Synthesis, Characterization and Application of New Functional Materials

This thesis comprises six chapters that deal with the synthesis, characterization, and application of new functional materials. The introduction is divided into two parts. The first gives a short overview of the definition and classification of polymers leading focus to stimuli-responsive polymers. The second part gives a brief overview of the classification and the functionality of gels. It provides a deeper insight into the literature of their use in chemistry as a confinement nanoreactor material. The first chapter describes the outcome of photosensitized [2 + 2]-cycloaddition reactions of various cinnamates compared in different reaction media, including homogeneous organic solutions under inert conditions, degassed water, and aerated physical gels. The reactions were performed under LED blue light (λmax = 455 nm) irradiation and [Ir{dF(CF3)ppy}2(dtb-bpy)]PF6 (1.0 mol%) as photocatalyst. The processes were optimized taking into consideration solvent, gelator, and substrate. Comparative kinetics analyses, as well as the effect of the reaction media on the diastereoselectivity of the process, were evaluated during this investigation. In a number of cases, carrying out the reaction in a less polar solvent, like toluene or highly polar solvent, like water had a tremendous impact on the diastereoselectivity of the process, pointing towards an effect on the stabilization of the putative diradical intermediate in this medium. The next five chapters are dealing with the synthesis, characterization, and functionalization of (bio)polymer materials. The second chapter describes a new methodology for the pH-triggered degradation of polymers or for the release of drugs under visible light irradiation based on the cyclization of ortho-hydroxy-cinnamates (oHC) to coumarins. The key oHC structural motif can be readily incorporated into the rational design of novel photocleavable polymers via click chemistry. This main-chain moiety undergoes a fast photocleavage when irradiated with 455 nm light provided that a suitable base is added. A series of polyethylene glycol-alt-ortho-hydroxy cinnamate (polyethylene glycol (PEG)n-alt-oHC)-based polymers were synthesized and the time-dependent visible-light initiated cleavage of the photoactive monomer and polymer was investigated in solution by a variety of spectroscopic and chromatographic techniques. The photo-degradation behavior of the water-soluble poly(PEG2000-alt-oHC) was investigated within a broad pH range (pH = 2.1-11.8), demonstrating fast degradation at pH 11.8, while the stability of the polymer is greatly enhanced at pH 2.1. Moreover, the neat polymer showed long-term stability under daylight conditions, thus allowing its storage without special precautions. In addition, two water-soluble PEG-based drug-carrier molecules (mPEG2000-oHC-benzhydrol/phenol) were synthesized and used for drug delivery studies, monitoring the process by UV-vis spectroscopy in an ON/OFF intermittent manner. The third chapter describes a novel methacrylate-based cross-linked polymer gel bearing an iridium photocatalyst that showed air tolerance and pumping recyclability features through its tunable swelling and deswelling ability. The photocatalytic activity of the polymer gel was demonstrated in an E-to-Z isomerization reaction and in an azide-alkene [2 + 3] cycloaddition. The fourth chapter describes the use of biopolymers, such as sodium alginate, as a sustainable adhesive binder for several metals and high-density polyethylene. A standard pull test and peel test was performed with disks, made of different material and size. Adhesive failure was investigated by varying the amount of applied alginate solution, drying time, drying temperature, the effect of surface area, and the nature of the adherend. Alginate adhesion was remarkably strong, relatively general, and sensitive to the presence of water. A brief comparison with other biopolymers is also provided. The fifth chapter describes the design and synthesis of a new series of hydrogel membranes composed of trialkyne derivatives of glycerol ethoxylate and bisphenol A diazide (BA-diazide) or diazide-terminated PEG600 monomer via a Cu(I)-catalyzed photoclick reaction. The water-swollen hydrogel membranes displayed thermoresponsive actuation and their lower critical solution temperature (LCST) values were determined by differential scanning calorimetry. Glycerol ethoxylate moiety served as the thermoresponsive component and hydrophilic part, while the azide-based component acted as the hydrophobic comonomer and most likely provided a critical hydrophobic/hydrophilic balance contributing also to the significant mechanical strength of the membranes. These hydrogels exhibited a reversible shape-memory effect in response to temperature through a defined phase transition. The swelling and deswelling behavior of the membranes are systematically examined. Due to the click nature of the reaction, easy availability of azide and alkyne-functional monomers, and the polymer architecture, the glass transition temperature (Tg) was easily controlled through monomer design and crosslink density by varying the feed ratio of different monomers. The mechanical properties of the membranes were studied by universal tensile testing measurements. Moreover, the hydrogels showed the ability to absorb a dye and release it in a controlled manner by applying heat below and above the LCST. The last chapter describes the design and synthesis of a water-soluble methionine (methionine methacryloyloxyethyl ester (METMA)) and poly(ethylene glycol methyl ether methacrylate) containing block-copolymer via reversible addition-fragmentation chain transfer (RAFT) polymerization and the attachment of the CORM (tricarbonyldichlororuthenium(II) dimer [Ru2Cl4(CO)6]) in the methionine side chain units. Inductively coupled plasma optical emission spectrometry (ICP-OES) and FT-IR confirmed the presence of CORM molecules in the polymer. The time-dependent CO release from CORM conjugated block-copolymer was investigated by a myoglobin assay. This CORM conjugated block-copolymer slowly and spontaneously released CO with sustained-release kinetics. Moreover, this CO-releasing polymer was able to prevent biofilm formation against Pseudomonas aeruginosa

