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

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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 38670-38677

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

Department of Energy and Chemical Engineering and ‡Innovation Center for Chemical Engineering, Incheon National University, Incheon 22012, Republic of Korea § Departmentof Energy Science, Sungkyunkwan University, Suwon, 16419, Republic of Korea

ACS Appl. Mater. Interfaces 2017.9:38670-38677. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/07/19. For personal use only.

S Supporting Information *

ABSTRACT: Designing and synthesizing the ordered lightharvesting 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 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 a mixture of appropriate ratio of 1,3,6,8tetrakis(p-benzoic acid)pyrene and [5,10,15,20-tetrakis(4carboxy-phenyl)porphyrinato]-Zn(II) 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 an efficient-energy transfer (EnT) process within the MLMs. We obtained the nanoscale maps of the fluorescence intensities and lifetimes of microsize MLM grains for unambiguous visualization of EnT phenomena occurring between two ligands in MLMs. Moreover, due to complementary absorption and energy transfer between the two ligands in the MLMs, our MLMs performed as superior photoinduced singletoxygen generators, verifying the enhanced light-harvesting properties of the pyrene- and porphyrin-based MLMs. KEYWORDS: metal−organic frameworks, energy transfer, singlet-oxygen generation, light harvesting, complementary absorption 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 singlecrystal 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

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 light-harvesting antenna systems absorb a wide wavelength of the sunlight reaching the earth and transfer the energy to the reaction center via resonance energy transfer (EnT) for conversion into chemical energy.1−3 To mimic these highly optimized natural EnT processes, artificial light-harvesting antenna assemblies based on covalently bonded porphyrin arrays,4 dendrimers,5 chromogenic polymers,6 and selfassembled donor−acceptor supramolecular systems7 have been demonstrated. Based on artificial and natural lightharvesting 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 metalcluster 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 © 2017 American Chemical Society

Received: September 18, 2017 Accepted: October 19, 2017 Published: October 19, 2017 38670

DOI: 10.1021/acsami.7b14135 ACS Appl. Mater. Interfaces 2017, 9, 38670−38677

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ACS Applied Materials & Interfaces Scheme 1. Synthesis of Mixed-Ligand MOFs from a Mixture of Zn-TCPP and H4TBAPy

tetrakis(4-carboxy-phenyl)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 lightharvesting properties of MLMs via an efficient EnT resulted in a superior photoinduced singlet-oxygen generation.

within MOFs containing porphyrin ligands. Because of their similarity to various chlorophylls, MOFs containing porphyrinbased 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 and co-workers reported an ideal photoswitching system via coordinative immobilization of a photochromatic ligand, bis(5-pyridyl-2-methyl-3-thienyl) cyclopentene (BPMTC), in a framework based on zinc tetra (4carboxyphenyl) porphyrin (Zn-TCPP). This system displayed a photoswitchable action through the control of 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 an 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 porphyrin-based 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 obtained by the solvothermal reaction between a Zr6 cluster and a mixture of an appropriate ratio of 1,3,6,8tetrakis(p-benzoic acid)pyrene (H4TBAPy) and [5,10,15,20-

2. EXPERIMENTAL SECTION 2.1. Materials. [5,10,15,20-Tetrakis(4-carboxy-phenyl)porphyrinato]-Zn(II) (Zn-TCPP),32 [5,10,15,20-tetrakisphenylporphyrinato]-Zn(II) (Zn-TPP),33 1,3,6,8-tetrakis(p-benzoic acid)pyrene (H4 TBAPy),31 PCN-222,32 and NU-100031 were synthesized according to the published procedures. 2.2. Synthesis. 2.2.1. 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 into the vial. After the resulting mixture was dissolved via sonication, the vial was placed in an oven at 80 °C for 1 h. After 1 h, 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 MLM2; and 12.8 mg, 0.015 mmol for MLM-3) 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) H4TBAPy were 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. 2.2.2. 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 into the vial. After the resulting mixture was dissolved via sonication, the vial was placed in an oven at 80 °C for 1 h. After 1 h, 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) Zn-TPP and (34.1 mg, 0.05 mmol for MLM-TPP-1; 32.8 mg, 0.048 mmol for MLM-TPP-2; and 30.7 mg, 0.045 mmol for MLM-TPP-3) H4TBAPy were 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 38671


