Investigation of Porphyrin-based MOFs as Porous Hosts for Generating Singlet Oxygen

Abstract Porous metal-organic frameworks (MOFs) consisting of crystalline coordination polymers are of interest as host materials for molecular sorption. MOFs exhibit permanent porosity, high thermal stability, and feature pores with high surface areas, large pore volumes, and properties that can be modified through synthesis. We are currently developing porous MOFs that incorporate photosensitizers in the MOF backbone in an effort to develop highly sorbent materials that generate singlet oxygen in order to oxidatively decompose adsorbed organic guest molecules present in the MOF. Ultimately, these materials will be used for applications involving environmental remediation and treatment of contaminated water sources. Toward this goal, the work in this project focused in three areas: (1) preparation of a porous MOF containing a metalloporphyrin as the photosensitizer, (2) investigation of a method to detect formation of singlet oxygen and (3) characterization of the activity of the MOF toward generating singlet-oxygen. A porous MOF containing a porphyrin as the photosensitizer was prepared via hydrothermal synthesis by reacting meso tetra(4-carboxyphenyl)porphyrin (TCPP) with zinc nitrate and 4,4’-bipyridine (BPY) in DMF at elevated temperature. Photolytic generation of singlet oxygen by the MOF was investigated by monitoring oxidative conversion of a model contaminant, 1,3-diphenylisobenzofuran (DPBF), to the corresponding diketone using UV-VIS absorption spectroscopy.

Generation & Detection of Singlet Oxygen

Key Findings

Porous Crystalline Solids

Acknowledgements References Prof. Chris Lambert & Morgan Stanton

N

NH N

HN

O OH

O

OH

OHO

O

HO

O OOO

O

O

hv

singlet oxygen

endoperoxide diketone

3O21O2

triplet oxygen

DPBF

+

Quartz cuvette with MOF and DPBF in DMF

480 nm cut-off filter

60 W Xe lamp

Oxidation of DPBF by singlet oxygen

Photolytic excitation setup & measurements

Absorption measurements were made using increasing concentrations from 50 µM to 250 µmolar DPBF in DMF to generate a calibration curve. The area of the peak at 405 nm directly correlates to the concentration of DPBF. A solution of 100 µM DPBF and 10 mg of MOF particles in DMF was excited for 80 minutes with a 60 W Xe lamp through a 480 nm cutoff filter to protect DBPF from UV degradation. Degradation of DPBF by singlet oxygen was determined at 10 minute intervals by monitoring the reduction in absorption by DPBF remaining in solution.

•  A MOF containing Zn, BPY & TCPP photosensitizer was synthesized hydrothermally and shown to be porous and thermally stable to above 300 °C.

•  Production of singlet oxygen by the MOF was demonstrated in DMF by monitoring degradation of DPBF spectroscopically.

Zeolites offer a limited number of structures and compositions, and smaller pore sizes (4-10 Å pores)

Structures, pore sizes and physical properties of metal-organic frameworks can be modified by changing the organic ligand (4 to 20 Å pores)

Zeolites Metal-organic frameworks (MOFs)

MOFs Actively Absorb Organic Guests

IRMOF-5 Surface area: 3774 m2/g Pore volume: 8.3 × 10-7 m3/g Channel dimensions: 12 Å × 12 Å

0

0.0005

0.001

0.0015

2 3 5

mol

es s

obed

(mm

ol)

C0(mM)

Naphthalene Phenanthrene

+

8.3 ×

8.5 ×

7.9 ×

We have shown previously that MOFs such as IRMOF-5 absorb a variety of organic guests from solution with high selectivity for larger guests that more closely match the pore dimensions.

Pillared Porphyrin Homologous Series: Intergrowth in Metal−Organic Frameworks, E.-Y.Choi, P. M. Barron, R. W. Novotny, H.-T. Son, C. Hu, and W. Choe, Inorganic Chemistry 2009 48, 426-428 Singlet oxygen generating nanolayer coatings on NiTi alloy for photodynamic application, K.-H. Choi, K.-K.Wang, S.-L.Oh, Ji-Eun Im, B.-J.Kim, J.-C.Park, D.-H. Choi, H.-K. Kim, Y.-Rok R. Kim, Surface and Coatings Technology 210 205, S62-S67

0   10   20   30   40   50   60   70   80   90  

Abs

orba

nce

at 4

05nm

Time (minutes)

DPBF  in  dark  

MOF  in  dark  

DPBF  in  light  

DPBF  +  MOF  in  light  

Degradation of DPBF

0.0  

0.5  

1.0  

1.5  

2.0  

2.5  

300   350   400   450   500  

Abs

orba

nce

Wavelength (nm)

250μM  DPBF  200μM  DPBF  150μM  DPBF  100μM  DPBF  50μM  DPBF  

Absorbance spectra of DPBF at different concentrations

•  Test additional MOFs containing different photosensitizers for activity toward generating singlet oxygen.

•  Investigate oxidatiive degradation of environmental contaminants such as BPA and benzo[a]pyrene by MOF-generated singlet oxygen in aqueous media.

Future Directions

Synthesis and Structure of TCPP MOF

N

NH N

HN

O OH

O

OH

OHO

O

HOCoCl2

pyridine150 °C2 days

Tetra(4-carboxyphenyl)porphyrin (TCPP) (photosensitizing ligand)

metal-binding sites

4,4’-bipyridine (BPY)

Zinc nitrate

N

NH N

HN

O OH

O

OH

OHO

O

HOCoCl2

pyridine150 °C2 days

DMF 80°C 24 Hours

TGA Data: loss of guest solvent and thermal stability

pore

Zn metal ion centers

BPY ligand

coordination of Zn metal ions by TCPP & BPY ligands

TCPP ligand

Structure of MOF

MOF composed of Zn, TCPP and BPY

Temperature (°C)

Wei

ght %

Loss of DMF solvent from pores in MOF

Decomposition of MOF

Francisco Javier Rosales Espinoza (Chemical Engineering), Jiawei Lu (Chemical Engineering) Advisor: Professor John C. MacDonald (Chemistry)