Paddle-Wheel BODIPYâHexaoxatriphenylene Conjugates

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A: Spectroscopy, Photochemistry, and Excited States

Paddle-Wheel BODIPY-Triphenylene Conjugates – Participation of Redox- Active Hexaoxatriphenylene in Excited State Charge Separation to Yield High-Energy Charge Separated States Robert Cantu, Sairaman Seetharaman, Eric M. Babin, Paul A. Karr, and Francis D'Souza J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b01192 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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The Journal of Physical Chemistry

Paddle-Wheel BODIPY-Triphenylene Conjugates – Participation of RedoxActive Hexaoxatriphenylene in Excited State Charge Separation to Yield HighEnergy Charge Separated States Robert Cantu,† Sairaman Seetharaman,† Eric M. Babin,† Paul A. Karr,‡ Francis D’Souza†,* †

Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, TX

76203-5017, United States. ‡Department of Physical Sciences and Mathematics Wayne State College, 1111 Main Street, Wayne, Nebraska, 68787, United States ABSTRACT: Hexaoxatriphenylene, a scaffold linker often utilized in building covalent organic frameworks, is shown to be electroactive and a useful entity to build light energy harvesting donor-acceptor systems. To demonstrate this, new donor-acceptor conjugates have been synthesized by employing BODIPY as a sensitizer. Excited state electron transfer leading to high energy charge separated states, useful to drive energy demanding photocatalytic reactions, from the electron rich hexaoxatriphenylene to 1BODIPY*, in the synthesized tri BODIPY-triphenylene ‘paddle-wheel’ conjugates, has been successfully demonstrated using femtosecond transient absorption spectroscopy. The measured rate of charge separation was in the range of ~3-10 x 1011 s-1 revealing ultrafast charge separation. INTRODUCTION Research on covalent organic frameworks (COFs) has witnessed rapid growth over the years as they are promising materials due to high specific surface area, defined pore size, and structural diversity.1-4 Apart from their traditional applications in gas storage and catalysis,5-8 they have also been sought out as potential candidates for organic electronics, light energy harvesting, and redox catalysis.9-19 COFs are made by combination of organic building blocks covalently linked into extended structures using ditopic, tritopic and tetratopic linkers.1-4,19 While the choice of building blocks depends upon the sought out applications and to control pore sizes of the super structures, the linkers are primarily used to create structures of different dimensionalities.

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Research on donor-acceptor systems that are capable of undergoing photoinduced electron transfer leading to charge separated states is of paramount importance in solar energy harvesting towards a sustainable society.20-30 By molecularly engineering broad-band capturing systems, long-lived charge separated states carrying enough energy to drive catalytic reactions to manufacture high energy products are being developed by a number of research groups across the globe. In this respect, demand for better electron donors and acceptors, and novel supramolecular architectures to promote efficient excited state charge separation continues to be increasing.2030

In the development of COFs, often the scaffold linkers are assumed to be electro- and photo-inactive, and this was also to be the case for 2,3,6,7,10,11-hexahydroxytriphenylene, a tritopic linker.1-4,19 However, as demonstrated in the present study, the reaction product of this linker, hexaoxatriphenylene is electron rich31 and when connected to a suitable photosensitizer engages itself in efficient excited state charge separation process. In the present study, we have employed BF2-chelated dipyrromethenes (BODIPYs) as sensitizers32-35 which are connected to triphenylene through the center boron, rendering a paddle-wheel type structures (see Scheme 1). Systematic electrochemical, computational and photochemical studies involving femtosecond transient spectroscopy have been performed to establish efficient charge separation in these novel supramolecular structures. EXPERIMENTAL SECTION Chemicals. All the reagents were from Aldrich Chemicals (Milwaukee, WI) while the bulk solvents utilized in the syntheses were from Fischer Chemicals. Tetra-n-butylammonium perchlorate, (n-Bu4N)ClO4, used in electrochemical studies was from Fluka Chemicals. General synthesis for BODIPY starting materials: In a 1 L round bottom flask, corresponding benzaldehyde (3 mmol) and 2,4-dimethylpyrrole (0.63 g, 6.6 mmol) in THF (90 mL) were dissolved. The reaction mixture was allowed to stir for 10 minutes, then 3 drops of trifluoroacetic acid was added and the reaction mixture was stirred overnight (for about 16 hours). At the end, a solution of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (0.68 g, 3 mmol) in THF (120 mL) was introduced and the mixture was stirred continuously for another 4 hours. Next, triethylamine (18 mL, 0.13 mmol) was added, followed by dropwise addition of BF3-diethyl ethereate (18 mL, 0.15 mmol) while cooling the reaction mixture in an ice bath. The mixture was 2 ACS Paragon Plus Environment

