Nanovesicular MOF with Omniphilic Porosity: Bimodal Functionality for

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 23140−23146

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Nanovesicular MOF with Omniphilic Porosity: Bimodal Functionality for White-Light Emission and Photocatalysis by Dye Encapsulation Debabrata Samanta, Parul Verma, Syamantak Roy, and Tapas Kumar Maji* Molecular Materials Laboratory, Chemistry and Physics of Materials Unit, School of Advanced Materials (SAMat), Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India

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S Supporting Information *

ABSTRACT: A new π-chromophoric and asymmetric bola-amphiphilic oligo-(p-phenylene ethynylene)-based tetracarboxylate (OPE-TC1) linker was designed, synthesized, and self-assembled with Zn(OAc)2. The resulting nanoscale metal−organic framework (MOF) {Zn2(OPE-TC1)}n (NMOF-1) showed a vesicular morphology and permanent porosity with omniphilic pore surface. NMOF-1 showed cyan emission with high quantum efficiency (49%). The omniphilicity of the pore was utilized to incorporate ambipolar dye sulforhodamine G (SRG) to tune the band gap as well as to get pure white-light emission. Furthermore, the polar pore surface of NMOF-1 allowed facile diffusion of the substrate for efficient photocatalytic activity. The dye-encapsulated framework further showed enhanced dihydrogen production by 1.75-fold compared to that from the as-synthesized NMOF-1 because of the modulated band gap and high excited state lifetime. As a control experiment, we have synthesized a MOF (MOF-OMe) with an OPE-TC2 linker having −OMe functional groups that did not show nanoscale architecture. This suggested the important role of unsymmetrical bola-amphiphilicity in nanostructuring. This rational design of a chromophoric linker resulted in a nanoscale MOF with omniphilic porosity to achieve bimodal functionality in clean energy applications. KEYWORDS: bimodal functionality, nanovesicular MOF, dye encapsulation, white-light emission, photocatalysis

1. INTRODUCTION The last one and a half decade has seen an upsurge in energy research because of the steady decline of non-renewable sources of energy we are dependent on.1 Solar energy provides an excellent alternative to the non-renewable sources, but the problem remains in efficiently harvesting and utilizing it.2,3 Therefore, materials which can harvest solar energy toward clean energy solutions are in high demand.4−7 The photocatalytic H2 production from water has attracted great attention as H2 is a source of clean energy.8−11 Except thermodynamic parameters, other three important requirements for an efficient water-splitting photocatalyst are as follows: (1) broad visible light absorption energy band; (2) high excited state lifetime; and (3) high charge carrier mobility.12,13 Research has therefore been concentrated on designing such materials.14−16 Similarly, materials that can display pure white-light emission with high quantum efficiency have direct applications in solid-state lighting and display, organic light-emitting diodes, organic field effect transistors, and photovoltaics.17,18 In this context, bimodality on energy generation like photocatalytic H2 production through water splitting19−22 and solid-state lighting17,18 can be comprehended by a semiconducting luminescent metal−organic framework (MOF)23−27 via encapsulating suitable guest dye molecules (Scheme 1). In this design approach, a suitable dye would not only allow the tuning of emission color by the © 2018 American Chemical Society

energy transfer process but also it can open a new visible light absorption window for tuning the band gap.28,29 Additional advantages of MOFs are that they are permanently porous that would help in facile diffusion of the substrate to the catalytic site and fast charge migration, which are of paramount importance for efficient catalytic activity.28,29 Advantageously, nanoscale MOFs (NMOFs) would further portray easy solution processability and also reduce the diffusion barrier for efficient substrate−catalyst interaction.30−34 Oligo-(p-phenylene ethynylenes) (OPEs) have excellent charge transport and optical output properties35−37 and seem to be good candidates for investigating bimodal functionality i.e., optoelectronics and photocatalysis. Recent studies by our group revealed that the introduction of bola-amphiphilicity into OPE, by adding long alkyl chains in the backbone, results in soft, processable, and emissive nanoscale MOFs.38,39 We therefore envisaged that OPE-based linkers with mixed polar side chains could generate a three-dimensional porous NMOF network with an omniphilic pore surface, which is required for ambipolar dye encapsulation to generate white-light emission. The omniphilic pore would also facilitate diffusion of the substrate, which is required for photocatalytic H2 evolution Received: April 19, 2018 Accepted: June 19, 2018 Published: June 19, 2018 23140

DOI: 10.1021/acsami.8b06363 ACS Appl. Mater. Interfaces 2018, 10, 23140−23146

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ACS Applied Materials & Interfaces Scheme 1. Graphical Representation of the Bimodal Functionalitya

a

White-light emission and photocatalysis in the present work.

