Synthesis and High Solid-State Fluorescence of Cyclic Silole

Dec 11, 2014 - Department of Chemistry, The Hong Kong University of Science .... Similar photophysical properties of 1–3 in the solid state were obser...
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Synthesis and High Solid-State Fluorescence of Cyclic Silole Derivatives Yuanjing Cai,*,†,‡ Kerim Samedov,‡ Brian S Dolinar,‡ Zhegang Song,§ Ben Zhong Tang,§ Chaocan Zhang,† and Robert West*,‡ †

School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, People’s Republic of China Department of Chemistry, University of Wisconsin−Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States § Department of Chemistry, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Cyclotetrasiloxanes 1−3 containing different silole-based fluorogenic units (silafluorene, 1,3-diphenyl-9-silafluorene, tetraphenylsilole) were synthesized by cohydrolysis and condensation reactions. Their optical properties in solution and as crystals were studied. These compounds have low quantum yields in solution (Φfl = 0.01−0.18) with fluorescence maxima at 359−375 nm for silafluorene-containing compounds 1 and 3 and at 491 nm for AEE-active tetraphenylsilole compound 2. However, 1−3 have high solid-state quantum yields (Φfl = 0.65−0.78) with fluorescence maxima at 377−390 nm for compounds 1 and 3 and at 517 nm for tetraphenylsilole- and silafluorene-containing compound 2. Packing analysis of 1−3 in the crystal structure and MO and excited-state calculations were performed to explore possible fluorescence mechanisms in these compounds.

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INTRODUCTION In recent years there has been considerable interest in electroand photoluminescent materials because of the great variety of their applications in light-emitting diodes, charge storage devices, field effect transistors, photodiodes, etc.1 Current research in this area has been driven by the ever-growing need of novel materials as well as further optimization of their electro- and photoluminescent properties. Siloles−a class of silicon-containing compounds analogous to cyclopentadiene with a remarkably small HOMO−LUMO gap,2 intrinsically high electron mobility,3 favorable photophysical properties (near UV to blue light emission peak range: 350−520 nm),4 and high fluorescence efficienciesare nowadays among the most popular and promising structural building blocks in molecular and polymer-based materials used for manufacturing high-performance electro- and photoluminescent devices.4b,5 Given the high degree of their structural variability,6 their electronic and optical properties can be tuned through a multitude of methods such as polymerization,6a,b attaching conjugating substituents7 or highly fluorescent groups to the silicon atom,8 and structural modification of peripheral phenyl moieties.6c Previously, we demonstrated that wellknown silole derivatives combined with highly fluorescent bis(diquinaldinatoalumino) ligands yield compounds displaying a 4- to 6-fold increase in solid-state fluorescence efficiency.8−10 Against the backdrop of our latest discovery that highly fluorescent ring-shaped siloxanes containing exclusively silolebased fluorophorstetraphenylsilole (Scheme 1a) or silafluor© XXXX American Chemical Society

Scheme 1. Different Silole Fluorogenic Units

ene (Scheme 1b)are easily accessible through a simple cohydrolysis reaction between corresponding dichlorosilole derivatives and dichlorosilane under ambient conditions,11,12 it appeared of interest to us to test if combining different fluorogenic units in this structural framework would have a similar effect on the fluorescence efficiencies of their derivatives. We thus set out to extend our studies of ring-shaped silole derivatives and their photophysical properties by synthesizing a series of near-UV or blue-light-emitting compounds containing well-known fluorogenic units such as phenyl, 1,3-diphenyl-9silafluorene (Scheme 1c), 9-silafluorene, and tetraphenylsilole in the siloxane framework. Here, we report the synthesis, characterization, and photophysical properties, including AEE (aggregation-enhanced emission: a phenomenon of enhanced emission upon Received: September 1, 2014

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dx.doi.org/10.1021/om500884b | Organometallics XXXX, XXX, XXX−XXX

