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

• Published in: Polymer journal Vol. 56; no. 6; pp. 577 - 588
• Main Authors: Arias, Jose Jonathan Rubio, Zhang, Zijing, Takahashi, Manae, Mahalingam, Paramasivam, Pimbaotham, Pimjai, Yodsin, Nuttapon, Unno, Masafumi, Liu, Yujia, Jungsuttiwong, Siriporn, Azoulay, Jason, Rammo, Matt, Rebane, Aleksander, Laine, Richard M.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 14-03-2024; Nature Publishing Group
• Subjects: 119/118; 140/131; 639/624/1111; 639/638; Absorption; Biomaterials; Bioorganic Chemistry; Cages; Carbon; Charge transfer; Chemistry; Chemistry and Materials Science; Chemistry/Food Science; Conjugation; Copolymers; Emission; Excitation; Molecular orbitals; Original Article; Polymer Sciences; Polymers; Polysiloxanes; Red shift; Science; Silicon; Surfaces and Interfaces; Terpolymers; Tetracyanoquinodimethane; Thin Films
• ISSN: 0032-3896; 1349-0540
• DOI: 10.1038/s41428-024-00899-5

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
(OSiMe
3
)
3
with a missing corner and edge-opened, end-capped [PhSiO
1.5
]
8
(OSiMe
2
)
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 (F
4
TNCQ). 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, (vinylMeSiO
2
)[PhSiO
1.5
]
4
(O
2
SiMevinyl), 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)
2
SQs. 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.
a Column separation under black light illumination. b Calculated overlap of dπ-pπ orbitals demonstrating conjugation