Scheme 2. Ligands OPE-TC1 and OPE-TC2 and Their Self-Assembly with ZnII

lation of sulforhodamine G (SRG), which was further stabilized by multiple noncovalent interactions (Scheme 1). SRG2@NMOF-1 showed tunable emission including whitelight through partial energy transfer from OPE-TC1 to SRG. Moreover, SRG encapsulation resulted in a 1.75-fold increase in photocatalytic H2 evolution from water.

from water. The incorporated dye and facile substrate diffusion could also lead to enhanced photocatalysis because of light absorption over a wide range, including the visible region. In this work, we have therefore designed and synthesized a novel asymmetric bola-amphiphilic linker, oligo-(p-phenylene ethynylene)tetracarboxylate (OPE-TC), containing dodecyl (non-polar) and triethyleneglycolmonomethylether (TEG, polar) side chains in the backbone as in OPE-TC1 (Scheme 2). The OPE-TC1 linker after self-assembly with Zn(II) resulted in a nanoscale MOF {Zn2(OPE-TC1)}n (NMOF-1) with vesicular morphology. The framework shows permanent omniphilic porosity as realized by the CO2 (195 K) and solvent vapor (water and benzene, 298 K) adsorption study. Furthermore, mesopores are also formed by the interconnected network formation by the nanovesicles of NMOF-1, which is realized by microscopic studies (field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM)) and supported by N2 adsorption at 77 K. This interparticle mesopore formation facilitated encapsu-

2. RESULTS AND DISCUSSION Synthesis of the newly designed ligand, OPE-TC1, was performed utilizing Sonogashira−Hagihara cross-coupling reactions (Scheme 2).40 A nanoscale MOF, NMOF-1, was synthesized by the coordination-directed self-assembly of ZnII and OPE-TC1 (2:1) in tetrahydrofuran (THF) for 2 days, yielding a green precipitate of NMOF-1 with 73% yield. Energy-dispersive X-ray spectroscopy and inductively coupled plasma mass spectrometry of NMOF-1 showed the presence of 13.6 and 13.8 wt % Zn, respectively. No significant weight loss in thermogravimetric analysis (TGA) up to 300 °C indicated the absence of any solvent/water in the framework (Figure 23141


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ACS Applied Materials & Interfaces

Figure 1. (a) Possible structure of NMOF-1 as reported in the literature (without side chains),41 (b) powder X-ray diffraction (PXRD) patterns, (c) CO2 uptake at 195 K, and (d) benzene and water vapor uptake at 298 K by NMOF-1.

Figure 2. Evolution stages of nanovesicle (NMOF-1) by TEM (top row) and FESEM microscopy (bottom row).

interparticle assemblies, which is very clear from TEM (Figure S1, SI) and FESEM (Figure S2, SI) of NMOF-1 (vide infra). Because of the presence of both hydrophobic (dodecyl) and moderately hydrophilic (TEG) side chains in the backbone, the network also showed significant uptake of water (49 cm3 g−1, at p = 1) and benzene (30 cm3 g−1, at p = 1), suggesting omniphilic nature of the pore surface (Figure 1d). Several high intense peaks were observed in powder X-ray diffraction (PXRD) of NMOF-1, signifying high degree of crystallinity of the frameworks (Figure 1b). The PXRD patterns were indexed utilizing TREOR-90 as implemented in Crysfire software (Table S2, SI). The framework showed an orthorhombic crystal system with the cell parameters of a = 21.67(5) Å, b = 13.13(2) Å, and c = 11.30(3) Å and cell volume of 3217 Å3. For instance, the very low angle peak at 2θ = 4.073° (d = 21.7 Å) is associated with the (100) plane and possibly indicates repeating ZnII centers connected by the OPE-TC1 linker to form the framework as shown in Figure 1a. The pattern at 2θ = 11.3° (d = 7.9 Å) could indicate the