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Figure 1. (a) Photographs of NU-1000, MLM-1−3, and MLM-TPP in white light and UV light (365 nm). (b) Powder XRD patterns of assynthesized NU-1000, MLM-1−3, and PCN-222 MOFs. (c) 1H NMR spectra of the as-synthesized NU-1000 and MLM-1−3 in DMSO-d6/D2SO4 (9:1). (d) N2 adsorption−desorption isotherms for NU-1000, MLM-1−3, and PCN-222 MOFs at 77 K. 2.4. Confocal Fluorescence Imaging and Lifetime Mapping. For optical measurements, MLM-1−3 in powder form were dispersed in acetone solutions, 0.5 mL of which was dropcasted on 0.15 μm thick glass substrate, which was then dried in ambient condition. Confocal fluorescence images and spectra were obtained by using a laboratorymade laser confocal microscope.34 375 nm wavelength diode laser (BDL-375, Becker & Hickl GmbH) in cw mode was used for photoexcitation of the samples. An oil-immersion objective lens with 1.3 numerical aperture was used for focusing the laser light to the sample placed on the scanning stage of the microscope, and the same objective lens was used for collecting the fluorescence signal. Collected light was focused to an optical fiber with a 100 μm core diameter that acted as a confocal detection pinhole, and then guided to a 30 cm long spectrometer equipped with a cooled charge-coupled device. The scanning sample stage was synchronized with the spectrometer so that a full fluorescence spectrum was obtained at each pixel position. Confocal images were generated by integrating the desired wavelength ranges of confocal fluorescence spectra at every pixel. The spatial resolution of the confocal imaging was estimated to be ∼400 nm.35 For the scanning fluorescence lifetime mapping, the same confocal microscope was combined with a commercial time-correlated single photon counting (TCSPC) system (Becker & Hickl GmbH). The light source was the same 375 nm wavelength diode laser operated in the pulsed mode with the repetition rate of 80 MHz and 60 ps pulse width. The photodetector was a high-speed photomultiplier tube detector (HPM-100, Becker & Hickl GmbH) with 120 ps time resolution. TCSPC system was electronically synchronized with the sample scanning stage of the confocal microscope and a fluorescence decay curve was obtained at every pixel position. As a result, 64 × 64 pixel image with total of 4096 data points was generated. The decay curve at each pixel was fitted with a two-component exponential curve, and the estimated mean lifetimes were rendered in false color scale in the 64 by 64 pixel image. Figure S1 depicts the layout of our fluorescence lifetime mapping setup.

temperature) to separate the solid from the mother solution. The resultant powder was washed three times with 40 mL of DMF. 2.2.3. Activation Procedure for MLMs. The solid obtained above was resuspended in 12 mL of DMF, and 0.5 mL 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 taking physical measurements, the samples were activated under dynamic vacuum at 120 °C until a vacuum level of ≤0.002 mmHg·min−1 was reached. 2.3. Instrumentation Used for Characterizations. Powder Xray diffraction (PXRD) measurements were carried out on a Rigaku smartlab with Cu Kα radiation over a range of 2 < 2θ < 40° in 0.02 steps with a 1 s counting time per step. 1H NMR spectra were recorded using Agilent 400-MR spectrometers. Scanning electron microscope (SEM) images were obtained using a field emission scanning electron microscope (JEOL, JSM-7800F) operated at an acceleration voltage of 15.0 kV. Samples were coated with a layer of Au (∼3 nm thickness) before imaging. N2 adsorption/desorption isotherms were measured volumetrically at 77 K in the range of 7.0 × 10−6 < P/P0 < 1.00 with an Autosorb-iQ outfitted with the micropore option from Quantachrome Instruments (Boynton Beach, FL) using the Autosorb-iQ Win software package. The samples were activated at 120 °C for 12 h using the outgas port of the Autosorb-iQ instrument. The specific surface areas for N2 were calculated using the Brunauer−Emmett−Teller (BET) model in the linear range, as determined using the consistency criteria. Fourier transform infrared (FTIR) spectrum was recorded on Bruker VERTEX 80 V spectrometer. The spectral resolution is 2 cm−1. 38672


