Materials Synthesis Athina Anastasaki,†,‡ Vasiliki Nikolaou,† Gabit Nurumbetov,† Paul Wilson,†,‡ Kristian Kempe,†,‡ John F. Quinn,‡ Thomas P. Davis,†,‡ Michael R. Whittaker,†,‡ and David M. Haddleton*,†,‡ †Chemistry Department, University of Warwick, Library Road, CV4 7AL, Coventry, United Kingdom ‡ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, Monash Institute of Pharmaceutical Sciences, Monash University (Parkville Campus), 399 Royal Parade, Parkville, Victoria 3152, Australia

Cu(0)-Mediated Living Radical Polymerization: A Versatile Tool for Materials Synthesis

Lignocellulose, the main component of agricultural and forestry waste, harbours tremendous potential as a renewable starting material for future biorefinery practices. However, this potential remains largely unexploited due to the lack of strategies that derive substantial value from its main constituents. Here, we present a catalytic strategy that is able to transform lignocellulose to a range of attractive products. At the centre of our approach is the flexible use of a non-precious metal catalyst in two distinct stages of a lignocellulose conversion process that enables integrated catalyst recycling through full conversion of all process residues. From the lignin, pharmaceutical and polymer building blocks are obtained. Notably, among these pathways are systematic chemo-catalytic methodologies to yield amines from lignin. The (hemi)cellulose-derived aliphatic alcohols are transformed to alkanes, achieving excellent total carbon utilization. This work will inspire the development of fully sustainable and economically viable biorefineries.

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Complete lignocellulose conversion with integrated catalyst recycling yielding valuable aromatics and fuels

A class of tunable visible and near-infrared donor–acceptor Stenhouse adduct (DASA) photoswitches were efficiently synthesized in two to four steps from commercially available starting materials with minimal purification. Using either Meldrum’s or barbituric acid “acceptors” in combination with aniline-based “donors”, an absorption range spanning from 450 to 750 nm is obtained. Additionally, photoisomerization results in complete decoloration for all adducts, yielding fully transparent, colorless solutions and films. Detailed investigations using density functional theory, nuclear magnetic resonance, and visible absorption spectroscopies provide valuable insight into the unique structure–property relationships for this novel class of photoswitches. As a final demonstration, selective photochromism is accomplished in a variety of solvents and polymer matrices, a significant advantage for applications of this new generation of DASAs.

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Tunable Visible and Near Infrared Photoswitches

Molecular photoswitches comprise chromophores that can be interconverted reversibly with light between two states with different photochemical and physicochemical properties. This feature renders them useful for diverse applications, ranging from materials science, biology (specifically photopharmacology) to supramolecular chemistry. With new and more challenging systems to control, especially extending towards biomedical applications, using visible or near-infrared light for photoswitch activation becomes vital. Donor-acceptor Stenhouse adducts are a novel class of visible light-responsive negative photochromes that provide a possible answer to current limitations of other photoswitch classes in the visible and NIR window. Their rapid development since their discovery in 2014, together with first successful examples of applications, demonstrate both their potential and areas where improvements are needed. A better understanding of DASA characteristics and its photoswitching mechanism has revealed that they are in fact a subset of a more general structural class of photochromes, namely Stenhouse photoswitches. This tutorial review aims at providing an introduction and practical guide on DASAs: it focuses on their structure and synthesis, provides fundamental insights for understanding their photoswitching behavior and demonstrates guiding principles for tailoring these switches for given applications.

The (photo)chemistry of Stenhouse photoswitches: guiding principles and system design.

Aryl halides are very useful electrophiles for synthesizing various substituted aromatic compounds via metal-catalyzed cross-coupling reactions. Because of the high cost associated with their synthesis and the stoichiometric halide waste produced when using aryl halide feedstocks, cheaper and more sustainable alternatives have been explored, such as phenols. However, phenols have a very reactive hydroxyl group and a C–O bond with high dissociation energy. To overcome such challenges, earlier studies focused on finding ways to reduce the energy of the C–O bond while removing the active proton by transforming phenols into phenol derivatives (e.g., sulfonates, esters, carbamates, ethers, and metal salts). A greater ambition is to directly cross-couple phenols with nucleophiles via C–O cleavage. In this Perspective, we briefly summarize efforts and accomplishments concerning the cross-coupling of phenol derivatives and phenols.

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An Adventure in Sustainable Cross-Coupling of Phenols and Derivatives via Carbon–Oxygen Bond Cleavage

We present a crosslinked polyurethane elastomer featuring a thermochromic molecular sensor for local temperature analysis. The thermochrome is a modified donor-acceptor Stenhouse adduct (DASA) that was dispersed homogeneously into the polymer blend in minuscule amounts. Rapid temperature jump measurements in a pyroprobe and impacts in a Hopkinson bar show that the DASA has suitable kinetics for detecting localized temperature increase following impact or rapid heating. The thermochrome retains a signature of the peak temperature in the elastomer, allowing post-mortem mapping of micron-scale temperature localization in materials such as explosive and propellant composites. We demonstrate the concept by using the kinetics of the DASA activation to determine peak temperatures reached during bullet perforation of the polyurethane.

A temperature-mapping molecular sensor for polyurethane-based elastomers

https://escholarship.org/content/qt6qn8w37j/qt6qn8w37j.pdf?t=o94807

Triazine-mediated controlled radical polymerization: new unimolecular initiators

A one-pot synthesis of anilines and nitrosobenzenes from phenols has been developed using an ipso-oxidative aromatic substitution ((i)SOAr) process. The products are obtained in good yields under mild and metal-free conditions. The leaving group effect on reactions that proceed through mixed quionone monoketals has also been investigated and a predictive model has been established.

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Direct synthesis of anilines and nitrosobenzenes from phenols

The development of mechanochromic molecules has opened new pathways for the study of localized stress and failure in polymers. Their application as stress or temperature diagnostics, however, requires suitable measurement techniques capable of detecting the force- and temperature-sensitive chemical species with high spatial resolution. Confocal imaging techniques offer excellent spatial resolution but the energy input during these measurements can itself affect the activation state of the mechanochromic species. Here, we present a systematic study of the effects of laser-based imaging on the activation and fluorescence behavior of mechanochromic spiropyran (SP) integrated into poly(methyl acrylate) (PMA) and poly(methyl methacrylate) matrices using a confocal Raman microspectrometer. Localized stress and temperature activation were studied by means of high-rate compressive loading and dynamic fracture. Laser illumination of SP in PMA revealed a strong excitation wavelength- and power-dependence. Suitable correction functions were established and used to account for the observed laser effects. The presented study demonstrates that confocal imaging using conventional Raman spectrometers is a powerful characterization tool for localized stress analysis in mechanochromic polymers, offering quantifiable information on the activation state with high spatial resolution. However, laser-mechanophore interactions must be well understood and effects of laser excitation and exposure times must be taken into consideration when interpreting the obtained results.

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J. Read de Alaniz

Papers

A temperature-mapping molecular sensor for polyurethane-based elastomers

Triazine-mediated controlled radical polymerization: new unimolecular initiators

Direct synthesis of anilines and nitrosobenzenes from phenols

Optical characterization and confocal fluorescence imaging of mechanochromic acrylate polymers