Luminescent Properties of Breathing MIL-53(Al) MOF for Sensing Applications

Metal organic frameworks" (MOFs) are a novel family of crystalline microporous materials that have a very high surface area with customizable functions of great impact in the field of green and renewable energies as a media of gas storages, chemical sensors, drug delivery systems [1]. While, in general, coordination bond between organic moieties and metal nodes provides structural integrity. Only some MOFs have a flexible structure, which can cause the breathing phenomenon or gate-opening effect: when external stimuli like pressure, temperature and solvents are added to or removed from MOFs, the pore diameter changes [2]. The uniqueness of MOF breathing attracts great interest both in fundamental and applied research, the tunable pore sizes are, for example, of practical use for separating and purifying the adsorbed gases. One of the main families of flexible MOFs is the MIL- 53(M) series, with M = Al, Sc, Cr, Fe, and Ga, that show a reversible structural transition (breathing) from large to narrow pore upon hydration-dehydration, respectively. It has been proposed that this 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 ligands, also known as " π-flipping," and (2) distortion mode from the corner- sharing octahedral MO6 (M = Al, Cr) clusters [3]. Herein, we deal with the MIL-53(Al) and we focus our attention on its ratiometric photoluminescence (PL) behavior under external stimuli, such as temperature and mechanical pressure. To this aim, we studied different samples, both in the form of powder and pellets obtained with a mechanical hydraulic press. 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 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. Our results showed that under UV excitation at 305 nm, MIL-53(Al) emits two PL bands, centered at 393 nm (violet) and 452 nm (blue), both decaying in a ns timescale. On varying the temperature from 300 up to 400 K, the violet PL is quenched while the intensity of the other band remains almost constant. The decrease of the violet band is accompanied by a shortening of its lifetime, due to the activation of non-radiative channels from the excited state. It is worth noting that this thermochromic response shows a reversible relationship of the emission intensity when the temperature is decreased from 400 to 300 K. Moreover, also under mechanical stress the emission at 393 nm changes: its intensity increases when the mechanical pressure increases up 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 thermally and mechanically deformed-based luminescent MOF sensors. References [1] Khan, N.A.; Hasan, Z.; Jhung, S.H. “Adsorptive removal of hazardous materials using metal- organic frameworks (MOFs): A review”. J. Hazard. Mater., 244–245, 444–456 (2013). [2] Beurroies, I.; Boulhout, M.; Llewellyn, P.L.; Kuchta, B.; Férey, G.; Serre, C.; Denoyel, R. “Using pressure to provoke the structural transition of metal-organic frameworks”. Angew. Chem. Int. Ed., 49, 7526–7529 (2010). [3] Alhamami, M.; Doan, H.; Cheng, C.-H. “A Review on Breathing Behaviors of Metal-Organic- Frameworks (MOFs) for Gas Adsorption”. Materials, 7, 3198-3250 (2014).