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

Abstract

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 arri