Maximizing Photoresponsive Efficiency by Isolating Metal-Organic

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Maximizing Photoresponsive Efficiency by Isolating MetalOrganic Polyhedra into Confined Nanoscaled Spaces Yao Jiang, Jinhee Park, Peng Tan, Liang Feng, Xiao-Qin Liu, Lin-Bing Sun, and Hong-Cai Zhou J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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Journal of the American Chemical Society

Maximizing Photoresponsive Efficiency by Isolating MetalOrganic Polyhedra into Confined Nanoscaled Spaces Yao Jiang,† Jinhee Park,‡ Peng Tan,† Liang Feng,§ Xiao-Qin Liu,† Lin-Bing Sun,†* and Hong-Cai Zhou§* † State

Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), College of Chemical Engineering, Nanjing Tech University, Nanjing 210009 (China) ‡ Department

of Emerging Materials Science, Daegu Gyeongbuk Institute of Science and Technology, Daegu 42988 (Republic of Korea) § Department

of Chemistry, Texas A&M University, College Station, Texas 77842-3012 (United States)

ABSTRACT: Photoresponsive metal-organic polyhedra (PMOPs) have attracted expanding interests due to their modular nature with tunable functionality and variable responsive behaviors tailored conveniently by external-stimulus. However, their photoresponsive efficiency is often compromised after activation because of desorption-triggered aggregation into bulk PMOPs, which limits their utility in stimuli-responsive applications. Here, we report a case system that can overcome the aggregation problem and achieve maximized photoresponsive efficiency by polyhedral isolation in the nanoscaled spaces of mesoporous silica (MS). Through confinement, amountcontrollable PMOPs are well dispersed in the nanoscaled spaces of MS, avoiding aggregation that commonly takes places in bulk PMOPs. Furthermore, reversible trans/cis isomerization of photoresponsive groups can be realized freely during ultraviolet/visible light irradiation, maximizing control over photoresponsive guest adsorption behaviors. Remarkably, after trans/cis isomerization, the confined PMOP-1 shows 48.2% of change in adsorption amount for propene with small molecular size and 43.9% for brilliant blue G (BBG) with large molecular size, which is significantly higher than that over bulk PMOP-1 with 11.2% for propene and 7.8% for BBG, respectively. Therefore, our work paves a way for the design and construction of multifunctional composite materials towards efficient stimuliresponsive needs. photoresponsive efficiency of PMOPs in solid state is highly desired.

INTRODUCTION Metal-organic polyhedra (MOPs), also known as porous coordination nanocages or nanoballs, are individual discrete structures constructed by rationally designed organic ligands and metal ions/clusters.1-9 Owing to their modular nature with tunable functionality, variable surface moieties and biocompatibility, MOPs have shown great potential in adsorption,10-12 catalysis,13-17 drug delivery,18-20 molecule inclusion,21-22 and as building blocks of metal-organic frameworks (MOFs).16,23-25 Particularly, photoresponsive metal-organic polyhedra (PMOPs) as optically responsive materials can function as a controllable and predictable molecular machine for the guest capture and release upon irradiation with light, which plays a vital role in stimuliresponsive drug delivery studies. PMOPs as light-responsive materials are considered as one of the most attractive stimuliresponsive systems because ultraviolet–visible (UV-Vis) light can function as a nondestructive, rapid and remotely controllable stimulus to trigger the responses. Additionally, light can also target desired positions precisely with high resolution and does not generate any undesired byproducts.26-32 For example, a copper-based PMOP-1 with photoresponsive azobenzene units, reported by our group previously, can undergo reversible photocontrollable trans/cis isomerization in solution to alter the molecular size upon the irradiation with UV/Vis lights.33 However, the applications of PMOPs are seriously restricted by the limited photoresponsive efficiency resulted from random aggregation problems in solid state.33 The aggregation of bulk PMOPs after solvent removal not only blocks the internal active site, but also hampers the trans/cis isomerization of azobenzene functional groups. Therefore, an efficient method to boost

Figure 1. Schematic illustrations of (a) the structure of PMOP-1 upon UV- and Vis- lights irradiation; (b) the bulk PMOP-1 (aggregation occurs in solid state and only few PMOP-1 molecules at the edges show photoresponsiveness upon UV- and Vis- lights irradiation); (c) PMOP-1 dispersed in the nanoscaled spaces of MS (the enhanced dispersity endows PMOP-1 with “real photoresponsiveness”). Color scheme in PMOP-1: Cu atoms, cyan; O atoms, red; N atoms, blue; C atoms, green. The large yellow sphere in a PMOP-1 molecule represents the free space inside the molecular cages.

