This work was supported by the Department of Energy under Grant DE-SC0012446.

1. Preparation of MOF suspensions using a polymer stabilizer
<ol>
	<li>Weigh out 50 mg of bis-amino-terminated polyethylene glycol (PNH2, Mn ~1,500) (see Table of Materials) and transfer to a one-dram vial (Table of Materials). Weigh out 1-5 mg of PCN-222(fb) (see synthetic protocol11) and place it into the same vial with PNH2. 
	NOTE: To attain the best possible MOF suspensions, the synthetic conditions req...

While the above results and protocol delineate general guidelines for minimizing scatter from MOFs in spectroscopic characterization, there is a wide variability in MOF particle size and structure that impacts spectroscopic results, and therefore blurs the methods of interpretation. To help clarify interpretation and ease the strain that comes with analyzing MOF spectroscopic data, finding a procedure to make the MOFs as small as possible is key. This is a limiting factor for most spectroscopy-related analyses of MOFs. B...

Metal-organic frameworks (MOFs) are composed of metal oxide nodes linked by organic molecules, that form hierarchical porous structures when their constituent parts react together under solvothermal conditions1. Permanently porous MOFs were first reported in the early 2000s, and since then, the burgeoning field has expanded to encompass a wide range of applications, given the unique tunability of their structural components2,3,4,5,6,7. During the growth of the field of MOFs, there have been a handful of researchers that have incorporated photoactive materials into the nodes, ligands, and pores of MOFs to harness their potential in light-driven processes, like photocatalysis8,9,10,11, upconversion12,13,14,15,16, and photoelectrochemistry17,18. A handful of the light-driven processes of MOFs revolve around energy and electron transfer between donors and acceptors17,19,20,21,22,23,24,25. The two most common techniques used to study energy and electron transfer in molecular systems are emission and transient absorption spectroscopy26,27.
A great deal of research on MOFs has focused on emission characterization, given the relative ease in preparing samples, performing measurements, and (relatively) straightforward analysis19,22,23,24,28. Energy transfer typically manifests itself as a loss in the donor emission intensity and lifetime and an increase in the emission intensity of the acceptor loaded into the MOF backbone19,23,28. Evidence of charge transfer in a MOF manifests as a decrease in emission quantum yield and lifetime of the chromophore in the MOF29,30. While emission spectroscopy is a powerful tool in the analysis of MOFs, it only addresses part of the necessary information to present a complete mechanistic understanding of MOF photochemistry. Transient absorption spectroscopy can not only provide support for the existence of energy and charge transfer, but the method can also detect spectral signatures associated with the non-emissive singlet and triplet excited state behaviors, making it one of the most versatile tools for characterization31,32,33.
