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Covalently Bound Nitroxyl Radicals in an Organic Framework Barbara K Hughes, Wade A, Braunecker, David C. Bobela, Sanjini U. Nanayakkara, Obadiah George Reid, and Justin C. Johnson J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01711 • Publication Date (Web): 01 Sep 2016 Downloaded from http://pubs.acs.org on September 4, 2016

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The Journal of Physical Chemistry Letters

Covalently Bound Nitroxyl Radicals in an Organic Framework Barbara K. Hughes,† Wade A. Braunecker,† David C. Bobela,† Sanjini U. Nanayakkara,† Obadiah G. Reid,‡ Justin C. Johnson†,* †

‡

National Renewable Energy Laboratory, 15013 Denver West Pkwy, Golden, CO 80401,

Renewable and Sustainable Energy Institute, University of Colorado, Boulder, CO 80309

ABSTRACT: A series of covalent organic framework (COF) structures is synthesized that possesses a tunable density of covalently bound nitroxyl radicals within the COF pores. The highest density of organic radicals produces an electron paramagnetic resonance (EPR) signal that suggests the majority of radicals strongly interact with other radicals whereas for smaller loadings the EPR signals indicate the radicals are primarily isolated but with restricted motion. The dielectric loss as determined from microwave absorption of the framework structures compared with an amorphous control suggests that free motion of the radicals is inhibited when more than 25% of available sites are occupied.

The ability to tune the mode of radical

interactions and the subsequent effect on redox, electrical, and optical characteristics in a porous framework may lead to a class of structures with properties ideal for photoelectrochemistry or energy storage.

*Corresponding author: [email protected]

ACS Paragon Plus Environment


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TOC graphic

Keywords: covalent organic framework, conductivity, organic radical


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Covalent organic frameworks (COFs) are a class of ordered modular structures containing molecular units that can possess particular functionality. COFs have been suggested as useful in gas sensing, storage, catalysis, etc.1-3 Recently, schemes for producing COFs that are stable in many chemical environments and those that exhibit fairly good ordering have been presented,4-7 expanding potential opportunities in new technological areas. Whereas pore filling is a natural avenue to explore in order to modify the native COF properties, the concept of pore surface engineering8 through chemical substitution has been more recently introduced to exert further control over properties like selectivity for molecular binding and catalysis.9-10 Using similar strategies for control of (photo)conductivity properties has the potential to be a powerful approach as COF applications expand into the realm of optoelectronics and batteries.11-12 For traditional organic photoelectrodes (e.g., polymers or molecular crystals), the task of post-synthetically adding chemical species to reliably and systematically alter the properties is challenging. One prominent example is the “doping” of polymers with chemical species that inject holes or electrons into the organic crystal or polymer.13 These methods are typically ad hoc and not well-understood, since the real chemical interaction that produces “doping” can be difficult to clarify. Moreover, the disruption of ordering or structure by such dopants can reduce charge mobility and introduce unwanted effects.14 COFs offer the advantage that once the frameworks are in place, the pores may be subsequently chemically modified in a controlled fashion, without a significant effect on the framework structure.8 For example, recent reports have suggested that the charge-transfer complex 7,7,8,8-tetracyanoquinodimethane (TCNQ) serves as an electrical dopant for a 2D copper-based metal-organic framework (MOF)15 or a tetrathiafulvalene-based COF16 when infiltrated into the framework pores. Doping with I2 was also demonstrated.17 Covalently attaching particular chemical agents might be even more



