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

The study addresses the challenge of delivering controlled, uniform light to liquid-state NMR samples located deep within the magnet bore, in order to enable in situ photo-NMR studies. Light triggers numerous chemical and biological processes, and integrating illumination with NMR can reveal mechanisms, kinetics, and enhance sensitivity (e.g., via photo-CIDNP). Traditional approaches using lasers, mirrors, and optical fibers are costly, complex, and can cause local heating, light losses, and magnetic field inhomogeneity. The authors propose a simple, universal illumination strategy (NMRtorch) where LEDs mounted in a lighthead at the tube top couple light into the tube wall, which acts as a light guide to uniformly illuminate the sample volume from outside. The purpose is to demonstrate feasibility, performance, wavelength flexibility (including UV), and utility across representative applications such as photo-CIDNP, actinometry, photodegradation, and photoswitch kinetics, while maintaining compatibility with standard NMR probes and workflows.

Prior work established multiple methods to illuminate NMR samples: external lasers directed via mirrors, glass rods, or optical fibers; probehead modifications integrating light sources; and more recently LED-based systems. These methods often suffer from high cost, safety and alignment complexity, local sample heating, limited wavelength availability, light coupling inefficiency (especially LED-to-fiber), and potential degradation of magnetic field homogeneity and shimming. Fiber-based inserts can reduce throughput and complicate handling. LED solutions placed outside often still rely on fibers, retaining drawbacks. Roughening of fiber tips and other approaches have been used to improve uniformity but at the expense of ease-of-use. Comprehensive reviews by Nitschke et al. and Ji et al. summarize these challenges and developments. The literature also reports photo-CIDNP with lasers and LEDs, LED-illuminated mechanistic studies, microfluidic and flow implementations, and in situ studies of azobenzenes. The present work builds on these by eliminating fibers and probe modifications, leveraging the tube wall as a light guide with engineered scattering to achieve uniform, controllable illumination.

Device concept and construction:

• NMRtorch consists of a specially prepared heavy-walled 5 mm NMR tube (borosilicate or quartz; wall ~1.2–1.4 mm) whose exterior surface around the sample region (bottom ~40–42 mm) is abrasively etched to create light-scattering centers; the tube wall acts as a light guide transporting light from the rim to the sample region and scattering it uniformly around the sample from outside.
• A lighthead mounted at the tube top houses one or more non-magnetic LEDs (single wavelength or multichannel arrays, e.g., RGBW). LEDs are positioned very close to the tube rim to maximize coupling; optional lenses were found largely unnecessary.
• The lighthead connects via a multicore cable to a control/power module with constant-current drivers (TTL-triggered) allowing up to four independent LED channels. PWM dimming is controlled directly by the NMR console via pulse programming to set duty cycle and timing relative to RF pulses.
• Cooling is provided by bore gas flow for lower-power LEDs (≤3 W) or dimmed operation; auxiliary compressed gas cooling and optional temperature sensing are used for higher-power arrays (e.g., 10 W UV).
• Transparent or semi-transparent caps (e.g., PTFE/PE-based, silicone dots, TEF film, or polished quartz plugs) allow light entry into the tube rim; caps were chosen for solvent compatibility and thermal tolerance.

Spectrometers and processing:

• Experiments were conducted on a Bruker Avance III 500 MHz spectrometer with a QCI-F cryoprobe (cooled 1H and 19F channels) and temperature control. Data processed with TopSpin 4.1, Dynamics Center 2.7, and plotted in GraphPad Prism 9.

Photo-CIDNP experiments:

• Samples: 6-fluoroindole (6FI) stock 250 mM in DMSO-d6; flavin mononucleotide (FMN) stock 10 mM in water; diluted to target concentrations in H2O/10% D2O. No degassing.
• Illumination: single blue LED (nominal 460 nm peak, 3 W, 0.7 A). PWM at 100 Hz implemented in the NMR pulse sequence to vary duty cycle (0–100%).
• Measurements: 19F NMR of 6FI; photo-CIDNP enhancement factor α = II/ID (II illuminated; ID dark). Light intensity at tube exterior measured as PPFD with Li-250A photometer and Li-190R quantum sensor. Imaging used zg with 19F detection and a 0.106 G cm−1 Gz during acquisition; position-dependent enhancement az = IzI/IzD to profile Z-axis light distribution while correcting for gradient nonlinearity.

Light distribution characterization:

• Tube etching patterns varied (unetched control; bottom-only etched; graded patterns). Pixel greyscale analysis of photographic images (Pixel 4A, ImageJ) under attenuated 460 nm illumination quantified scattering along Z; compared to az profiles from photo-CIDNP imaging. Fluorescein fluorescence images qualitatively assessed XY uniformity.

NMR actinometry:

• Actinometer: diarylethene (DAE; 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene) 5 mM in CD2Cl2. Photocyclization between closed form (CF) and open form (OF): CF→OF under visible (550 nm); OF→CF under UV (280 nm).
• Lighthead: two channels (280 nm and 550 nm LEDs, each nominal 3 W at 0.7 A). Pseudo-2D pulse program cycled wavelengths and acquired single scans every 5 s to monitor concentrations via characteristic 1H (and 19F) integrals.
• By pre-setting initial CF via controlled UV exposure and then measuring initial visible-light CF→OF rates under 550 nm, a plot of rate vs initial [CF] was fitted to d[CF]/dt = −I Φ ε (1 − 10^(−ε b c [CF])) to extract light intensity. Quantum yield Φ assumed 0.02 mol Einstein−1. PFD at tube exterior measured with PG200N spectral PAR meter.

