Design of MOFs Containing Porphyrins as Porous Hosts for Generating Singlet Oxygen

Transcription

1 Design of MOFs Containing Porphyrins as Porous Hosts for Generating Singlet Oxygen A Major Qualifying Project Report Submitted to the Faculty of the WORCESTER POLYTECHNIC INSTITUTE in partial fulfillment of the requirements for the Degree of Bachelor of Science By: Kevin Garcia Date: May 1, 2014 Approved: i

2 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 porous MOFs featuring layers of metalloporphyrin photosensitizers separated by bridging dipyridyl ligands, (2) synthesis and characterization of 5,15-diphenyl-10,20-di-(4-pyridyl)porphyrin (DPyDPP), a porphyrin with two pyridyl groups capable of coordinating to Zn metal ions, and (3) characterization of the activity MOFs toward generating singlet-oxygen. Porous MOFs were prepared via hydrothermal synthesis by reacting meso tetra(4-carboxyphenyl)porphyrin (TCPP) with zinc nitrate and 4,4 - bipyridyl (BP) or 5,15-diphenyl-10,20-di-(4-pyridyl)porphyrin (DPyDPP) in DMF/HNO 3 /EtOH 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. ii

3 Acknowledgements I would like to my MQP Advisor Professor John C. MacDonald for the research opportunities as well as the wisdom that he has provided me. Not only do I see him as a mentor with the patience only a father can muster from which to learn from but, by having the chance to see him in a different light, I can also see a friend that I hope to one day aspire to. iii

4 Table of Contents Abstract... ii Acknowledgements... iii Table of Contents... iv Table of Figures... v Introduction... 1 Background... 2 Porous solids... 2 Metal-organic frameworks (MOFs) Singlet Oxygen... 4 Current research in the MacDonald group Experimental and Characterization... 7 Methodology for Characterization of Products... 7 Synthesis of Ligands... 8 Meso (4-pyridyl)dipyrrlmethane ,15-diphenyl-10,20-di-(4-pyridyl) porphyrin... 8 Hydrothermal Synthesis of MOFs... 9 Zn-TCPP-BPY MOF Synthesis... 9 ZN-TCPP-DPyDPP MOF Synthesis in one step... 9 Zn-TCPP-DPyDPP MOF Synthesis in two steps Methodology for Monitoring Generation and Detection of Singlet Oxygen Results and Discussion Synthesis and characterization of 5,15-diphenyl-10,20-di-(4-pyridyl)porphyrin (DPyDPP) Synthesis and Characterization of MOFs Activity of MOFs toward generating singlet oxygen Conclusions References iv

5 Table of Figures Figure 1 Examples of several zeolite frameworks generated by different arrangements of sodalite cage Figure 3 An illustration of the generation of excited photosensitizer states and reactive dioxygen species. 9,... 6 Figure 4 The chemical structure of meso 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin Figure 5 Illustration of the setup used to photoexcite the MOF materials and monitor generation of singlet oxygen Figure 6 An overall reaction scheme for the synthesis of meso-5,15-diphenyl-10,20-di(4- pyridyl)porphyrin Figure 7 An illustration of the two dimensional layer or sheet that is formed as TCPP binds with metal ions (Blue) through coordination via their carboxylate groups (Red) Figure 8 An illustration of the crystal structure and stacking arrangement of Zn-TCPP-BPY MOF Figure 9 A graphical representation of the TGA data collected of the Zn-TCPP-BPY MOF (PPF- 4) Figure 10 A graph representing the data collected by TGA of the presumed Zn-TCPP-DPyDPP MOF Figure 11 TGA data reported in the Literature of the porphyrinic MOFs with a variety of metal ions used in their construction. The graph on the left depicts TGA of the MOF used in our comparison in Black. The graph on the right portrays how the porosity of this material is maintained by a test via TGA by comparison of before activation, post activation, and after resolvation of the material Figure 12 An illustration of the reported solved crystal structure of a MOF similar to our target MOF. This is a MOF designated as ZnZn-RPM (Robust Porphyrinic Materials) which incorporates TCPP and 5,15-di-(pentaflurophenyl)-10,20-di-(4-pyridyl)porphyrin ligands Figure 13 An illustration of the solved crystal structure of the Zn-DPyDPP MOF Figure 14 A graph representing the TGA data collected of the Zn-DPyDPP MOF Figure 15 Mechanisim of oxidation of DPBF by singlet oxygen Figure 16 UV-Vis Spectra of DPBF solutions of varying concentrations creating a calibration curve for comparison purposes Figure 17 A graph depicting the data collected during the monitoring of singlet oxygen generation v