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kept stirring at room temperature overnight, and then filtered through a bed of Celite. The residue was washed with methylene chloride (DCM), and the combined filtrate was rotary evaporated to dryness. The crude was re-dissolved in DCM (100 mL) and the solution was washed with saturated aqueous NaHCO3 solution (100 mL) followed by water (100 mL x 2). The organic layer was isolated, dried over anhydrous Na2SO4, and rotary evaporated to dryness. The crude product was purified by flash chromatography over silica using DCM to yield desired BODIPY product as an orange solid. General synthesis of (BODIPY)3-triphenylene conjugates, 1 and 2: In a round bottom flask, BODIPY (6.67 mmol) was dissolved in 150 mL of freshly distilled methylene chloride (DCM) and purged with N2 for 15 minutes. Dry AlCl3 (1.3 g, 48 mmol) was then added to the flask

and

allowed

to

stir for

an

additional

15

minutes.

Then,

2,3,6,7,10,11-

hexahydroxytriphenylene (324 mg, 1.0 mmol) was added to the solution and reaction was monitored using TLC until completion (approx. 20-30 minutes). Reaction mixture was then poured over a neutral alumina plug, washed with DCM, and roto-evaporated to dryness. The crude product is then purified by flash chromatography over silica gel (DCM:Hexane 1:1 v/v) to yield the desired (BODIPY)3-triphenylene as a dark orange solid. (BODIPY)3-Triphenylene, 1: Reaction Yield: 12% 1

H NMR (CDCl3, 400 MHz): δ (ppm): 1.38 (s, 18H, CH3), 2.17 (s, 18H, CH3), 5.95 (s, 6H,

CH-pyrrole), 7.33 (dd, J = 7.4, 1.6 Hz, H, Ar-H), 7.50 (t, J = 6.7 Hz, 9H, Ar-H), 7.88 (s, 6H, ArH). 13

C NMR (CDCl3, 100 MHz): δ (ppm): 157.9, 143.5, 139.8, 134.0, 132.4, 130.8, 129.8, 123.2,

122.5, 68.1, 38.7, 30.3, 28.9, 23.7, 23.0, 15.4, 14.8, 14.0, 10.9. HR-MS (MALDI): m/z calculated for C75H63B3N6O6 1176.5088, found 1176.5125. (Br-BOBIPY)3-Triphenylene, 2: Reaction Yield: 30%. 1

H NMR (CDCl3, 400 MHz): δ (ppm): 1.43 (s, 18H, CH3), 2.15 (s, 18H, CH3), 5.97 (s, 6H,

CH-pyrrole), 7.23 (d, J = 8.2 Hz, 6H, Ar-H), 7.67 (d, J = 8.2 Hz, 6H, Ar-H), 7.86 (s, 6H, Ar-H).



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C NMR (CDCl3, 100 MHz): δ (ppm): 157.9, 151.7, 143.4, 139.8, 133.7, 132.5, 131.9, 130.8,

128.8, 123.2, 122.6, 101.2, 68.1, 38.6, 30.4, 28.9, 23.7, 22.9, 15.4, 14.8, 14.0, 11.0. HR-MS (MALDI): m/z calculated for C75H60B3Br3N6O6 1410.240, found 1413.2433. Spectral measurements:

