Conjugation in polysiloxane copolymers via unexpected Si-O-Si dπ-pπ overlap, a second mechanism?

We previously reported that functionalized phenyl- and vinyl-silsesquioxanes (SQs) and [RSiO_1.5]_8,10,12 (R = Ph or vinyl) exhibited redshifted absorption and emission, suggesting 3-D conjugation via a cage-centered lowest unoccupied molecular orbital (LUMO). The functionalized [PhSiO_1.5]₇(OSiMe₃)₃ with a missing corner and edge-opened, end-capped [PhSiO_1.5]₈(OSiMe₂)₂ (double decker, DD) analogs also exhibit emission redshifts, indicating 3-D conjugation. DD [PhSiO_1.5]₈(OSiMevinyl)₂ and R-Ar-Br copolymers exhibit polymerization (DP)-dependent emission λ_max and integer charge transfer (ICT) to 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TNCQ). The terpolymer-averaged redshifts all suggest conjugation with two (O-Si-O) endcaps, possibly via a cage-centered LUMO. In assessing conjugation limits, it was anticipated that copolymers of the ladder (LL) SQ, (vinylMeSiO₂)[PhSiO_1.5]₄(O₂SiMevinyl), with Br-Ar-Br and without a cage would eliminate LUMO formation and a redshift. The λ_max values observed were greater for analogous copolymers, which requires a different explanation. Here, we assess the photophysical behavior of copolymers closer to polysiloxanes, namely, the expanded cage (MeVinylSiO)₂[PhSiO_1.5]₈(OSiMeVinyl)₂SQs. Copolymers with Br-Ar-Br exhibit redshifted absorption and emission, which supports conjugation via Si-O-Si bonds rather than cage-centered LUMOs, contrary to traditional views of Si-O-Si copolymers. One- and two-photon photophysical probes showed that XDD copolymers exhibit multiple fluorescence-emitting excited states, in violation of Kasha’s rule stating that emission should occur only from the lowest excited state. Finally, new modeling studies suggested that conjugation derives from Si-O-Si bond dπ-pπ interactions, an unexpected result for polysiloxanes that supports two forms of conjugation.