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A possible solution to the problems may lie in the immobilization/dispersion methods in porous materials including zeolites, MOFs, porous aromatic frameworks (PAFs), nanoporous membranes, mesoporous carbon and silica.34-40 For instance, the azobenzene-type molecular switches have been incroporated into MOFs and exhibited efficient trans/cis isomerization in the solid state.35,37-39 Also, an attractive way to satisfy the conformational freedom of spiropyran moieties within the nanopores of PAFs as well as flexible coordination cages have been reported.36,40 Dispersion of traditional unstable homogenous catalysts or active molecular species inside these porous materials have been documented as an effective method to ensure both the homogenous dispersion and improved stability.41 Among them, mesoporous silica (MS) shows advantages such as high chemical stability, tunable symmetries and ordered pore structures, making them excellent candidates as supports to disperse photoresponsive polyhedral units.42 It is proved to be feasible to realize the isomerization of azobenzene and their derivaties within the nanostructured silica.43 Here, we report a powerful strategy, polyhedral isolation in MS, in order to overcome the traditional aggregation problem and boost photoresponsive efficiency. Through confinement, the structural integrity of PMOPs is well preserved and high dispersity of MOPs in nanoscaled spaces of MS is achieved. Upon light exposure, the isolated, amountcontrollable PMOPs can achieve reversible trans/cis isomerization freely, leading to maximized control over photoresponsive guest adsorption behaviors. In comparison, bulk PMOPs after desorption-triggered aggregation can only achieve a limited level of light responses since the trans/cis isomerization of major internal PMOPs is limited by random packing. As a proof of concept, PMOP-1 [Cu24(C16H12N2O4)24], composed of surface photoresponsive azobenzene units, was dispersed insides nanoscaled spaces of MS via wet-impregnation. Furthermore, upon the irradiation with UV/Vis lights, the confined PMOP-1 presents 48.2% of change in adsorption amount for propene with a small molecular size, which is more than four times higher than that over bulk PMOP-1 (11.2%). Similar results are also observed for the adsorption of brilliant blue G (BBG) with a large molecular size. Therefore, the present strategy provides fresh insights into the design and construction of multifunctional composite materials towards efficient photoresponsive needs, which subsequently sheds light on controlling adsorption behaviors for the development of efficient, energy-saving separation processes.

Thermal gravimetric analysis (TGA) shows that 10.3 wt% [PMOP(0.1)@MS], 19.6 wt% [PMOP(0.2)@MS], and 39.7 wt% [PMOP(0.4)@MS] of PMOP-1 are introduced respectively (Figure S2a and Table S1), which are consistent with the feeding amounts (10 wt%, 20 wt%, and 40 wt%) of PMOP-1 in the experiments. Various characterization techniques were employed to verify the presence of PMOP-1 in the PMOP@MS composites. The UVVis diffuse reflectance spectrum (UV-Vis DRS) of PMOP-1 exhibits four obvious absorption peaks in the UV-Vis range (Figure S3). There is no detectable absorption peaks in MS, since the silicon-oxygen tetrahedron tetrahedral (SiO4) units have no absorption in the UV-Vis range. For PMOP-1@MS composites, there are four absorption peaks originated from PMOP-1 can be detected in the same range of UV-Vis DRS.48-51 With the increasing loading amount of PMOP-1, the characteristic peaks become stronger, suggesting that PMOP-1 molecules are successfully introduced to MS. In the infrared (IR) spectra (Figure S4), all of the characteristic bands assigned to PMOP-1 are observable in the PMOP-1@MS samples, and the band intensity increases gradually with the increase of PMOP-1 loading, which is in good agreement with the UV-Vis DRS results.