6 Introduction Porous metal-organic frameworks (MOFs) are a class of highly-ordered crystalline solids that has attracted considerable attention over the last two decades. 1 MOFs are synthesized through the use of metal ions or clusters that coordinate to rigid organic ligands to form a continuous, multidimensional framework. These frameworks, otherwise known as coordination polymers, result in solids that are permeated by channels along different axis that impart permanent porosity. These channels have defined pore sizes that can range within the average range of 4-20 Å 1 in diameter to the extremes of pores sizes with 61 Å by 72 Å dimensions. 2 MOFs can thought of as organic analogues of zeolites, which are naturally occurring or synthetically made inorganic porous solids usually consisting of aluminosilicate minerals. Both types of materials exhibit exceptionally high pore volumes and surface area. The contrasting factor is that MOFs, unlike zeolites, have the ability to be customized via modification to the organic ligands from which they are constructed. This key difference allows the structures and properties of MOFs (i.e. metal ions, ligand dimensions and, functional groups) to be tailored to specific applications. MOF materials and their customization seem only to be limited by the creativity and ingenuity of the chemists seeking to use these materials towards real world applications. The MacDonald group has been focusing on the applications of MOFs towards environmental remediation by exploring the design of a class of MOFs that incorporate porphyrin ligands into the framework. Porphyrins belong to a class of compounds known as photosensitizers, which are dyes that absorb light in the UV and visible spectrum exciting them into their triplet state allowing for an energy transfer to free molecular oxygen resulting in the generation of singlet oxygen. Singlet oxygen is a highly reactive species of free oxygen that is a means to oxidize or oxidatively decompose organic molecules containing susceptible unsaturated hydrocarbons. By synthesizing a sorbent material like a MOF containing theses photosensitizers built directly into the backbone of their structure, one can create a catalytic material capable of generating singlet oxygen in order to oxidatively decompose organic guest molecules that it may absorb, passively allow the product to pass through the material via its channels, and turn over to repeat the process with newly absorbed guests molecules. It is known that porphyrins featuring four metal binding sites can form both two- and threedimensional MOFs in the presence of metals ions. For example, meso-5,10,15,20-tetrakis(4- carboxyphenyl)porphyrin (TCPP) coordinates one metal ion at the center of the porphyrin ring and to two additional metal ions at each of the four carboxyl groups to form ordered sheets of porphyrin called paddlewheel layers. 11 In the absence of additional linking ligands, the paddlewheel layers stack at van der Waals contact distance to form a solid exhibiting low porosity. Addition of a linking ligand with two metal binding sites such as 4,4 bipyridine (BPY) leads to formation of highly porous three-dimensional frameworks with BPY acting as a bridging ligand between paddlewheel layers of TCPP. 1

7 Toward expanding on our research developing porous photosensitizing MOFs, this project focused on investigating how modifying the dipyridine linker would affect the structure, porosity and reactivity of TCPP paddlewheel MOFs toward generating singlet oxygen. Important questions we wanted to address are whether increasing the size of the linking ligand would result in larger channels (i.e. increased porosity) that enhance diffusion of guests into the MOF, whether increasing the dimensions of channels would alter the stability or architecture of the crystalline lattice of the MOF, and whether increasing porosity would affect generation of singlet oxygen and the rate of oxidation of guest contaminant. Accordingly, the aims of this project were (1) to design, synthesize and characterize meso-5,15-diphenyl-10,20-di(4-pyridyl)porphyrin (DPyDPP) as a longer structural analog of BPY, (2) synthesize a paddlewheel MOF composed of TCPP, DPyDPP and Zn metal ions and characterize its structure, porosity and thermal stability, and (3) test the activity of that MOF toward generating singlet oxygen by monitoring oxidative degradation of 1,3-diphenylisobenzofuran (DPBF) as a model guest contaminant using UV- Visible spectroscopy and then compare the activity to the TCPP-BPY-Zn MOF. Background Porous solids Porous solids are a class of materials that have pores or channels permeating their structures with dimensions that are large enough to facilitate the diffusion of guest molecules. These materials also exhibit the ability to maintain their porosity even after the evacuation of solvent and guest molecules. The porosity of porous materials, defined as the ratio of void volume in the channels to the total volume of the solid, ranges widely from with correspondingly high surface area that is directly proportional to their porosity. These properties make porous solids ideal as sorbent materials for applications involving molecular adsorption, storage and separation.[refs] A well-known example of porous solid is activated carbon, which has played host to a wide range of applications from simple water filtration, 4 catalysis, 5 and medical applications. 6 Porous materials that result from irreproducible, non-repeating patterns are classified as disordered porous solids. Such solids have no defined structure leaving the dimensions of their pores and void volumes difficult to characterize. Ordered porous solids differ from disordered porous solids in that they exhibit crystalline properties that include well-defined pore structures, dimensions and topology that can be controlled and reproduced. 3 A prime example of a class of ordered porous solids is inorganic zeolites. These materials, which occur naturally or are produced synthetically via hydrothermal or sol-gel preparations, 7 are composed of hydrated, crystalline tectoaluminosilicates consisting of TO 4 (T= Si, Al) tetrahedral bridged by oxygen atoms. 3 Zeolites commonly are used for applications involving ion-exchange 7 due to the presence of negatively charged aluminum ions, which strongly attract cations of alkali and earth alkali metals within the channels of zeolites in 2

8 order to neutralize the overall charges. As shown in Figure 1, zeolites have a wide variety of crystalline structures with different channel topologies as well as surface properties that can be controlled by varying the ratios of silicon to aluminum during synthesis. This variation of reagent ratios alters the arrangements of the sodalite cage, the repeating unit in zeolites. Zeolites possess a restricted range of pore sizes (4-12 Å) that only allow zeolites to act as porous hosts for small ions and small organic molecules. Figure 1 Examples of several zeolite frameworks generated by different arrangements of sodalite cage. 7 Metal-organic frameworks (MOFs). MOFs are crystalline coordination polymers that consist of metal ions or ion clusters that coordinate with polyfunctional rigid organic ligands via electron rich moieties forming complexes that self-assemble into one-,two- or three dimensional frameworks. 3 MOFs exhibit exceptional porosity and surface areas in comparison to zeolites and other porous materials. MOFs have shown to have surface areas in the range of m 2 /g compared to disordered carbon in the 2000 m 2 /g and most porous zeolites within the range of only 900 m 2 /g. 16, 17 It is these properties that allow for high accessibility for organic guest molecules within the channels of MOFs compared to zeolites. 18 Because organic molecules are the building blocks used to assemble MOFs, the structures and properties of MOFs can be tuned to a multitude of different dimensions and topologies as well as exhibit different catalytic, chemical, and physical properties. Isoreticular MOF-5 (IRMOF-5), first synthesized by Omar Yaghi in 1999, is perhaps the most well-known example of a highly porous and stable MOF with cubic architecture that serves as a standard to which the porous properties of all new MOFs are compared. 3, 18 IRMOF-5 (far left in Figure 2) consists of 1, 4-benzene dicarboxylic acid (BDC) ligands that coordinate to tetrahedral 3

