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

The photophysics of [ o -4-RStyrPhSiO 1.5 ] 8  [R = Me, OMe, NBoc, and CN] was reported previously. Here we report studies on [ o -4-RStyrPhSiO 1.5 ] 10,12 , [ o -4-RStyrPhSiO 1.5 ] 3– [PhSiO 1.5 ] 7 , and [ o -4-RStyrPhSiO 1.5 ] 6 [PhSiO 1.5 ] 6  to explore cage size, geometry, and partial substitution effects on photophysical properties. All compounds were characterized by traditional methods including solution spectroscpy and two-photon absorption (TPA) cross sections and except R = NBoc offer  T d5%  ≥ 400 °C/air. All exhibit absorption and emission spectra similar to the T 8  cages but with some important differences in TPA cross sections. The R-stilbenes appear to interact in the excited state through the cage, exhibiting emission spectra red-shifted from the parent stilbenes. TPA studies show novel behavior that is functional group, geometry, and substitution number dependent. Thus, NBoc TPA  cross sections/moiety increase ,  with decreasing numbers  of functional groups from 8 to 3 for PhT 10  and 10 to 6 for PhT 12  where [NBocStyrPhSiO 1.5 ] 8 TPA/moiety ≈0 . In contrast, CN cages offer TPA/moiety values slightly greater on going from 3 to 8 (PhT 10 ) and 6 to 10 (PhT 12 ). NBoc TPA data are best explained if bromination occurs asymmetrically, leading to asymmetric functionalization and exceptional polarization in partially substituted cages as symmetrically substituted cages exhibit opposing polarizations.  In sum, all the individual induced transition dipoles on excitation mutually cancel . In contrast, both the cage and CN are strongly electron withdrawing such that no significant polarization is observed/expected when asymmetrically functionalized. Both NBoC and CN substituents offer red shifts greater than Me and MeO T 10,12 , suggesting extended conjugation without polarization. Asymmetric bromination is supported by DFT modeling studies where initial  o -Br/ o- H bonding stabilizes incoming Br 2  by 300 mEv.