RESULTS AND DISCUSSION Synthesis and Characterization. PMOP-1 was synthesized from Cu(OAc)2 and organic ligand 5-((2,4dimethylphenyl)diazenyl)isophthalic acid with a 120o bridging angle. Structurally, PMOP-1 has a cuboctahedral geometry when the dicopper units are viewed as vertices and the ligands are viewed as edges.33 The PMOP-1 has a diameter of 4.04 nm in trans conformation and 3.11 nm in cis conformation, respectively (Figure S1). MS is a type of porous material possessing ordered pore structures and tunable pore spaces in nanoscale, which should be an ideal accommodation for PMOP-1.42,44-47 MS shows a pore size of 9.30 nm that is appropriate confined spaces for the accommodation of PMOP-1. A wet-impregnation strategy was utilized to introduce PMOP-1 to the nanoscaled spaces of MS. By simply tuning the feeding amounts of PMOP-1, the composites with controllable loading amounts of PMOP-1 in nanoscaled spaces can be obtained and denoted as PMOP(n)@MS (n = 0.1, 0.2, or 0.4, where n denoted as the loading amounts of PMOP-1).

Figure 2. (a) TEM, (b) HRTEM, and (c) STEM and elemental mapping of the composite PMOP(0.1)@MS. The arrows in (b) show several examples of PMOP-1. (d) N2 sorption isotherms and (e) BJH pore size distributions of the samples MS, PMOP(0.1)@MS, PMOP(0.2)@MS, PMOP(0.4)@MS, and PMOP-1. The N2 sorption data are obtained before UV-light irradiation.

The low-angle X-ray diffraction (XRD) patterns of PMOP1@MS show similar diffraction peaks to MS, suggesting that the ordered mesoporous MS structures are well maintained after the introduction of PMOP-1 (Figure S5). Scanning electron microscopy (SEM) images of different samples are displayed in Figure S6. All samples show the short rod-like morphology, indicating that the morphology of MS is well preserved after the introduction of PMOP-1. Transmission electron microscopy (TEM) plays a vital role to characterize the structure of materials


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and the dispersion of PMOP-1. The TEM and scanning transmission electron microscopy (STEM) images illustrate that the ordered mesoporous structures of PMOP-1@MS are well preserved (Figures 2, S7-S9), which is comparable to pristine MS (Figure S7). Moreover, as shown in the high resolution TEM (HRTEM) of PMOP(0.1)@MS (Figure 2b), the PMOP-1 molecules can be directly observed as dark dots in the pores of MS. But for pristine MS, there is no observable substance from their inner pores (Figure S7b). In addition to Si and O derived from MS, the elements C, N, and Cu stemmed from PMOP-1 are detected in the samples from energy dispersive X-ray (EDX) spectroscopy (Figure S10) and are homogeneously distributed from elemental mapping images (Figure 2c). The wide-angle XRD pattern of MS exhibits a single broad scattering peak at about 23o ascribed to amorphous silica walls (Figure S11).52 The sample PMOP(0.1)@MS shows almost the same pattern as that of MS, indicating that PMOP-1 molecules are well-dispersed in the nanoscaled spaces of MS with phase purity. Some new scattering peaks become observable with further enhancement of PMOP-1 loading. A series of scattering peaks can be identified in the wide-angle XRD patterns of the samples PMOP(0.2)@MS and PMOP(0.4)@MS. In addition, these peaks are in line with those of bulk PMOP-1, suggesting that the aggregation of PMOP-1 molecules in the samples with high loading. In this case, there are two phases in the samples, i.e. dispersed PMOP-1 and aggregated PMOP-1. From the N2 sorption analysis at 196 oC (Figure 2d), the MS exhibits a typeIV isotherm with type-H1 hysteresis loop, which are typical characteristics of materials with cylindrical mesopores.53 The PMOP-1 presents minor N2 uptake because of the bulk aggregation after activation. After confinement of PMOP-1 in MS, the N2 uptake and the pore size decrease. As show in Figure 2e, with the increase of PMOP-1 loading, the pore size of the samples decreases gradually from 9.3 nm (MS) to 8.9 nm [PMOP(0.1)@MS], 8.7 nm [PMOP(0.2)@MS], and 8.5 nm [PMOP(0.4)@MS]. These results demonstrate that the PMOP-1 molecules are successfully confined in the nanoscaled spaces of MS. The TGA and derivative thermo-gravimetric (DTG) analysis show that the decomposition of PMOP-1 starts at 277 oC (Figure S2b). Interestingly, when PMOP-1 is confined in the nanoscaled spaces of MS, the decomposition temperature increases to 289 °C for the sample PMOP(0.4)@MS. However, with the decreasing content of PMOP-1, the decomposition temperature keeps increasing to 309 °C [PMOP(0.2)@MS] and even to 317 °C [PMOP(0.1)@MS]. This mirrors that the confined PMOP-1 exhibits improved thermal stability in comparison with the bulk analogues.

