The design of efficient radical photoinitiating systems requires a systematic and detailed evaluation of their photochemical characteristics. Correlating absorbance and the corresponding electronic transitions of a photoinitiator is critical for understanding its photoinduced reaction pathways. In the current contribution, we provide an in-depth investigation into the photochemistry and photophysics of two oxime ester derivatives (O-benzoyl-α-oxooxime, OXE01, and O-acetyloxime, OXE02), known for their excellent performance in pigmented formulations. In particular, we shed light on their wavelength-dependent photopolymerization properties. We utilized a combination of UV–vis spectroscopy, density functional theory (DFT) calculations, photochemically induced dynamic nuclear polarization spectroscopy (photo-CIDNP), and pulsed-laser polymerization with a wavelength-tunable laser with subsequent size exclusion chromatography coupled to high-resolution electrospray ionization mass spectrometry (PLP-SEC-ESI-MS) for obtaining detailed insights. Both photoinitiators have high molar extinction coefficients (ε) of greater than 1.75 × 104 L mol–1 cm–1 at close to 330 nm, with the n−π* and π–π* transitions, relevant for cleavage of the N–O bond, at approximately 335 nm according to DFT calculations. We have probed the wavelength-dependent initiation behavior of both OXE01 and OXE02 in the presence of methyl methacrylate (MMA) via PLP with a wavelength-tunable laser between 285 and 435 nm at constant photon counts. Surprisingly, the highest conversions of MMA were found at a wavelength of 405 nm, even though the molar extinction coefficients of the photoinitiators are low (ε405 of 45 and 2 L mol–1 cm–1 for OXE01 and OXE02, respectively) compared with shorter wavelengths. Accordingly, the absorption spectrum of a photoinitiator is not a straightforward guide for selecting the most efficient excitation wavelength.

The wavelength-dependent conversion of two rapid photoinduced ligation reactions, i.e., the light activation of o-methylbenzaldehydes, leading to the formation of reactive o-quinodimethanes (photoenols), and the photolysis of 2,5-diphenyltetrazoles, affording highly reactive nitrile imines, is probed via a monochromatic wavelength scan at constant photon count. The transient species are trapped by cycloaddition with N-ethylmaleimide, and the reactions are traced by high resolution mass spectrometry and nuclear magnetic resonance spectroscopy. The resulting action plots are assessed in the context of Beer-Lambert's law and provide combined with time-dependent density functional theory and multireference calculations an in-depth understanding of the underpinning mechanistic processes, including conical intersections. The π → π* transition of the carbonyl group of the o-methylbenzaldehyde correlates with a highly efficient conversion to the cycloadduct, showing no significant wavelength dependence, while conversion following the n → π* transition proceeds markedly less efficient at longer wavelengths. The influence of absorbance and reactivity has critical consequences for an effective reaction design: At high concentrations of o-methylbenzaldehydes (c = 8 mmol L), photoligations with N-ethylmaleimide (possible for λ ≤ 390 nm) are ideally performed at 330 nm, whereas at high light penetration regimes at lower concentrations (c = 0.3 mmol L), 315 nm irradiation leads to the highest conversion. Activation and trapping of 2,5-diphenyltetrazoles (possible for λ ≤ 322 nm) proceeds best at a wavelength shorter than 295 nm, irrespective of concentration.

Emulating nature's protein paradigm, single-chain nanoparticles (SCNP) are an emerging class of nanomaterials. Synthetic access to SCNPs is limited by ultralow concentrations, demanding reaction conditions, and complex isolation procedures after single-chain collapse. Herein, we exploit the visible light photodimerization of styrylpyrene units as chain folding mechanism. Critically, their positioning along the polymer chain creates a confined environment, increasing the photocycloaddition quantum yields dramatically, enabling single-chain folding at unrivaled high concentrations without subsequent purification. Importantly, the enhanced photoreactivity allows for single-chain folding at λ = 445 nm LED-irradiation within minutes as well as via ambient light, enabling an unprecedented folding system. The herein demonstrated enhancement of quantum yields by steric confinement serves as a blueprint for all photochemical ligation systems.

We introduce a light induced sequence enabling λ-orthogonal star polymer formation via an arms-first approach, based on an α,ω-functional polymer carrying tetrazole and o-methyl benzaldehyde moieties, which upon irradiation can readily undergo cycloaddition with a trifunctional maleimide core. Depending on the wavelength, the telechelic strand can be attached to the core at either photo-reactive end.

