Sample illumination device facilitates in situ light-coupled NMR spectroscopy without fibre optics

Light plays a crucial role in numerous chemical and biological reactions, making its controlled incorporation into analytical procedures essential. Nuclear Magnetic Resonance (NMR) spectroscopy, a powerful analytical technique, often benefits from in situ illumination for studying light-dependent phenomena. Existing methods for illuminating NMR samples, such as using lasers guided by mirrors or optical fibers, suffer from drawbacks including high cost, localized sample heating, limited wavelength variety, complex adjustments, safety concerns, light intensity losses, and difficulty with magnetic field homogeneity. While LEDs offer an inexpensive and versatile alternative, their use often still relies on optical fibers, inheriting similar limitations. This research presents a novel approach to overcome these challenges, using a sample illumination device to facilitate in situ photo-NMR experiments without the need for fiber optics or probehead modifications.

Several methods for illuminating NMR samples have been developed and reviewed by Nitschke et al. and Ji et al., enabling studies of various light-sensitive systems like photocatalyzed reactions and polymerization. Direct in situ illumination is generally preferred over ex situ methods, but the sample's location within the magnet presents significant technical hurdles. Previous approaches employing lasers, mirrors, or optical fibers have limitations related to cost, heating, wavelength range, and light loss. The increasing availability of LEDs provides a more accessible light source, but their coupling into optical fibers remains inefficient and problematic, affecting light intensity and magnetic field homogeneity. This new design aims to resolve these limitations by eliminating fiber optics entirely.

The NMRtorch device consists of a lighthead containing one or more LEDs positioned directly atop a specialized NMR tube. The tube's wall acts as a light guide, with light-scattering centers (created by etching) on the exterior surface around the sample volume to ensure uniform illumination. The lighthead connects to a control box with constant-current power supplies, triggered by the NMR console, enabling control over multiple LEDs of varying wavelengths. Cooling is provided by the magnet bore's gas flow, supplemented by a compressed gas line for higher-power LEDs. The design allows for easy sample handling, filling, and capping, minimizing magnetic field inhomogeneity. Several experiments were conducted to assess the device's performance and efficacy: Photo-CIDNP experiments using 6-fluoroindole (6FI) and flavin mononucleotide (FMN) to assess signal enhancements and light uniformity; assessment of light intensity using an LED duty cycle controlled by the NMR console and an ex situ photometer; characterization of light distribution in tubes with varying etching patterns through photographic image analysis and photo-CIDNP imaging; determination of light intensity using a diarylethene (DAE) actinometer involving reversible photoisomerisation with UV and visible light, tracked by ¹H and ¹⁹F NMR; evaluation of UV-induced degradation of quinine hydrochloride under ICH Q1B guidelines; and investigation of multi-colour triggered photoisomerisation of 4-aminoazobenzene (AAB) using an RGBW LED array to assess the kinetics of photoswitching. NMR spectroscopy was performed using a Bruker 500 MHz Avance III spectrometer with a QCI-F cryoprobe.

NMRtorch achieved a remarkable 64-fold signal enhancement in 19F photo-CIDNP experiments with 6FI and FMN, exceeding previously reported enhancements. Light intensity and photo-CIDNP enhancement demonstrated linear dependence on LED duty cycle. The etching patterns on the NMRtorch tubes significantly impacted light distribution, with non-uniform etching patterns providing more uniform illumination. DAE actinometry provided accurate measurement of light intensity within the sample. The NMRtorch setup delivered sufficient UV dosage within 2 hours to meet ICH Q1B guidelines for photostability testing, with simultaneous online NMR monitoring of quinine hydrochloride degradation. Multi-color experiments on AAB revealed distinct kinetics and equilibria for isomerisation triggered by different wavelengths, including the unexpected discovery of a bi-exponential thermal cis-to-trans relaxation process. The study also revealed that the proposed approach is compatible with any typical NMR probehead, including cryoprobes, and that it enabled experiments with optically dense liquid samples or suspensions.

The NMRtorch device successfully addresses the challenges associated with in situ illumination in NMR spectroscopy. The elimination of fiber optics improves light intensity, simplifies sample handling, and enhances magnetic field homogeneity. The high signal enhancement in photo-CIDNP experiments, the precise control of light intensity, and the ability to perform simultaneous online monitoring of photoreactions under UV and visible light illumination highlight the device's versatility and power. The findings demonstrate the effectiveness of the NMRtorch for studying a wide range of light-dependent phenomena, making photo-NMR experiments more accessible and efficient. The discovery of a previously unknown bi-exponential thermal relaxation mechanism in AAB further underscores the method's ability to uncover subtle details of photochemical processes.

The NMRtorch provides a universal and convenient solution for in situ illumination in NMR spectroscopy, eliminating the limitations of fiber optic-based systems. Its effectiveness in photo-CIDNP, UV degradation, and photoswitching studies showcases its broad applicability. Future research could focus on optimizing etching patterns for different tube sizes and wavelengths, developing automated control systems for high-throughput experiments, and exploring its applications in other areas of light-dependent chemistry and biology.

The current NMRtorch tubes were manually etched, potentially leading to variations in light distribution. While a range of lighthead designs were tested, customized combinations of LEDs might require more advanced manufacturing techniques. The sample volume is reduced compared to standard tubes due to the thicker walls needed for effective light guiding. Further research might investigate different methods for creating the light-scattering patterns on the NMR tubes in order to ensure higher uniformity and consistency.