Photoresponsive Properties. To obtain insights into photoresponsive properties of the isolated PMOP-1@MS composites, isomerization of azobenzene groups in PMOP-1 was first investigated in ethylene glycol via UV-Vis absorption spectra (Figures S12-S14). UV-light irradiation causes decreased absorbance at 340 nm (ππ* transition of the trans-azobenzene isomer) and increased absorbance at 438 nm (nπ* transition of the cis-azobenzene isomer). The maximum yield of cis isomer is 95.4% after UV-light irradiation. In the reverse direction, the absorbance at 340 nm recovers after Vis-light irradiation.54-56 This result demonstrates that the trans/cis isomerization of PMOP-1 is reversible in ethylene glycol via alternating irradiation with UVand Vis- lights. Furthermore, the isomerization from cis to trans of the samples by thermal activation were conducted as shown in Figures S14 and S18, the cis isomers of PMOP-1 (or PMOP@MS) can be converted to trans isomers efficiently after thermal activation.56 Similarly, the UV-Vis absorption spectra of the composites PMOP-1@MS exhibit the same behavior as PMOP-1 (Figures S16-S18). As shown in Figure 3a, the sample of PMOP(0.1)@MS presents a signature absorbance at 336 and 434 nm corresponding to the ππ* and nπ* transition of the azobenzene group in PMOP-1. After irradiation of PMOP(0.1)@MS under 365 nm of UV-light (or 450 nm of Vislight), it shows a decreased (or increased) absorbance at 360 nm. The maximum yield of cis isomer is 92.3% after UV-light irradiation (Figure S19). Because of the formation of Haggregates, the maximum of absorption of trans-azobenzene is blue-shifted, which can be observed in UV-Vis absorption spectra shown in Figure S20.57 Besides, the efficient trans/cis isomerization between trans-PMOP(0.1)@MS and cisPMOP(0.1)@MS is totally reversible through alternating UV- and Vis- lights irradiation (Figure 3b). Additionally, the UV-Vis DRS was employed to further explore the photoresponsive properties of PMOP(0.1)@MS (Figure S21). The results show that irradiation of PMOP(0.1)@MS in solid state with UV- or Vis- light leads to cis or trans isomerization, indicating that the changes in isomerization can be observed in solid state via DRS. In general, azobenzene undergoes trans/cis isomerization in both solution and solid state, while the conversion of azobenzene in solution is more obvious due to the better dispersion of samples in solution.58 Furthermore, N2 sorption isotherms are employed to monitor the change of pore structure by comparing one specific sample upon UV- and Vis- lights irradiation (Figure S22 and Table S1). The cis-isomer is formed after UV-light irradiation for PMOP-1@MS samples, and the Brunauer-Emmett-Teller (BET) specific areas decrease while the pore volumes and Barrett-Joyner-Halenda (BJH) pore sizes increase. The decreased BET specific areas upon trans to cis transformation of PMOP-1@MS can be ascribed to the twist conformation of the cis-azobenzene groups, which prevents N2 molecules from accessing to the inner cages of PMOP-1 (Figure 1a). Nevertheless, the trans-PMOP-1 in the nanoscaled spaces of MS shows a stretching conformation with a large molecular size, thus leading to decreased pore volumes and pore sizes.

