.