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attractive18 because the exact configurations that the linked molecules adopt can be controlled. Additionally, through-bond interactions, such as those that induce charge-transfer, can be engendered. (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) is a well-studied organic radical because of its relative stability and its role in catalysis and polymerization. The use of the radical character as a marker for structural information is also highly advantageous. Utilization of TEMPO units in a variety of materials has been shown to facilitate charge conduction, storage, and electrocatalytic activity.19-23 The potential benefits of TEMPO radicals could be further leveraged in an ordered framework. Controlled TEMPO arrangements and binding motifs may create specific environments for interactions with chemical agents flowing through the pores, such as gases or ions, which would further enable chemical or charge storage applications.24 The redox properties of TEMPO may also allow it to serve as a catalyst in reactions such as oxygen reduction for fuel cells.22 Finally, by appropriate interaction with the framework and controlling the relative stability of the radical, TEMPO may serve as an electrical dopant. Here, we adapt a known conjugated COF structure using highly efficient click chemistry to covalently attach TEMPO units to the interior of the COF pores. The amount of TEMPO is controlled by producing COFs with varying molar amounts of reactive building units (Scheme 1). After the crystalline COF is formed, the reaction with the TEMPO linker proceeds to completion, resulting in COFs with different TEMPO loadings. Through various structural and analytical measurements, we investigate the relative arrangements of radical units and their influence on the framework structure. In particular, the unpaired spins provide a marker for the interactions between the covalently bound TEMPO units in the pores through EPR, which allows us to understand the conformations and interactions of these “pore surface” substituents more


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generally.10 Steady-state microwave absorption is used to quantify the change in effective conductivity induced by TEMPO loading, quantifying its efficacy as a dopant. By varying the interaction of TEMPO with other radicals and with the framework, we are aiming for controllable modulation of electronic conductivity and redox behavior. The

synthesis

of

TPB-DMTP-COF

(TPB,

triphenylbenzene,

DMTP,

dimethoxyterephthaldehyde), upon which our novel COFs are based, has been previously reported.9

For simplicity, we refer to this structure from here on as COF-0.

COF-0 is

synthesized through the condensation of 2,5-dimethoxyterephthalaldehyde (DMTA) and 1,3,5tri-(4-aminophenyl)benzene (TAPB). It possesses fully conjugated rings, tight π-stacking, and excellent chemical stability. In modified versions of COF-0, a three component system is employed with TAPB, DMTA, and 2,5-bis(2-propynyloxy)terephthalaldehyde (BPTA), shown in Scheme S1b. The stoichiometry of DMTA and BPTA is varied to produce the COF series (COFx, x = 0, 25, 50, 75, 100). The number x indicates the percentage of linkers with the acetylene vs. the ether linkage based upon the ratios of reactants.



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Scheme 1. a) Synthesis of azide-functionalized TEMPO, b) synthesis of COF-x and T-COF-x structures, and c) idealized structure of COF fully loaded with TEMPO (T-COF-100).

These COFs were washed with supercritical CO2 and characterized by X-ray diffraction (XRD) and found to have similar crystallinity among the series and compared to literature


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(Figure S1). FTIR spectroscopy confirms the successful addition of the acetylene species (C-H stretch near 3300 cm-1) in place of the ethers for COF-25 and higher (Figure 1). The chemical species containing a TEMPO unit with an azide functional group, TEMPOa, was synthesized according to Scheme 1a. A propyl chain was added to increase the C/N ratio in the TEMPO-a, thereby reducing the danger in its handling. 4-Hydroxy-TEMPO was ionized with NaH and reacted with 3-bromopropanol to produce (1). A tosyl leaving group was introduced to this compound, and it was subsequently reacted with NaN3 to generate TEMPO-a (2). A slight excess of TEMPO-a was reacted with the aforementioned COF-x series. Click chemistry of this type is known to proceed very efficiently, and FTIR spectroscopy confirms the presence of TEMPO in the COF (C-O stretch near 1200 cm-1) after this reaction as well as the loss of the azide group (stretch near 2100 cm-1) and absence of acetylene functionality, suggesting a quantitative reaction. The resulting series of frameworks (T-COF-x) is shown by elemental analysis to have a composition similar to predictions (Table 1).

Figure 1. FTIR spectra of selected COFs. TEMPO-a spectrum is shown in red.