9 clusters of zinc (II) ions. As shown at the top in Figure 2, lengthening the diacid ligand preserves the topology and cubic symmetry of IRMOF-5, allowing for channels with larger dimensions to be designed. This modularity with respect to the linking ligand gives rise to family of MOFs with similar in crystal structure to IRMOF-5 that exhibit larger pore and channel dimensions, larger void volumes, and increased surface area. As shown on the bottom in Figure 2, functionality can be introduced on the BDC ligand without altering the cube structure of IRMOF-5. Substituents such as bromine or amine groups present on the MOF backbone protrude into the channels making it possible to alter the surface properties and to introduce chemically reactive functional groups inside the void volume of the MOF where additional functionality can be introduced via post-synthetic modification. 19 Figure 2 Comparison of the cubic structures of IRMOFs formed when linear aromatic dicarboxylic acids are reacted with Zn (II) ions. Top: Increasing the length of the aromatic dicarboxylic acids (highlighted in orange) gives IRMOFs with larger channels. Bottom: introducing substituents (highlighted in red) onto benzenedicarboxylic acid gives IRMOFs with cubic frameworks identical to that of IRMOF-5 (far left) in which substituents protrude into the channels. 3,8 Singlet Oxygen Singlet oxygen is an excited form of the molecular oxygen that became the focus of intense study in 1963 after liberated singlet oxygen was discovered as the cause of the chemiluminescence of the hypochlorite-peroxide reaction by Khan and Kasha. 9,14 The properties of this highly reactive form of molecular oxygen caught the attention of reseachers looking to utilize singlet oxygen for applications such as photooxidation, photodynamic therapy of cancer and polymer science. 9 4

10 Molecular oxygen normally resides in the ground state, otherwise known as its triplet state. This triplet state has an electron configuration that is shown below. O2 (core)(πx )(πy )(πx* )(πy* ) The point of interest in this configuration is the spin directions of the valence electrons shown below. O2 (πx* )(πy* ) Molecular oxygen has two singlet excited states, 1 Δ g and 1 Σ g +. 1 Δ g lays 95 kj mol -1 above the triplet state and 1 Σ g + is 158 kj mol -1 above. 9 As shown below, both states have different electron configurations. 1 Δ g has its last two electrons paired in the same π-antibonding orbital where 1 Σ g + has those same electrons in the orbitals as the ground state except they have antiparallel spins. 1 Σ g + (πx* )(πy* ) 1 Δ g (πx* ) or (πy* ) Once oxygen is converted to the singlet excited state, other molecules can deactivate singlet oxygen and return it to its ground state. This process known as quenching can take place via two routes. 15 Physical quenching is a process that only leads to the deactivation of singlet oxygen via an interaction with another species present. This results in no consumption of molecular oxygen and does no yield any oxidation products. The second route is known as chemical quenching which is the result of an oxidation reaction occurring between singlet oxygen and another species yielding a new product. Singlet oxygen, because of it increased electrophilicity, has the ability to rapidly react with species containing unsaturated carbon-carbon bonds, sulfides, and amines, just to name a few. 9 As described later in this report, we take advantage of the ability of porphyrins in the backbone of MOFs act as photosensitizers to covert dissolved triplet oxygen into singlet oxygen catalytically as a source of oxidant to react with guest contaminants sorbed in the MOF. Current research in the MacDonald group. Our group has been has been focusing on developing MOFs that incorporate ligands that are dyes and can act as photosensitizers. Photosensitizers are a class of molecules that include of wide variety of UV-Vis absorbing molecules that have the ability to generate singlet oxygen. By absorbing an appropriate wavelength of light, these photosensitizers are most commonly excited via a one-photon transition between the ground state, S 0, and a singlet excited state S n. Relaxation of this S n state yields the lowest of the singlet states S 1. However, by the process of an intersystem crossing generates the triplet state of the sensitizer, T 1. As shown in Figure 3, T 1 has a lifetime that is magnitudes longer than S 1 which allows for this excited state to react in two different ways, defined as Types I and II mechanisms. 9 Type I mechanisms involve either the 5

11 abstraction of a hydrogen atom or the transfer of an electron between the excited substrate and the substrate, yielding free radicals. Figure 3 An illustration of the generation of excited photosensitizer states and reactive dioxygen species. 9, We are interested in the Type II mechanism. Type II involves the generation of singlet oxygen through an energy transfer between the excited sensitizer and triplet oxygen during a collision. Using this particular mechanism, we have been exploring the synthesis of MOFs that incorporate photosensitizers into the framework. This would allow the backbone of the MOF itself to generate singlet oxygen through photoexcitation. Figure 4 shows the chemical structure of the particular photosensitizers we have been using to conduct our research thus far, meso 5,10,15,20- tetrakis(4-carboxyphenyl) porphyrin (TCPP). In addition to containing a porphyrin ring capable of acting as a photosensitizer, TCPP features four carboxyl groups that are known to coordinate to transition metal ions making it a suitable ligand for generating MOFs. 6

