Issue 24, 2024

Conjugation through Si–O–Si bonds, silsesquioxane (SQ) half cage copolymers, extended examples via SiO0.5/SiO1.5 units: multiple emissive states in violation of Kasha's rule

Abstract

We previously reported that phenyl- and vinyl-silsesquioxanes (SQs), [RSiO1.5]8,10,12 (R = Ph or vinyl) functionalized with three or more conjugated moieties show red-shifted absorption- and emission features suggesting 3-D conjugation via a cage centered LUMOs. Corner missing [PhSiO1.5]7(OSiMe3)3 and edge opened, end capped [PhSiO1.5]8(OSiMe2)2 (double decker, DD) analogs also offer red shifted spectra again indicating 3-D conjugation and a cage centered LUMO. Copolymerization of DD [PhSiO1.5]8(OSiMevinyl)2 with multiple R–Ar–Br gives copolymers with emission red-shifts that change with degree of polymerization (DP), exhibit charge transfer to F4TNCQ and terpolymer averaged red-shifts suggesting through chain conjugation even with two (O–Si–O) end caps possibly via a cage centered LUMO. Surprisingly, ladder (LL) SQ, (vinylMeSiO2)[PhSiO1.5]4(O2SiMevinyl) copolymers offer emission red-shifts even greater for analogous copolymers requiring a different explanation. Here we assess the photophysical behavior of copolymers of a more extreme SQ form: the half cage [PhSiO1.5]4(OSiMe2Vinyl)4, Vy4HC SQs. We again see small red-shifted absorptions coupled with significant red-shifted emissions, even with just a half cage, thus further supporting the existence of pπ–dπ and/or σ*–π* conjugation through Si–O–Si bonds and contrary to most traditional views of Si–O–Si linked polymers. These same copolymers donate an electron to F4TCNQ generating the radical anion, F4TCNQ−. as further proof of conjugation. Column chromatographic separation of short from longer chain oligomers reveals a direct correlation between DP and emission λmax red-shifts as another indication of conjugation. Further, one- and two-photon absorption and emission spectroscopy reveals multiple excited fluorescence-emitting states in a violation of Kasha's rule wherein emission occurs only from the lowest excited state. Traditional modeling studies again find HOMO LUMO energy levels residing only on the aromatic co-monomers rather than through Si–O–Si bonds as recently found in related polymers.

Graphical abstract: Conjugation through Si–O–Si bonds, silsesquioxane (SQ) half cage copolymers, extended examples via SiO0.5/SiO1.5 units: multiple emissive states in violation of Kasha's rule

Supplementary files

Article information

Article type
Paper
Submitted
26 feb. 2024
Accepted
05 may. 2024
First published
30 may. 2024
This article is Open Access
Creative Commons BY license

Dalton Trans., 2024,53, 10328-10337

Conjugation through Si–O–Si bonds, silsesquioxane (SQ) half cage copolymers, extended examples via SiO0.5/SiO1.5 units: multiple emissive states in violation of Kasha's rule

Z. Zhang, J. J. R. Arias, H. Kaehr, Y. Liu, R. Murata, M. Unno, N. Yodsin, P. Pimbaotham, S. Jungsuttiwong, M. Rammo, J. Heo, J. Kim, A. Rebane and R. M. Laine, Dalton Trans., 2024, 53, 10328 DOI: 10.1039/D4DT00567H

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