Chapter 8
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Silole-Based Cyclosiloxanes with High Solid-State Fluorescence Quantum Yields and Their AIE Properties Yuanjing Cai and Robert West* Department of Chemistry, University of Wisconsin-Madison, 1101 University Ave., Madison, Wisconsin 53706, United States *E-mail:
[email protected] or
[email protected] A series of fifteen novel cyclosiloxanes containing various silole-based fluorogenic units (silafluorene, 1,3-diphenyl9-silafluorene and tetraphenylsilole) was synthesized by facile cohydrolysis and condensation reactions and their photophysical properties (UV-vis, PL) in solution and in solid state were studied. All fifteen cyclosiloxanes display strong fluorescence in the uv to greenish blue region. While they generally show relatively low quantum yields in solution (Φfl = 0.00−0.18), they all have unusually high solid-state quantum yields (Φfl = 0.35−0.99). Extensive studies of their AIE properties were undertaken. While silafluorene-based cyclosiloxanes do not display any significant AIE effect, tetraphenylsilole-containing cyclosiloxanes have turned out to be remarkably AIE-active with the roughly 100-fold increase of PL intensities being observed for some of them. A thorough analysis of molecular packing patterns in the solid state of these compounds and theoretical calculations were performed to rationalize observed high solid-state quantum yields and gain more insight into the fluorescence mechanism operative in these compounds. Finally, several potential real life applications are introduced and discussed.
© 2016 American Chemical Society Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.
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1. Introduction Organic light emitting diodes (OLEDs) have been one of the major foci of industrial and academic research alike since the late 1980s, when the low voltage organic electroluminescence in small and polymeric organic molecules, was reported. Since that time, OLEDs have found a wide use in displays and eco-friendly lighting applications. The OLED technology offers full color displays at significantly reduced manufacturing costs, larger viewing angles, more flexibility, lower power consumption, better contrast, and can be used to make screens large enough for laptops, cell phones, desktop computers, TVs, etc. It holds an endless potential for future application (e.g. OLED materials deposited on plastic and/or other customary construction materials could be used to create wall-size video panels, roll-up screens for laptops, head wearable displays, etc.) and opens up a plethora of completely new design possibilities for lighting applications. While OLEDs in general outperform LCDs and LEDs in terms of flexibility, brightness and energy efficiency, they do have certain disadvantages when it comes to lifetime, manufacturing costs and environmental resistance of the organic semiconducting materials that are at the heart of each OLED device. Research targeting development and discovery of novel more efficient, more durable OLED materials is thus of utmost importance. Interestingly, most organic luminescent materials do not display any photoor electroluminescence in the solid state due to the ACQ (aggregation-caused quenching) phenomenon and thus are not suitable for OLED applications. It’s mainly due the so-called AIE (aggregation-induced quenching) effect discovered and reported by B. Z. Tang in 2001 that some fluorescent materials not only remain fluorescent but also display up to a hundred-fold increase in PL or EL in the solid state (thin films) and thus are ideal for OLED applications (1). However, the handful of materials known to show such an “abnormal” emissive behavior is based only on a few fluorogenic fragments: Silole, Tetraphenylethylene and 8-Hydroxyquinoline. Among them, siloles derivatives of an unsaturated five-membered silacycle, are the ones with the most promising optoelectronic properties, the broadest potential for their further optimization and possible applications (e.g. fluorescent probes, molecular thermometers, drug delivery carriers, chemosensors, etc). In structural terms, siloles may be classified as two types, siloles of type I with the silole ring fused with other aromatic rings – silafluorenes - and siloles of type II – with multiple substituents in 2,3,4,5 positions of the silole ring (e.g. tetraphenylsiloles). While siloles of type I do not display any significant AIE effect, they usually emit in the UV or deep blue regions (2, 3) which makes them highly desirable as solid-state blue emitting devices for realization of full-color displays or host materials for green or red dopant (4). Siloles of type II are known to be extremely AIE active due to the floppy phenyl substituents, and are thus among the molecular silole-based compounds with the most promising opto-electronic properties. The incorporation of siloles in conjugated polymers is further of interest and importance for practical applications as some opto-electronic properties, not inherent to siloles on the individual molecular level, can be realized in silole-containing polymers (SCPs). 138 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.