Figure 3. (a) Alteration in the UV-Vis spectra of PMOP(0.1)@MS upon UV-light and then Vis-light irradiation. The isosbestic points at 285 nm and 408 nm are observed as for the trans/cis isomerization. (b) Reversible change in absorbance as a function of cycle upon alternating UV- and Vislights irradiation.



Figure 4. (a) C3H6 sorption isotherms of the sample PMOP(0.1)@MS. (b) C3H6 adsorption capacities of the samples MS, PMOP(0.1)@MS, PMOP(0.2)@MS, and PMOP(0.4)@MS. (c) The change amount of C3H6 over different samples caused by trans/cis isomerization. (d) Adsorption cycles of C3H6 over trans/cis-PMOP(0.1)@MS.

Adsorption Behavior. Two adsorbates with relatively small and large molecular sizes, respectively, were employed to probe adsorption performances of the PMOP@MS composites. The adsorbate with a small molecular size, propene (C3H6), is regarded as one of the most important industrial raw chemicals, and much attention has been paid on the adsorption of C3H6 (Figure S23).59 Because of the aggregation of PMOP-1 and the blockage of active sites by adjacent ones, only a relatively small amount of C3H6 is adsorbed in bulk PMOP-1 (9.9 mol C3H6/mol MOP) (Figure 4b and Figure S24). Remarkably, the isolated PMOP-1 in nanoscaled spaces exhibits superior C3H6 adsorption capacity compared with the bulk PMOPs. Note that the adsorption capacity of C3H6 is based on the molar amounts of MOPs, so the uptake correlates well with the dispersion extent of MOPs. In the sample PMOP(0.1)@MS, the PMOP-1 molecules are highly dispersed as demonstrated above, and the adsorption capacity of transPMOP(0.1)@MS can reach 20.6 mol C3H6/mol MOP, indicating that almost one copper site captures one C3H6 molecule. The results here presented that most of the active sites are accessible in trans-PMOP(0.1)@MS and photoresponsive efficiency is maximized by isolation (Figure 4a). Bulk PMOP-1 inclines to aggregate with each other, thus blocks the active sites and obstructs the isomerization of azobenzene groups. Hence, there is only a minor difference in adsorption capacity on C3H6 for PMOP-1 after respective irradiation with UV- and Vis- lights (11.2%). However, the difference in adsorption capacity reaches 48.2% for trans- and cis-PMOP(0.1)@MS (Figure 4c). The interesting adsorption behavior can be attributed to the dispersion and facile trans/cis isomerization of PMOP-1 in the nanoscaled spaces. In trans state, the azobenzene groups in PMOP-1 show a stretching conformation and the active sites are exposed and accessible for the adsorbates. Inversely, the azobenzene groups in cis-PMOP-1 exhibit a twist conformation, which blocks the active sites. As a result, quite different adsorption behaviors are observed for PMOP-1 with trans/cis isomers (Figure S24). In addition, the composites PMOP(0.2)@MS and PMOP(0.4)@MS display 33.6% and 20.1% of change in adsorption amount during trans/cis isomerization (Figures S25 and S26). The reversibility of the

