Abstract
The entrapping of physicochemical active molecules inside mesoporous networks is an appealing field of research due to the myriad of potential applications in optics, photocatalysis, chemical sensing, and medicine. One of the most important reasons for this success is the possibility of optimizing the properties that a free active species displays in solution but now trapped inside a solid substrate. Additionally it is possible to modulate the textural characteristics of substrates, such as pore size, specific surface area, polarity and chemical affinity of the surface, toward the physical or chemical adhesion of a variety of adsorbates. In the present document, two kinds of non-silicon metal alkoxides, Zr and Ti, are employed to prepare xerogels containing entrapped tetrapyrrolic species that could be inserted beforehand in analogue silica systems. The main goal is to develop efficient methods for trapping or binding tetrapyrrole macrocycles inside TiO2 and ZrO2 xerogels, while comparing the properties of these systems against those of the SiO2 analogues. Once the optimal synthesis conditions for obtaining translucent monolithic xerogels of ZrO2 and TiO2 networks were determined, it was confirmed that these substrates allowed the entrapment, in monomeric form, of macrocycles that commonly appear as aggregates within the SiO2 network. From these experiments, it could be determined that the average pore diameters, specific surface areas, and water sorption capacities depicted by each one of these substrates, are a consequence of their own nature combined with the particular structure of the entrapped tetrapyrrole macrocycle. Furthermore, the establishment of covalent bonds between the intruding species and the pore walls leads to the obtainment of very similar pore sizes in the three different metal oxide (Ti, Zr, and Si) substrates as a consequence of the templating effect of the encapsulated species.
Highlights
During the last decades of the 20th century, the sol-gel process emerged as a new and interesting option for synthesizing and producing new types of hybrid materials [1]
The addition of some acetylacetone is necessary to lower the rates of the hydrolysis and polycondensation reactions, through the formation of the respective metallic acetylacetonate intermediates (M(OPrn)3·acac adduct species), which allowed the formation of translucent xerogel pore networks with the macrocyclic species trapped in stable and monomeric forms
This last supposition was verified by monitoring the UV-Vis spectrum of the (OH)AlTSPc molecules during all the sol-gel synthesis process until the final ZrO2 pore network was achieved
Summary
During the last decades of the 20th century, the sol-gel process emerged as a new and interesting option for synthesizing and producing new types of hybrid materials [1]. The intensities of the UV-vis and fluorescence spectroscopic signals of the trapped macrocycle depend on the identity and size of the alkyl or aryl groups attached to the pore walls [39,40] Through this combined trapping strategy, it has been possible to optimize the fluorescence of synthetic tetrapyrrolic species, as for instance the free bases of ortho-, meta- or para-substituted meso-tetraphenylporphyrins (H2T(o-, mor p-S)PP) (Figure 1c), as well as the coordination and fluorescence of natural tetrapyrrolic species, such as chlorophyll a [41]. Due to all the above characteristics, in this contribution we will explore the possibility of synthesizing hybrid materials containing physically trapped or chemically bonded tetrapyrrolic macrocycles inside non-siliceous networks, created through the sol-gel method. Our investigation will attempt to propose an explanation with respect to the observed differences existing among the three systems and most importantly, to suggest the best substrates to use for preserving the properties of different chemical species when trapped inside xerogel networks, and those related to tetrapyrrolic macrocycles
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