The primary reason why more robust characterization techniques like transient absorption spectroscopy are seldom applied toward MOFs is due to the difficulty in preparing samples with minimal scatter, especially with suspensions34. In the few studies successfully performing transient absorption on MOFs, the MOFs are &#60;500 nm in size, with some exceptions, stressing the importance of reducing particle size to minimize scatter15,21,25,35,36,37. Other studies make use of MOF thin films17 or SURMOFs38,39,40 to circumvent the scatter issue; however, from an applicability stand-point, their use is quite limited. Additionally, some research groups have taken to making polymer films of MOFs with Nafion or polystyrene34, the former raising some concerns for stability given the highly acidic sulfonate groups on Nafion. Gaining inspiration from the preparation of colloidal semiconductor suspensions41,42, we have found great success using polymers to help suspend and stabilize MOF particles for spectroscopic measurements11. In this work, we establish widely applicable guidelines to follow when it comes to preparing MOF suspensions and characterizing them with emission, nanosecond (ns), and ultrafast (uf) transient absorption (TA) spectroscopy techniques.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>1 cm cuvette sample mount (SM1)</td>
			<td>Edinburgh Instruments</td>
			<td>n/a</td>
			<td>Contact company</td>
		</tr>
		<tr>
			<td>1 mL disposable syringes</td>
			<td>EXELINT</td>
			<td>26044</td>
			<td></td>
		</tr>
		<tr>
			<td>10 mL disposable syringes</td>
			<td>EXELINT</td>
			<td>26252</td>
			<td></td>
		</tr>
		<tr>
			<td>1-dram vials</td>
			<td>FisherSci</td>
			<td>CG490001</td>
			<td></td>
		</tr>
		<tr>
			<td>20 nm syringe filters</td>
			<td>VWR</td>
			<td>28138-005</td>
			<td>The filters are made by Whatman/Cytiva, and their catalog number is 6809-1002</td>
		</tr>
		<tr>
			<td>200 nm syringe filters</td>
			<td>Cytiva, Whatman</td>
			<td>6784-1302</td>
			<td></td>
		</tr>
		<tr>
			<td>Absorption spectrophotometer</td>
			<td>Agilent&#160;</td>
			<td>Cary 5000 Spectrophotometer</td>
			<td>Contact company</td>
		</tr>
		<tr>
			<td>Acetronitrile (ACN)</td>
			<td>FisherSci</td>
			<td>AA36423</td>
			<td></td>
		</tr>
		<tr>
			<td>Ar gas tank</td>
			<td>Linde/PraxAir</td>
			<td>P-4563</td>
			<td></td>
		</tr>
		<tr>
			<td>bis amino-terminated polyethylene glycol (PNH2)</td>
			<td>Sigma-Aldrich</td>
			<td>452572</td>
			<td>MOF suspending agent</td>
		</tr>
		<tr>
			<td>Clamping sample mount for nsTA (SM2)</td>
			<td>Ultrafast Systems</td>
			<td>n/a</td>
			<td>Contact company</td>
		</tr>
		<tr>
			<td>Concave lens for telescope(CCL1)</td>
			<td>Thorlabs</td>
			<td>LD1613-A-ML</td>
			<td></td>
		</tr>
		<tr>
			<td>Convex lens for telescope (CVL1)</td>
			<td>Thorlabs</td>
			<td>LA1708-A-ML</td>
			<td></td>
		</tr>
		<tr>
			<td>Custom 1 cm optical cell with 24/40 outer joint</td>
			<td>QuarkGlass</td>
			<td>QSE-1Q10-2440 (Spectrosil Cat #1-Q-10</td>
			<td>We requested the 1 cm cell to have a joint</td>
		</tr>
		<tr>
			<td>Custom 2mm optical cell with 14/20 outer joint</td>
			<td>QuarkGlass</td>
			<td>QSE-1Q2-1420 (Spectrosil Cat # 1-Q-2)</td>
			<td>We requested the 2 mm cell to have a joint</td>
		</tr>
