Metal–Organic Framework Photoconductivity via Time-Resolved

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Cite This: J. Am. Chem. Soc. 2019, 141, 9793−9797

Metal−Organic Framework Photoconductivity via Time-Resolved Terahertz Spectroscopy Brian Pattengale,*,†,§ Jens Neu,*,‡,§ Sarah Ostresh,† Gongfang Hu,† Jacob A. Spies,† Ryotaro Okabe,†,⊥ Gary W. Brudvig,† and Charles A. Schmuttenmaer*,† †

Department of Chemistry and Yale Energy Sciences Institute, Yale University, New Haven, Connecticut 06520-8107, United States Department of Molecular Biophysics and Biochemistry and Yale Microbial Sciences Institute, Yale University, New Haven, Connecticut 06520-8107, United States

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‡

S Supporting Information *

the-art systems reaching conductivities on the order of 103 S/cm.22−25 Typically, this is achieved in one of two ways. One method is to design the system such that charges can move between the ligand and the node with negligible energetic barriers. The concept of redox matching controls this behavior, with an important criterion being that both of the repeating units must have similar redox potentials such that the charge does not prefer to localize on a specific unit.22,26,27 In this manner, a continuous band, or extended molecular orbital, is formed along polymeric ligand−metal chains within the MOF, providing a charge transport pathway.26,28−30 A second method is to develop charge transport bands that extend through identical units in the structure. This through-space mechanism relies on utilizing the crystalline structure of the MOF to orient often highly conjugated or heteroatomcontaining linkers such that their electronic states delocalize. This strategy has been achieved with linkers composed of moieties such as tetrathiafulvalene and anthracene, among others.31−33 This through-space mechanism describes the charge transport of the MOF investigated in this work. The most popular methods to measure charge transport in such MOFs have been probe-based methods and noncontact spectroscopic methods.22 If synthesis allows, single crystals can be prepared and measured with probe-based methods. In cases where such crystals cannot be produced, or crystals are of poor quality, samples are subject to anisotropy. Spectroscopic methods that are sensitive to mobile electrons are able to overcome such anisotropy in materials. Time-resolved microwave techniques have been used to measure the photoconductivity of various MOFs; however, the time resolution is on the order of hundreds of nanoseconds.26,31 An ideal method overcomes both anisotropy and time resolution issues, while having excellent sensitivity. For these reasons, time-resolved terahertz (THz) spectroscopy (TRTS) is an ideal measurement technique for reliably measuring the photoconductivity of MOF materials. TRTS has been widely used to investigate photoinduced conductivity in a large variety of materials and chemical systems due to the sensitivity of THz to mobile carriers.34−41 To date, there has been one report of using TRTS to determine the intrinsic photoconductivity of a 2-dimensional MOF.41 The specific study measured a MOF

ABSTRACT: While metal−organic frameworks (MOFs) have been under thorough investigation over the past two decades, photoconductive MOFs are an emerging class of materials with promising applications in light harvesting and photocatalysis. To date, there is not a general method to investigate the photoconductivity of polycrystalline MOF samples as-prepared. Herein, we utilize timeresolved terahertz spectroscopy along with a new sample preparation method to determine the photoconductivity of Zn2TTFTB, an archetypical conductive MOF, in a noncontact manner. Using this technique, we were able to gain insight into MOF photoconductivity dynamics with subpicosecond resolution, revealing two distinct carrier lifetimes of 0.6 and 31 ps and a long-lived component of several ns. Additionally, we determined the frequency dependent photoconductivity of Zn2TTFTB which was shown to follow Drude−Smith behavior. Such insights are crucially important with regard to developing the next generation of functional photoconductive MOF materials. etal−organic frameworks (MOFs) have firmly established themselves as an important class of functional materials.1,2 Due to their high porosity and large internal surface area, they have been frequently utilized for gas separation, gas storage, and as membranes.3−8 Their structural diversity and tunability have led to the development of architectures that perform various functions, ranging from molecular recognition and sensing to visible light harvesting and photocatalysis.9−13 Visible light harvesting mechanisms in MOFs differ greatly by their design, with some having inherent absorption that is ligand-based or charge-transfer based, while others have modifications such as photosensitizer incorporation.14−21 In photocatalytic applications, efficient photogenerated charge separation is an important parameter. This implies that charge transport pathways, i.e. photoconductivity, must be present. The porosity of a MOF is achieved by creating open space between organic linkers and inducing nodes between linkers via metal oxide clusters. As a direct result of the rather large voids and nodes within their polymeric chains, MOFs tend to be insulators. There has been increased interest in overcoming this issue in the past decade, with researchers working to develop conductive and photoconductive MOFs, with state-of-