12 Figure 4 The chemical structure of meso 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin. Experimental and Characterization Reagents and solvents were purchased from Alfa Aesar, Acros, Frontier Scientific, Pharmco or Avocado and used without further purification. Samples of MOF crystals were characterized using a combination of optical microscopy, thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD), and single-crystal X-ray diffraction (SXRD). Bulk samples of all crystals were isolated and examined under a low-power optical polarizing stereomicroscope to determine the homogeneity of samples based on crystal morphology (paddlewheel MOFs generally form square plates), and to ensure that single crystals analyzed by SXRD were not twinned or cracked. Infrared spectra were examined for crystals of MOFs and spectra were obtained using a Bruker Optics FT-IR spectrometer equipped with a Vertex70 attenuated total reflection (ATR) accessory by collecting 64 scans over a scan range from 4000 to 400 cm -1 at 4 cm -1 resolution. PXRD data were collected on a Bruker-AXS D8-Advance diffractometer using Cu-K α radiation with X-rays generated at 40kV and 40mA. Bulk samples of crystals were placed in a 20 x 16 cm x 1 mm well in a glass sample holder and scanned at RT from 5-45 (2θ) in 0.05 steps at a scan rate of 2 /min. Samples of all crystals were analyzed by TGA to determine if crystalline samples were porous and to quantify the amount of guest solvent contained within MOFs based on loss of mass during heating. Analysis by TGA was carried out using a TA Instruments 2920 thermogravimetric analyzer. Bulk samples of crystals (5-10 mg) were placed in Pt sample pans and then heated from RT to 500 C at a rate of 10 C /min under a nitrogen atmosphere. UV/Vis spectra were collecting on an Evolution 300 UV/Vis Spectrophotometer. Methodology for Characterization of Products 7

13 ThermoGravimetric Analysis (TGA) Samples of MOF crystals were left in the growth solution prior to analysis to prevent guest solvent from evaporating out of the MOF crystals on exposure to air. Samples of MOF crystals were removed from the growth solution, blotted dry to remove solvent on the exterior, 5-10 mg of sample loaded into a tared Pt TGA pan, and then heated in the TGA instrument at a rate of 10 /minute from RT to 700 C under a nitrogen atmosphere. Synthesis of Ligands Meso (4-pyridyl)dipyrrlmethane 4- Pyridylcarboxaldehyde (20 mmol, 1.9 ml) was placed into a round bottom flask containing pyrrole (20 ml). The flask was sealed and evacuated of air and purged using N 2. The mixture was heated to 85 o C and left stirring overnight, approximately 17 hours. The reaction was left to cool to room temperature. The excess pyrrole was removed by rotary evaporation. The resulting crude was then purified by column chromatography (100 g of mesh silica gel, 75% ethyl acetate/25% hexanes). Two yellow bands eluted within a tight R f of each other (<0.05) TLC analysis (75% ethyl acetate/25% hexanes) revealed the second band appeared to be the desired product by comparison against a 4- Pyridylcarboxaldehyde sample proving the first band to be the aldehyde reagent. The fractions were then collected, combined, and concentrated to yield 1.1 grams of a tan solid (23% yield). 1 H and 13 C NMR analysis revealed the solid was pure and did not require recrystallization. Characterization 1 H NMR (500 MHz, DMSO-d6) d ppm 5.30 (s, 1 H) (m, 2 H) 5.84 (q, J=2.61 Hz, 2 H) 6.55 (td, J=2.60, 1.65 Hz, 2 H) (m, 2H) (m, 2 H) (br. s., 2 H). 13 C NMR (126 MHz, DMSO-d6) d ppm (s, 1 C) (s, 1 C) (s, 1 C) (s, 1 C) (s, 1 C) (s, 1 C) (s, 1 C) (s, 1 C). 5,15-diphenyl-10,20-di-(4-pyridyl) porphyrin 5-(4-Pyridyl)dipyrrlmethane (2.24 mmol, 500 mg) and benzaldehyde (2.24 mmol, 238 mg, ml) were dissolved in dry dichloromethane (350 ml) in a 500 ml round bottom flask. The reaction flask was then purged of air and placed under an atmosphere of N 2. The solution was stirred until the reactants were completely dissolved as the solution was cooled to 0 0 C using an ice/water bath to form a clear yellow solution. Trifluoroacetic acid (43 eq., 6.89 ml) was added 8

14 drop-wise to the solution, which turned from clear yellow to a progressively darker red/purple, indicating formation of porphyrin. After stirring for 30 minutes, 2,3-dichloro-5,6-dicyano-1,4,- benzoquinone (DDQ) (2 eq., 4.48 mmol, g) was added to the solution and the flask was removed from the ice bath to return to room temperature and stir for 1 hour. The reaction solution was then wash with saturated aqueous sodium bicarbonate (200 ml), deionized water (3 x 500 ml), then brine (100 ml). The organic layer was then dried over magnesium sulfate, vacuumed filtered and concentrated in vacuo. The resulting dark red/purple crude solid was purified by column chromatography (60 g mesh silica gel, 0/100% EtOH/CHCl 3-3/97% EtOH/CHCl 3 ). The dark red/purple band was collected until the eluting band darkened to an opaque black color, which was indicative of a side product that followed right after the porphyrin. The dark red/purple product was then recrystallized by concentrating it in a solution of chloroform (50 down to 10 ml), then adding hexanes (15 ml) 143 mg (20% yield) of the desired porphyrin as a purple solid. Characterization 1 H NMR (500 MHz, CHLOROFORM-d) d ppm (s, 2 H) (m, 6 H) (m, 4 H) (m, 4 H) 8.76 (d, J=4.77 Hz, 4H) 8.85 (d, J=4.77 Hz, 4 H) (m, 4 H) Hydrothermal Synthesis of MOFs Two different MOFs, Zn-TCPP-BPY and Zn-TCPP-DPyDPP, were prepared via hydrothermal synthesis. Zn-TCPP-BPY was synthesized using a modified procedure adapted from a method described previously by Choe. 11 Zn-TCPP-DPyDPP was synthesized using two different modified procedures adapted from methods described independently by Choe 11 and Hupp. 13 Zn-TCPP-BPY MOF Synthesis A 3:1 (v:v) solution of diethylforamide (1.5 ml) and ethanol (0.5 ml) was used to dissolve 5,10,15,20-tetra(4-carboxyphenyl)porphyrin (7.9 mg, 0.01 mmol), zinc nitrate hexahydrate (8.2 mg, 0.03 mmol), and 4,4-bipyridine (3.3 mg, 0.02 mmol). This mixture was sealed in a microwave vial and placed into an oven at 80 o C for 24 hours. After the allotted time, the oven was turned off to allow the solution to cool slowly to room temperature. Purple square blocks were produced. ZN-TCPP-DPyDPP MOF Synthesis in one step 9