Recyclable, Immobilized Transition-Metal Photocatalysts

[EN] The use of transition-metal complexes as photocatalysts have allowed the performance of diverse organic transformations in an outstanding manner, characterized not only by high yields, TOF, and selectivity values, but also by modulating and providing access to novel molecular structures that, without them, would be difficult if not impossible. However, one of the biggest concerns regarding the use of these photocatalysts relies on the difficulties associated with their isolation from reaction media and reutilization once the chemical process ends. The above, besides contaminating reaction products and requiring out tedious purification processes, prompts the inevitable loss of the catalyst, directly affecting its recyclability. In addition, this situation results in negative outcomes from an economic and environmental perspective, since transition-metal complexes are usually expensive materials, and their unsuccessful recovery could result in leakage into the environment. These drawbacks served as inspiration for the elaboration of the present review focused on the development of novel strategies developed during the last decade for the successful recovery of these species. The strategies summarized herein, whether for homogeneous or heterogeneous systems, are based on the use of thermotropic solvents, changes in the hydrophilic balance of the catalyst, the employment of polymers, copolymers, porous macromolecular structures, and inorganic nanostructures as support of these entities. Moreover, the use of organized and confining media, such as micelles and gels in this context, is also discussed. We hope that this review will motivate the search for new strategies to develop novel catalytic systems, understanding that high performance is based not only on yields but also on recyclability, sustainability, and responsibility to the environment.Financial support from the Universitat Regensburg, the Elitenetzwerk Bayern (SYNCAT), the Spanish Ministry of Science and Innovation (PID2019-105391GB-C21, PID2019-105391GB-C22), is gratefully acknowledged. S.B. thanks MINECO for a Juan de la Cierva-Formacion contract (FJC2019-039515-I). R. P.-R. thanks Generalitat Valenciana for a Distinguished Researcher contract (CIDEGENT/2018/044). D.D.D. thanks the DFG for the Heisenberg Professorship Award and the Spanish Ministry of Science, Innovation and Universities for the Senior Beatriz Galindo Award (Distinguished Researcher, BEAGAL18/00166). D.D.D. also thanks NANOtec, INTech, Cabildo de Tenerife and ULL for laboratory facilities.Pérez-Ruiz, R.; Abramov, A.; Bonardd, S.; Díaz Díaz, D. (2022). Recyclable, Immobilized Transition-Metal Photocatalysts. Advanced Synthesis & Catalysis. 364:2-17. https://doi.org/10.1002/adsc.20210104821736