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displayed peak at around 982 cm−1, which can be assigned to the C−N stretching. This peak corresponding to the C−N stretching was not seen in NU-1000, which further confirms that MLMs contain porphyrin (Figure S5). The morphology and the size of the as-synthesized MOFs were further characterized by scanning electron microscope (SEM). The SEM images in Figure S6 clearly show that the MLMs have similar shapes, similar to rice grains, with an individual size of around 6−7 μm. The shape of the MLM crystals also differs to those of the NU-1000 MOFs, which have a rod-type morphology, indicating that the incorporation of porphyrin ligand induced a different mode of crystal growth. The porosities of the MLMs were examined via nitrogen adsorption−desorption isotherm at 77 K. The porosities of the MLMs were defined as type IV N2 isotherms with a “knee” part appearing at low relative pressures. Brunauer−Emmett−Teller (BET) analyses of the N2 adsorption−desorption isotherms confirm that the materials have a high surface area. The surface areas are 2253, 2360, 2291, 2411, and 2523 m2·g−1 for NU1000, MLM-1, MLM-2, MLM-3, and PCN-222, respectively. The surface areas of the MLMs were found to increase compared to NU-1000. However, the surface area decreased slightly in comparison to that of PCN-222 (Figure 1d). Besides their high surface areas, the Zr-based MOFs are also known to be extraordinarly stable relative to all of the other MOFs. Consequently, we expected our new MLMs to be highly stable under different chemical conditions. Thus, we conducted a stability test by immersing our MLMs in different media, such as 8 M HCl, water, and 1 mM NaOH solutions, for around 12 h. The MOFs treated in this way were then washed with water, DMF, and acetone for a period (12 h). The washing solvent remained colorless during the washing process, indicating that the pyrene and porphyrin moieties remained in the MOF during the harsh washing process. This confirms that no phase transition or collapse of the frameworks took place during these treatments. In addition, the MOFs were observed to be stable under acidic, neutral, and basic conditions because no change was observed in the PXRD data obtained after these treatments (Figure S7). The probable reason for its high stability could be the Zr6 cluster, which forms strong Zr−O bonds and is considered to be one of the most stable building units for MOF construction.32 It is clear from the above experiments that porphyrin moieties are strongly coordinated in the MLMs, rather than just being encapsulated in the MOFs. The efficient energy transfer in the MLMs was visualized with confocal imaging and time-resolved spectroscopy. Because of the superb spatial and temporal resolution provided by confocal imaging and spectroscopy,38 individual MLM grains could be identified individually, and their spectral and spatial fluorescence characteristics can be systematically studied. The absorption band of PCN-222 is partially overlapped with the emission band of NU-1000 (Figure S8), suggesting that the resonant energy transfer can occur from pyrene (as the donor) to porphyrin (as the acceptor). Preliminarily, we found that the emission intensities of the MLM powders under UV illumination gradually decreased as the porphyrin contents increased (Figure 1a). Figure 2a shows the representative fluorescence spectra of MLM-1−3. Here, for the normalization of fluorescence spectra, the sum of two peak heights at 470 and 660 nm was set to be the same for MLM-1−3. The normalized contrast of Figure 2b was also set accordingly. Fluorescence spectra of NU-1000 and PCN-222 are shown in the inset for reference, which indicates that the fluorescence emissions were

2.5. Singlet-Oxygen Generation. 4 mg MOF was added to 100 mL acetonitrile containing 1,5-dihydronaphthalene (DHN, 1 × 10−4 M) solution. The solution was bubbled with oxygen for 30 min before the measurements. Visible irradiation was provided using a fiber optic coupled halogen lamp (150 W, SCHOTT, KL 1500 compact), and the reaction was started. As the reaction went on, 2 mL reaction solution was taken out at regular intervals to measure the UV−vis spectra.

