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

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.