15 Solutions consisting of a 3:1 ratio of diethylforamide and ethanol were used to dissolve varying ratios of 5,10,15,20-tetra(4-carboxyphenyl)porphyrin, zinc nitrate hexahydrate, and 5,15- diphenyl-10,20-di-(4-pyridyl)porphyrin in an attempt to determine optimal stoichiometry of the components for generating a MOF. The respective ratios of reactants used in this series of reactions are shown in Table 1. Each solution was sonicated for 1 minute to dissolve the solid components, sealed in a microwave vial, placed in an oven at 80 o C for 24 hours, and then allowed to slowly cool back to room temperature. A Heterogeneous mixture of smaller thin, red square-shaped plates and larger purple, rhombic crystals was produced. Attempt TCPP(mmol) Zn(NO 3 ) 2 6H 2 O (mmol) DPyDPP (mmol) DEF (ml) EtOH (ml) (TGA Purity) Table 1. Ratios of reagents used in the one step hydrothermal synthesis of Zn-TCPP-DPyDPP MOF. 11 Zn-TCPP-DPyDPP MOF Synthesis in two steps Step 1: 5,10,15,20-tetra(4-carboxyphenyl)porphyrin (11.8 g, mmol) and zinc nitrate hexahydrate(1708 mg, 0.06 mmol) were placed in DMF (2 ml), the solution was sonicated for 2 minutes, then sealed in a microwave vial and placed in a oven at 80 o C for 2 hours. Step 2The hot vial was removed from the oven and carefully uncapped. 5,15-Diphenyl-10,20-di- (4-pyridyl)porphyrin (9.24 mg, mmol) and 0.03 M HNO 3 in EtOH (2 ml) were added, the vial was capped, placed back into the oven at 80 o C for an additional 20 hours, then allowed to slowly cool to room temperature. This synthesis produced thin, red, square-shaped plates. Methodology for Monitoring Generation and Detection of Singlet Oxygen As shown in Figure 6, a 60 watt xenon lamp was used as the light source to irradiate MOF particles in solutions of DMF containing 1,3-diphenylisobenzofuran (DPBF) as a guest contaminant susceptible to oxidation by singlet oxygen to assess the activity of the MOF toward generating single oxygen. The lamp was placed behind a 480 nm cut-off filter to prevent degradation of the DPBF through exposure to ultraviolent light. This filtered light reflected off a right angle mirror to irradiate the surface of the solution contained in a 100 ml beaker resting on a stir plate. This experimental set up was housed in a dark, curtained room and the outside of the 10

16 beaker covered in aluminum foil to eliminate exposing the MOF and DPBF to light from outside sources. Figure 5 Illustration of the setup used to photoexcite the MOF materials and monitor generation of singlet oxygen. To determine whether irradiating the MOF produced singlet oxygen and to quantify the rate of reactivity of singlet oxygen with DPBF, 35 ml of 100 µm DPBF in DMF solution containing10 mg of activated MOF crystals, which were left intact without grinding by mortar and pestle, that were heated to 200 C for 120 minutes prior to adding them to the solution to remove guest DMF solvent. The solution was stirred and irradiated continuously under the xenon lamp for 90 minutes. Aliquots of 3 ml of the solution were removed by syringe at 10 minute intervals, the solution was passed through a 0.2 micron filter to remove particles of MOF, and the absorbance measured using UV-Visible spectroscopy to determine the concentration of unoxidized DPBF remaining in solution. Upon removal from the reaction flask, exposure of samples containing DPBF to ambient light was minimized by storing and transporting the solution in a covered container prior to analysis by UV-Visible spectroscopy. Results and Discussion Synthesis and characterization of 5,15-diphenyl-10,20-di-(4-pyridyl)porphyrin (DPyDPP) The purpose of this part of the project was to synthesize DPYDPP as a structural analog of 4,4 - bipyridine (BPY) in order to test our hypothesis that it would be possible to replace BPY with DPyDPP as a structural bridging ligand in order to increase spacing between layers of TCPP, ultimately leading to paddlewheel MOFs with larger channels. Synthesis of the target ligand meso-5,15-diphenyl-10,20-di(4-pyridyl) porphyrin was carried out according to the reaction 11