1

H NMR (400 MHz) and

13

C NMR (100 MHz) spectra were

recorded on a Bruker Avance instrument by using CDCl3 as solvent. 1H NMR chemical shifts are reported in parts per million (ppm) relative to the solvent residual peak. Multiplicities are given as s (singlet), d (doublet), t (triplet), q (quartet), dd (doublet of doublets), and m (multiplet), and the coupling constants, J, are given in Hz. 13C NMR chemical shifts are reported relative to the solvent residual peak and HRMS was recorded on a Bruker-Daltonics micrOTOF-Q II mass spectrometer. The UV-visible spectral measurements were carried out with a Shimadzu Model 2550 double monochromator UV-visible spectrophotometer. The fluorescence emission was monitored by using a Horiba Yvon Nanolog coupled with time-correlated single photon counting with nanoLED excitation sources. A right angle detection method was used. Differential pulse and cyclic voltammograms were recorded on an EG&G PARSTAT electrochemical analyzer using a three electrode system. A platinum button electrode was used as the working electrode. A platinum wire served as the counter electrode and an Ag/AgCl electrode was used as the reference electrode. Ferrocene/ferrocenium redox couple was used as an internal standard. All the solutions were purged prior to electrochemical and spectral measurements using nitrogen gas. Femtosecond pump-probe transient spectroscopy: Femtosecond transient absorption spectroscopy experiments were performed using an Ultrafast Femtosecond Laser Source (Libra) by Coherent incorporating diode-pumped, mode locked Ti:Sapphire laser (Vitesse) and diodepumped intra cavity doubled Nd:YLF laser (Evolution) to generate a compressed laser output of 1.45 W. For optical detection, a Helios transient absorption spectrometer coupled with femtosecond harmonics generator both provided by Ultrafast Systems LLC was used. The source for the pump and probe pulses were derived from the fundamental output of Libra (Compressed output 1.45 W, pulse width 100 fs) at a repetition rate of 1 kHz. 95% of the fundamental output of the laser was introduced into a TOPAS-Prime-OPA system with 290-2600 nm tuning range from Altos Photonics Inc., (Bozeman, MT), while the rest of the output was used for generation of white light continuum. Kinetic traces at appropriate wavelengths were assembled from the time4 ACS Paragon Plus Environment

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resolved spectral data. Data analysis was performed using Surface Xplorer software supplied by Ultrafast Systems. All measurements were conducted in degassed solutions at 298 K. The estimated error in the reported rate constants is +10%.

RESULTS AND DISCUSSION The synthesis of BODIPY-triphenylene conjugates involved replacing the fluorine atoms of the central BF2 fragment of BODIPY with vicinal oxygen atoms of a catechol fragment.36 In the present study, a one-step reaction between meso-aryl functionalized BODIPY with 2,3,6,7,10,11hexahydroxytriphenylene in the presence of dry AlCl3 in freshly distilled dichloromethane, followed by flash chromatographic purification over silica gel to yield the desired product was followed (see SI for synthetic details). The structural integrity was arrived from 1H, 13C NMR and HR-MALDI mass techniques (see Scheme 1 inset and Figure S1-S3 in the supporting information). Scheme 1. Synthesis of BODIPY-triphenylene donor-acceptor conjugates

Figure 1a shows the normalized UV-visible spectra of the BODIPY-triphenylene conjugates, 1 and 2 in o-dichlorobenzene (DCB) along with the control BODIPY monomer. All of them exhibited the typical BODIPY peak34-35 in the 500 nm range while the peak of 2 was slightly red-shifted by 3 nm due to electron withdrawing bromo substituents on the meso-phenyl 5 ACS Paragon Plus Environment


groups of BODIPY. The hexaoxatriphenylene peak appeared in the 325 nm range that was overlapped with the BODIPY peak in this spectral range. Pristine BODIPY exhibited strong fluorescence with a peak maxima at 516 nm, however, for both 1 and 2, this peak was fully quenched (over 98%, Figure 1b) suggesting occurrence of excited state processes such as energy and electron transfer in these conjugates.37

Figure 1. (a) Normalized UV-vis, and (b) fluorescence spectra of the indicated compounds in DCB. The samples were excited at the peak maxima of the visible band.