S11a, Supporting Information (SI)). Finally, elemental analysis suggested the molecular formula of NMOF-1 as {Zn2(OPETC1)}n. Fourier-transform infrared spectroscopy revealed intense signals at 1378 and 1568 cm−1, corresponding to symmetric and asymmetric stretching vibrations with a difference of 190 cm−1 (Figure S12a, SI). This difference is indicative of a bidentate coordination mode of the carboxylate anion with the ZnII ion. NMOF-1 showed typical type-I CO2 uptake profile at 195 K with a final amount of 27 cm3 g−1 at p = 1 atm, suggesting permanent porosity of the framework (Figure 1c). The low uptake of CO2 can be attributed to the occupancy of the pores by long dodecyl and TEG side chains. Interestingly, the framework also showed a type-II isotherm of N2 adsorption at 77 K with smaller uptake at low-pressure regions (Figure S15a, SI). The pore size distribution analysis by the nonlocal density functional theory (NLDFT) method suggests that NMOF-1 has wide mesopore distributions centered at ∼3.2 nm (Figure S15b). This mesopore distribution arises due to the network formation based on 23142


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Figure 3. (a) Normalized UV−vis and photoluminescence (PL) spectra of SRG and NMOF-1, respectively. (b) PL spectra (λex = 376 and 529 nm) and excitation spectrum (λem = 600 nm) of SRG2@NMOF-1. (c) Fluorescence decay profile of SRG2@NMOF-1 upon excitation at 373 nm (filled) and 555 nm (empty) and monitored at 600 nm. (d) Commission internationale de l’éclairage (CIE) plot and emission colors of NMOF-1, SRG1@NMOF-1, SRG2@NMOF-1, and SRG3@NMOF-1.

suggested the necessity of mixed polar side chains to control the particle size in the nano regime and also vesicular morphology. As NMOF-1 contains an OPE backbone in the framework, it was found to be highly emissive with 49% fluorescence quantum yield. Some recent reports demonstrate that the OPE backbone can act as an energy donor with a suitable chromophoric acceptor. 27,35 The UV−vis spectrum of NMOF-1 showed absorption maximum at 376 nm and the corresponding emission maximum appeared at 482 nm. SRG (15 × 12 Å2) was chosen for encapsulation to achieve an energy donor−acceptor system as (a) it can be encapsulated very easily inside the mesopore formed by the interparticle void space of NMOF-1 by noncovalent interactions such as π−π stacking (SRG and OPE backbone); H-bonding, between TEG side chains and polar groups (sulfonate, imine, and secondary amine); and van der Waals interactions between dodecyl chains and alkyl groups of SRG and (b) its absorption spectrum (λmax = 529 nm) in methanol merges partially with the emission spectrum of NMOF-1 (Figure 3a). Such a spectral overlap is essential for partial energy transfer toward color tunability. In a general procedure of encapsulation, a methanolic dispersion of NMOF-1 was treated with a measured amount of SRG and kept overnight (Table S1, SI). Three different amounts of SRG-loaded MOF compounds were achieved: 1.4 mol % (SRG1@NMOF-1), 3.6 mol % (SRG2@NMOF-1), and 8.2 mol % (SRG3@NMOF-1). Notably, 8.2 mol % is the maximum achievable loading of dye into NMOF-1 under the experimental conditions. Upon SRG loading, the absorption band of SRG was blue-shifted by 7 nm (from 529 to 522 nm), whereas the band corresponding to NMOF-1 was red-shifted by 8 nm (from 376 to 384 nm) because of strong noncovalent interactions between SRG and

distance between two adjacent secondary building units. The framework structure of {Zn2(OPE-TC1)}n has been modeled on the basis of a literature report on the ZnII-OPE tetracarboxylate-based linker having no side chain.41 The morphology of the nanoscale MOF was investigated by transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). TEM and FESEM images represent vesicular morphology of NMOF-1 with diameter ranging from 100 to 300 nm (Figure 2). As the reaction was performed in an intermediate polar solvent (THF), NMOF-1 got folded to a vesicular morphology to reduce the surface energy by keeping dodecyl chain outside and TEG inside. To understand the evolution stages of the nanovesicles, we performed the reaction by varying the reaction time, 2, 12, and 24 h, to obtain NMOF-1a, NMOF1b and NMOF-1c, respectively. Both NMOF-1a and NMOF1b revealed a cross-linked nanofiber morphology, which tether easily with each other over time. As a result, the fiber width is longer in NMOF-1b (60−80 nm) than in NMOF-1a (20−30 nm). After 24 h (NMOF-1c), the fibers grown wider and started folding to reduce surface energy. Interestingly, the TEM image also showed a folding step toward the formation of nanovesicles (Figure 2). The FESEM images of NMOF-1c showed an island-like network (300−400 nm), which eventually folds to form vesicles. The PXRD patterns of NMOF-1 and NMOF-1a match well, indicating that packing remains same throughout the evolution of nanovesicle (Figure S13, SI). To investigate the effect of mixed polar side chain functionality on the morphology, OPE-TC2 was prepared without TEG and dodecyl side chains and self-assembled with ZnII under similar reaction conditions to form MOF-OMe. FESEM images of the framework revealed microscopic particles with irregular morphology (Figure S7, SI). This 23143