3. RESULTS AND DISCUSSION The new MLMs, MLM-1−3, were synthesized by conventional methods, as described in Scheme 1. The solvothermal reaction between ZrOCl2·8H2O and benzoic acid in DMF for 1 h, followed by the addition of Zn-TCPP and H4TBAPy, and stirring at 100 °C for 24 h resulted in the formation of MLM1−3 as dark crystalline products. The synthetic procedures are described in detail in Experimental Section. During the synthesis of these MOFs, the amount of zirconium precursor and benzoic acid was kept constant. However, the amounts of Zn-TCPP and H4TBAPy were systematically increased and decreased, respectively. The MOFs were obtained as dark red, highly crystalline solids. Interestingly, the MOFs became darker with the increasing amount of porphyrin used during synthesis (Figure 1a). Microscopy images of the MLMs also indicate that individual crystals became redder when the porphyrin content increased (Figure S2). A comparison has been made with the well-known MOF, NU-1000, which was also synthesized by a conventional method using a pyrene-based ligand and yielded a yellow product. Figure 1b shows the powder XRD diffraction patterns of the as-synthesized MOFs. The XRD patterns confirm the presence of phase-pure MOFs, MLM-1−3, whose phase purity is also comparable to that of NU-1000. This further confirms that the crystalline properties of these MOFs are preserved even after the coordination of porphyrin molecule in different ratios. Several control experiments were performed to verify the coassembly of two ligands in a single MOF. Zinc porphyrin is easily demetalated in hydrochloric acid solution and exhibits a green color due to the protonation of pyrrole moieties in porphyrin.36,37 Obvious color changes from purple to green were observed in the MLMs during the activation step, where an HCl solution was involved. Moreover, individual crystals also became greenish, as shown in the microscopy images (Figures S2 and S3). These results indicate the presence of the porphyrin moiety in the MOF crystal. In addition, [5,10,15,20tetrakisphenylporphyrinato]-Zn(II) (Zn-TPP) was added instead of Zn-TCPP, which was used as a porphyrin ligand, to eliminate the part that could bind to the metal cluster. In the case of the MOFs made with Zn-TPP (denoted as MLM-TPP), a yellow powder was obtained, and no red product was formed (Figures 1a and S4). This result indicates that the ligand of the metal cluster is coordinated rather than being encapsulated in the NU-1000. 1 H NMR spectra of the as-synthesized MOFs were obtained to analyze the ratio of pyrene and porphyrin molecules contained in these MLMs. The 1H NMR spectra of the digested MLMs measured in DMSO-d6/D2SO4 (9:1) confirm that the ratio of porphyrin to pyrene was 10% for MLM-1, 16.6% for MLM-2, and 25% for MLM-3 (Figure 1c). New peaks between 8.4 and 8.7 ppm can be assigned to the protons in the porphyrin molecules present in these MLMs. The presence of these new peaks, which are absent in NU-1000, further confirms the successful coordination of the porphyrin molecules in the MLMs. In addition, the FTIR spectrum 38673


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Generally, in an energy transfer system consisting of a donor and an acceptor, the fluorescence yield of the donor is given as24 fluorescence yield of donor ∼

kf k f + k nr + kET

Here, kf and knr represent the transition rates of the radiative and the nonradiative decay channels, respectively, and kET represents the energy transfer rate. Therefore, with increasing probability of energy transfer, the fluorescence yield of the donor is expected to decrease but that of the acceptor should increase, consistent with our observations shown in Figure 2a,b. The energy transfer between the pyrenes and porphyrins in the MLMs are also manifested in the time-resolved fluorescence measurements. In Figure 3a, maps of fluorescence

Figure 2. (a) Representative fluorescence spectra of MLM-1−3. The inset shows the fluorescence spectra of NU-1000 and PCN-222. (b) Fluorescence intensity image of MLM-1−3 obtained at energy ranges of pyrene emission (upper panels) and porphyrin emission (lower panels). Vertical scales represent the fluorescence intensity.

located between 400 and 630 nm, originating from the pyrene moieties. The peaks between 630 and 765 nm result from the fluorescence of the porphyrin moieties. In the normalized fluorescence spectra of the MLMs in Figure 2a, we note that the relative intensity of pyrene emission decreased, whereas that of the porphyrin emissions increased with increasing incorporation of porphyrin molecules. This trend of decreasing (increasing) pyrene (porphyrin) emission was confirmed in the normalized confocal fluorescence images of the MLMs obtained at pyrene and porphyrin emission energies of 400− 630 and 630−765 nm, respectively, as shown in Figure 2b. Individual grains of the MLMs a few microns in size are clearly visible in the confocal fluorescence images, both for pyrene emission (upper panels) and porphyrin emission (lower panels). Notably, the relative fluorescence intensity of pyrene is strongest in the MLM-1 sample, decreasing with increasing amount of incorporated porphyrin. In contrast, the relative intensity of porphyrin fluorescence showed a monotonic increase with increasing amount of incorporated porphyrin. This systematic trend of fluorescence intensity variation in pyrene and porphyrin emissions with the amount of porphyrin was observed in all of the MLMs grains and, thus, indicates that energy transfer from pyrene (as a donor) to porphyrin (as an acceptor) occurred in our MLMs and could be reliably tunable by controlling the amount of incorporated porphyrin.