17 scheme shown in Figure 5. Procedures utilized for the individual reaction steps were adopted from Boccalon. 10 Figure 6 An overall reaction scheme for the synthesis of meso-5,15-diphenyl-10,20-di(4-pyridyl)porphyrin. 10 This procedure was designed to synthesize the exact porphyrin ligand desired by this project. 143 mg of DPyDPP were obtained for and overall yield of 4.6% from initial starting materials. This product was shown to be pure by 1 H NMR. This synthesis was reproduced twice in order to acquire an amount necessary to further later MOF syntheses. Although the yields of DPYDPP reported by Boccalon were not attained, the compound was successfully synthesized using their procedure. Synthesis and Characterization of MOFs Zn-TCPP-BPY Hydrothermal synthesis was used to obtain this MOF as it is a method that promotes crystal growth and size through higher concentrations of ligand to solvent. A conventional approach to the creation of these structures at room temperature may be feasible but, crystal size is vital for characterization by Single Crystal X-ray Diffraction (SXRD). For this particular MOF, any further modification to the procedure reported by Choe was not necessary because it was designed for this particular framework. After the synthesis time period, predetermined by the Choe procedure, the result was a vial containing purple, square crystals. It was expected to see crystals consisted mostly of TCPP ligand to have this morphology due to how TCPP polymerizes into a two dimensional sheet via zinc-carboxylate coordination as show below in Figure #. Because of the square planar orientation of the carboxylate moieties on each TCPP ligand, these crystals will visually manifest as thin, red square plates. The TCPP only morphologically contributes to the square facet of these crystals. It is the thickness of these crystals that allows us to visually determine the inclusion of the BPY. Not only do we see increased thickness but, the crystals are purple rather than the expected red of the Zn-TCPP plates. 12

18 Figure 7 An illustration of the two dimensional layer or sheet that is formed as TCPP binds with metal ions (Blue) through coordination via their carboxylate groups (Red). By SXRD of a larger crystal, the structure of the framework as seen in Figure # below, was determined. BPY acts as a bridging ligand between TCPP paddlewheel layers by coordination of the nitrogen in the BPY to not only the axial sites of the zinc ions at the paddlewheel sites, the zinc ions which coordinate to the carboxylates from two opposing TCPP ligands, but at the axial sites at zinc ions coordinating with the pyrrole nitrogens in the center of each porphyrin ring. Due to the nature of the Zinc metal ions in use, even though there is an octahedral coordination arrangement at the paddlewheel zinc ions, the zinc ions chelated by the porphyrins rings coordinate in only a square pyramidal configuration as they usually prefer. This difference in coordination geometries alters the motif of the framework, forming a pattern of larger and smaller pores. Figure 8 An illustration of the crystal structure and stacking arrangement of Zn-TCPP-BPY MOF. 13

19 With the crystal structure data providing the idea that this MOF should have channels that are about 16 Å by 7 Å in dimension based on the length of the TCPP and BPY ligands, respectively, thermogravimetric analysis (TGA) was done in order to obtain additional supporting evidence. TGA provides data on porosity of solid materials by determining how much relative mass is lost as function of temperature. This loss of relative mass is reflects directly how much of the material is void volume. As stated in the methodology for TGA in an earlier section, MOF crystals were prepared by blotting sample crystals dry on filter paper for the sole purpose of removing excess solvent from the surface of the crystals. This allows the TGA results to reveal a more accurate picture as to the extent of the porosity of the MOFs by the assumption that what we see in the TGA results is a result of the guest solvent, DMF and EtOH, escaping from the channels of the framework. The graph in Figure # below shows the TGA results collected on the Zn-TCPP-BPY MOF crystals Thermogravimetric Analysis: Loss of Guest Solvent TCPP- BP- Zn Weight % Temperature ( o C) Figure 9 A graphical representation of the TGA data collected of the Zn-TCPP-BPY MOF (PPF-4). Looking from 25 o -75 o C, a 50% loss of mass is shown to occur at a substantial rate with an additional 10% loss occurring from 75 o -150 o C finishing at a plateau that steadily degrade until a sharp downslope as it approaches 500 o C. The rapid loss of over 60% of the relative mass by the 150 o C, above the boiling point of the guest solvent DMF, is evidence that supports the claim that this is a porous material that can be readily evacuated of its guest solvents. However, it is hard to determine if these were the expected results. Returning to supporting data reported by Choe, although we do see the drop is percent mass plateau at the same temperature, 150 o C, there 14

20 is a large difference of 60% versus 10% between our data and the data reported by Choe, respectively. This can be caused by the difference in handling the preparation of the sample crystals when placing the crystals onto the pan. While the procedure used to collect the TGA data we collected can be seen as written in an earlier section, it is unclear as to how the Choe group prepared and handled their samples. We believe the cause of this inconsistency may lie here. On the other hand, we do see some consistency to reports that this material appears to show thermal stability up to 200 o C. The data collected shows the beginning of not immediate but, slow degradation at 200 o C up until its more extreme decomposition as temperatures approach 500 o C. In the end, the collected data proves to show that it is possible to synthesize a material containing channels with dimensions large enough to allow passive diffusion of oxygen and guest contaminants into the channels via interaction with the walls of these channels. By this process know n as sorption can now facilitate the proximity necessary for these contaminants to react with singlet oxygen generated when TCPP is irradiated. Zn-TCPP-DPyDPP The synthesis of the Zn-TCPP-DPyDPP MOF was one of the three main goals of this project. With the first being the successful synthesis of the DPyDPP ligand, the second involved incorporating said ligand as a bridging ligand as a way to replace the BPY ligand in order to create channels with larger dimensions and void volume. Two methods were modified in attempts to synthesize this particular MOF. The first was a procedure modified from the Choe method. This involved a single step that only required the replacement of the BPY with the DPyDPP as modification. This resulted in the creation of Zn-DPyDPP MOF crystals which will discussed in the next section. As of result of this one step method provided by Choe only providing a means to synthesize a heterogeneous mixture of Zn-TCPP plates and the newly discovered Zn-DPyDPP crystals, a new method had to be attempted in order to obtain the desired Zn-TCPP-DPyDPP MOF. Further researched led to the discovery of a two-step method designed by the Hupp group which involved the synthesis of MOFs from only porphyrinic ligands, both of which were TCPP and 5,15-di-(4-pyridyl)-10,20-di-(pentafluorophenyl)porphyrin (DPyDPFPP). It was these similarities in ligands that gave enough reason to adopt this new procedure. The Choe method is a one-step procedure that places all reagents in the same solvent system at the same time and because of the difference in ligand sizes and coordination moieties will determine how the framework will self-assemble. This method was designed specifically for self-assembly involving TCPP and BPY, not for a dual porphyrinic ligand system. The Hupp method is a twostep procedure that allows for the assembly of the TCPP paddlewheel layers to begin before the second step which introduces the second ligand, the dipyridylporphyrin, under acidic conditions believed to promote the coordination and incorporation of the second ligand. This second method 15