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Whereas trends between predicted and observed C-H-N values for COFs with increasing TEMPO functionalities are consistent (and is particularly pronounced for N values), some deviation in overall C-H-N values from those predicted in Table S1 is noted. Similar deviation in elemental analysis data has been observed for other COF systems employing click chemistry,25 where FTIR also confirmed quantitative consumption of the acetylene functionality, yet elemental analysis of the found nitrogen content was generally a full 1-2% lower than calculated values. It is likely that the presence of a small amount of residual solvent (methanol) remains in these systems and skews the data, even after 2 days of drying under vacuum. XRD confirms that the overall crystallinity of the COF remains intact following the TEMPO-a reaction. BrunauerEmmet-Teller (BET) analysis shows that the highly porous COF-x exhibits lower BET surface area (34 m2/g for T-COF-100 compared to 1473 m2/g for COF-0) after TEMPO-a incorporation, as expected. Table 1. Elemental composition of COFs. Calc.

Meas.

C

H

N

C

H

N

COF-100

81.80

4.58

6.36

78.55

4.68

6.14

T-COF-25

74.48

5.97

10.53

68.65

5.43

8.46

T-COF-50

71.51

6.45

12.51

65.21

5.79

10.13

T-COF-75

69.56

6.78

13.81

66.02

5.94

10.85

T-COF-100

68.19

6.99

14.73

64.74

6.12

11.94


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The predicted structure of one ring of T-COF-100 is shown in Scheme 1c. The steric hindrance of the TEMPO units upon increasing loading is apparent. For a 3.3 nm pore size, the likelihood of TEMPO-TEMPO interactions is fairly large for T-COF-50 and higher loadings. For T-COF-100, conformational changes in the linkages attached to the TEMPO units are likely to occur to minimize the energy of the structure. Ring stacking in the third dimension out of the plane is only slightly influenced by additional TEMPO loading as evidenced by the 0.02° shift in the characteristic XRD peak assigned to the (001) direction through the T-COF-x series (Figure S1). The COFs absorb in the blue portion of the visible spectrum, with increasing absorption at longer wavelengths as TEMPO is added (Figure S2). Electron paramagnetic resonance (EPR) spectroscopy was employed to further elucidate the TEMPO packing within the voids of the T-COF-x structure. The powder EPR spectra for a series of COFs, taken at room temperature, are shown in Figure 2. The spectra show a gradual trend of lineshape narrowing as more TEMPO units are added to the COF, similar to trends observed in other radical-polymer and radical-dendrimer systems.26-27 At lowest loading, the TCOF-25 spectrum shows features of isolated TEMPO species, but with considerable broadening due either to motional averaging of the nitrogen hyperfine interaction, and/or exchange broadening from nearby radicals. The spectra show an asymmetric broadening on the low gvalue (high magnetic field) component, characteristic of constrained motional averaging,28 suggesting radicals conform in a way that is not fully mobile. In T-COF-50, the signature of isolated TEMPO is present, but nearly overwhelmed by a broader, exchanged narrowed feature indicative of strongly interacting radicals.29 The fully loaded T-COF-100 possesses the largest spin exchange, leading to a spectrum with a single, narrow Lorentzian lineshape.



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Figure 2. Room temperature powder EPR spectra of several COFs. Magenta, reference isolated TEMPO radical spectrum obtained from a 100 µM radical solution in toluene. We have performed microwave absorption experiments on the T-COF-x series. Similar techniques have been used on framework materials to determine photoconductivity.30 Here we are measuring the equilibrium “dark” effective conductivity of bulk powder samples by measuring their steady-state absorption of microwave energy and comparing these results with a set of control samples and electromagnetic simulations. The samples were prepared for microwave experiments by pressing a known amount of material into a loose pellet encapsulated in a Parafilm disk. The mass of each COF used was adjusted such that the different samples contained the same number of ring repeat units, ensuring that trends cannot be the result of differing sample volumes. The sample or empty Parafilm disk was mounted in a microwave