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Figure 2. (a) DPVs of the indicated compounds in DCB containing 0.1 M (TBA)ClO4. The * represents ferrocene used as an internal reference. (b) Spectral changes observed during first oxidation of 1 (Eapplied = 0.90 V vs. Ag/AgCl) in DCB containing 0.2 M (TBA)ClO4. Next, electrochemical studies were performed using cyclic (CV) and differential pulse voltammetry (DPV) techniques. Figure 2a shows DPVs of the investigated compounds in which the first oxidation and first reduction were found to be fully reversible (see Figure S4 in SI for CV). The first oxidation and first reduction of the control BODIPY were located at 1.35 and -1.12 V vs. Ag/AgCl. In contrast, both 1 and 2 revealed an additional oxidation process at 0.82 V prior to BODIPY ring oxidation, and by comparison with the oxidation potential of another control compound, 2,3,6,7,10,11-hexahydroxytriphenylene, this peak was assigned to oxidation of hexaoxatriphenylene entity. Reduction of BODIPY in 1 and 2 were located at -1.05 and -1.00 V, respectively, while oxidations of BODIPY in these conjugates (irreversible) were in the 1.30-1.40 V, not significantly different from the pristine BODIPY. These studies unequivocally proven that the spacer group used in building COFs is indeed electron rich with a facile oxidation potential. Spectroelectrochemical studies were also performed to characterize the one-electron oxidized product of 1 and 2. As shown in Figure 2b, during first oxidation corresponding to the triphenylene donor, peaks corresponding to neutral compound experienced a slight decrease in intensity of BODIPY peak at 500 nm with the appearance of a new peak at 824 nm and broad spectral features in the 400 nm range. Appearance of such radical cation signal in transient spectral measurements provide evidence of charge separation in the conjugates. In order to visualize the geometry and electronic structure, the conjugates were optimized at the B3LYP/6-311G(d,p) level.38 True minima were obtained on a Born-Oppenheimer potential energy surface. Figure 3a shows the optimized structure of 1 in two orientations. Due to bonding via the central boron (tetrahedral), the BODIPY entities assumed an orthogonal geometry to the central triphenylene entity, resulting in paddle-wheel type structures. It may also be pointed out here that the BODIPY and triphenylene were spatially very close, that is, a distance of ~2.3 Å was calculated from the central boron to the nearest carbon of triphenylene entities. The frontier HOMO was delocalized on the triphenylene entity while the LUMO was on the BODIPY entities 7 ACS Paragon Plus Environment


(Figure 3b). These results along with the earlier discussed electrochemical results establish the donor-acceptor nature of the conjugates and predict formation of BODIPY•--TP•+ (TP=hexaoxatriphenylene) charge separated states upon light illumination to excite the BODIPY. In fact, free-energy calculations performed using Rehm-Weller approach39,40 revealed photoinduced electron transfer from TP to 1BODIPY* is exothermic with ∆GCS values of -0.60+0.02 eV and ∆GCR values of -1.85+0.02 eV, respectively (where CS and CR represent charge separation and charge recombination). Here, ∆GCR = -ERIP (where ERIP is the energy stored in the radical ion pair). The magnitude of ERIP suggests high potential radical ion-pair generation in these conjugates, making them suitable to drive energy demanding photocatalytic reactions.

Figure 3. (a) B3LYP/6-311G(d,p) optimized structure of 1 in two orientations, and (b) frontier HOMO and LUMO. An energy level diagram depicting the photochemical events in the paddle-wheel BODIPYhexaoxatriphenylene donor-acceptor conjugates is shown in Figure 4. Supported by the freeenergy calculations, the 1BODIPY* produced by direct excitation of one of the BODIPY entities 8 ACS Paragon Plus Environment

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promotes charge separation to yield (BODIPY)2-BODIPY•--TP•+ charge separated state. The 1

BODIPY* thus generated could also populate the 3BODIPY* state via intersystem crossing pro-

cess, however, as demonstrated below from femtosecond transient absorption spectral studies, no evidence of 3BODIPY* via intersystem crossing was observed. Once formed, the (BODIPY)2BODIPY•--TP•+ charge separated state could charge recombine to yield the neutral (BODIPY)3TP or could populate the 3BODIPY* state prior returning to the ground state. As demonstrated below the latter path of populating 3BODIPY* seems to dominate over direct charge recombination to the ground state.