21 is designed for the incorporation of more than a single porphyrin ligand in the backbone of the framework. It is this reason why the Hupp method was believed to be the more ideal procedure for our purposes. With the new method adopted, the results of using this method with the small modification of the ligands used, TCPP/DPyDPP combination rather than the reported TCPP/DPyPFPP, were the appearance of red, square plates. The appearance of these plates tells us that Zn-TCPP paddlewheels layers are present but, this facet cannot prove nor disprove the incorporation of DPyDPP. By rotating the crystals onto its side in attempts of seeing the thickness of these plates, it can be seen that most of the plates did not have the thickness we were expecting. These crystals turned out to be rather thin. This was a cause for concern as the crystals appeared to be too thin for SXRD. Attempts to mount multiple crystals proved this hypothesis as a solved crystal structure could not obtained. Without SXRD for characterization, alternative methods must be used to provide an accurate picture of the properties of these particular crystals. One hopeful observation was the appearance of a homologous mixture of crystals that resulted from this method. Most if not all the crystals within the observed samples appeared to have the same morphology indicative of Zn-TCPP paddlewheel layers, red, square plates. The second glimmer of hope came from the collected TGA data shown in figure 10 below. 100 Thermogravimetric Analysis: Loss of Guest Solvent Zn- TCPP- DPyDPP 80 Weight % Temperature ( o C) Figure 10 A graph representing the data collected by TGA of the presumed Zn-TCPP-DPyDPP MOF. 16

22 The data shows a 65% percent loss of mass that appears to plateau at about 200 o C with degradation only beginning around 500 o C. With the assumption of the lost mass consisting of only guest solvent thanks to the method of preparing the sample, this data shows a MOF with high void volume and thermal stability. Both of these properties are within the expected as we the goal was to create a MOF that had the dimensions attainable by large porphyrinic ligands which in turn would provide larger void volumes. To further solidify to results thus far, Hupp reports TGA data as shown, in black, in Figure 11 below. The similarity between the observed and reported TGA data is striking. Figure 11 TGA data reported in the Literature of the porphyrinic MOFs with a variety of metal ions used in their construction. The graph on the left depicts TGA of the MOF used in our comparison in Black. The graph on the right portrays how the porosity of this material is maintained by a test via TGA by comparison of before activation, post activation, and after resolvation of the material. 13 Although it is also unclear as to the handling of the sample crystals, similar to the TGA data reported for the Zn-TCPP-BPY MOF, it is the appearance of similar to almost identical porous behavior observed through TGA that leads us to believe we have obtained the desired framework which can be seen in Figure 12. The illustration shown is of the Zn-TCPP-DPyDPFPP MOF structure as reported by Hupp. This structure is believed to be analogous to the structure created here with the exception of our DPYDPP ligand rather than its fluorinated counterpart. 17

23 Figure 12 An illustration of the reported solved crystal structure of a MOF similar to our target MOF. This is a MOF designated as ZnZn-RPM (Robust Porphyrinic Materials) which incorporates TCPP and 5,15-di-(pentaflurophenyl)- 10,20-di-(4-pyridyl)porphyrin ligands. 13 An unfortunate yet intriguing phenomenon occurred during the preparation of these crystals for the monitoring of singlet oxygen generation experiment. Prior to activation of the crystals, they were washed with clean DMF in order to remove any remaining reagents that remain. The usually colored solvent was then decanted and replenished with fresh DMF until the solvent ran clear. The problem was that the solution never went clear by this method after excessive repetitions. A more rigorous method of cleaning by placing the MOF particles in a 100 ml of DMF while slowly stirring and gently heating the bath only proved to exacerbate the problem by visual confirmation that TCPP appeared to be dissolved in the washing solvent through the appearance of the DMF turning a clear pink/red color. Recovery of the MOF particles also showed a loss of the total initial amount of MOF present. A sample of this solution was examined by UV-Vis to reveal a peak at 423 nm with a smaller pair of peaks shown at around 560 nm and 600 nm. These results are indicative of a metalated porphyrin present in solution. Normally this wouldn t pose a problem as DPBF adsorbs at 416 nm however, given the high solubility and coloration of TCPP, a concentration of about 0.01 mm of TCPP is relatively easy to obtain and overshadows the DPBF which renders our particular test unfit to monitor the activity of this system as it stands now. It would prove beneficial in our pursuits to recreate the exact MOF structure created by Hupp to observe any similar behaviors as they report to also do that same method of washing the crystals prior to any further testing. Zn-DPyDPP 18

