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 conclusiona spherical LUMO exists inside the cage.
Bromination
and iodination of title double decker (DD) phenylsilsesquioxane
macromonomers occurs at ortho and para positions, respectively, as
in PhT8,10,12 cages. Heck cross-coupling with 4-Me/CNstyrene
gives the corresponding 4-Me/CNstilbene-substituted cages. All compounds
were characterized by FTIR, MALDI-TOF, TGA, NMR and GPC. These compounds
show UV–vis absorptions very similar to individual stilbene
analogues. However, emission for all macromonomers, except p-MeStil2Ph6DD(OTMS)4,
is redshifted 50–70 nm as seen before in full and partial cages
centered LUMOs conjugated to all the stilbene moieties suggesting
semiconducting behavior. Cage-centered LUMO formation even occurs
in a DD cage where two of four Si–O–Si bridges are broken
suggesting that LUMO formation is an extremely common phenomenon.
These results are supported by both modeling studies and nonlinear
light scattering in which magnetic dipole moments form and scatter
light in the cage centers. The exception behaves like p-methylstilbene in both absorption and emission indicating that the
onset of semiconducting behavior requires a minimum number of substituents
and points to the potential to tailor band gaps and therefore multiple
photophysical properties.
Ultrafast dynamics and third-order nonlinear optical studies of tetraoxa[22]porphyrin(2.1.2.1)s are reported using a single beam Z-scan technique with femtosecond (fs) and nanosecond (ns) pulses at respective near-IR and visible wavelengths.
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