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The nitrile imine-mediated tetrazole–ene cycloaddition is a widely used class of photoligation. Optimizing the reaction outcome requires detailed knowledge of the tetrazole photoactivation profile, which can only partially be ascertained from absorption spectroscopy, or otherwise involves laborious reaction monitoring in solution. Photodissociation action spectroscopy (PDAS) combines the advantages of optical spectroscopy and mass spectrometry in that only absorption events resulting in a mass change are recorded, thus revealing the desired wavelength dependence of product formation. Moreover, the sensitivity and selectivity afforded by the mass spectrometer enable reliable assessment of the photodissociation profile even on small amounts of crude material, thus accelerating the design and synthesis of next-generation substrates. Using this workflow, we demonstrate that the photodissociation onset for nitrile imine formation is red-shifted by ca. 50 nm with a novel N-ethylcarbazole derivative relative to a phenyl-substituted archetype. Benchmarked against solution-phase tunable laser experiments and supported by quantum chemical calculations, these discoveries demonstrate that PDAS is a powerful tool for rapidly screening the efficacy of new substrates in the quest toward efficient visible light-triggered ligation for biological applications.

We introduce a highly efficient photoligation system, affording a pro-fluorescent Diels–Alder product that, on demand, converts into an intensively fluorescent naphthalene via E1 elimination in the presence of catalytic amounts of acid. The Diels–Alder reaction of the photocaged diene (o-quinodimethane ether or thioether) with electron-deficient alkynes is induced by UV or visible light. In contrast to previously reported ligation techniques directly leading to fluorescent products, the fluorescence is turned on after the photoligation. Thus, the light absorption of the fluorophore does not undermine the photoligation via competitive absorption, and as a result, photobleaching or side reactions of the fluorophore are not observed. Critically, the gated generation of a fluorescent product allows for fluorometric determination of the conversion. We employ a simple synthesis strategy for heterobifunctional electron-deficient alkynes allowing for facile functionalization of payload molecules.

We introduce a catalyst-free, highly efficient, ambient temperature Diels–Alder reaction employing o-methylbenzaldehyde derivatives as photocaged dienes as an ideal approach for forming three-dimensional insoluble networks for inkjet printing of OLED emissive layer. Herein, poly(methyl methacrylate) based polymers containing 4-(9H-carbazol-9-yl)-2-(3′-hydroxy-[1,1′-biphenyl]-3-yl)isoindoline-1,3-dione as a blue-green (λmax= 495–500 nm) thermally activated delayed fluorescence (TADF) emitter and a photochemically active maleimide/o-methylbenzaldehyde cross-linker couple were synthesized and their photo-cross-linking behavior was studied. Time resolved fluorescence measurements confirm that the TADF properties are maintained upon integration in a polymer network and HOMO/LUMO levels of the emitter species remain unchanged by the photo-cross-linking at 365 nm of the polymer chains. The network formation of the fluorescent films is evidenced by solvent resistance tests and monitored by Fourier transform infrared (FT-IR) spectroscopy as well as time of flight secondary ion mass spectroscopy (ToF-SIMS), showing the consumption of maleimide and o-methylbenzaldehyde groups with increasing irradiation time. The surface roughness is investigated via atomic force microscopy (AFM) and found to be unchanged by a solvent wash after the cross-linking. Furthermore, confirmation that the polymer solution can be printed on an inkjet-printer and subsequently photo-cross-linked for multilayer OLED device fabrication is obtained.

We introduce the hybrid copolymerization of two disparate monomer classes (vinyl monomers and ring-strained cyclic olefins) via living photopolymerization. The living character of the polymerization technique (metal-free photo-ROMP) is demonstrated by consecutive chain-extensions. Further, we propose a mechanism for the copolymerization and analyze the copolymer structure in detail by high-resolution mass spectrometry.

