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

Abstract

The photophysics of [o-4-RStyrPhSiO1.5]8 [R = Me, OMe, NBoc, and CN] was reported previously. Here we report studies on [o-4-RStyrPhSiO1.5]10,12, [o-4-RStyrPhSiO1.5]3–[PhSiO1.5]7, and [o-4-RStyrPhSiO1.5]6[PhSiO1.5]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 Td5% ≥ 400 °C/air. All exhibit absorption and emission spectra similar to the T8 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 PhT10 and 10 to 6 for PhT12 where [NBocStyrPhSiO1.5]8 TPA/moiety ≈0. In contrast, CN cages offer TPA/moiety values slightly greater on going from 3 to 8 (PhT10) and 6 to 10 (PhT12). 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 T10,12, suggesting extended conjugation without polarization. Asymmetric bromination is supported by DFT modeling studies where initial o-Br/o-H bonding stabilizes incoming Br2 by 300 mEv.