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Carbazole-Based Porous Organic Frameworks for Visible Light PhotocatalysisDepartment of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588
Patrick Brady, Jingzhi Lu and Jian Zhang*
Introduction
Synthesis Scheme of Monomers
Oxidative Polymerization
Characterization
Catalytic ReactionsConclusion and Future Work
References
• Traditional organic photochemical processes use ultraviolet (UV) light as the energy source to drive chemical reactions. However, solar irradiation on earth only contains 3% UV light. Therefore, it is important to design new catalysts systems that efficiently utilize ubiquitous visible light to promote chemical reactions.
• This research will design and synthesize heterogeneous, porous materials as new photocatalysts that use visible light to assist the transformation of organic compounds. We will also control the porosity and light absorbance of the catalysts to promote organic reactions.
• Carbazole-based porous organic frameworks (Cz-POFs) represent a new generation of green, sustainable photocatalysts because of the following features:1) Do not contain noble metals (metal free)2) Tunable porosity which allows for access of different sized substrates3) Heterogeneous in solution (reusable)4) Can be easily modified with different substituents, which modifies the HOMO-LUMO energy levels, photoredox potential, and light absorbance.
1)Nowakowska, M.; White, B.; Vogt, S. and Guillet, J. E. Studies of the antenna effect in polymer molecules. XVII. Synthesis and photocatalytic activity of poly(sodium styrenesulfonate-co-N-vinylcarbazole) and poly[sodium styrenesulfonate-co-N-(acryloyloxyhexyl)carbazole]. J. Polym. Sci. A Polym. Chem., 1992, 30, 271–277.2)Lee, Y.T.; Chang, Y.T.; Lee, M.T.; Chiang, P.H.; Chen, C.Ti and Chen, C.Ts. Solution-processed bipolar small molecular host materials for sing-layer blue phosphorescent organic light-emitting diodes. J. Mater. Chem. C. 2014, 2, 382.3)Chen, Q.; Luo, M.; Hammershøj, P.; Zhou, D.; Han, Y.; Laursen, B.W.; Yan, C.G.; Han, B.H. Microporous polycarbazole with hight specific surface area for gas storage and separation. J. Am. Chem. Soc. 2012, 134 (14), 6084-6087.
Catalytic Ability
We have designed and synthesized four carbazole based monomers with different substituents, which are confirmed using NMR analysis. The four monomer species were then polymerized and characterized by IR, UV-Vis spectroscopy, and gas adsorption analysis. Both carbazole monomers and polymers were tested in three different catalytic reactions. For all reactions, the polymer species exhibits a higher catalytic efficiency. Specifically, the polymer catalyst was at least two times more effective than the monomer for the debromination reaction. CN-Cz-POF showed a higher conversion (69%) compared to monomer (3%) for the amine oxidative coupling. For [2+2] cycloaddition, the polymer showed an increased selectivity also. In the future, we plan to analyze the electrochemical properties of the polymers to determine HOMO-LUMO energy levels and to propose the catalytic reaction mechanisms for their use in other catalytic reactions.
AcknowledgementThis material is based upon work supported by the National Science Foundation under CHE–1156560. A special thanks to the Zhang Group and the Department of Chemistry at The University of Nebraska-Lincoln for their assistance.
Figure 10. TLC plate of catalytic product from four monomers compared with one polymer (Trial 1)
Me Br NO2 CN BP
Flash Column Chromatography
Figure 1. Silica gel column used for purification (eluent: 6:1 HEX:DCM)
Byproduct
Disubstituted Product
Single Substitute Product
Figure 8. Catalysis reaction under blue LED
0.0 0.2 0.4 0.6 0.8 1.00
50100150
050
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050
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100150200250
P/P0
Me-Cz-POF
N 2 Upt
ake (c
m3 /g)
Br-Cz-POF
NO2-Cz-POF
SABET= 482 m2/g
SABET= 475 m2/g
SABET= 447 m2/g
CN-Cz-POFSABET= 500 m2/g
1 100.0
0.5
1.00.0
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1.00.0
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Pore Width (nm)
Me-Cz-POF
Nor
mal
ized
Inte
nsity
(a.u
.)
Br-Cz-POF
NO2-Cz-POF
CN-Cz-POF
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Wavenumber (cm-1)
Me-Cz-POF
Br-Cz-POF
NO2-Cz-POF
CN-Cz-POF
Polymers
Monomers
Photoluminescence
Figure 2. Digital photographs of suspensions of Cz monomers and polymers in DMF:Water (1:1, v:v) irradiated with UV lamp
Me Br CN NO2
Infrared Spectra
Figure 4. Infrared Spectra of Cz-POFs with different substituent groups
N2 Uptake
Figure 5. N2 uptake at 77 K and BET surface area for Cz-POFs with different substituents
Pore Width
Figure 6. Pore size distribution for Cz-POFs with different substituents
• Debromination
• Amine Oxidative Coupling
CN NO2 Br Me0
10
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70
Debromination
Substituent Attached
Yiel
d of
Pro
duct
(%)
CN NO2 Br Me0
10
20
30
40
5060
70
80
Amine Oxidative Coupling
Substituent Attached
Yiel
d of
Pro
duct
(%)
8.8 8.6 8.4 8.2 8.0 7.8 7.6 7.4 7.2 7.0
Chemical shift (ppm)
Me-Cz-Mon
Br-Cz-Mon
NO2-Cz-Mon
CN-Cz-Mon
NMR Spectra
Figure 3. Nuclear Magnetic Resonance (NMR) spectra for monomers
Me Br NO2CN MP BP
Figure 11. TLC plate of catalytic product from four monomers compared with two polymers (Trial 2)
[2+2] Cycloaddition
• [2+2] Cycloaddition
Monomer Polymer
Monomer Polymer
Figure 9. Catalysis reaction under white fluorescent light bulb
Figure 7. Ultraviolet-Visible light absorbance for Cz-POFs with different substituents
Me Methyl MonomerBr Bromo Monomer
NO2 Nitro Monomer
CN Cyano MonomerMP Methyl PolymerBP Bromo Polymer
UV-Vis Spectra
300 400 500 600 700 8000.0
0.2
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Nor
mal
ized
Abs
orba
nce
(a.u
.)
Wavelength (nm)
Me-Cz-POF
300 400 500 600 700 8000.0
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mal
ized
Abs
orba
nce
(a.u
.)
Wavelength (nm)
Br-Cz-POF
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1.0
Nor
mal
ized
Abs
orba
nce
(a.u
.)
Wavelength (nm)
NO2-Cz-POF
300 400 500 600 700 8000.0
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Nor
mal
ized
Abs
orba
nce
(a.u
.)
Wavelength (nm)
CN-Cz-POF
CN Starting materialStarting
material
NO2 Br Me