24 As mentioned early in the previous section, while attempting to synthesize the Zn-TCPP- DPyDPP MOF via Choe s one step method, a heterogeneous mixture of crystals appeared in the resulting batch of crystals. Most of the smaller crystals had the resemblance of the Zn-TCPP that was known to manifest as red, square plates. The larger crystals contrasted not only in size but in clarity and morphology. Very little light passed through the larger crystals and only allowed light through while shifting the crystal. This gave the crystals more of a purple appearance and only revealed its red hue when light was able to shine through various sections of the crystal. The crystals also appeared rhombic in shape. The sheer size of these crystals yielded enough diffraction to obtain a crystal structure by SXRD. Figure 13, below, shows the solved crystal structure of what appears to be a MOF structure consisting of only DPyDPP ligands that have zinc ions only at the center of the porphyrin rings allowing for coordination of pyridyl nitrogens from adjacent DPyDPP. This particular coordination appears to aggregate into stacked sheets of this DPyDPP polymer. An interesting point of interest to note is the lack of TCPP anywhere in the solved structure. It appears that the metalated DPyDPP self-selective during assembly. A hypothesis as to why this is requires the comparison between DPyDPP and the fluorinated analog DPyDPFPP. It is possible that due to the lack of electron withdrawing groups in DPyDPP like the pentafluorophenyl or carboxyphenyl groups in DPyDPFPP and TCPP, respectively, is the cause for increased basicity in not only the pyridyl nitrogens but the pyrrole nitrogens at the center of the porphyrin rings as well. If this is the case, DPyDPP may coordinate with zinc ions faster than TCPP causing the Zn-DPyDPP to be favored kinetically. This could be the rationale behind the two step method designed by Hupp that allows for the formation of the TCPP paddlewheel layers initially then introducing the dipyridyl porphyrin under acidic conditions in order to slow the coordination of the pyridyl- moieties by protonation. This could also be another reason for the incorporation of DPyDPFPP rather than DPyDPP, in order to slow down the coordination even further. 19

25 Figure 13 An illustration of the solved crystal structure of the Zn-DPyDPP MOF. Further characterization was done on this peculiar framework as investigation into the structure reveals what appears to be winding, narrow channels between the Zn-DPyDPP sheets. Figure 14 shows the results from TGA. This particular MOF exhibits interesting behavior as it shows an initial loss of 35% relative mass that plateaus around 75 o C until an additional 20% beginning at 200 o C. This is followed by another plateau at 250 o C until the drastic degradation around 500 o C Thermogravimetric Analysis: Loss of Guest Solvent Zn- DPyDPP Weight % Temperature ( o C) Figure 14 A graph representing the TGA data collected of the Zn-DPyDPP MOF. This MOF structure appears to be porous but, these channels appears to not permeate through the entirety of the crystal. The series of moss being lost followed by a plateau only to lose additional 20

26 mass gives rise to the idea that while there are in fact channels open to pore on the surface of the crystal supported by the first series of mass loss and plateau, the second drop and plateau seems to indicate pockets of solvent trapped between the sheets of Zn-DPyDPP only to be released as temperatures increase past the point of the energy of the Van der Waals forces that may be keeping the Zn-DPyDPP aggregates together, allowing the sheets to separate. As seen while cleaning Zn-TCPP-DPyDPP crystals, the Zn-DPyDPP MOF also exhibits the same instability in DMF. A possible hypothesis that may explain this occurrence is that both types of synthesized crystals are only aggregates sheets of their respective ligands. Without any bridging ligands incorporated between layers, it is possible that the layers could dissociate and dissolve back into solution. Further investigation will have to be done by not only recreation of Hupp s Zn-TCPP-DPyDPFPP MOF but also successful synthesis and characterization by SXRD of the Zn-TCPP-DPyDPP MOF. Activity of MOFs toward generating singlet oxygen The final goal of this project was to establish whether the incorporation of a porphyrin into the backbone of a porous MOF would allow the MOF to act as a photosensitizing host capable of generating singlet oxygen in order to oxidize organic contaminants sorbed within the MOF. Generation of singlet oxygen was assessed by irradiating the solution of DPBF and MOF particles and monitoring oxidative degradation of DPBF. DPBF was selected as a model contaminant because the pi system in the furan ring is known to react with singlet oxygen and readily undergo oxidation to form an unstable endoperoxide intermediate that rapidly rearranges to form the corresponding diketone, as shown in Figure 15. In addition, DPBF in DMF shows an absorption maximum in the UV at 416 nm where the TCPP photosensitizer does not absorb, thus allowing the concentration of DPBF and by inference, the presence of singlet oxygen to be measured using UV-Visible spectroscopy. Lastly, we reasoned that the largely nonpolar, hydrocarbon structure and size of DPBF would make that compound a good test candidate for modeling sorption and oxidation of known contaminants such as benzo[a]pyrene and estrogens that exhibit similar structures and properties. 21

27 Figure 15 Mechanisim of oxidation of DPBF by singlet oxygen. A calibration curve was created by measuring absorption by solutions of DPBF in dimethylforamide (DMF) with varying concentrations ranging from 50 to 250 µm in 50 µm intervals. A stacked plot of the UV-Vis absorption spectra in the region from nm is shown in Figure 16. The data shows that DPBF absorbs intensely at 416 nm and that the concentration of DPBF (C) varies linearly as a function of the absorption (A) at 416 nm according to Beer s law, A = εlc (where ε = the extinction coefficient for DPBF (4.36 M -1 cm -1 ), l = the path length (1 cm) and C = concentration). Figure 16 UV-Vis Spectra of DPBF solutions of varying concentrations creating a calibration curve for comparison purposes. We then examined the activity of the Zn-TCPP-BPY MOF toward generating singlet oxygen using the method as described in the experimental section. In order to verify that singlet oxygen produced by the irradiated MOF was responsible for the depletion of DPBF from the solution, a series of control experiments were carried out to rule out particular factors. DPBF in solution 22