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Construction of Porphyrin-containing Metallacycle with Improved Stability and Activity within Mesoporous Carbon Li-Jun Chen, Shangjun Chen, Yi Qin, Lin Xu, Guang-Qiang Yin, JunLong Zhu, Fan-Fan Zhu, Wei Zheng, Xiaopeng Li, and Hai-Bo Yang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02386 • Publication Date (Web): 06 Apr 2018 Downloaded from http://pubs.acs.org on April 6, 2018
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Construction of Porphyrin-containing Metallacycle with Improved Stability and Activity within Mesoporous Carbon Li-Jun Chen†, Shangjun Chen‡, Yi Qin†, Lin Xu†,*, Guang-Qiang Yin†,║, Jun-Long Zhu†, Fan-Fan Zhu†, Wei Zheng†, Xiaopeng Li║, Hai-Bo Yang†,* †
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Chang-Kung Chuang Institute, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, P. R. China. ‡ Department of Chemistry, Shanghai Normal University, Shanghai 200234, P. R. China. ║ Department of Chemistry, University of South Florida Tampa, FL 33620, USA. Supporting Information Placeholder ABSTRACT: The successful construction of porphyrin func-
above-mentioned shortcomings (Scheme 1). The advent of mesoporous carbons, which possess ordered pore structures with tunable volumes and symmetries,9 brings a good candidate providing the confined environment within the support matrix. Herein, we present the fabrication of porphyrin-containing metallacycle with well-defined shape and size confined in the cavity of mesoporous carbon FDU-16, which was found to exhibit much superior 1O2 generation efficiency compared to that of free metallacyces in solution. More importantly, the stability of the resultant metallaycles within the confined cavity is greatly improved, which allows the resultant hybrids to work as a heterogeneous catalyst for photooxidation of sulfides.
Self-assembly in a confined space is ubiquitous in nature. For example, self-assembly of biological macromolecules such as DNA, RNA, and proteins is often facilitated by a confinement enviorment inside cells, which has proven to play an important role in many important life processes.1 Abiological molecular self-assembly within a confine space has received extensive attention in recent years.2 In particular, self-assembly in a confined space renders the resultant self-assembled species robust and persistent features under the confined conditions, thus allowing for the construction of novel hybrid functional materials.3 Therefore, investigation on self-assembly in a confined space and their functionality has evolved to be one of the most attractive topics within supramolecular chemistry. Discrete metallacycles4 with well-defined shapes and sizes, which are predominated by organic donor/metal acceptor paradigm with spontaneous formation of coordination bonds5, have been extensively explored for the construction of functional metallosupramolecular architectures with wide applications in sensing, catalysis, etc.6 However, the dynamic nature of coordination bonds sometimes lead to the instability of the obtained metallacycles, i.e. metallacycles may decompose under relatively harsh conditions.7 Moreover, in some cases, especially for fluorescent metallacycles, the aggregation-caused quenching (ACQ) effect may blockade and compromise their photophysical functionality.8 Thus the construction of isolated functional metallacycles with improved stability and dispersity is particularly necessary. Inspired by self-assembly of biological molecules in nature, we envision that the confinement of discrete metallacycles in welldefined porous matrices may provide an option to overcome the
Scheme 1. Schematic Presentation of Abiological Selfassembly within a Confined Space.
tionalized metallacycle in the confined cavity of mesoporous carbon FDU-16 (3⊂ ⊂C) is presented in this study. Because of high dispersity of metallacycles within the mesoporous cavities, the stability and activity of porphyrin-containing metallacycles were obviously improved. For example, 1O2 generation efficiency of 3⊂ ⊂C is ca. 6-fold faster than that of free metallaycles in solution. Thus the resultant hybrid material has been successfully employed as a heterogeneous catalyst for photooxidation of sulfides.
The 120° donor precursor 1 substitued with porphyrin moiety was easily synthesized in a few steps as indicated in Scheme S1. A typical 120° diplatinum-(II) acceptor ligand 2 was adopted to interact with ligand 1 to afford the metallacycle 3 (Scheme S2). Self-assembly process was first conducted in solution to confirm the formation of tris-porphyrin metallacycle 3 as monitored by 31P and 1H NMR, two-dimensional (2-D) COSY and 1H-1H NOESY NMR spectroscopy, ESI-TOF-MS spectrum, as well as the solidstate 13C and 31P NMR spectroscopy (Figure S1-S5). The optimized structure of metallacycle 3 featured a roughly planar hexagonal structure with an internal diameter of ~3.0 nm and a diagonal distance of ~5.0 nm (Figure S6a-b). Moreover, transmission electron microscopy (TEM) images of a very dilute solution of 3 (~10.0 nM) exhibited discrete particles with a measured diameter of ~5.0 nm (Figure S6c), which was consistent with the theoretical size.
