Efficient Energy Transfer (EnT) in Pyrene and Porphyrin-Based Mixed

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Efficient Energy Transfer (EnT) in Pyrene and PorphyrinBased Mixed-Ligand Metal-Organic Frameworks Kyoung Chul Park, Changwon Seo, Gajendra Gupta, Jeongyong Kim, and Chang Yeon Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14135 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017

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ACS Applied Materials & Interfaces

Efficient Energy Transfer (EnT) in Pyrene and Porphyrin-Based Mixed-Ligand Metal-Organic Frameworks Kyoung Chul Park,†a Changwon Seo,†b Gajendra Gupta,a Jeongyong Kim,*b Chang Yeon Lee*ac a

Department of Energy and Chemical Engineering, Incheon National University, Incheon

22012, Republic of Korea. Email: [email protected] b

Department of Energy Science, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon-

si, Gyeonggi-do 16419, Republic of Korea. Email: [email protected] c

Innovation Center for Chemical Engineering, Incheon National University, Incheon 22012,

Republic of Korea † These authors contributed equally

KEYWORDS: metal-organic frameworks, energy transfer, singlet oxygen generation, lightharvesting, complementary absorption.

ABSTRACT: Designing and synthesizing the ordered light-harvesting systems, possessing complementary absorption and energy transfer process between the chromophores, are essential steps to accomplish successful mimicking of the natural photosynthetic systems. Metal–organic

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frameworks (MOFs) can be considered as an ideal system to achieve this due to their highly ordered structure, superior synthetic versatility and tailorable functionality. Herein, we have synthesized the new light-harvesting mixed-ligand MOFs (MLMs, MLM-1–3) via solvothermal reactions between a Zr6 cluster and mixture of an appropriate ratio of 1,3,6,8-tetrakis(p-benzoic acid)pyrene (H4TBAPy) and [5,10,15,20-tetrakis(4-carboxy-phenyl)porphyrinato]-Zn(II) (ZnTCPP) ligands. The identical symmetry and connectivity of the two ligands of the MLMs was the key parameter of successful synthesis as a single MOF form and the ample overlap between the emission spectrum of pyrene and the absorption spectrum of porphyrin provided the ideal platform to design efficient energy transfer (EnT) process within the MLMs. We obtained the nanoscale maps of the fluorescence intensities and lifetimes of micro-size MLM grains for unambiguous visualization of EnT phenomena occurring between two ligands in MLMs. Moreover, due to complementary absorption and energy transfer between two ligands in the MLMs, our MLMs performed as superior photo-induced singlet oxygen generators, verifying the enhanced light-harvesting properties of the pyrene and porphyrin-based MLMs.

1. INTRODUCTION

Photosynthesis in nature is promoted by an antenna molecule, such as chlorophyll, and connected accessory pigment (carotenoid) present in the thylakoid membranes. These lightharvesting antenna systems absorb wide wavelength of the sunlight reaching the earth and transfer the energy to the reaction center via resonance energy transfer (EnT) for conversion into


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chemical energy.1-3 To mimic these highly optimized natural EnT processes, artificial lightharvesting antenna assemblies based on covalently bonded porphyrin arrays,4 dendrimers,5 chromogenic polymers,6 and self-assembled donor–acceptor supramolecular systems7 have been demonstrated. Based on artificial and natural light-harvesting arrays, EnT antenna behavior is most effectively achieved by assembling an ordered network of chromophores.8

Recently, metal–organic frameworks (MOFs), which consist of multidentate organic building blocks and metal or metal-cluster secondary building units, have rapidly emerged as new porous materials.9 MOFs have been used in diverse applications including catalysis,10-12 gas storage,13 sensing,14 separation,15 drug delivery,16 and electrocatalysis17 because of their superior porosity and surface area, structural diversity, and tailorable functionality.

In particular, MOFs have received significant attention as a suitable platform for research into directional EnT phenomena because the distances and angles between chromophores in the regular MOF structure can be easily determined by single crystal X-ray crystallography. These parameters can be systematically tuned by the rational design of organic ligands.18-19

In recent years, huge progress has been made by several groups in the design and development of MOFs that can be utilized in EnT. EnT in MOFs takes place through the directional pathways, including metal to ligand, metal to metal, host to guest, and ligand to ligand.20-25 Several groups have reported intriguing strut-to-strut energy transfer pathways within MOFs containing porphyrin ligands. Because of their similarity to various chlorophylls, MOFs containing porphyrin-based linker can serve as light-harvesting functional struts, as well as achieve efficient EnT. Recently, Lee et al. demonstrated that boron dipyrromethene (bodipy) and porphyrin-based MOFs could be used as architectures for investigating EnT behavior.26 More recently, Shustova


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and co-workers reported ideal photo-switching system via coordinative immobilization of a photochromatic ligand, bis(5-pyridyl-2-methyl-3-thienyl) cyclopentene (BPMTC), in a zinc tetra (4-carboxyphenyl)

porphyrin

(Zn-TCPP)-based

framework.

This

system

displayed

photoswitchable action through the control the presence or absence of strut-to-strut EnT.27 Around the same time, Zhou and co-workers utilized the same MOF architecture to control the production of singlet oxygen.28-29 To obtain outstanding coverage for the broad range of light wavelengths reaching the planet’s surface, new types of chromophores that can absorb light of complementary wavelengths are urgently required.