		<tr>
			<td>Dimethylformamide (DMF)</td>
			<td>FisherSci</td>
			<td>D119</td>
			<td></td>
		</tr>
		<tr>
			<td>Dye laser (Nd:YAG pumped) for 415 nm output</td>
			<td>Sirah</td>
			<td>CobraStretch</td>
			<td></td>
		</tr>
		<tr>
			<td>Dye laser dye, Exalite 417</td>
			<td>Luxottica</td>
			<td>4170</td>
			<td></td>
		</tr>
		<tr>
			<td>Femtosecond laser</td>
			<td>Coherent</td>
			<td>Astrella</td>
			<td></td>
		</tr>
		<tr>
			<td>Fluorimeter&#160;</td>
			<td>Photon Technology Inc. (Horiba)</td>
			<td>QuantaMaster QM-200-4E</td>
			<td></td>
		</tr>
		<tr>
			<td>Fluorimeter arc lamp, 75 W</td>
			<td>Newport</td>
			<td>6251NS</td>
			<td></td>
		</tr>
		<tr>
			<td>Fluorimeter PMT</td>
			<td>Hamamatsu</td>
			<td>1527</td>
			<td></td>
		</tr>
		<tr>
			<td>Fluorimeter Software</td>
			<td>PTI/Horiba</td>
			<td>FelixGX</td>
			<td></td>
		</tr>
		<tr>
			<td>Fluorimeter TCSPC Module</td>
			<td>Becker &#38; Hickl GmbH</td>
			<td>PMH-100</td>
			<td></td>
		</tr>
		<tr>
			<td>lens mounts for telescope</td>
			<td>Thorlabs</td>
			<td>LMR1</td>
			<td></td>
		</tr>
		<tr>
			<td>Long purging needles</td>
			<td>STERiJECT</td>
			<td>PRE-22100</td>
			<td></td>
		</tr>
		<tr>
			<td>Magnetic stirrer</td>
			<td>Ultrafast Systems</td>
			<td>n/a</td>
			<td>Contact company</td>
		</tr>
		<tr>
			<td>mirror 1 (MM1) 350-700 nm</td>
			<td>Newport</td>
			<td>10Q20BB.1</td>
			<td></td>
		</tr>
		<tr>
			<td>MM1 mount</td>
			<td>Thorlabs</td>
			<td>KM100</td>
			<td></td>
		</tr>
		<tr>
			<td>MM1 post</td>
			<td>Thorlabs</td>
			<td>TR2</td>
			<td></td>
		</tr>
		<tr>
			<td>MM1 post holder</td>
			<td>Thorlabs</td>
			<td>PH1.5</td>
			<td></td>
		</tr>
		<tr>
			<td>MM2 mount</td>
			<td>Thorlabs</td>
			<td>MFM05</td>
			<td></td>
		</tr>
		<tr>
			<td>MM2,3 mirrors</td>
			<td>thorlabs</td>
			<td>BB03-E02</td>
			<td></td>
		</tr>
		<tr>
			<td>MM2,3 post</td>
			<td>Thorlabs</td>
			<td>MS3R</td>
			<td></td>
		</tr>
		<tr>
			<td>MM2,3 post bases</td>
			<td>Thorlabs</td>
			<td>MBA1</td>
			<td></td>
		</tr>
		<tr>
			<td>MM2,3 post holders</td>
			<td>Thorlabs</td>
			<td>MPH50</td>
			<td></td>
		</tr>
		<tr>
			<td>MM3 mount</td>
			<td>Thorlabs</td>
			<td>MK05</td>
			<td></td>
		</tr>
		<tr>
			<td>mounting posts for telescope optics</td>
			<td>Thorlabs</td>
			<td>TR4</td>
			<td></td>
		</tr>
		<tr>
			<td>Nanosecond TA Nd:YAG lasers</td>
			<td>Spectra-Physics</td>
			<td>QuantaRay INDI Nd:YAG</td>
			<td></td>
		</tr>
		<tr>
			<td>Nanosecond TA spectrometer</td>
			<td>Edinburgh Instruments</td>
			<td>LP980</td>
			<td></td>
		</tr>
		<tr>
			<td>nsTA ICCD camera</td>
			<td>Oxford Instruments</td>
			<td>Andor iStar ICCD camera</td>
			<td>Contact company</td>
		</tr>
		<tr>
			<td>nsTA PMT&#160;</td>
			<td>Hamamatsu</td>
			<td>R928</td>
			<td></td>
		</tr>
		<tr>
			<td>Optical parametric amplifier</td>
			<td>Ultrafast Systems</td>
			<td>Apollo</td>
			<td></td>
		</tr>
		<tr>
			<td>Parafilm</td>
			<td>FisherSci</td>
			<td>S37440</td>
			<td></td>
		</tr>
		<tr>
			<td>Pinhole wheel</td>
			<td>Thorlabs</td>
			<td>PHW16</td>
			<td></td>
		</tr>
		<tr>
			<td>Pinhole wheel post base</td>
			<td>Thorlabs</td>