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Figure 4. (a) UV−vis spectra of SRG, NMOF-1, and SRG2@NMOF-1 as a water dispersion. Photocatalytic activity of NMOF-1 and SRG2@ NMOF-1 using (b) full range light (290−750 nm) and (c) visible light (420−750 nm). (d) Cumulative H2 production upon intermittent degassing by NMOF-1 and SRG2@NMOF-1 (290−750 nm).

OPE-TC. This also confirms that SRG is encapsulated in the mesopore rather than existing as a physical mixture of SRG and NMOF-1. Successive incorporation of SRG into NMOF-1 resulted in tunable emissive materials. The emission spectrum (λex = 376 nm) of NMOF-1 covers the spectral range of 425−575 nm with a maximum at 482 nm, and the corresponding CIE coordinate was found to be (0.17, 0.31), indicating cyan emission (Figure 3). SRG1@NMOF-1 showed additional emission at 562 nm for encapsulated SRG, resulting in a shift of the CIE coordinate (0.27, 0.31) toward white-light emission. Interestingly, 3.6 mol % SRG loading (SRG2@NMOF-1) provided sufficient emission intensity at 472 and 562 nm for generating pure white-light emission and the corresponding CIE coordinate appeared at (0.31, 0.35). SRG3@NMOF-1 exhibited yellow emission with the CIE coordinate of (0.33, 0.44). Because of the absence of the TEG side chain in OPETC2, SRG encapsulation in MOF-OMe was achieved only up to 1.1 mol %, leading to a CIE value of (0.25, 0.34). We have studied a possible excitation energy transfer process in SRG2@ NMOF-1. As the UV−vis spectrum of SRG showed no absorption at 376 nm, observation of its emission at 562 nm (λex = 376 nm) is indicative of a partial energy transfer process from NMOF-1 to SRG. Upon direct excitation at the dye (λex = 529 nm), the emission band at 562 nm was observed with a marked decrease in intensity when compared with that of the excitation at NMOF-1 (λex = 376 nm). The excitation spectrum (λem = 600 nm) showed two notable bands at 529 and 395 nm for both SRG and NMOF-1. Fluorescence decay profiles of NMOF-1 and SRG2@NMOF-1 were also measured to obtain further evidence for the energy transfer. The decay profile of NMOF-1 monitored at 472 nm, upon excitation by a 373 nm laser, revealed the excited state lifetime of 6.2 ns, whereas SRG2@NMOF-1 showed a decreased

lifetime of 5.1 ns for the same. Furthermore, direct excitation at SRG showed a lower lifetime of 0.53 ns when compared to that from excitation at the dye-incorporated donor scaffold (373 nm laser), where the lifetime was found to be 2.3 ns (Figure 3c). All of the above studies clearly indicate the presence of a partial energy transfer from the NMOF-1 backbone to SRG. The energy transfer efficiency was calculated to be 93% with the energy transfer rate constant 1.46 × 109 s−1. The absolute quantum yields were measured to be 49 and 52% for NMOF-1 and SRG2@NMOF-1, respectively. Notably, the emission band of NMOF-1 was blue-shifted from 482 to 470 nm upon SRG encapsulation, indicating significant interaction between the dye and OPE-TC backbone. The OPE backbone in NMOF-1 provides a high absorption coefficient and excited state lifetime, which are essential for photocatalysis. The high excited state lifetime implies a sluggish electronic recombination process, which would facilitate photocatalytic H2 evolution from water. The UV− vis spectra revealed optical band gaps of 2.9 and 2.3 eV for NMOF-1 and SRG2@NMOF-1, respectively. Moreover, density functional theory (DFT) computations on a model system of NMOF-1 at the B3LYP/6-31+G* level (LAN2DZ: basis set and ECP for Zn) also revealed that the energy difference between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is 2.92 eV (Figure S23, SI). Therefore, we decided to explore the ability of NMOF-1 and SRG2@NMOF-1 as a heterogeneous catalyst for photochemical H2 evolution from water in the presence of a sacrificial electron donor. In a general procedure, 15 mL aqueous solution of each 0.1 M sodium sulfate and 0.2 M sodium sulfide was added to a well-dispersed solution of 5.00 mg of catalyst in 10 mL of water. We performed photocatalysis using both full range (290−750 nm) 23144


