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

Summary

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.

Abstract

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.

Introduction

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 components^2,3,4,5,6,7. During the gr....

Protocol

1. Preparation of MOF suspensions using a polymer stabilizer

1. Weigh out 50 mg of bis-amino-terminated polyethylene glycol (PNH₂, M_n ~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 required to make the MOF particle sizes need to be at or below 1 µm.
2. Find a suitable solvent (if not water, use an anhydrous solvent such as dime....

Results

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.......

Discussion

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.......

Materials

List of materials used in this article

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