Synthesis and Photophysical Properties of Stilbeneoctasilsesquioxanes. Emission Behavior Coupled with Theoretical Modeling Studies Suggest a 3-D Excited State Involving the Silica Core

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 chan...

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

“…17,39 Similar blue shifts are not seen for the analogous [p-MeStyrPhSiO 1.5 ] 8 . 18 The objectives of the current study are to (1) Expand our previous study to the larger cages, 19,53 [PhSiO 1.5 ] 10,12 possessing equivalent chemical structures, but with different symmetries and degrees of functionality. We report here on structure−property relationships for selected multifunctional derivatives.…”

“…52 The 4-R electron donor moieties were Me, MeO, and NBoc for comparison with our previous work. 17,18 In future studies, these baseline systems can be compared with cages with push−pull or pull−pull electronic/photonic interactions across cage faces and/or through cage structures.…”

“…This "dimer" peak arises from small amounts of coupled SQ cores, i.e., double Heck reactions on the same double bond. 18 TGAs of RStyr x Ph y SQ (except for the NBoc moiety) offer excellent thermal stabilities with T d5% ≥ 400°C/air. Figure 7 provides representative TGAs with analogous data presented in Figures S21−S24 (SI).…”

“…The most important driver was the fact that in a number of our studies we found strong evidence for interaction of cage LUMOs with conjugated moieties attached directly to cage silicons. 17,18 We also have now determined in a number of these studies and on modeling the bromination reactions of these cages 19 that the LUMOs stick out beyond the cage face and as such can interact with segments of the functional groups that likely lie above cage faces.…”

Mechanistic Investigations of Organic Aggregates with Different Topologies for Optical Limiting Applications

“…[21][22][23][24] Novel cubic 3-D systems based on octasilsesquioxanes investigated in my lab demonstrated strong enhancement in two-photon absorption cross-section as compared to single stilbene building block (corner). 14,25 This result opens the way to the development real 3-D building bocks which are capable of forming highly dense threedimensional structures with strong nonlinear optical absorption.…”