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absorption and excited state lifetime and reducing the optical band gap. Our result could help in fabrication of host−guest systems for economic clean energy applications.

and visible light (420−750 nm) irradiation. Light irradiation was carried out using a 290 MW xenon lamp (Oriel Instruments), and the amount of hydrogen evolution was quantified every hour by gas chromatography (Agilent Technologies, 7890B). Both the NMOF-1 and SRG2@ NMOF-1 catalysts produced 0.6 and 1.08 mmol g−1 H2, respectively, upon irradiation with full range light (290−750 nm) for 5 h (Figure 4b). For both the samples, effective saturation in catalysis occurs after 3 h in each cycle. However, complete regeneration of the catalytic activity was observed by degassing the solution with argon for 30 min. On intermittent degassing the reaction mixture in every 3 h, NMOF-1 and SRG2@NMOF-1 produced 4.8 and 8.4 mmol g−1 H2, respectively, after 24 h of light irradiation (290−750 nm). The average rate of H2 production is 0.1 mmol g−1 h−1 for NMOF-1, whereas SRG2@NMOF-1 provided an enhanced catalytic rate of 0.35 μmol g−1 h−1 (Figure S10, SI). Therefore, SRG loading enhanced the amount of H2 production as well as the catalytic rate by 1.75-fold because of additional absorption. Although the excited state lifetime was slightly reduced (6.2 → 5.1 ns) after SRG loading, the higher light absorption corresponding to SRG resulted in the enhancement of photocatalytic activity.42,43 DFT computations also revealed that electron transfer is feasible from the conduction band of SRG to the conduction band of NMOF-1 (Figure S24, SI). The recyclability of the catalyst was checked up to four cycles without significant loss of activity. We further investigated the catalytic efficiency of both the catalysts in visible light keeping the other reaction conditions similar. NMOF-1 produced only 32 μmol g−1 H2 after 5 h of irradiation, demonstrating inefficient catalysis in visible light. SRG2@NMOF-1 provided an improved catalytic effect with 56 μmol g−1 after 5 h of irradiation (420−750 nm). OPE-TC1, OPE-TC2, and OPETC1 mixed with SRG (3.6 mol %) showed minimal catalytic activity (Figure S14a, SI). Moreover, the PXRD pattern of SRG2@NMOF-1 remained very similar after performing photocatalytic experiments (Figure S14b, SI). These observations excluded any possibility of hydrolysis of the framework during photocatalysis. In the absence of a sacrificial reagent, the catalytic activity was found to be insignificant. Notably, the catalysts retain the emission property even in the presence of a sacrificial reagent. The DFT computation showed that the HOMO and LUMO are situated on the OPE backbone. Therefore, we assume that the OPE backbone acts as both a photosensitizer and a catalyst, whereas ZnII remains inactive (d10 system).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b06363. Synthetic procedure, UV−vis and PL spectra, PXRD, gas adsorption, TGA profiles, DFT data, lifetime measurement, and FESEM and TEM images (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tapas Kumar Maji: 0000-0002-7700-1146 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Subi George and Suman Kuila for lifetime measurements and Dr. K. S. Narayan and Anaranya Ghorai for the quantum yield measurements. P.V. and S.R. acknowledge UGC, India, for financial support. T.K.M. and D.S. are grateful to the DST, India (Project Nos. MR-2015/ 001019 and TRC-DST/C.14.10/16-2724, JNCASR), and JNCASR for funding.

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3. CONCLUSIONS In summary, we have put forward a design strategy to synthesize an omniphilic NMOF via asymmetrization of bolaamphiphilic OPE side chains. The resultant NMOF showed tunable and white-light emission upon encapsulation of ambipolar sulforhodamine G. The polar pore surface also allowed for facile diffusion of water. Therefore, both NMOF-1 and SRG@NMOF-1 showed significant photocatalytic activity for H2 evolution from water. The work demonstrates an energy-saving bimodal activity of the material, which showed white-light emission and photocatalytic activity, simultaneously, upon light irradiation. The findings also open up the door for OPE-based materials as water-splitting photocatalysts and, notably, without using any costly metal co-catalyst. SRG encapsulation resulted in the enhancement of the photocatalytic activity possibly due to increased visible light 23145


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Nanovesicular MOF with Omniphilic Porosity: Bimodal Functionality for

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