Laser-based spectroscopy and fluorescence microscopy of metal-organic frameworks and their composites: relevance to photonics and photocatalysis
- Autores: Elena Caballero Mancebo
- Directores de la Tesis: Abderrazzak Douhal (dir. tes.), Boyko Yuda Koen (codir. tes.)
- Lectura: En la Universidad de Castilla-La Mancha ( España ) en 2020
- Idioma: español
- Tribunal Calificador de la Tesis: Johan Hofkens (presid.), Juan Ángel Organero Gallego (secret.), Cristina Flors Ong (voc.)
- Programa de doctorado: Programa de Doctorado en Nanociencia y Nanotecnología por la Universidad de Alicante; la Universidad de Castilla-La Mancha; la Universidad de La Laguna; la Universidad Jaume I de Castellón y la Universitat de València (Estudi General)
- Materias:
- Enlaces
- Tesis en acceso abierto en: RUIdeRA
- Resumen
Metal-organic frameworks (MOFs) are hybrid porous materials formed by coordination bonding between organic ligands and metal ions or clusters. Their rich and tuneable structural and photodynamical properties make them ideal materials in cutting-edge devices in photonics, catalysis and medicine. The aim of this Ph.D. thesis is to elucidate the photophysical properties of selected MOFs using fast and ultrafast time-resolved spectroscopic techniques and fluorescence lifetime imaging microscopy.
First, a brief overview of the photophysical behaviour of the MOFs is explained, remarking the relevance of the materials composition and structure on their resulting photoproperties. In addition, some photo-applications of these materials are also shown. The results of the research carried out first show studies on how the interaction of Nile Red (NR) with the 2D M-ITQ-R family of MOFs affects its photophysical behaviour. When NR is encapsulated within the Al-ITQ-HB (R = HB, heptane benzoic acid), its photodynamics is affected by two MOF environments characterized by different polarities: a) when it is closed to the metal centres they undergo an intramolecular charge transfer reaction in ~1 ps from its locally excited state, while b) when it is close to the apolar linkers, the energy barrier for this reaction is higher and the reaction does not take place. The study of single crystals under the confocal fluorescence microscope reveals a homogenous distribution of the emitters within the composites, and it confirms the encapsulation of NR molecules within the material. The adsorption of NR molecules on Al-ITQ-HB surface favours efficient energy transfer processes between neighbouring dye molecules. This photoevent is affected by NR concentration, varying from ~2 to ~0.5 ns when the concentration is largely increased. This photoreaction becomes faster when the length of the alkyl chain of the MOF linker decreases in the NR@Al-ITQ-EB composite (R = EB, ethyl benzoic acid). This is explained in terms of a higher polarity of this material and translated in stronger dye-dye interactions. Finally, when the substituent group is changed in the linker, Al-ITQ-AB composites (R = AB, amino benzoic acid), the energy transfer is inhibited due to an electron transfer from the amino group of the linkers to the adsorbed dye molecules. We also studied the effect of the metal cluster nature on the photobehaviors of the formed composites, by replacing the Al atoms by Zr ones. This exchange leads to a slower photodynamics due to a higher organic content in the MOF. These results show how changes in the structure of the 2D-MOF affect the photobehavior of organic molecules, opening possibilities for research on the photoinduced processes in relevant molecules supported on this 2D-MOF family and their application in photocatalysis and photonics.
Secondly, the effect of the topology of the framework on the photophysical properties of the MOFs was studied in two Ce-based MOFs. Both, Ce-NU-1000 and Ce-CAU-24-TBAPy, have the same linkers and metal clusters, but present different structures, resulting in different cluster orientations, and different distances and angles between the linkers. The studies show the presence of charge transfer reactions between the ligands and the metal clusters, as well as within the linker itself. Ce-NU-1000 MOF exhibits excimer formation, contrary to Ce-CAU-24-TBAPy. Furthermore, e--h+ recombination times in Ce-CAU-24-TBAPy (1.6 and 13.4 μs) are longer than those of Ce-NU-1000 (0.6 and 4.9 μs), which suggests different structural configurations of the e--acceptor metal clusters. Single crystals fluorescence microscopy of Ce-CAU-24-TBAPy shows the presence of defects within its framework. In addition to these promising results for the application of these materials in photocatalysis, the potential of Ce-CAU-24-TBAPy as explosive sensor was tested, revealing a high selectivity for nitroaromatic molecules.
The obtained knowledge from the previous studies and the analysis of the state-of-the-art led to the participation in the writing of a review article about the photophysics of MOFs. The review also shows that the strong interaction of MOFs with light makes them very versatile materials, with applications in a variety of scientific and technological fields, such as medicine or optoelectronics.
Finally, the photoionization of a Zr-based MOF, as well as its behaviour under the laser trapping microscope, was studied during a 3-months stay at the group of Prof. Hiroshi Miyasaka at the University of Osaka (Japan).