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crystal X-ray diffraction (see SI, Table S1 for details). The bond angles and bond distances of silole units in these structures are all within the range of those found in the Cambridge Structural Database (SI, Table S2).13 The crystal structures of trans/cis-1, 2, and 3 are shown in Figure 1. Crystals of 1 (2:1 ratio of stereoisomers) and 2 and 3 were used in UV and fluorescence studies. Figure 2 displays the photophysical properties of 1−3 in THF solution. The UV−vis absorption maxima, ascribed to the π−π* transition of the silole fluorophores, are at 279, 291 nm (silafluorene fluorophore), 265 nm (1,3-diphenyl-9-silafluorene fluorophore), 367 nm (tetraphenylsilole fluorophore), respectively. Compared to the emission peak at 359 nm for 1, containing silafluorene fluorophores, compound 3, containing phenyl-substituted silafluorene fluorophores, displays a ∼15 nm red-shift of the fluorescence emission peak in solution. Although 2 has a very small peak at 388 nm resulting from the silafluorene fluorophores, the emission peak of 2 in solution is mainly attributed to the tetraphenylsilole moieties (491 nm), and its intensity is ∼9-fold larger than the intensity of the peak at 388 nm. Although the solution quantum yields for silafluorenecontaining compounds are usually around 0.15,11 as mentioned previously, the emission intensity of the silafluorene unit in the compound 2 is quite low and the solution quantum yield of 2 is close to zero. The spectral overlap between the absorption band of the tetraphenylsilole (maximum at 367 nm) and the emission band of the silafluorene unit (maximum at 388 nm) and relatively short distance between these two neighboring fluorogenic units ( 300 °C. Anal. Calcd (%) for C80H56O4Si4: C 80.50, H 4.73. Found: C 80.39, H 4.70. UV (THF, 0.88 × 10−5 mol/L, rt), λmax (nm)/εmax (mol−1 L cm−1): 279/3.75 × 104, 291/3.09 × 104, 326/1.10 × 104, 367/1.25 × 104. IR (νmax/cm−1): 3074w (C−HAr), 3061w (C−HAr), 3020w (C−HAr), 2993w (C−HAr), 1597m (CCAr), 1572w (C CAr), 1562w, 1535w, 1487m, 1460w, 1441m, 1433m, 1308w, 1132s (Si−CAr), 1092vs (Si−O−Si), 1037s (Si−O−Si), 1026s, 1007s, 995s, 941m, 908m, 844m, 792s, 785s, 762s, 742s. 1H NMR (400.182 MHz, CDCl3), δ (TMS, ppm): 7.70−7.68 (d, Ar−H), 7.36−7.24 (m, Ar− H), 6.98−6.84 (m, Ar−H), 6.58−6.57 (d, Ar−H), 6.43−6.41 (d, Ar− H). 13C{1H} NMR (125.743 MHz, CDCl3), δ (TMS, ppm): 154.09, 146.14, 138.54, 137.18, 132.04, 131.24, 131.01, 129.62, 128.91, 128.37, 128.25, 127.19, 126.40, 125.73, 120.19. 29Si{1H} NMR (99.3796 MHz, CDCl3), δ (TMS, ppm): −38.57, −40.89. EMM-ESI-TOF-MS: m/z calcd. for [C80H56O4Si4+H]+ 1193.3329; found 1193.3307. Synthesis of 2,6-Dimethyl-2,6-diphenyl-4,4,8,8-bis(2,2′-(3,5diphenylbiphenyl))cyclotetrasiloxane (3). A solution of 9,9-dichloro-1,3-diphenyl-9-silafluorene (3.82 g, 9.48 mmol) and dichloro(methyl)phenylsilane (1.54 mL, 9.48 mmol) in THF (20 mL) was added dropwise at room temperature into a 100 mL round-bottom flask containing water (0.68 mL, 37.92 mmol), triethylamine (5.28 mL, 37.92 mmol), and Et2O (40 mL). The mixture was stirred overnight. The reaction mixture was filtered and washed with THF (80 mL). After removing the solvent under reduced pressure, toluene (30 mL) and hexane (10 mL) were added, and colorless crystals of compound 3 were grown upon slow evaporation of solvent. Yield: 0.32 g, 7% (based on 9,9-dichloro-1,3-diphenyl-9-silafluorene). Mp > 300 °C. Anal. Calcd (%) for C62H48O4Si4: C 76.82, H 4.99. Found: C 76.87, H 5.03. UV (THF, 1.05 × 10−5 mol/L, rt), λmax (nm)/εmax (mol−1 L cm−1):

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ASSOCIATED CONTENT

S Supporting Information *

NMR, single-crystal X-ray crystallography, IR, UV−vis, and fluorescence spectra, packing interactions analysis, MO and excited-state calculation details, a text file of all computed molecule Cartesian coordinates in a format for convenient visualization. The crystal structures are available free of charge from the Cambridge Crystallographic Data Center. The CCDC codes are 1010363, 1010364, 1010365, 1010366, and 1010367 for compounds 1-a, 1-b, 2, 3, and S1, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail (Y. Cai): [email protected]; gardencyj@ hotmail.com. *E-mail (R. West): [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y. Cai thanks the China Scholarship Council. We acknowledge Haley Albright for helping with synthesis and Prof. Trisha Andrew (University of Wisconsin−Madison) for providing instruments for fluorescence measurements. We also thank Yuning Hong (Hong Kong University of Science & Technology) for the solid-state fluorescence quantum yield measurements. We acknowledge Ilia A. Guzei (Molecular Structure Laboratory, University of Wisconsin−Madison) for his contribution to single-crystal related matters. NMR spectra were financed by NSF CHE-9208463 and NSF CHE-9629688. Mass spectrometry was partially funded by NSF Award CHE9974839 to the Department of Chemistry. Molecular orbital calculations were supported in part by National Science Foundation Grant CHE-0840494.

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