Figure 4. Energy level diagram showing different photochemical events occurring in the paddlewheel BODIPY- hexaoxatriphenylene donor-acceptor conjugates (TP=hexaoxatriphenylene). Finally, femtosecond transient absorption spectral studies were performed to establish the occurrence of photoinduced charge separation in these conjugates. The samples were excited using ultrafast pulsed laser (100 fs) tuned to 505 nm corresponding to BODIPY excitation. Figure S5 in SI shows the femtosecond spectra of the control BODIPY in DCB. The instantaneously formed 1BODIPY* revealed a depleted peak at 508 nm having contributions from both ground 9 ACS Paragon Plus Environment


state bleaching and stimulated emission. The recovery of this peak was slow that was in agreement with its relatively long lifetime of 2.8 ns.41 No new peaks, especially in the region of 825 nm where TP•+ was expected was observed.

Figure 5. Femtosecond transient absorption spectra at the indicated delay times of (a) 1 and (b) 2 in deaerated DCB. The samples were excited at 505 nm corresponding to BODIPY. Inset in each figure shows the time profiles of recovery of deplete peak at 505 nm (blue lines) and 826 nm peak corresponding to radical cation (red lines).


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Figure 6. Femtosecond transient absorption spectra at the indicated delay times of (a) 1 and (b) 2 in deaerated benzonitrile. The samples were excited at 505 nm corresponding to BODIPY. Inset in each figure shows the time profiles of recovery of deplete peak at 505 nm (blue lines) and 826 nm peak corresponding to radical cation (red lines).

Figure 7. Femtosecond transient absorption spectra at the indicated delay times of (a) 1 and (b) 2 in deaerated toluene. The samples were excited at 505 nm corresponding to BODIPY. Inset in each figure shows the time profiles of recovery of deplete peak at 505 nm (blue lines) and 826 nm peak corresponding to radical cation (red lines). In contrast, the BODIPY-triphenylene conjugates revealed transient spectral features supportive of the occurrence of ultrafast charge separation leading to BODIPY•--TP•+ charge separated states in both conjugates. Figures 5a, 6a and 7a show the transient spectra at the indicated delay times of 1 upon selective excitation of BODIPY entity in DCB, benzonitrile and toluene, respectively. The instantaneously formed 1BODIPY* within 1 ps revealed a depleted peak at 505 nm peak primarily due to ground state bleaching. Faster recovery of this peak led to new peaks at 432, 590 and 827 nm characteristic of BODIPY•--TP•+ charge separated state. Decay of the radical ion peaks was accompanied by another peak at 527 nm, characteristic of 3BODIPY*. These result suggest that the BODIPY•--TP•+ charge separated state with an estimated high energy of about 1.85 eV populates the low-lying 3BODIPY* (ET ~1.7012 eV) prior returning to ground state. Similar results were also obtained for compound 2, that is, faster recovery of the 11 ACS Paragon Plus Environment


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transient peaks corresponding to 1BODIPY* with concurrent appearance of peaks corresponding to radical ion-pairs, and finally, the charge separated state populating low-lying 3BODIPY* prior returning to the ground state (Figures 5b, 6b and 7b, respectively in DCB, benzonitrile and toluene). In order to evaluate the rate constants for charge separation, kCS, and charge recombination, kCR, the growth and decay of the 827 nm peak of the radical cation was monitored (see Figure insets for the time profiles). Such analysis yielded kCS and kCR values of 2.8 x 1011 s-1 and 7.8 x 1010 s-1 for 1, and 3.0 x 1011 s-1 and 9.2 x 1010 s-1 for 2 in DCB, respectively. Further, the electron transfer properties were investigated in four solvents of varying polarity, viz., acetonitrile (ε=37.5), benzonitrile (ε=26.0), dichlorobenzene (ε=2.8) and toluene (ε=2.38) as electron transfer properties depend upon polarity of solvent media. As shown in in Table 1, polar solvent facilitated the electron transfer rate constants, however, between 1 and 2 with little changes in their redox potentials, the measured rate constants were within the experimental error in a given solvent. The higher charge separation rate constants are understandable owing to close proximity between the donor-acceptor entities. Similarly, the close proximity, large Columbic attraction, and the low-lying triplet excited state of BODIPY would explain the higher charge recombination rate constants. Table 1. Kinetics of charge separation, kCS and charge recombination, kCR for the investigated BODIPY-triphenylene conjugates, 1 and 2, in solvents of different polarity (error = +10%). Compound 1