M

© 2019 American Chemical Society

Received: April 22, 2019 Published: June 9, 2019 9793

DOI: 10.1021/jacs.9b04338 J. Am. Chem. Soc. 2019, 141, 9793−9797

Communication

Journal of the American Chemical Society that was synthesized as a free-standing film with no support. Another report used TRTS to investigate MOF structures that are insulating as-prepared; however, they become conductive after infiltration with redox active molecules.42 Despite the limited usage of TRTS to investigate conductive MOFs, THz time-domain spectroscopy (THz-TDS) has been well-established as a technique to study MOF materials.43−47 Given these promising preliminary results, there is need to develop and demonstrate a generalizable method to measure and determine the photoconductivity of polycrystalline MOF samples via TRTS. In this report, we fill this need by developing and demonstrating a general method to determine the photoconductivity of a polycrystalline 3-dimensional photoconductive MOF sample, Zn2TTFTB (where TTFTB is tetrathiafulvalene tetrabenzoate), via TRTS. With such a tool in-hand, researchers can investigate and engineer photoconductive MOF materials, thereby tailoring them toward effective photocatalytic applications. Zn2TTFTB was synthesized according to a published literature procedure.31 The phase-purity of the crystalline MOF sample was confirmed via powder X-ray diffraction (pXRD, Figure 1a). The dark maroon powder sample was

Figure 2. (a) Real part of the refractive index and (b) imaginary part. Inset of panel a shows a photograph of a tape cell containing Zn2TTFTB. Inset of panel b shows THz transmission data scanned over the sample holder which is aligned to the data.

suspension of the MOF in another material, or matrix, avoiding any risk of matrix effects. THz-TDS was performed to determine the complex frequency-dependent refractive index for Zn2TTFTB using the calculation methods described in the data analysis section of the SI. The imaginary part of the refractive index (Figure 2b) shows a relatively sharp peak at approximately 0.6 THz, corresponding to a resonant absorption, along with a weaker shoulder at 1.0 THz and broad absorption beyond 2 THz. The real part of the refractive index for Zn2TTFTB is shown in Figure 2a and exhibits the corresponding dispersions close to the resonances. The refractive index for a tape cell with no sample is shown in Figure S3. One advantage of the tape cell preparation is that the as-prepared polycrystalline MOF sample can be measured without applying pressure to the sample, which is often done to prepare homogeneous pressed pellets for THz-TDS measurements. To investigate the effects of applied pressure, a tape cell was measured after applying 11 kbar with a hydraulic press, which is a conventional pressure for preparing pressed pellet samples.48,49 Due to pressure, the imaginary part of the refractive index changes (Figure S4) and a splitting of the resonance at 0.6 THz is observed. Interestingly, however, the photoconductivity dynamics are unchanged (Figure S5) while the frequency-dependent photoconductivity exhibits a slight increase in magnitude that we ascribe to improved grainto-grain contact after pressing. A pressed PTFE pellet was also prepared and measured via THz-TDS (Figure S6), where a change in the resonant absorption was also observed. These experiments, together, highlight that caution is warranted if a MOF sample is subjected to any preparation treatments prior to measurement. However, in this work, we focus on the asprepared sample in a tape-cell, avoiding the need for any postsynthesis treatments such as applied pressure. Time-resolved THz experiments were performed using 400 nm pump-pulses (0.35 mJ, 8.56 mm 1/e2 diameter) which can be delayed with respect to the THz probe pulse (1.02 mm 1/e2 diameter) as described in the SI. This corresponds to a normalized pump fluence of approximately 16 mJ cm−2 within the probe spot. Optical pump terahertz probe (OPTP) traces34−41,50,51 were measured by changing the pump delay at the peak of the THz signal, and a representative trace is shown in Figure 3a. The THz amplitude is attenuated after photoexcitation due to the generation of mobile carriers in the photoconductive Zn2TTFTB MOF. There is a recovery of the THz attenuation at short delay times (subps) followed by an additional slower (ps) recovery process that leads into a small amount of photoconductivity remaining within the 0.7 ns time window. The OPTP traces for four independent samples were

