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.