Effect of Reaction Media on Photosensitized [2+2]- Cycloaddition of Cinnamates

The outcome of photosensitized [2+2]-cycloaddition reactions of various cinnamates has been compared in different reaction media, including homogeneous organic solutions under inert conditions, degassed water, and aerated physical gels. The reactions were performed under LED blue light (λmax=455 nm) irradiation and [Ir{dF(CF3)ppy}2(dtb-bpy)]PF6 (1.0 mol%) as photocatalyst. The processes were optimized taking into consideration solvent, gelator, and substrate. Comparative kinetics analyses, as well as the effect of the reaction media on the diastereoselectivity of the process, were evaluated during this investigation. In a number of cases, carrying out the reaction in a less polar solvent, like toluene or highly polar solvent, like water had a tremendous impact on the diastereoselectivity of the process, pointing towards an effect on the stabilization of the putative diradical intermediate in this medium. Moreover, while for reactions run in homogeneous solution oxygen needs to be excluded, no erosion in yield is observed when the photoadditions were run in aerated gel media.Deutsche ForschungsgemeinschaftElitenetzwerk BayernMinisterio de Ciencia, Innovación y Universidade

Aerobic Visible-Light-Driven Borylation of Heteroarenes in a Gel Nanoreactor

[EN] Heteroarene boronate esters constitute valuable intermediates in modern organic synthesis. As building blocks, they can be further applied to the synthesis of new materials, since they can be easily transformed into any other functional group. Efforts toward novel and efficient strategies for their preparation are clearly desirable. Here, we have achieved the borylation of commercially available heteroarene halides under very mild conditions in an easy-to-use gel nanoreactor. Its use of visible light as the energy source at room temperature in photocatalyst-free and aerobic conditions makes this protocol very attractive. The gel network provides an adequate stabilizing microenvironment to support wide substrate scope, including furan, thiophene, selenophene, and pyrrole boronate esters.Financial support from the Generalitat Valenciana (CIDEGENT/2018/044) and the Spanish Ministry of Science and Innovation (PID2019-105391GB-C21, PID2019-105391GBC22, BEAGAL18/00166, and BES-2017-080215) is gratefully acknowledged. We thank the Electron Microscopy Service from the UPV and Prof. Julia Perez-Prieto for spectroscopy facilities. D.D.D. also thanks NANOtec, INTech, Cabildo de Tenerife, and ULL for laboratory facilities.Herrera-Luna, JC.; Díaz Díaz, D.; Abramov, A.; Encinas Perea, S.; Jiménez Molero, MC.; Pérez-Ruiz, R. (2021). Aerobic Visible-Light-Driven Borylation of Heteroarenes in a Gel Nanoreactor. Organic Letters. 23(6):2320-2325. https://doi.org/10.1021/acs.orglett.1c004512320232523

Biopolymers as Sustainable Metal Bio-Adhesives

We describe the use of biopolymers, such as sodium alginate, as an sustainable adhesive binder for several metals and high‐density polyethylene. A standard pull test and peel test was performed with disks, made of different material and size. Adhesive failure was investigated by varying the amount of applied alginate solution, drying time, drying temperature, effect of surface area, and the nature of the adherend. Alginate adhesion was remarkably strong, relatively general, and sensitive to the presence of water. A brief comparison with other biopolymers is also provided

Biopolymers as sustainable metal bio-adhesives

We describe the use of biopolymers, such as sodium alginate, as an sustainable adhesive binder for several metals and high-density polyethylene. A standard pull test and peel test was performed with disks, made of different material and size. Adhesive failure was investigated by varying the amount of applied alginate solution, drying time, drying temperature, effect of surface area, and the nature of the adherend. Alginate adhesion was remarkably strong, relatively general, and sensitive to the presence of water. A brief comparison with other biopolymers is also provided.Universität RegensburgUniversidad de La LagunaDeutsche ForschungsgemeinschaftMinisterio de Ciencia, Innovación y Universidade