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cavity and the reflection coefficient of the cavity measured as a function of frequency about its resonance (Figure S3); the characteristics of the resonance curve are used to calculate the dielectric constant and effective conductivity σ of each material through numerical simulation of the cavity response. We term this the “effective” conductivity because in the absence of an appropriate control sample it is impossible to distinguish dielectric and electronic contributions; e.g. rotating dipoles vs. mobile charges. Here, our control is (poly(2,2,6,6-tetramethylpiperidine4-yl-1-oxyl methacrylate) PTMA), consisting of an amorphous poly(methyl methacrylate) backbone with attached TEMPO units in varying concentrations.21, 31 Being a saturated polymer, any effective conductivity measured in this material will be a good measure of dielectric loss. Figure 3 shows that the microwave reflection for both control and T-COF-x series is similar, but the resulting change in conductivity per mole of added TEMPO is different. This similarity in effective conductivity for the PTMA and COF samples at low TEMPO loadings indicates that even in the COF, the doping induced change in effective conductivity is dominated by dielectric effects: TEMPO units rotating in the field. The data show that, contrary to our expectation, TEMPO is likely not an effective electronic dopant in this COF; we have measured an effective conductivity of 0.6 S/cm at the highest TEMPO loading, which is entirely accounted for by dielectric effects; any extant electrical contribution must be much smaller than this value. This is particularly interesting in light of the fact that the COF-0 sample showed an effective conductivity of 0.05 S/cm, which is likely to be a reasonable measure of the true electrical conductivity of the un-doped COF, given the absence of small dipolar molecules or substituents in the structure. Despite this negative result, the different effective conductivity trends between PTMA and COF samples as a function TEMPO loading remains interesting. For the PTMA samples, the



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conductivity is linear in the amount of added TEMPO, (constant conductivity per TEMPO unit), as one expects for the response of the dipole moment of TEMPO to the 9 GHz microwave field. For the T-COF-x series, the conductivity is sublinear in added TEMPO, showing that the constraints put on the TEMPO in the fully loaded pores reduces available degrees of freedom that

can

respond

to

the

microwave

field.

Figure 3. Effective conductivity per mol of TEMPO measured at 9 GHz for the T-COF-x and PTMA-x series. Both EPR and microwave experiments reveal an increasing degree of radical-radical interactions for the frameworks T-COF-x with x > 25, which may signify a reduced interaction with the framework as the TEMPO units are further crowded into the pores. This could lead to a saturation of any influence on conductivity or other framework electronic properties at relatively small loadings. A subtle change in the stacking structure of the COF does occur, but only becomes noticeable for the highest loadings (Figure S1). COF structures with larger pores32-33 that can accommodate an overall higher loading of radicals may facilitate observations of trends that arise when TEMPO radicals are either non- or weakly interacting, which may more clearly reveal any interaction between TEMPO and the framework.


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Using our knowledge gained from the interactions of radical units covalently bound to COFs, we are building toward secondary investigations that will uncover the relevance of the systematically varying radical and framework properties on gas or charge storage applications. No evidence for significantly enhanced conductivity in TEMPO-loaded COFs was observed, but providing a conjugated bridge between the radical and framework may facilitate the interaction that gives rise to charge transfer, for which EPR would be a sensitive probe. For maximum sensitivity in a microwave absorption experiment of conductivity, doping strategies involving nonpolar substituents could be explored.

This combination of techniques provides unique

information about substituent interactions, restricted motions, and charge-transfer in COFs that have undergone pore surface engineering.

Depending on the desired COF functionality, a

balance needs to be met between maintaining porosity, minimizing dopant disruption of structural integrity, and producing effective electronic interaction between dopants and framework. Success in this area will facilitate greater use of COFs not only in the traditional fields of gas storage and catalysis but also batteries34 and solar energy conversion.35 ASSOCIATED CONTENT Supporting Information Supporting Information includes detailed synthesis and methods information, X-ray diffraction data, optical absorption, BET analysis details, and microwave absorption data. AUTHOR INFORMATION Corresponding Author [email protected]



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Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We acknowledge support from the Laboratory Directed Research and Development program at NREL. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes. Work on some synthetic schemes was supported by the Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, under contract DE-AC3608GO28308. We thank Katherine Hurst and Todd Vinzant for assistance with sample preparation and analysis.

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