Regiochemical Substituent Switching of Spin States in Aryl(trifluoromethyl)carbenes

Design, Exploitation, and Rational Improvements of Diazirine-Based Universal Polymer Crosslinkers

ConspectusAddition of new covalent bonds between the chains of thermoplastic polymers (i.e., crosslinking) provides improved mechanical strength and enhanced high-temperature performance while also providing an effective strategy for photopatterning. Traditionally, however, crosslinking of each polymer substrate has required the use of a specific crosslinking technology (hydrosilylation for PDMS, vulcanization for rubber, etc.). The lack of a general solution to the challenge of polymer crosslinking means that there are many thermoplastics (e.g., polypropylene or polyhydroxyalkanoates) that have desirable properties, but which cannot be upgraded by traditional crosslinking technologies.Our lab developed the first universal crosslinkers for aliphatic polymers by leveraging trifluoromethyl aryl diazirine motifs, functional groups that have been widely used in chemical biology for >30 years, but which have seldom been exploited in materials science. These novel reagents work (via C-H insertion) on essentially any commodity polymer that contains aliphatic C-H bonds, including industrial plastics like polypropylene (the crosslinking of which has been an outstanding challenge in the field for >50 years), as well as commercially important elastomers (e.g., polydimethylsiloxane), biodegradable polymers (e.g., polycaprolactone), and green polymer materials derived from biomass (e.g., polyhydroxyalkanoates).Subsequent structure-function work from our group led to crosslinkers that were >10-fold more effective in undergoing C-H insertion with aliphatic substrates. We then developed an improved synthesis of our electronically optimized diazirines and incorporated them into a family of cleavable crosslinker reagents, which permit the on-demand generation of reprocessable thermosets. At the same time, other groups replaced the perfluoropropyl linker in our first-generation crosslinker with a series of dynamic linkages; these permit the ready generation of vitrimeric materials and can be used in the reactive compatibilization of immiscible plastic waste.Since the publication of our initial Science paper in 2019, this burgeoning field of diazirine-based polymer crosslinkers has experienced an explosion of interest. Publications from our lab and others have described the use of these reagents in covalent adhesion, photopatterning of low dielectric materials for microelectronics, and direct optical printing of quantum dots. Our crosslinkers have also been shown to heighten the robustness of ice-phobic coatings and improve the performance of woven ballistic fabric, while─perhaps most unexpectedly─substantially improving the stability of high-performance perovskite solar cells. Electronically optimized diazirines can also be used to covalently link proteins to polymer surfaces, suggesting a broad range of applications in the biocompatibilization of medical devices. This Account will summarize the development of trifluoromethyl aryl diazirine reagents for materials science over the past 5 years. A brief comparison will also be made, in the Summary and Outlook section at the end of the Account, to competing (and often complementary) reagents based upon azide and diazoalkyl motifs. Finally, we have compiled a Frequently Asked Questions list that covers many practical aspects of crosslinker design and application; this is appended as Supporting Information.

Crosslinking Vinyl-Addition Polynorbornenes via Difunctional Diazirines to Generate Low Dielectric-Constant and Low Dielectric-Loss Thermosets

Thermosets having low dielectric constant (Dk < 3) and low dielectric dissipation factor (Df < 0.003), high glass transition temperature (Tg > 150 °C), and good adhesion to copper are desirable for the low loss layers of the copper clad laminates (CCL) in next generation printed circuit boards. Three different difunctional diazirines are evaluated for both thermal and photochemical crosslinking of a high Tg vinyl-addition polynorbornene resin: poly(5-hexyl-1-norbornene) (poly(HNB)). The substrate polymer, crosslinked by the carbenes generated from the activated diazirines, forms thermosets with Dk < 2.3 and Df < 0.001 at 10 GHz depending on the identity of the diazirine and the loading. The Dk and Df values for one composition are stable for 1600 h at 125 °C in air and for 1400 h at 85 °C and 85% relative humidity, suggesting good long-term reliability of this thermoset. Adhesion of poly(HNB) to copper can be enhanced by priming the copper surface with a diazirine prior to high temperature lamination; peel strength values of greater than 7.5 N cm-1 are achieved. Negative-tone photopatterning of poly(HNB) with diazirines upon exposure to 365 nm light is demonstrated.

Functionalization of Polydimethylsiloxane with Diazirine-Based Linkers for Covalent Protein Immobilization

Biomolecule attachment to solid supports is critical for biomedical devices, such as biosensors and implants. Polydimethylsiloxane (PDMS) is commonly used for these applications due to its advantageous properties. To enhance the biomolecule immobilization on PDMS, a novel technique is demonstrated using newly synthesized diazirine molecules for the surface modification of PDMS. This nondestructive process involves a reaction between diazirine molecules and PDMS through C-H insertion with thermal or ultraviolet activation. The success of the PDMS modification is confirmed by various surface characterization techniques. Bovine serum albumin (BSA) and immunoglobulin G (IgG) are strongly attached to the modified PDMS surfaces, and the amount of protein is quantified using iodine-125 radiolabeling. The results demonstrate that PDMS is rapidly functionalized, and the stability of the immobilized proteins is significantly improved with multiple types of diazirine molecules and activation methods. Confocal microscopy provides three-dimensional images of the distribution of immobilized IgG on the surfaces and the penetration of diazirine-based linkers through the PDMS substrate during the coating process. Overall, this study presents a promising new approach for functionalizing PDMS surfaces to enhance biomolecule immobilization, and its potential applications can extend to multimaterial modifications for various diagnostic and medical applications such as microfluidic devices and immunoassays with relevant bioactive proteins.

