Photophysical Properties of Partially Functionalized Phenylsilsesquioxane: [RSiO_1.5]₇[Me/nPrSiO_1.5] and [RSiO_1.5]₇[O_0.5SiMe₃]₃ (R = 4-Me/4-CN-Stilbene). Cage-Centered Magnetic Fields Form under Intense Laser Light

Abstract

Macromonomers [RPhSiO 1.5 ] 8,10,12 and [RCH CHSiO 1.5 ] 8,10,12 , where R is a conjugated group, have previously been shown to offer photophysical properties wherein excitation promotes an electron from the HOMO to an excited-state LUMO that sits in the center of the cage and allows communication between all conjugated groups, suggesting 3-D delocalization. In the current work, we explore replacing one conjugated group in [RPhSiO 1.5 ] 8 with either Me or nPr or s i m p l y r e m o v i n g o n e c o r n e r f r o m t h e c a g e , [RPhSiO 1.5 ] 7 (O 0.5 SiMe 3 ) 3 , and examine its effect on any potential LUMO that might form. We report here that such changes seem to have no effect on the existence of a 3-D LUMO-derived delocalization as witnessed by emission redshifts from the R = 4-Me-/4-CN-stilbene moieties essentially identical to those for the original [RPhSiO 1.5 ] 8 macromonomers. Of particular importance is the fact that removing one corner from the cage also has little effect on the photophysics, indeed significantly improving fluorescence emission quantum efficiencies. However, removing most of the conjugated groups on the corner missing cage (from 7 to 2), e.g., [MeStilSiO 1.5 ] 2 [PhSiO 1.5 ] 5 (O 0.5 SiMe 3 ) 3 , eliminates the red-shift, implying the absence of a LUMO inside the cage. This suggests a minimum number of groups are needed to form such a LUMO. Also, for the first time, the radiation patterns for nonlinear, optically induced magnetic scattering at elevated light intensities are reported for these compounds and shown to support the same conclusiona spherical LUMO exists inside the cage.

Conjugated Copolymers That Shouldn't Be

“…Our initial presumption was that these compounds should represent the limiting case where not only is there no 3D cage, but there are also two vinyl(Me/Ph)Si(O-) 2 bridges per cage in any oligomers/ polymers we might synthesize. Contrary to our original thoughts, the resulting compounds actually offer emission further red-shifted than any analogous compounds previously studied, [1][2][3][4][5][6][7]9] seeming to imply conjugation and semiconducting behavior where there should be none. In the following sections, we begin by characterizing a set of ladder copolymers and thereafter assess their photophysical properties.…”

Conjugated Copolymers That Shouldn't Be

“…Our initial presumption was that these compounds should represent the limiting case where not only is there no 3D cage, but there are also two vinyl(Me/Ph)Si(O‐) 2 bridges per cage in any oligomers/polymers we might synthesize. Contrary to our original thoughts, the resulting compounds actually offer emission further red‐shifted than any analogous compounds previously studied, [1–7, 9] seeming to imply conjugation and semiconducting behavior where there should be none . In the following sections, we begin by characterizing a set of ladder copolymers and thereafter assess their photophysical properties.…”

“…offer photophysical properties not at all reflective of organic decorated silicas but commensurate with cage centered LUMOs. [1][2][3][4][5] We recently extended this concept of excited-state 3D conjugation to sets of incomplete and modified cages including corner-missing T 8 , [R'Stilbene-SiO 1.5 ] 7 [O 1.5 SiMe/nPr], [R'StilbeneSiO 1.5 ] 7 [O 0.5 SiMe 3 ] 3 , [6] and newly developed double decker (DD) [R'StilbeneSiO 1.5 ] 8 -[OSiMe 2 ] 2 and [R'StilbeneSiO 1.5 ] 8 [O 0.5 SiMe 3 ] 4 . [7] Recent efforts on double decker SQs follow two parallel paths in their development exploring either molecular or macromolecular hybrid materials.…”

Photophysical Properties of Functionalized Double Decker Phenylsilsesquioxane Macromonomers: [PhSiO1.5]8[OSiMe2]2 and [PhSiO1.5]8[O0.5SiMe3]4. Cage-Centered Lowest Unoccupied Molecular Orbitals Form Even When Two Cage Edge Bridges Are Removed, Verified by Modeling and Ultrafast Magnetic Light Scattering Experiments

“…These redshifts reflect the formation of a cage-centered LUMO that allows excited state communication between all the appended moieties leading to 3-D conjugation suggesting semiconducting behavior. We recently reported that such LUMOs also form in SQs where one phenyl is replaced with Me/ n Pr or is completely removed (corner missing SQ) . These results suggest that 3-D LUMOs may form inside multiple types of SQ cage macromonomers, as recently also seen in germanium analogues …”