Figure 1. (a) pXRD pattern calculated from single crystal data (CSD: 913459)31 compared to the as-synthesized Zn2TTFTB sample. Inset shows a small portion of the Zn2TTFTB crystal structure. (b) Diffuse reflectance UV−visible spectra. Inset of panel b shows an SEM image of Zn2TTFTB.

measured by diffuse reflectance UV−visible spectroscopy (Figure 1b), showing broad absorption throughout the visible region of the spectrum. Scanning electron microscopy (SEM) images (inset of Figure 1b and Figure S1) show that the crystallites of the polycrystalline sample are hexagonalpyramidal prisms with widths of 1−2 μm and lengths up to 15 μm. A segment of the Zn2TTFTB crystal structure is shown in Figure 1a, inset. The MOF framework orients the TTFTB linkers in a helical pattern that overlaps sulfur moieties, leading to conductive channels in the MOF.31 Tape cells were constructed as described in the experimental section (SI). An example photograph of a prepared cell is shown in Figure 2a, inset. In order to confirm the homogeneity of the material within the tape cell, spatially resolved THz-TDS was employed. The tape reference was mounted in one location and samples in two other locations of the sample holder. A micropositioning stage was used to obtain a pointwise line scan of the THz transmission which is shown in the inset of Figure 2b. From the scan, it is apparent that the two samples show almost identical THz transmission, and do not have significant variation at their position center, evidencing that the sample preparation method is reproducible. We also note that the preparation method does not include 9794

DOI: 10.1021/jacs.9b04338 J. Am. Chem. Soc. 2019, 141, 9793−9797

Communication

Journal of the American Chemical Society

The magnitude of Zn2TTFTB photoconductivity was found to be approximately 0.5 mS/cm. This value is 2 orders of magnitude larger than the reported probe-based measurements.31,32 Such a large discrepancy is not surprising given that current−potential probe measurements involve long-distance, time-averaged charge transport through an entire crystal whereas TRTS is a more local probe of photoconductivity on a fs-time scale. Therefore, TRTS is a better suited technique regarding subpicosecond photophysical events occurring in light harvesting and photocatalytic applications. In conclusion, we have measured the frequency-dependent TRTS photoconductivity of as-synthesized, polycrystalline conductive MOF samples. We report the first TRTS photoconductivity results for an intrinsically conductive 3-dimensional MOF, Zn2TTFTB. The observed photoconductivity at a 500 fs pump delay is approximately 0.5 mS/cm, 2 orders of magnitude larger than the previously reported value. The photoconductivity decays on two different time scales within 50 ps after photoexcitation into a smaller long-lived photoconductivity signal as determined by OPTP. Using the subpicosecond time resolution and high sensitivity of TRTS, there is great opportunity to understand and engineer the photoconductivity of MOF materials, thereby developing them for effective functional applications.