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However, although a handful of silole-based molecular and polymeric materials is known to date, only a few of them possess photophysical properties (such as very high solid-state quantum yields) required to be considered for real-life OLED applications. Against this backdrop we set out to develop a facile and robust chemical approach to extend the scope of silole-based derivatives with optimized photophysical properties (high solid-state quantum yields) that would be suitable for use in the OLEDs and/or promising precursors for fluorescent polymers for a wider range of possible applications. This chapter gives a detailed account of the syntheses, isolation, characterization and superior photophysical properties of these highly fluorescent ring-shaped silole derivatives. A tentative explanation for the extraordinary high solid-state quantum yields of the compounds based on theoretical calculations and detailed X-ray studies is provided (5–8).
2. Design of Silole-Based Cyclosiloxanes with High Solid-State Fluorescence Quantum Yields 2.1. The Synthesis and Characterization of Silole-Based Cyclosiloxanes The silole-based cyclosiloxanes discussed in this chapter contain silafluorene, phenyl-substituted silafluorene and tetraphenylsilole as fluorogens. The synthetic routes to these compounds are shown in Scheme 1. 9-Dichloro-9-silafluorene was synthesized starting from 2,2′-dibromobiphenyl via its lithiation and susequent quenching of the intermediate organolithium compound with SiCl4 (9). It was used together with dichlorosilanes as starting materials in the cohydrolysis reactions to synthesize cyclosilafluorene derivatives. Using Et2O and Et3N in the reaction favors the formation of cyclic silafluorene derivatives due to their poor solubilities. Sublimation of the crude product gives the final compound as a white solid. 9,9-Dichloro-1,3-diphenyl-9-silafluorene was synthesized according to a literature procedure (10–12). The white solid phenyl-substitued cyclosiloxane was synthesized by cohydrolysis reaction and purified by crystallization. 1,1-Dichloro-2,3,4,5-tetraphenylsilole was synthesized according to a literature procedure (13) and purified by crystallization in Et2O. The hydrolysis of this compound results in a yellow powder of 1,1-dihydroxyl2,3,4,5-tetraphenylsilole (14). Condensation reaction between dihydroxyltetraphenylsilole and dichlorosilanes gives tetraphenylsilole cyclosiloxanes. All these compounds were purified by crystallization in Et2O. The characterization of the above mentioned silole-based cyclosiloxanes includes 1H-NMR, 13C-NMR, and 29Si-NMR spectroscopy, single-crystal X-ray diffraction, UV-Vis, ESI-MS/MALDI-MS and elemental analysis. Their crystal structures and CCDC numbers are shown in Scheme 2. The crystal structure of the compound cyclotetraphenylsilole (CCDC number 1010357) is a solvatomorph of the crystal of the same compound (CCDC number 1010359). The solvatomorphism means that systems in which the crystal structures of the substance are defined by different unit cells, but wherein these unit cells differ in their elemental composition through the inclusion of one or more 139 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.
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molecules of solvent. In general, most bond lengths and angles of the silole unit in these cyclosiloxanes are very similar to those found in other silole-containing counterparts in the CSD database. The C-Sitetraphenylsilole-C bond angles (93.80°-94.77°) are all larger than C-Sisilafluorene-C bond angles (92.6°-93.43°). The planarity of these compounds was determined by fitting a plane to the Si and O atoms making up the siloxane ring and measuring the root mean square deviation (RMSD) of those atoms from the plane. Except cyclotetrasilafluorene (CCDC No.1008136, RMSD=0.45), all the other silole-based cyclosiloxanes have nearly planar cyclosiloxane rings (RMSD