We pioneer the synthesis of well-defined high molar mass segmented copolymers, employing a unique combination of step-growth and reversible addition–fragmentation chain transfer (RAFT) polymerization. The step-growth precursor polymer is obtained via the ambient temperature UV-light-induced Diels–Alder reaction of 6′-(propane-1,3-diylbis­(oxy))­bis­(2-methyl­benzaldehyde) (AA monomer) and di­(isopropionic ethyl ester fumarate) trithio­carbonate (BB monomer). Unconventional off-stoichiometric conditions (r = [AA]0:[BB]0 = 1.5–1.75) are employed to ensure a sufficiently high incorporation of BB in the step-growth product (1200 ≤ M n/g mol–1 ≤ 3950). The optimum r value is based on a detailed product distribution analysis, comparing experimental and bivariate kinetic Monte Carlo generated data, using a scheme of over 200 reactions. The analysis highlights the unexpected occurrence of AA homopolymerization and the ligation of the resulting AA segments at higher reaction times. The precursor step-growth polymer is successfully transformed into a segmented copolymer via insertion of styrene by RAFT polymerization at 60 °C (11 200 ≤ M n/g mol–1 ≤ 53 400), as confirmed both experimentally and through simulations.

We report light-induced reactions in at wo-chromophore system capable of sequence-independent l-orthogonal reactivity relying solely on the choice of wavelength and solvent. In as olution of water and acetonitrile,L ED irradiation at l max = 285 nm leads to full conversion of 2,5-diphenyltetrazoles with N-ethylmaleimide to the pyrazoline ligation products.S imultaneously present o-methylbenzaldehyde thioethers are retained. Conversely,L ED irradiation at l max = 382 nm is used to induce ligation of the o-methylbenzaldehydes in acetonitrile with N-ethylmaleimide via o-quinodimethanes, while 2,5-diphenyltetrazoles also present are retained. This unprecedented photochemical selectivity is achieved through control of the number and wavelength of incident photons as well as favorable optical properties and quantum yields of the reactants in their environment. Photoinducedligationreactionsinaone-potsystemthatcanbe controlled by the choice of irradiation wavelength without the need for additives or catalysts are highly desirable tools in the fields of organic, biological, and macromolecular chemistry as well as materials science.Until recently,this orthogonality,a lso described as l-orthogonality, [1] wavelength selectivity, [2] or chromatic orthogonality, [3] has-with the exception of photocleavage or photodeprotection systems, [4] the use of additional protecting groups, [5] added catalysts, [6] release from plasmon resonant liposomes, [7] and photoisomerization systems (photo switches) [3,8] -been achieved in only asequencedependent fashion. [1,2, 9] Clearly,t he design of ac atalyst-free, sequence-independent orthogonal ligation system is af ormidable challenge. [10] Generally,s equence-dependent l-orthogonality is possible,i fo ne photoactivatable chromophore absorbs light in ar egion of the ultraviolet-visible spectrum where the other chromophore does not absorb (long-wavelength irradiation induces the first reaction). Thei nverted order (short-wavelength irradiation induces the first reaction) of photoreactions usually results in undesired nonselective activation of both chromophores.I n2 000, Bochet and coworkers introduced ap air of protecting groups that can be cleaved with different wavelengths in ao ne-pot system with acceptable selectivity: [4a,b] One of the photoactivatable substances was transformed with 254 nm light to 92 %c onversion, while the other photocleavable compound was retained to adegree of 83 %. In the wavelength-inverted approach, the latter substance can be transformed by 420 nm light to 93 % conversion, while 92 %o fthe first compound is retained. While this selectivity for ap hotodeprotection is notable,itis far from being quantitative and addresses bond cleavage rather than formation. Examples that show control over ligation reactions with different colors of light without the limitation of sequence dependence are scarce:I tw as shown indirectly by combining av isible-light-triggered "off switch" of athermal ligation reaction with aUV-light-induced ligation reaction. [11] Orthogonal la...

We report a visible light responsive moiety capable of generating highly reactive thioaldehydes. Upon irradiation with visible light (420 nm) the pyreneacyl sulfide species undergoes a Norrish II elimination yielding thioaldehydes capable of being trapped by nucleophiles (amines, aminoxys, and thiols), as well as Diels-Alder processes, representing a new versatile ligation platform.

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.