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Scheme 2. Schematic Presentation of Self-assembly of Trisporphyrin Metallacycle 3 in Cavities of Mesoporous Carbon FDU-16.
The mesoporous carbon material (FDU-16) with a threedimensional (3-D) cage-type cubic (Im3 m) structure containing large cavities (~6.0 nm) interconnected by small pore entrances (~3.0 nm) 10 was selected as substrate. Since the pore windows are smaller than the dimension of 3 (∼5.0 nm), it can prohibite the migration and leaching of 3 after the metallacycles was formed, thus allowing for the metallacycles being encapsulated in the pore. The dipyridyl ligand 1 was preloaded within the mesoporous matrix FDU-16 before the organoplatinum acceptor 2 was added. After 1.5 h of stirring, the dark solids (3⊂ ⊂C) were separated by centrifugation and washed with a large quantity of dichloromethane until no Pt traces was detected by using inductively coupled plasma (ICP) in the filtrate. ICP was firstly employed to characterize the Pt content doped in 3⊂ ⊂C, which revealed a doping mass fraction of metallacycle 3 to be ca. 15% (Figure S8a and Table S1). Then the solid-state 13C and 31P NMR spectroscopy was explored to confirm the formation of 3⊂ ⊂C, which displayed similar characteristic signals with free 3 despite the lower peaks intensity due to the low content of 3 confined in the composites (Figure S9). Additionally, compared to the free metallacycle 3 in solution, no obvious chemical shifts were observed in the case of 3⊂ ⊂C, demonstrating the successful construction and confine of metallacycle 3 in FDU-16. In order to obtain the further structural insight into the hybrid materials of 3⊂ ⊂C, low-angle powder X-ray diffraction (PXRD) and small-angle X-ray scattering (SAXS) were employed. In both PXRD (Figure 1a) and SAXS patterns (Figure S11), 3⊂ ⊂C showed the similar diffraction peaks to the parent pristine FUD-16, demonstrating the well preservation of crystalline long range pore
ordering mesostructure. TEM images of 3⊂ ⊂C viewed along the [100], [110], and [111] directions together with the corresponding Fourier diffractograms (Figure 1d-f and S12), also displayed the well-ordered body-centered cubic mesoporous structure. Moreover, 3⊂ ⊂C exhibited a type-IV isotherm with a H2-type hysteresis loop in N2 adsorption measurement, which was analogous to that of the parent FDU-16 (Figure 1b and S13), indicating the maintainess of prorous structures. Meanwhile, compared to FDU-16, the obvious decrease from 566 m2/g to 494 m2/g in the BET surface areas was observed (Table S2), which was caused by the incorporation of metallacycles within the FDU-16 cavities. X-ray photoelectron spectroscopy (XPS) was further conducted to investigate the potential interactions between metallacycle 3 and mesoporous carbon matraix. The XPS survey spectrum of 3⊂ ⊂C presented the obvious N, Pt, and P elements (Figure S14). The Pt XPS spectra of both 3 and 3⊂ ⊂C (Figure 1c) displayed typical characteristics of Pt(II) ions at ~ 73.0 eV (Pt 4f7/2) and ~ 76.3 eV (Pt 4f5/2)11 with no appreciable differences between two materials, indicating no strong electron interaction between the metallacycle 3 and FDU-16.