Figure 3. (a) Representative OPTP trace. (b) Photoconductivity data collected at a pump delay of 500 fs with Drude−Smith fit results.

globally fit with a Gaussian-convoluted triexponential function to account for the instrument response function (IRF) and recovery processes (Figure S7, Table S1). Evidently, a fast decay process, τ1, dominates the early part of the OPTP signal that fit to 0.60 ± 0.03 ps, which is on the IRF time scale. Following this decay process, τ2 fit to a value of 31 ± 0.3 ps. Finally, τ3 was fixed to 10 ns, a value much larger than the experimental time window, to account for remaining photoconductivity after 0.7 ns. The spectrally resolved THz photoconductivity of Zn2TTFTB was measured at 500 fs after photoexcitation using TRTS as previously described34,52 which provides information on the quasi-instantaneous photoinduced conductivity in the sample. Using the experimentally determined penetration length (6.9 μm, Figure S8) and the equations presented in the SI, the frequency-dependent photoconductivity is determined and displayed in Figure 3b along with the average Drude−Smith fit performed on three independent samples (Figure S9). The region from 0.25 to 0.75 THz and above 2 THz exhibit strong resonances (Figure S10), which are not related to the photoconductivity. These regions are therefore excluded from the Drude−Smith fit, and shaded gray in the figure. The pXRD pattern of the sample after TRTS experiments (Figure S11) confirms that the MOF structure retains its crystallinity during the measurement. The resulting conductivity is well-described with the Drude−Smith model,40,53 validating that the MOF exhibits photoconductivity.54 The Drude−Smith model exhibits the lowest number of fit-parameters compared to other commonly used models like the Drude−Anderson model.55 This reduced number of fit parameters, while slightly limiting the fit agreement, provides a direct physical interpretation of the result and is therefore desirable and the agreement is typical for TRTS experiments.41,56,57 The Drude−Smith parameters obtained are shown in the SI (Table S2). We obtain a carrier scattering time of 17 ± 2 fs, which supports that THz is an excellent technique for investigating this material, as the inverse of the scattering time corresponds to the Drude plasmon resonance frequency. This frequency is higher than the used experimental frequency range and therefore, TRTS probes the nonresonant response of free charges (i.e., the photoconductivity). Furthermore, the backscattering parameter, c, is determined to be −0.86 ± 0.01. A c parameter value close to −1 is typical for nanoparticle materials in which the backscattering at interfaces is dominant. Therefore, there is some insight into the effect of grain boundaries on the photoconductivity, because the micrometer-sized Zn2TTFTB crystallites exhibit backscattering typical for much smaller particles.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b04338. Experimental methods and details, photoconductivity calculation details and equations, TDS of reference tape cell and pressed cell, OPTP and TRTS supplementary figures and fit results (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] *[email protected] ORCID

Brian Pattengale: 0000-0002-1749-4081 Jens Neu: 0000-0002-1054-0444 Gongfang Hu: 0000-0002-0387-9079 Gary W. Brudvig: 0000-0002-7040-1892 Charles A. Schmuttenmaer: 0000-0001-9992-8578 Present Address ⊥

School of Life Science and Technology, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama, Kanagawa 226-8503, Japan Author Contributions §

B.P. and J.N. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Energy Biosciences, and from the Department of Energy, under contract DE-FG02-07ER15909, as well as from a generous donation from the TomKat Foundation. R.O. acknowledges funding from the Nakatani Foundation for 9795

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performing research at Yale University in the Schmuttenmaer group.

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Communication

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(56) Regan, K. P.; Koenigsmann, C.; Sheehan, S. W.; Konezny, S. J.; Schmuttenmaer, C. A. Size-Dependent Ultrafast Charge Carrier Dynamics of WO3 for Photoelectrochemical Cells. J. Phys. Chem. C 2016, 120, 14926−14933. (57) Richter, C.; Schmuttenmaer, C. A. Exciton-like trap states limit electron mobility in TiO2 nanotubes. Nat. Nanotechnol. 2010, 5, 769.

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DOI: 10.1021/jacs.9b04338 J. Am. Chem. Soc. 2019, 141, 9793−9797