Figure 3. (a) Fluorescence lifetime maps of NU-1000, MLM-1 and -3, and PCN-222 grains. (b) Average fluorescence time decay curves of NU-1000, MLM-1 and -3, and PCN-222 obtained from the cross hair locations shown in (a) at the emission wavelengths of 400−630 nm (pyrene) and 630−765 nm (porphyrin).

lifetime (τ) measured at the fluorescence emission wavelength ranges of pyrene (400−630 nm) and porphyrin (630−765 nm) are shown (upper and lower panels, respectively). In these fluorescence lifetime maps, the local τ values are rendered with the color scale shown on the sides of the maps (i.e., the brightness corresponds to the fluorescence intensity). The τ maps of NU-1000 and PCN-222 are also shown for reference. The representative fluorescence time decay curve averaged from the locations indicated by cross hairs in the fluorescence lifetime maps are shown in Figure 3b. The time decay curves, F(t), were fitted with the following decay curve equation. F(t ) = A1 e−t / τ1 + A 2 e−t / τ2 38674


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Figure 4. (a) Reaction scheme for the photooxidation of DHN with a singlet-oxygen photosensitizer (MLM-1−3). (b) Photographs of DHN solution in the presence of MOFs during the irradiation at 0 min (left) and 480 min (right). (c) UV−vis absorption spectra of photooxidation of DHN in CH3CN catalyzed by various MOFs. (d) Plot of conversion rate (%) against irradiation time (min) for the photooxidation of DHN using NU-1000, MLM-1−3, PCN-222, and NU-1000/PCN-222 (4:1) MOFs as singlet-O2 photosensitizers.

fluorescence images of MLM-3 shown in Figure 2b and lifetime maps of MLM-3 in Figure 3a is high; specifically, in the center of the grain, the fluorescence of pyrene (porphyrin) is lower (higher), confirming that, at these locations, the energy transfer between the donor and acceptor ligands is more active. Our nanoscale confocal imaging and fluorescence lifetime measurements reveal the systematic modulation in the emission and decay profiles of the donor and acceptor molecules, visualized at the level of single MOF crystal grains. A control experiment was carried out with a physical mixture of NU-1000 and PCN-222. The mixing ratios were determined to be equal to the ratio of the two ligands in the MLMs. In the case of the physical mixture of NU-1000 and PCN-222, only the emission of pyrene (400−630 nm) appears, and the emission spectrum resulting from the porphyrin moiety between 630 and 765 nm is absent (Figure S9). This observation confirms that EnT process only occurred within the ordered MOFs, where donors and acceptors were adjacent. Because of their excellent photochemical properties and high efficiency for light-harvesting, porphyrin-based complexes are widely used for singlet-oxygen (1O2) generation.28,40,41 Taking advantage of our porphyrin-containing MLMs, we conducted the experiments to study if the efficiency of this energy transfer affects the production of singlet oxygen. 1,5-Dihydroxynaphthalene (DHN) was used to evaluate the singlet-oxygen production. As shown in the scheme above (Figure 4a) and the literature, DHN acts as an efficient 1O2 scavenger to produce its oxidized product, 5-hydroxy-1,4-naphthalenedione (juglone).42 The formation of singlet oxygen can be easily monitored by observing the disappearance of the DHN absorption band at 298 nm and the appearance of a new peak for juglone at 420 nm in the UV−vis spectrum (Figures 4c and S10). A 4 mg sample of each MLM was placed in a 0.1 mM DHN solution containing 100 mL of acetonitrile. The photoreaction was performed using a 150 W halogen lamp for up to 480 min. As shown in Figures 4c and S10, upon photooxidation, the absorbance at 298 nm decreased with time and the absorption at 420 nm was found to increase with time.