2

kCS, s-1

kCR, s-1

Acetonitrile

~ 1012

3.4 x 1011

Benzonitrile

~ 1012

1.1 x 1011

DCB

2.8 x 1011

7.8 x 1010

Toluene

0.9 x 1011

7.4 x 1010

Acetonitrile

~ 1012

2.8 x 1011

Benzonitrile

0.9 x 1012

1.3 x 1010

solvent


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DCB

3.0 x 1011

9.2 x 1010

Toluene

0.8 x 1011

9.5 x 1010

The kCS and kCR evaluated for 1 and 2 are comparable to values reported earlier for closely held donor-acceptor conjugates in literature with BODIPY being the sensitizer,36,43-44 except for BODIPY-C60 dyads where the kCR values were 1-2 orders of magnitude lower44 due to the charge stabilizing property of C60 in donor-acceptor systems.30 CONCLUSION In summary, we have been able to demonstrate hexaoxatriphenylene, often used as a spacer in the construction of COFs, as an electron rich entity capable of producing high energy charge separated states at ultrafast time scale when connected to an appropriate photosensitizer. The newly synthesized paddle-wheel BODIPY-triphenylene conjugates were able to full-fil these predictions. The electron donating property of hexaoxatriphenylene and acceptor property of BODIPY were established from electrochemical and computational studies. Spectroelectrochemical studies provided signature peak of TP radical cation, assisting in characterizing the electron transfer products. Using femtosecond transient absorption spectroscopic technique, we were able to characterize the electron transfer products and evaluate the solvent polarity dependent rate constants of charge separation and recombination. The measured rate constants revealed ultrafast electron transfer processes. Further studies to build COFs employing photosensitizer donor-acceptor system to generate high energy charge separated states of appreciable lifetimes useful for photocatalytic applications, are underway in our laboratory. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at: DOI: xxxx 1

H, 13C and HRMS spectra of the conjugates, cyclic voltammograms of the conjugates, and

transient absorption spectra of the control compound (PDF) AUTHOR INFORMATION 13 ACS Paragon Plus Environment


Corresponding Author * E-mail: [email protected] ORCID F. D’Souza: 0000-0003-3815-8949 Author Contributions The manuscript was written through contributions of all authors. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is financially supported by the National Science Foundation (Grant No. 1401188 to FD). We thank Prof. Guido Verbeck and the Laboratory for Imaging Mass Spectrometry at the University of North Texas for MALDI-Orbitrap Mass Spectrometry data. REFERENCES (1) Coté, A. P.; Benin, A. I.; Ockwig, N. W.; O'Keeffe, M.; Matzger, A. J.; Yaghi, O. M. Porous, Crystalline, Covalent Organic Frameworks, Science 2005, 310, 1166–1170. (2) Waller, P. J.; Gandara, F.; Yaghi, O. M. Chemistry of Covalent Organic Frameworks, Acc. Chem. Res. 2015, 48, 3053-3063. (3) Feng, X.; Ding, X.; Jiang, D. Covalent Organic Frameworks, Chem. Soc. Rev. 2012, 41, 6010-6022. (4) Colson, J. W.; Dichtel, W.R. Rationally Synthesized Two-Dimensional Polymers, Nat. Chem. 2013, 5, 453-465. (5) Furukawa, H.; Yaghi, O. M. Storage of Hydrogen, Methane, and Carbon Dioxide in Highly Porous Covalent Organic Frameworks for Clean Energy Applications, J. Am. Chem. Soc. 2009, 131, 8875–8883. (6) Han, S. S.; Furukawa, H.; Yaghi, O. M.; Goddard, W. A., III Covalent Organic Frameworks as Exceptional Hydrogen Storage Materials, J. Am. Chem. Soc. 2008, 130, 11580– 11581. (7) Doonan, C. J.; Tranchemontagne, D. J.; Glover, T. G.; Hunt, J. R.; Yaghi, O. M. Exceptional Ammonia Uptake by a Covalent Organic Framework, Nat. Chem. 2010, 2, 235– 238. 14 ACS Paragon Plus Environment

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