			<td>CF125C</td>
			<td></td>
		</tr>
		<tr>
			<td>Pinhole wheel post holder</td>
			<td>Thorlabs</td>
			<td>PH1.5</td>
			<td></td>
		</tr>
		<tr>
			<td>Pinhole wheel post/mount assembly</td>
			<td>Thorlabs</td>
			<td>NDC-PM</td>
			<td></td>
		</tr>
		<tr>
			<td>post bases for telescope optics</td>
			<td>Thorlabs</td>
			<td>CF125C</td>
			<td></td>
		</tr>
		<tr>
			<td>post holders for telescope optics</td>
			<td>Thorlabs</td>
			<td>PH4</td>
			<td></td>
		</tr>
		<tr>
			<td>Power detector for ns TA</td>
			<td>Thorlabs</td>
			<td>S310C</td>
			<td></td>
		</tr>
		<tr>
			<td>Prism assembly (P2,3)</td>
			<td>Edinburgh Instruments</td>
			<td>n/a</td>
			<td>Contact company</td>
		</tr>
		<tr>
			<td>Prism mount (P1)</td>
			<td>OWIS</td>
			<td>K50-FGS</td>
			<td></td>
		</tr>
		<tr>
			<td>Prism post (P1)</td>
			<td>Thorlabs</td>
			<td>TR4</td>
			<td></td>
		</tr>
		<tr>
			<td>Prism post base (P1)</td>
			<td>Thorlabs</td>
			<td>CF125C</td>
			<td></td>
		</tr>
		<tr>
			<td>Prism post holder (P1)</td>
			<td>Thorlabs</td>
			<td>PH4</td>
			<td></td>
		</tr>
		<tr>
			<td>Quartz prisms (P1-P3)</td>
			<td>Newport</td>
			<td>10SR20</td>
			<td></td>
		</tr>
		<tr>
			<td>Rubber outer joint septa (14/20)</td>
			<td>VWR</td>
			<td>89097-540</td>
			<td></td>
		</tr>
		<tr>
			<td>Rubber outer joint septa (24/40)</td>
			<td>ChemGlass</td>
			<td>CG-3022-24</td>
			<td></td>
		</tr>
		<tr>
			<td>Sonication tip</td>
			<td>Branson</td>
			<td>product discontinued</td>
			<td>Closest alternative is 1/8&#34; diam. tip from iUltrasonic</td>
		</tr>
		<tr>
			<td>Square ND filters</td>
			<td>Thorlabs</td>
			<td>NEK01S</td>
			<td></td>
		</tr>
		<tr>
			<td>Stir bars</td>
			<td>StarnaCells/FisherSci</td>
			<td>NC9126395</td>
			<td></td>
		</tr>
		<tr>
			<td>Thorlabs power detector for ufTA</td>
			<td>Thorlabs</td>
			<td>S401C</td>
			<td></td>
		</tr>
		<tr>
			<td>Thorlabs power meter</td>
			<td>Thorlabs</td>
			<td>PM100D</td>
			<td></td>
		</tr>
		<tr>
			<td>Tip sonicator</td>
			<td>Branson</td>
			<td>Digital Sonifer 450, product discontinued</td>
			<td>Closest alternative is SFX550 from iUltrasonic</td>
		</tr>
		<tr>
			<td>Tygon tubing</td>
			<td>Grainger</td>
			<td>8Y589</td>
			<td></td>
		</tr>
		<tr>
			<td>ufTA ND filter wheel</td>
			<td>Thorlabs</td>
			<td>NDC-25C-2-A</td>
			<td></td>
		</tr>
		<tr>
			<td>ufTA ND filter wheel mount</td>
			<td>Thorlabs</td>
			<td>NDC-PM</td>
			<td></td>
		</tr>
		<tr>
			<td>ufTA ND filter wheel post</td>
			<td>Thorlabs</td>
			<td>PH2</td>
			<td></td>
		</tr>
		<tr>
			<td>ufTA ND filter wheel post base</td>
			<td>Thorlabs</td>
			<td>CF125C</td>
			<td></td>
		</tr>
		<tr>
			<td>ufTA pump alignment mirror</td>
			<td>Thorlabs</td>
			<td>PF10-03-F01</td>
			<td></td>
		</tr>
		<tr>
			<td>Ultrafast TA telescope assembly</td>
			<td>Ultrafast Systems</td>
			<td>n/a</td>
			<td>Contact company</td>
		</tr>
		<tr>
			<td>Ultrafast transient absorption spectrometer</td>
			<td>Ultrafast Systems</td>
			<td>HeliosFire</td>
			<td></td>
		</tr>
		<tr>
			<td>Xe arc probe lamp</td>
			<td>OSRAM</td>
			<td>4050300508788</td>
			<td></td>
		</tr>
	</tbody>
</table>