We introduce a powerful approach based on the combination of size exclusion chromatography with high-resolution mass spectrometry to selectively follow the compaction of discrete polymer chains that have uniform elemental composition. Single-chain nanoparticles (SCNP) have attracted considerable interest for a wide range of applications associated with their adjustable morphology. However, the precise characterization of morphological changes during the compaction is still challenging using existing analysis techniques. We employ a polystyrene backbone functionalized with tetrazole and fumarate moieties to utilize the nitrile imine-mediated tetrazole–ene cycloaddition for compaction. As every compaction step is associated with an elimination of one nitrogen molecule, it can be monitored via high-resolution electrospray ionization mass spectrometry. The combination with size exclusion chromatography enables the direct correlation of changes in mass with changes in morphology associated with the compaction. By establishing a calibration between the retention time and the hydrodynamic radius, ion chromatograms of discrete chains can be directly applied to determine the reduction in hydrodynamic radius associated with each cross-linking event. Therefore, accessing the compaction of discrete polymer chains becomes possible for the first time.

A powerful, simple and efficient method for photochemical crosslinking, exploiting the self-dimerization of photo-caged dienes based on o-methylbenzaldehydes (o-MBAs) is introduced. Using a small molecule model system it is possible to identify the intermediate and final products of this light induced dimerization process. The bi-functional photocurable monomers contain o-MBA as well as vinylic units, which enables radical copolymerization with other monomers to tune the mechanical properties of the resulting networks. Crosslinked materials with reduced E-moduli ranging from 0.29 to 5.76 GPa and a hardness between 52 and 329 MPa were obtained, thus spanning a wide regime of different ‘soft’ to ‘hard’ material properties. The functional groups can be addressed through simultaneous reaction or in a λ-orthogonal fashion. This orthogonality can be achieved by inducing the free radical polymerization independently of the dimerization either thermally or viaphotoinitiation at a wavelength higher than the wavelength used for the non-radical dimerization process (>400 nm). Critically, this allows for a spatially defined adjustment of both the E-modulus and hardness within the respective material by controlling the irradiation parameters, such as wavelength, curing time and intensity. The introduced self-dimerizing resists thus represent a highly controllable combined radical/non-radical and λ-orthogonal curing system.

We break new ground in the in-depth mechanistic understanding of the molecular behavior of tert-butyl methacrylate (t-BMA) and N-isopropylacrylamide (NIPAM) copolymers upon thermal treatment and correlate this understanding with the mechanical properties of the resulting materials. We explore—via nuclear magnetic resonance (NMR) and infrared (IR) spectroscopic measurements—the formation of cyclic anhydride and imide structures and exploit high-resolution mass spectrometry (HRMS) coupled to size-exclusion chromatography (SEC) as well as tandem MS (MS/MS) to correlate the time and temperature dependent cyclization processes. Critically, we propose a reshuffling mechanism describing dynamic ring-opening events involving the anhydride and imide structures, which enables the in-depth understanding of the dynamic material properties. Drawing on information from low-molecular-weight model systems, we translate our findings to high-molar-mass polymers, showing that our mechanistic model enables the understanding of the material properties of the polymers.

We introduce a photocaged diene system (o-quinodimethane thioethers) based on o-methylbenzaldehydes (o-MBAs) that can be activated with visible light. The pioneered system is accessible in a single step from commercially available starting materials in excellent yields. Variable synthetic handles can be attached to the photocaged diene, often without elaborate protecting group chemistry. Full conversion of various o-methylbenzaldehydes to the Diels–Alder adduct is achieved in the presence of maleimides under catalyst-free conditions triggered by visible light irradiation with LEDs under flow conditions. Unlike the previously reported UV-induced ligation of o-quinodimethanes, the reaction can be conducted both in organic solvents and in aqueous solution. We further demonstrate the ability of the photocaged dienes to ligate two polymer blocks by visible light. The [4+2] nature of the reaction makes it a powerful orthogonal ligation platform.