Figure 1. (a) PXRD patterns and (b) N2 sorption isotherms of 3⊂ ⊂C, and mesoporous matrix FDU-16, (c) XPS spectrum of metallacycle 3 and 3⊂ ⊂C. TEM image of ultrathin cuts from hybrid materials 3⊂ ⊂C recorded along the [100] (d), [110] (e), and [111] (f) directions, (g) HAADF-STEM image and (h) the corresponding elemental mapping images. Subsequently, elemental mappings were employed to gain insight into the distribution of metallacycles in the mesoporous carbon matrix. Besides C element from the matrix, the elements Pt and P stemmed from metallacycle 3 were detected (Figures 1h and S15). Moreover, no distinctly congregates (Figures 1g and S16) were observed through high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and TEM analysis of 3⊂ ⊂C, demonstrating a uniform homogeneous distribution of metallacycle 3 in the whole carbon matrix. These
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results were further confirmed by scanning electron microscopy (SEM) and cooresponding mapping analysis (Figures S17) of 3⊂ ⊂C. In order to elucidate that the above-mentioned selfassembly happened in the confined space rather than on the surface of mesoporous carbon, a control experiment, in which mesoporous carbons FDU-16 were directly soaked into the solution of metallacycle 3, was carried out. TEM, HAADF-STEM images, and elemental mapping (Figure S18) revealed that metallacycles 3 were aggregated on the surface of FDU-16 (3/C). After washing 3/C several times with dichloromethane, the obtained materials presented an extremely low loading of metallacycle 3 (0.6%) (Figure S8b and Table S1). All collected data suggested that, in the case of 3⊂ ⊂C, metallacycles 3 were mainly located inside the pores of mesoporous matrix without accumulation.
Figure 2. (a) Fluorescence response of SOSG upon treatment with 3⊂ ⊂C (irradiation by white LED, λex = 504 nm, λem = 532 nm); (b) Singlet oxygen generation by metallacycle 3, 3⊂ ⊂C, and 3/C. The dots are experimental data and the solid lines are fitted curves. Porphyrin derivatives are well known to be capable of producing copious yield of singlet molecular oxygen with widespread applications including catalytic oxidation.12 Nevertheless, many porphyrin derivatives trend to aggregate even at low concentration, thus decreasing their efficiency as photosensitizers.13 Immobilization of porphyrin-contaning complexes in solid matrices, wherein the porphyrin can be isolated and protected, may provide a promising approach to overcome such drawback.14 As demonstrated above, the porphyrin-containing metallacycles 3 were uniformly distributed inside the pores of mesoporous matrix, thus allowing for an enhancement of producing singlet oxygen and their catalytic ability. Therefore, photosensitization efficiency and catalytic property of the 3⊂ ⊂C hybrids were then examined. The 1 O2 generation efficiencies were detetected by using Singlet Oxygen Sensor Green (SOSG), which can react with the generated 1O2 to give green fluorescence (Figure 2a, see discussion in SI).15 For comparison, the 1O2 generation efficiencies of metallacycle 3 and composites 3/C were also investigated. Notably, the hybrid 3⊂ ⊂C was about six times as efficient as metallacycle 3 in generating 1 O2 (Figure 2b, S22-24, Table S3). Such enhenment was presumably owing to the enhanced stability and dispersion of metallacycle 3 in the confined spaces. To explain, the photochemical properties, photostability and aggregation behavior of metallacycle 3 in solution were then examined as a control experiment. As shown in Figure S25, the emission intensity of 3 was found to decrease when the concentration was above 6.0 µM due to the ACQ effect. TEM (Figure S26) and atomic force microscopy (AFM) images (Figure S27) of metallacycle 3 provided the direct evidence for the formation of aggregates upon increasing concentration. Moreover, both 1H NMR and 31P NMR disclosed a decomposition of metallacycle 3 upon long time irradiation (24 h, Figure S28), indi-
cating the limited stability of metallacycle 3 in solution at relatively high concentration. Thus, the improvement of 1O2 generating efficency of 3⊂ ⊂C can be ascribled to the improved disperisity and stability of metallacycle 3 in a confined space.
Figure 3. (a) Scheme for the photooxidation of sulfides (3⊂ ⊂C as catalyst); (b) photooxidation profile of sulfides; (c) reusability hybrids 3⊂ ⊂C and metallacycle 3. To futher demonstrate that the stability and activity of metallacycle 3 can be enhanced in the confined cavity, the catalytic performance of the 3⊂ ⊂C hybrids (0.5 mol %) in photooxidation of sulfides was conducted (Figure 3a). As monitored by NMR (Figure S29) and GC-MS (Figure S30), full conversion from sulfides to sulfoxides catalyzed by 3⊂ ⊂C was observed after 4 h white LED irradiation. The blank reaction without a catalyst gave very poor conversion efficiency (