a technical guide for performing spectroscopic measurements on metal-organic frameworks

Metal-organic frameworks (MOFs) offer a unique platform to understand light-driven processes in solid-state materials, given their high structural tunability. However, the progression of MOF-based photochemistry has been hindered by the difficulty in spectrally characterizing these materials. Given that MOFs are typically larger than 100 nm in size, they are prone to excessive light scatter, thereby rendering data from valuable analytical tools like transient absorption and emission spectroscopy nearly uninterpretable. To gain meaningful insights of MOF-based photo-chemical and physical processes, special consideration must be taken toward properly preparing MOFs for spectroscopic measurements, as well as the experimental setups that garner higher quality data. With these considerations in mind, the present guide provides a general approach and set of guidelines for the spectroscopic investigation of MOFs. The guide addresses the following key topics: (1) sample preparation methods, (2) spectroscopic techniques/measurements with MOFs, (3) experimental setups, (3) control experiments, and (4) post-run stability characterization. With appropriate sample preparation and experimental approaches, pioneering advancements toward the fundamental understanding of light-MOF interactions are significantly more attainable.

The electronic absorption spectra of PCN-222(fb) with and without PNH2 and filtering are shown in Figure 4. The MOF without PNH2 was just tip-sonicated and diluted. When comparing the two spectra, the biggest difference is the minimization of baseline scatter, which shows up as a broad upward absorption with decreasing wavelengths and also broadens the electronic transitions quite noticeably. For further comparison, the PCN-222(fb) ligand in solution, tetracarboxyphenyl...

Watch this Scientific Journal Video about A Technical Guide for Performing Spectroscopic Measurements on Metal-Organic Frameworks at JoVE.com

A Technical Guide for Performing Spectroscopic Measurements on Metal-Organic Frameworks

Here, we use a polymer stabilizer to prepare metal-organic framework (MOF) suspensions that exhibit markedly decreased scatter in their ground-state and transient absorption spectra. With these MOF suspensions, the protocol provides various guidelines to characterize the MOFs spectroscopically to yield interpretable data.

Metal-organic frameworks (MOFs) offer a unique platform to understand light-driven processes in solid-state materials, given their high structural ...