Here, A1 and A2 are the amplitudes, and τ1 and τ2 are two decay components (short and long, respectively). The average decay time, τmean, was calculated using24 τmean =

A1τ1 + A 2 τ2 A1 + A 2

In an energy transfer system, the fluorescence lifetime of the donor is given as24 τ of donor =

1 k r + k nr + kET

Therefore, the fluorescence lifetime of the donor decreases on energy transfer.24 Indeed, as shown in Figure 3b, the fluorescence lifetime of NU-1000 was measured to be 0.39 ns, which decreased to 0.24 ns in MLM-1 and further decreased to 0.20 ns in MLM-3, indicating the occurrence of energy transfer in our MLMs. Although the effect of the fluorescence lifetimes of the acceptor molecules of the onset of energy transfer has rarely been investigated, a recent study of the energy transfer in binary CdSe nanoplatelets reported that decay of the acceptor emission slows with active energy transfer.39 As shown in the τ maps and fluorescence decay curves of the porphyrin emissions in Figure 3a,b, respectively, we also observed a clear increase in the porphyrin fluorescence lifetime in the MLMs: PCN-222 (1.56 ns), MLM-1 (2.65 ns), and MLM-3 (4.50 ns), which confirms the effective energy transfer active in our MLMs. Here, for the MLM-3 sample, we observed that the local fluorescence lifetime value is spatially variable, even in the same grain. In the pyrene (porphyrin) emission, the center or the end of the grain had longer (shorter) τ values than the rest of the grain. This inhomogeneity of the fluorescence lifetime is believed to originate from the spatially nonuniform incorporation of porphyrin molecules in the MLM-3 sample, where the region of longer (shorter) lifetime in porphyrin (pyrene) emission represents the area of successful incorporation of porphyrin molecules. We note that spatial correlation between the 38675


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With the passage of time, the color of the solution also gradually changed from transparent to yellow (Figures 4b and S11). The complete disappearance of the peak at 298 nm and the appearance of a new peak 420 nm confirm the generation of singlet oxygen in our system. In addition, the juglone calibration curve was used to determine that the MLMs had conversion efficiencies as high as 60%. To compare the photoinduced singlet-oxygen generation ability, we performed control experiments using NU-1000, PCN-222, and a physical mixture of NU-1000 and PCN-222. NU-1000 showed only 24% efficiency after 480 min, and the physical mixture of NU1000 and PCN-222 also exhibited an inferior conversion rate (almost identical to that of NU-1000) during the same period of time. Notably, MLMs showed an even better efficiency than PCN-222, which is composed exclusively of porphyrin-based ligand. These results verify that the enhancement of lightharvesting through complementary absorption and energy transfer between the two ligands in the MLMs is responsible for the increase in photoinduced singlet-oxygen production. As described in Introduction, the EnT phenomenon is an essential process in the photosynthesis to achieve high efficiency. Thus, the highly efficient EnT process in our new MLMs can be considered as an ideal mimic of a natural system.

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Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b14135. Detailed experimental procedures and characterizations: layout of the fluorescence lifetime mapping setup; optical microscopic images of MOFs; photographs of MOFs (dispersed in acetone); photographs of MLM-TPPs, SEM images of MOFs, chemical stability test; emission spectrum of NU-1000 and absorption spectrum of PCN222; emission spectrum of physically mixed MOFs; timedependent absorption spectra; photographs of DHN solution (PDF)

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This research was supported by two mid-career researcher programs of the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2016R1A2B4010376), and this work was also supported by an Incheon National University Research Grant in 2014.

4. CONCLUSIONS The solvothermal reaction between a Zr6 cluster and a mixture of an appropriate ratio of H4TBAPy and Zn-TCPP yielded the mixed-ligand MOFs (MLMs, MLM-1−3). Efficient EnT process between pyrene and porphyrin moieties occurred in the MLMs, and the EnT processes were monitored by confocal fluorescence mapping and lifetime maps in a single-crystal domain. EnT in the MLMs led to the enhancement of their light-harvesting properties; thus, the MLMs exhibited a superior photoinduced singlet-oxygen generation capability. Our MLMs are mimics of the natural photosynthesis process and could be promoted to various applications including photocatalysis, photovoltaics, and photodynamic therapy.

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K.C.P. and C.S. contributed equally.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.K.). *E-mail: [email protected] (C.Y.L.). ORCID

Gajendra Gupta: 0000-0002-0098-8288 Jeongyong Kim: 0000-0003-4679-0370 Chang Yeon Lee: 0000-0002-1131-9071 38676


Research Article

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

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