Stimuli-responsive Photoluminescent and Structural Properties of MIL-53(Al) Metal-Organic Framework

The term "metal-organic frameworks" (MOFs) refers to a novel family of crystalline microporous materials that have vast surfaces with customizable functions such as the ability to separate and purify gases [1]. Coordination bonds between organic moieties and metals or metal oxides provide these materials their structural integrity [2]. MOFs may have a flexible structure, which can result in the unusual breathing phenomenon or gate-opening effect: as external stimuli like pressure, temperature, solvents, or gas molecules are added to or removed from MOFs, the pore diameter of the material changes [3]. The class of MOFs that exhibits breathing phenomena has enormous potential in green and renewable energy as media of gas storage, chemical sensors, drug delivery systems, and is therefore at the center of attention of both fundamental and applied research [4]. This framework, MIL-53(M) series, with M = Al, Sc, Cr, Fe, and Ga, is a prototypical family of flexible MOFs, exhibiting a reversible, structural transition (breathing) from large pore (LP) to narrow pore (NP) configurations upon hydration-dehydration, respectively. Generally, the structure of MIL-53(M), is composed of connections of corner-sharing MO4(OH)2 octahedra connected by 1,4-benzenedicarboxylic (BDC) acids. The channels of as- synthetized MIL-53 are filled with disordered BDC and H2O molecules, revealing the NP form of MIL-53 (Al, Cr), which is produced hydrothermally and is referred to as MIL-53 as. The hydrogen-bond interactions between the oxygen atoms of the carboxylic group and the 2-hydroxo group, as well as the hydrogen atoms of the water molecules, are what give rise to this NP structure [5]. Due to the lack of contact, the MIL-53 develops a LP kind of porous structure when dehydrated at high temperatures. Temperature changes, in addition to guest molecules, can cause the framework transition, which results in the dehydration-rehydration cycle. MIL-53 has been demonstrated to exhibit both the LP and NP forms at high and low temperatures (>300 K and 300 K), respectively. The framework transition takes place through two different mechanisms in the absence of van der Waals force interactions between the adsorbent and the adsorbate: (1) twisted benzene groups of benzenedicarboxylate (BDC) ligands, also known as " π-flipping," and (2) distortion mode from the corner- sharing octahedral MO6 (M = Al, Cr) clusters [6]. This work focuses on photophysical characteristics of MIL-53(Al) that includes important properties: it is stable, highly active, and well suited for CO2/CH4 separation, to exhibit good adsorption behavior of organic dyes [7], and to be fluorescent sensors for Fe3+ [8]. Additionally, Al carboxylate-based MOFs were demonstrated to be photoresponsive and to exhibit photocatalytic activity towards RhB degradation and CO2 reduction [9]. Herein, we investigated the photoluminescence (PL) and structural properties of MIL-53(Al) under different stimuli, such as temperature and mechanical pressure, and based on which, selectivity and reusability tests of this material are also performed under different heating-cooling cycles, respectively. EXPERIMENTAL RESULTS We prepared different pellets of MIL-53(Al)-activated powder using mechanical hydraulic press with the pressure ranging from 0.044 to 0.22 GPa and then analyzed their PL properties comparing with the powder samples. Time-resolved PL spectra were carried out under a tunable laser excitation, provided by an optical parametric oscillator (VIBRANT OPOTEK) pumped by the third harmonic (3.49 eV) of a Nd:YAG laser (pulse width 5 ns, repetition rate 10 Hz). The emitted light was analyzed by a monochromator equipped with a grating of 150 lines/mm and blaze wavelength 300 nm,and acquired by an intensified CCD camera driven by a delay generator (PIMAX Princeton Instruments) setting the acquisition time window, TW, and the delay, TD, with respect to the arrival of laser pulses. All the emission spectra were detected with a bandwidth of 10 nm and corrected for the monochromator dispersion. We demonstrated that under UV excitation at 305 nm, MIL-53(Al) exhibits two emission bands, both decaying in a ns timescale. The first band is centered at 393 nm and is associated with an Intra-ligand charge transfer mechanism of the BDC linkers; the second is peaked at 452 nm and is related to LMCT transition. When subjected to temperature changes, this luminescent MOF exhibits a unique Ratiometric fluorescence behavior. As shown in Figure 1, the emission at 393 nm is quenched when the sample is heated up to 400 K. It is worth noting that this thermochromic response exhibits reversible relationship of emission intensity with respect to temperature which is shown to be reproducible. Moreover, under higher mechanical stress MIL-53(Al) displays turn-on behavior in PL emission intensity, which also restored towards the original intensity once after the removal of stress, hence offering a thrilling avenue for the application in mechanically deformed-based luminescent sensors. Figure 1 – PL emission spectra of MIL-53(Al) powder recorded under UV excitation at 300 to 400K (in left) and, in right PL emission spectra of MIL-53(Al) powder + pellets pressed at pressure, P = 0.12 to 0.22 GPa In conclusion, the reported results evidence the stimuli-responsive properties of MIL-53(Al) and are relevant in the development of MOFs based sensors. REFERENCES [1] Lee, J.; Farha, O.K.; Roberts, J.; Scheidt, K.A.; Nguyen, S.T.; Hupp, J.T. “Metal-organic framework materials as catalysts”. Chem. Soc. 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