Pyrene is a versatile π-conjugated polyaromatic hydrocarbon (PAH). It absorbs UV light and emits visible light with a high fluorescence quantum yield. Another PAH, porphyrin, is a blue and red absorber with a high molar extinction coefficient. Because of the good overlap between the emission spectrum of pyrene and the absorption spectrum of porphyrin,30 facile EnT is expected within a co-assembly of these complementary components. Pyrene and porphyrinbased MOFs have been independently reported as NU-100031 and PCN-222,32 respectively. Interestingly, because of the identical symmetry and connectivity of the pyrene-based ligand in NU-1000 and porphyrin-based ligand in PCN-222, the two MOFs have equivalent structures (csq network topology), in which eight edges of Zr6 octahedron cluster are linked by the square planar tetratopic ligands.

These two independent investigations encouraged us to synthesize novel structures containing two complementary ligands in a single MOF structure to study strut-to-strut EnT. The mixed ligand approach is the ideal method to co-assemble two ligands in a single MOF structure. Thus, we herein report new zirconium-based mixed-ligand MOFs (MLMs, MLM-1–3), which were


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obtained by the solvothermal reaction between a Zr6 cluster and a mixture of an appropriate ratio of 1,3,6,8-tetrakis(p-benzoic acid)pyrene (H4TBAPy) and [5,10,15,20-tetrakis(4-carboxyphenyl)porphyrinato]-Zn(II) (Zn-TCPP) (Scheme 1). The mixed-ligand strategy successfully yielded MLMs because of the identical symmetry and connectivity of the two ligands (see above). The efficient strut-to-strut EnT in MLMs was confirmed by confocal fluorescence mapping and the fluorescence lifetime mapping. The enhanced light-harvesting properties of MLMs via an efficient EnT resulted in superior photo-induced singlet oxygen generation.

Scheme 1. Synthesis of mixed-ligand MOFs from a mixture of Zn-TCPP and H4TBAPy.

2. EXPERIMENTAL SECTION Materials (Zn-TCPP),32

[5,10,15,20-

1,3,6,8-tetrakis(p-benzoic

acid)pyrene

[5,10,15,20-Tetrakis(4-carboxy-phenyl)porphyrinato]-Zn(II) Tetrakisphenylporphyrinato]-Zn(II)

(Zn-TPP),33

(H4TBAPy),31 PCN-22232 and NU-100031 were synthesized according to the published procedures. Synthesis


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Synthesis of MLM-1–3 : 0.097 g of ZrOCl2·8H2O (0.30 mmol) and 2.7 g (22 mmol) of benzoic acid were placed in a 20 ml vial and 8 ml of dimethylformamide (DMF) was poured to above vial. The resulting mixture was dissolved via sonication, after that vial was placed in an oven at 80°C for 1 h. After 1h the vial was taken out from the oven, allowed to cool to room temperature and then (8.5 mg, 0.01 mmol for MLM-1; 10.3 mg, 0.012 mmol for MLM-2 and 12.8 mg, 0.015 mmol for MLM-3) of Zn-TCPP and (34.1 mg, 0.05 mmol for MLM-1; 32.8 mg, 0.048 mmol for MLM-2 and 30.7 mg, 0.045 mmol for MLM-3) of H4TBAPy was added to the solution. The mixture was sonicated to disperse the organic ligand to an even suspension, which was then stirred in a hot plate at 100°C for 24 h. The reaction mixture was cooled to room temperature and then centrifuged (50-ml capped tube, at 2000 rpm for 10 min at room temperature) to separate the solid from the mother solution. The resultant powder was washed three times with 40 ml of DMF.

Synthesis of MLM-TPP-1–3 : 0.097 g of ZrOCl2·8H2O (0.30 mmol) and 2.7 g (22 mmol) of benzoic acid were placed in a 20 ml vial and 8 ml of dimethylformamide (DMF) was poured to above vial. The resulting mixture was dissolved via sonication, after that vial was placed in an oven at 80°C for 1 h. After 1h the vial was taken out from the oven, allowed to cool to room temperature and then (6.8 mg, 0.01 mmol for MLM-TPP-1; 8.1 mg, 0.012 mmol for MLM-TPP2 and 10.2 mg, 0.015 mmol for MLM-TPP-3) of Zn-TPP and (34.1 mg, 0.05 mmol for MLMTPP-1; 32.8 mg, 0.048 mmol for MLM-TPP-2 and 30.7 mg, 0.045 mmol for MLM-TPP-3) of H4TBAPy was added to the solution. The mixture was sonicated to disperse the organic ligand to an even suspension, which was then stirred in a hot plate at 100°C for 24 h. The reaction mixture was cooled to room temperature and then centrifuged (50-ml capped tube, at 2000 rpm for 10


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min at room temperature) to separate the solid from the mother solution. The resultant powder was washed three times with 40 ml of DMF.

Activation procedure for MLMs : The solid obtained above was re-suspended in 12 ml of DMF and 0.5ml of 8 M HCl was added to the solution and swirled. The vial was placed in an oven at 100°C for 24 h. The reaction mixture was cooled down to room temperature and then centrifuged (50-ml capped tube, at 2000 rpm for 10 min at room temperature) to separate the solid from the mother solution. The resultant powder was then washed three times with 40 ml of DMF. After the centrifugation was over, the supernatant was discarded and fresh acetone was added. The MOF was soaked in fresh acetone for 12 h and exchanged the solvent with fresh acetone. This process was repeated twice, soaking for 12 h each time. Before physical measurements the samples were activated under dynamic vacuum at 120°C until a vacuum level of ≤0.002 mmHg • min-1 was reached.

Instrumentation used for characterizations Powder X-ray diffraction (PXRD) measurements were carried out on a Rigaku smartlab with Cu Kα radiation over a range of 2°

Efficient Energy Transfer (EnT) in Pyrene and Porphyrin-Based Mixed

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