Electronically optimized diazirine-based polymer crosslinkers

Electronically optimized bis-diazirine crosslinkers allow aliphatic polymers to be crosslinked with up to 10-fold improved efficacy, relative to earlier designs. Activation is achieved using modest temperatures or through UV or visible light.

Structure-function relationships in aryl diazirines reveal optimal design features to maximize C-H insertion

Diazirine reagents allow for the ready generation of carbenes upon photochemical, thermal, or electrical stimulation. Because carbenes formed in this way can undergo rapid insertion into any nearby C-H, O-H or N-H bond, molecules that encode diazirine functions have emerged as privileged tools in applications ranging from biological target identification and proteomics through to polymer crosslinking and adhesion. Here we use a combination of experimental and computational methods to complete the first comprehensive survey of diazirine structure-function relationships, with a particular focus on thermal activation methods. We reveal a striking ability to vary the activation energy and activation temperature of aryl diazirines through the rational manipulation of electronic properties. Significantly, we show that electron-rich diazirines have greatly enhanced efficacy toward C-H insertion, under both thermal and photochemical activation conditions. We expect these results to lead to significant improvements in diazirine-based chemical probes and polymer crosslinkers.

Recent Advances in Chemical Biology Using Benzophenones and Diazirines as Radical Precursors

The use of light-activated chemical probes to study biological interactions was first discovered in the 1960s, and has since found many applications in studying diseases and gaining deeper insight into various cellular mechanisms involving protein-protein, protein-nucleic acid, protein-ligand (drug, probe), and protein-co-factor interactions, among others. This technique, often referred to as photoaffinity labelling, uses radical precursors that react almost instantaneously to yield spatial and temporal information about the nature of the interaction and the interacting partner(s). This review focuses on the recent advances in chemical biology in the use of benzophenones and diazirines, two of the most commonly known light-activatable radical precursors, with a focus on the last three years, and is intended to provide a solid understanding of their chemical and biological principles and their applications.

Insights into the Stability of Siloxy Carbene Intermediates and Their Corresponding Oxocarbenium Ions

Siloxy carbenes, formed thermally or photochemically from acyl silanes via a 1,2-Brook rearrangement, are intriguing reactive intermediates that partake in a range of chemical reactions. To gain further insight into the properties of this class of carbenes, the thermodynamic stabilities of a series of known siloxy carbenes were explored on the basis of hydrogenation enthalpies. Calculations were conducted at the B3LYP-D3(BJ) level (using dispersion-corrected DFT) on siloxy carbenes (X-C-OSiR₃, singlet and triplet state), oxocarbenium ions (X-CH-OSiR₃⁺), and their hydrogen addition products (X-CH₂-OSiR₃). Overall, strong correlation between singlet-triplet gaps and hydrogenation enthalpies was observed. Carbene stabilization enthalpy (CSE) values were also determined to provide additional insight into the structural features that influence the stability of siloxy carbenes.

Fishing for Drug Targets: A Focus on Diazirine Photoaffinity Probe Synthesis

Target identification is a high-priority, albeit challenging, aspect of drug discovery. Diazirine-based photoaffinity probes (PAPs) can facilitate the process by covalently capturing transient molecular interactions. This can help identify target proteins and map the ligand's interactome. Diazirine probes have even been incorporated by cellular machinery into proteins. Embarking on the synthesis of customized PAPs, containing either an aliphatic or trifluoromethyl phenyl diazirine, can be a considerable endeavor, particularly for medicinal chemists and chemical biologists new to the field. This review takes a synthetic focus, aiming to summarize available routes, propose new avenues, and illuminate recent advances in diazirine synthesis. Select examples of diazirine photoaffinity labeling applications have been included throughout to provide instructive definition of the advantages and limitations of the technology while simultaneously highlighting how these reagents can be applied in a practical sense.

Photoactivation of (p-methoxyphenyl)(trifluoromethyl)diazirine in the presence of phenolic reaction partners

Shine light on your chemistry! Irradiating 3-(4-methoxyphenyl)-3-(trifluoromethyl)-3H-diazirine in the presence of equimolar solutions of phenol and tyrosine derivatives leads to Friedel-Crafts alkylations (see scheme), which suggests a strategy for the development of "cleaner" diazirines for chemical biology.

Photoinduced carbene generation from diazirine modified task specific phosphonium salts to prepare robust hydrophobic coatings

3-Aryl-3-(trifluormethyl)diazirine functionalized highly fluorinated phosphonium salts (HFPS) were synthesized, characterized, and utilized as photoinduced carbene precursors for covalent attachment of the HFPS onto cotton/paper to impart hydrophobicity to these surfaces. Irradiation of cotton and paper, as proof of concept substrates, treated with the diazirine-HFPS leads to robust hydrophobic cotton and paper surfaces with antiwetting properties, whereas the corresponding control samples absorb water readily. The contact angles of water were determined to be 139° and 137° for cotton and paper, respectively. In contrast, water placed on the untreated or the control samples (those treated with the diazirine-HFPS but not irradiated) is simply absorbed into the surface. Additionaly, the chemically grafted hydrophobic coating showed high durability toward wash cycles and sonication in organic solvents. Because of the mode of activation to covalently tether the hydrophobic coating, it is amenable to photopatterning, which was demonstrated macroscopically.