Organic electrode materials are a highly promising and environmentally benign class of battery materials with radical polymers being at the forefront of this research. Herein, we report the first example of the 1,1,3,3-tetramethylisoindolin-2-yloxyl class of nitroxides as an organic electrode material and the synthesis and application of a novel styrenic nitroxide polymer, poly(5-vinyl-1,1,3,3-tetramethylisoindolin-2-yloxyl) (PVTMIO). The polymer was synthesized from the precursor monomer, 2-methoxy-5-vinyl-1,1,3,3-tetramethylisoindoline, and subsequent oxidative deprotection yielded the electroactive radical species. Cyclic voltammetry revealed a high oxidation potential of 3.7 V versus Li, placing it among the top of the nitroxide class of electrode materials. The suitability of PVTMIO for utilization in a high-voltage organic radical battery was confirmed with a discharge capacity of 104.7 mAh g–1, high rate performance, and stability under cycling conditions (90% capacity retention after 100 cycles), making it one of the highest reported organic p-dopable cathode materials.

The nitrile imine‐mediated tetrazole‐ene cycloaddition reaction (NITEC) is introduced as a powerful and versatile conjugation tool to covalently ligate macromolecules onto variable (bio)surfaces. The NITEC approach is initiated by UV irradiation and proceeds rapidly at ambient temperature yielding a highly fluorescent linkage. Initially, the formation of block copolymers by the NITEC methodology is studied to evidence its efficacy as a macromolecular conjugation tool. The grafting of polymers onto inorganic (silicon) and bioorganic (cellulose) surfaces is subsequently carried out employing the optimized reaction conditions obtained from the macromolecular ligation experiments and evidenced by surface characterization techniques, including X‐ray photoelectron spectroscopy and FT‐IR microscopy. In addition, the patterned immobilization of variable polymer chains onto profluorescent cellulose is achieved through a simple masking process during the irradiation.

A simple and efficient route for the synthesis of cyclic polymer systems is presented. Linear furan protected α‐maleimide‐ω‐cyclopentadienyl functionalized precursors (poly(methyl methacrylate) and poly(tert‐butyl acrylate)) were synthesized via atom transfer radical polymerization (ATRP) and subsequent substitution of the bromine end‐group with cyclopentadiene. Upon heating at high dilution, deprotection of the dieneophile occurs followed by an intramolecular Diels–Alder reaction yielding a high purity cyclic product.

A series of high‐performance polycarbonates have been prepared with glass‐transition temperatures and decomposition temperatures that are tunable by varying the repeat‐unit chemical structure. Patterning of the polymers with extreme UV lithography has been achieved by taking advantage of direct photoinduced chain scission of the polymer chains, which results in a molecular‐weight based solubility switch. After selective development of the irradiated regions of the polymers, feature sizes as small as 28.6 nm have been printed and the importance of resist‐developer interactions for maximizing image quality has been demonstrated.

The synthesis of the first example of an azaphenalene-based fused aromatic nitroxide TMAO, [1,1,3,3-tetramethyl-2,3-dihydro-2-azaphenalen-2-yloxyl, (5)], is described. This novel nitroxide possesses some of the structural rigidity of the isoindoline class of nitroxides, as well as some properties akin to TEMPO nitroxides. Additionally, the integral aromatic ring imparts fluorescence that is switched on by radical scavenging reactions of the nitroxide, which makes it a sensitive probe for polymer degradation.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Predicting wavelength-dependent photochemical reactivity is challenging. Herein, we revive the well-established tool of measuring action spectra and adapt the technique to map wavelength-resolved covalent bond formation and cleavage in what we term “photochemical action plots”. Underpinned by tunable lasers, which allow excitation of molecules with near-perfect wavelength precision, the photoinduced reactivity of several reaction classes have been mapped in detail. These include photoinduced cycloadditions and bond formation based on photochemically generated o-quinodimethanes and 1,3-dipoles such as nitrile imines as well as radical photoinitiator cleavage. Organized by reaction class, these data demonstrate that UV/vis spectra fail to act as a predictor for photochemical reactivity at a given wavelength in most of the examined reactions, with the photochemical reactivity being strongly red shifted in comparison to the absorption spectrum. We provide an encompassing perspective of the power of photochemical action plots for bond-forming reactions and their emerging applications in the design of wavelength-selective photoresists and photoresponsive soft-matter materials.

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