C3H6 uptake upon trans/cis isomerization was studied as shown in Figure 4d. After the fresh sample PMOP(0.1)@MS was saturated with C3H6, regeneration was conducted. The regenerated adsorbent was irradiated with UV- or Vis- light, and then reused for adsorption. The adsorption capacity of C3H6 in four cycles was recorded. The adsorption capacity and the change between trans and cis isomerization almost keep at the same level, thus demonstrating the complete reversibility of trans/cis isomerization of PMOP-1 confined in nanoscaled spaces. In addition to C3H6 with a small molecular size, brilliant blue G (BBG) with a large molecular size was selected to investigate the adsorption behavior towards photo-induced isomerization of PMOP-1.60 As shown in Figures S27-S29, regardless of the irradiation with UV- or Vis- light, MS and PMOP-1 exhibit the negligible adsorption capacity on BBG because of no suitable active sites for MS and the blocked active sites by random aggregation for PMOP-1. After the confinement of PMOP-1 in the nanoscaled spaces, the BBG adsorption capacity on transPMOP(0.1)@MS was 0.72 mol BBG/mol MOP. However, with increasing loading of PMOP-1, the BBG adsorption capacity exhibits a decreasing trend because of the reduced dispersity of PMOP-1, namely 0.56 mol BBG/mol MOP for transPMOP(0.2)@MS and 0.50 mol BBG/mol MOP for transPMOP(0.4)@MS. Furthermore, because of the blocked active sites caused by conformation changes, the cis isomerization makes the adsorption capacity of PMOP(0.1)@MS [cisPMOP(0.1)@MS] decrease to 0.50 mol BBG/mol MOP, which corresponds to 43.9% of change in BBG uptake (Figure S29). This is obviously higher than the change occurred in bulk PMOP1 upon irradiation with UV- and Vis- lights (7.8%). The dispersed PMOP-1 can realize reversible trans/cis isomerization freely, resulting in more controllable adsorption properties; on the contrary, trans/cis isomerization is limited in aggregated PMOP-1, leading to less controllable adsorption properties. For example, the difference in adsorption capacity of BBG for trans- and cis-PMOP(0.1)@MS with dispersed PMOPs is 43.9%, while that for trans- and cis-PMOP(0.4)@MS with aggregated PMOP-1 is only 14.0%. The relatively poor adsorption of BBG versus propene is related to a solution versus gas phase. There may exist competitive adsorption between adsorbates and solvent molecules in liquid, thus leading to the poor adsorption of BBG. On the other hand, the molecular size of C3H6 is relatively small (6.44 Å×4.65 Å, Figure S23), and each active site in PMOP-1 is available for C3H6 capture. However, for BBG with a large molecular size (19.8 Å×18.3 Å), steric hindrance becomes obvious and only partial sites in PMOP-1 are available. As a result, the adsorption of BBG is lower as compared with propene. On the basis of the above results, it is apparent that the confined PMOP-1 in nanoscaled spaces exhibits obviously better adsorption capacity on adsorbates with different molecular sizes than the bulk PMOP-1. The enhanced dispersity of PMOP-1 makes trans/cis isomerization of azobenzene groups occur freely. In the trans state, PMOP-1 shows a stretching conformation and makes the active sites accessible for adsorption. Inversely, cisPMOP-1 exhibits a twist conformation which blocks active sites, and thus the adsorption capacity decreases. Furthermore, the increasing loading of PMOP-1 reduces the dispersity and thus restrains the trans/cis isomerization of azobenzene groups, which leads to decreased changes in adsorption capacity.

CONCLUSIONS In summary, we have demonstrated the successful confinement of a PMOP in nanoscaled spaces of MS to avoid the aggregation problems commonly observed in bulk PMOPs. The confined


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PMOP-1 exhibits enhanced dispersity and stability in contrast to their bulk counterparts. The confinement strategy endows photoresponsive MOPs with “real photoresponsiveness” and maximizes control over their photoresponsive efficiency. Particularly, the free trans/cis isomerization of photoresponsive groups results in exposure/blockage of active sites, which makes the adsorption behavior controllable despite of the molecular size of adsorbates. This is impossible to realize in bulk PMOP-1 because the isomerization of photoresponsive groups is hampered by aggregation. Looking forward, the present strategy might impart new interests to achieve the full potential of stimuliresponsive materials, and subsequently develop efficient, energysaving adsorption processes.

ASSOCIATED CONTENT Supporting Information Experimental details and additional data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21722606, 21676138, and 21576137), the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions and the Robert A. Welch Foundation through a Welch Endowed Chair to HJZ (A-0030). J. Park acknowledges a National Research Foundation of Korea (NRF) grant funded by the Korea government (No. NRF2016R1C1B2009987 and No. NRF-2016M2B2A9912217).

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Maximizing Photoresponsive Efficiency by Isolating Metal-Organic

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