Thermoresponsive Shape-Memory Hydrogel Actuators Made by Phototriggered Click Chemistry

This article describes the design and synthesis of a new series of hydrogel membranes composed of trialkyne derivatives of glycerol ethoxylate and bisphenol A diazide (BA-diazide) or diazide-terminated PEG600 monomer via a Cu(I)-catalyzed photoclick reaction. The water-swollen hydrogel membranes display thermoresponsive actuation and their lower critical solution temperature (LCST) values are determined by differential scanning calorimetry. Glycerol ethoxylate moiety serves as the thermoresponsive component and hydrophilic part, while the azide-based component acts as the hydrophobic comonomer and most likely provides a critical hydrophobic/hydrophilic balance contributing also to the significant mechanical strength of the membranes. These hydrogels exhibit a reversible shape-memory effect in response to temperature through a defined phase transition. The swelling and deswelling behavior of the membranes are systematically examined. Due to the click nature of the reaction, easy availability of azide and alkyne functional-monomers, and the polymer architecture, the glass transition temperature (Tg) is easily controlled through monomer design and crosslink density by varying the feed ratio of different monomers. The mechanical properties of the membranes are studied by universal tensile testing measurements. Moreover, the hydrogels show the ability to absorb a dye and release it in a controlled manner by applying heat below and above the LCST.Universität RegensburgAgència de Gestió d’Ajuts Universitaris i de RecercaB. Braun Surgical, S.A.Ministerio de Ciencia, Innovación y Universidade

Hydrogen‐Bond‐Modulated Nucleofugality of SeIII Species to Enable Photoredox‐Catalytic Semipinacol Manifolds

Chemical bond activations mediated by H-bond interactions involving highly electronegative elements such as nitrogen and oxygen are powerful tactics in modern catalysis research. On the contrary, kindred catalytic regimes in which heavier, less electronegative elements such as selenium engage in H-bond interactions to co-activate C−Se σ-bonds under oxidative conditions are elusive. Traditional strategies to enhance the nucleofugality of selenium residues predicate on the oxidative addition of electrophiles onto SeII-centers, which entails the elimination of the resulting SeIV moieties. Catalytic procedures in which SeIV nucleofuges are substituted rather than eliminated are very rare and, so far, not applicable to carbon-carbon bond formations. In this study, we introduce an unprecedented combination of O−H⋅⋅⋅Se H-bond interactions and single electron oxidation to catalytically generate SeIII nucleofuges that allow for the formation of new C−C σ-bonds by means of a type I semipinacol process in high yields and excellent selectivity

Visible‐Light‐Triggered Degradation of pH‐Responsive Micelles Based on ortho‐Hydroxy Cinnamates

Micelles made by connecting hydrophobic and hydrophilic chains via photo-responsive ortho-hydroxy cinnamate (oHC) units undergo complete degradation within 10 min under visible light irradiation at basic pH values. This proof of concept relies on the controlled cyclization of the oHC moiety affording the corresponding coumarin with the concomitant separation of the hydrophobic portion. Furthermore, the neat polymeric micelles are stable under daylight conditions, thus allowing its storage without special precautions at neutral pH. The amphiphilic nature of mPEG(750)-oHC-dociecyl, expressed as the ability to self-assemble into micellar aggregates in aqueous media, was corroborated by the measurement of its critical micellar concentration (CMC) by fluorescence analysis, as well as the successful visualization of these aggregates using transmission electron microscopy (TEM). Moreover, the size distribution and suspension stability of these aggregates obtained at different pH values was analyzed in terms of dynamic light scattering (DLS) and zeta potential measurements