Often, measuring how light interacts with metal organic frameworks, or MOFs, is difficult to their highly scattering nature. This protocol is a simple and effective guide for preparing measurable samples for highly insightful spectroscopic techniques. The procedure is loosely based on prior systems using colloidal semiconductors stabilized with polymers.So, it can be applied to various systems that require the suspension of materials. The biggest issue with the procedure is that it needs to be tuned to the MOF type. The best approach is to systematically screen this procedure's variables for the MOF.Begin by preparing a suspension of freebase PCN 222 containing bis amino terminated polyethylene glycol, or aminated PEG, in a suitable solvent. Using a tip sonicator, sonicate the suspension for two to five minutes at 20 to 30%amplitude with two seconds on and two seconds off intervals. Ensure proper dispersion and homogeneity of the suspension after sonication.Draw the suspension into a fresh 10 milliliter plastic syringe. Remove the syringe needle and replace it with a 200 nanometer polytetrafluoroethylene, or PTFE, mesh syringe filter. Pass the metal organic framework, or MOF, suspension through the syringe filter into a new clean vial.To decrease the beam spot size, hitting the two millimeter cuvette, set up a Galilean telescope with first a concave lens, or CCL one, followed by a convex lens, or CVL one, hitting the laser. Ensure the distance between the two lenses is approximately the difference between the two focal distances of the lenses. Open both the laser and probe shutters and replace the first sample mount door, SM one, with the second sample mount door, SM two.And place a note card into the SM two clamping mount such that its orientation is completely facing the probe beam. Then, set up a series of three mini mirrors named MM one, two, three. Direct the incoming laser beam by approximately adjusting the turning knobs on the P three kinematic mount onto the center of MM one.To minimize laser beam expansion from mirror to mirror, place MM two in front of MM one to lower the angle of reflection between the two mirrors. When the beam hits approximately the center of MM one, rotate MM one so that the reflected laser beam hits MM two in the center. Similarly, when the beam hits the center of MM two, rotate it so the reflected laser beam hits MM three in the center.When the beam hits approximately the center of MM three, rotate MM three to make the reflected laser beam hit the alignment note card in the same spot as the probe beam. Using the vertical and horizontal knobs on the mirrors, fine tune the laser beam positions on each mirror and the note card, ensuring the beam has little to no clipping throughout its path. Repeat the beam alignment as demonstrated before, using a two millimeter cuvette with a 14 by 20 inner joint, or SC two, and 14 by 20 rubber septum.Insert the sample into a clamping sample mount, or SM two, completely facing the probe beam path. Next, fine tune the laser beam positions on each mirror and SM two, with the vertical and horizontal knobs on the mirrors. With a low profile stirrer, stir the sample moderately and perform transient absorption, or TA, measurements.To align the pump and probe beams for ultra fast transient absorption, or ultra fast TA measurements, first, prepare the chromophore solution without purging. Turn on the ultra fast laser pump source and spectrometer. Open the optical parametric amplifier software and set it to the desired excitation wavelength.Open the ultra fast TA spectrometer software and choose a probe window. Place the standard cuvette in the sample holder in line with the probe beam. Adjust the pump source power with a neutral density or ND filter wheel to see the pump beam if necessary.Place a white note card against the cuvette side facing the pump and probe beam. Adjust the pump spot on the note card with the turning knobs on the kinematic mount, such that vertically, it is at the same height as the probe beam and horizontally, it is within one or two millimeters next to the probe beam. Without the note card, fine tune the positions of the pump beam to obtain the highest TA spectral signal.With the pump and probe beams aligned, replace the sample cell holder with a mounted pinhole wheel having 2000 thousand to 25 micron holes at the focal point of the laser beam. Ensure that the pinhole wheel is near, if not exactly, perpendicular to the path of the laser beam. Set up the pinhole wheel so the laser beam passes through the 2000 micron pinhole.Then, set up a detector attached to a power meter closely on the other side of the pinhole wheel so that the whole laser beam hits the detector. Rotate the wheel to smaller sizes, measuring the power at each size to determine the beam spot size. To do a linear power response check, once the pump and probe beams are aligned and the MOF sample is stirred in the sample holder, measure and record the average pump power with a power meter attached to a detector in the pump beam path.Remove the detector from the beam path. In the live view TA mode, record the delta OD signal of the MOF sample at different points in the TA spectrum right after the chirp response of about two to three picoseconds. Plot the recorded data points as delta OD versus incident power in data analysis software.If there is a linear power response, the resulting plot forms a straight line, with the Y-intercept at zero. If there is a non-linear power response, as expected, significant deviations from a linear curve are typically observed. When the electronic absorption spectrum of freebase PCN 222 is compared to aminated PEG, the spectrum of PCN 222 without aminated PEG and filtering showed a broader electronic transition and considerable baseline scatter.Without the use of aminated peg, the excitation and emission spectra of freebase PCN 222 and the linker, H2TCPP in DMF, aligned quite well. The differences in emission lifetimes were attributed to energy transfer quenching of proteinated and proteinated H2TCPP linkers. The TA spectra of freebase PCN 222 without aminated PEG right after the sort band excitation at 415 nanometers showed a substantial scattering, causing the TA spectrum to become increasingly negative with decreasing wavelength.This starkly contrasted the spectrum of H2TCPP in solution. The kinetics of H2TCPP and freebase PCN 222 without aminated PEG were also starkly different. However, the spectrum of freebase PCN 222 with aminated PEG and its lifetime aligned much better with the H2TCPP TA spectrum.The ultra fast TA spectrum of freebase PCN 222 with aminated PEG resembled that of the linker in solution, showing a ground state bleach at about 420 nanometers and excited state absorptions on either side of the bleach. All these observations indicated the observed signal was from the MOF and not due to scattering. It is critical to measure the spectra and kinetics of the solvated MOF linker to understand what to expect when probing the spectra and kinetics of the MOF itself.This technique allows researchers to truly focus on understanding a sample's behavior when exposed to light, instead of figuring out ways to adequately prepare a sample for measurements.

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JoVE Journal

Chemistry

2.0K vues.  Virginia Polytechnic and State Institute. Souvent, mesurer comment la lumière interagit avec les structures organiques métalliques, ou MOF, est difficile à leur nature très dispersée. Ce protocole est un guide simple et efficace pour préparer des échantillons mesurables pour des techniques spectroscopiques très perspicaces. La procédure est vaguement basée sur des systèmes antérieurs utilisant des semi-conducteurs colloïdaux stabilisés avec des polymères.Ainsi, il peut être appliqué à divers systèmes qui né...