UV photodegradation (ICH Q1B):

• Quinine hydrochloride dihydrate 2% w/v in H2O/10% D2O. Illumination with 365 nm UV LED array (10 W) using either borosilicate or quartz NMRtorch tubes. 1H NMR monitored degradation in situ; UV–Vis (Nanodrop 2000) measured ΔA400 for ICH Q1B actinometry (foil-wrapped control kept in dark at same temperature).

Azobenzene photoswitching kinetics:

• 4-aminoazobenzene (AAB) 1 mM in DMSO-d6. Lighthead with red, green, blue, and white LEDs, each driven at 3 W (0.7 A). Pre-equilibrate with blue light for 5 min; record 1H spectra every 5 s (pseudo-2D zgesgp, 1 scan, 0 dummy). Toggle colors to drive trans↔cis isomerization; quantify isomer populations from distinct amine resonances (trans −6.10 ppm; cis −5.76 ppm); fit mono- or bi-exponential kinetics as appropriate.

Operational notes:

• Light coupling is maximized by placing the LED emitter close to the tube rim; several LEDs can be positioned slightly off-axis without large losses. Heavy-wall tubes reduce sample volume (~170–200 µL) but enhance guiding and uniformity. Etching extent and distribution tune Z-uniformity and overall intensity. No adverse effects observed on probe tuning, shimming, lineshape, or water suppression; no extra re-equilibration delay needed after light switching.

• NMRtorch feasibility and performance: The tube wall functions as an efficient light guide; etched exterior scattering creates uniform illumination around the sample. The setup avoids fiber optics and probe modifications and is compatible with standard probes, including cryoprobes.
• Photo-CIDNP sensitivity gains: 6-fluoroindole (6FI) with FMN under 460 nm, 3 W LED illumination at 11.7 T exhibited emissive 19F photo-CIDNP with a maximum 64-fold enhancement (negative peaks), to the authors’ knowledge the largest reported for 19F. Enhancements increased with illumination time and plateaued beyond ~6 s; highest absolute illuminated signal at 6 mM 6FI with 0.2 mM FMN.
• Linear control via PWM: Both the 6FI photo-CIDNP response and measured PPFD at the sample surface were linear with LED PWM duty cycle, enabling precise intensity control directly from the NMR console.
• Light distribution engineering: Etching patterns control Z-axis intensity profiles. Pixel greyscale scattering profiles agreed with position-dependent photo-CIDNP enhancement az across the NMR-active volume. Heavier etching increased overall intensity (e.g., Tube 4: 260.9 ± 44.9 µmol m−2 s−1 vs Tube 3: 117.2 ± 33.3 µmol m−2 s−1 at the surface) and could bias illumination towards targeted regions. XY illumination was largely uniform due to the diffuse extended scattering surface.
• In situ NMR actinometry: Using DAE reversible photoisomerization with dual LEDs (280/550 nm), the visible-light intensity inside the sample was I = 438 ± 21 µEinstein L−1 s−1, corresponding to a surface photon flux density of 251 ± 12 µmol m−2 s−1 under 550 nm. Actinometry was achieved in a single sample/run with wavelength cycling embedded in the pulse program.
• UV photodegradation at regulatory doses: A 10 W 365 nm LED delivered ICH Q1B-equivalent UV-A dosage in situ; quinine hydrochloride showed ΔA400 = 0.74 ± 0.02 after only 2 h. Quartz tubes increased degradation rates by ~3× versus borosilicate. Concurrent 1H NMR tracked decreases in intact quinine signals (more pronounced for quinoline moiety), spectral broadening, and appearance of new peaks.
• Photoswitch kinetics with multi-color illumination: For AAB, blue light rapidly drives trans→cis (k = 65 ± 6 µM min−1). Re-equilibration to trans is faster under green (k = 108 ± 3 µM min−1) than red (k = 9.3 ± 0.1 µM min−1), with red establishing higher trans equilibrium. Cis→trans under green and white had similar rates (~111 ± 5 µM min−1) but different equilibria. Thermal dark cis→trans relaxation required bi-exponential fits (kfast = 17.9 ± 0.9 µM min−1; kslow = 3.5 ± 0.1 µM min−1), suggesting a multistage mechanism.
• Instrumental compatibility and throughput: No detectable interference with tuning, shimming, water suppression, or lineshape. Sample handling remains conventional (filling/capping, glovebox compatibility). PWM and multichannel control streamline complex illumination protocols and increase throughput.

The work demonstrates that a simple LED lighthead coupled directly to the NMR tube rim can deliver controlled, high-intensity, and spatially uniform illumination within the spectrometer bore without fiber optics or probe modifications. This addresses the core challenge of illuminating samples deep in the magnet while preserving magnetic field homogeneity and ease of sample handling. The linearity of photo-CIDNP response with PWM duty cycle and the agreement between photographic scattering profiles and position-resolved CIDNP enhancement validate both the efficiency of light delivery and the ability to engineer Z-uniformity through tailored etching. In situ actinometry using a reversible photoswitch (DAE) shows that absolute light intensities can be quantified in a single experiment and integrated with pulse programs, enabling accurate, reproducible illumination control. High-power UV illumination enables regulatory-grade photostability testing with concurrent NMR monitoring, offering richer mechanistic insight than standalone UV irradiation chambers. Multicolor illumination allows rapid, programmable toggling of photoswitch states, revealing kinetic and equilibrium behaviors (including a previously undescribed biexponential thermal cis→trans relaxation in AAB) that would be difficult to capture with ex situ or single-wavelength approaches. Collectively, these results establish NMRtorch as a versatile, user-friendly, and widely compatible platform to augment NMR with in situ photochemistry, expanding the scope of analyzable light-driven phenomena and improving experimental throughput and control.

This study introduces NMRtorch, a universal LED-based device that uses the NMR tube wall as a light guide with engineered exterior scattering to achieve efficient, uniform in situ illumination. The platform delivers record 19F photo-CIDNP enhancements (64-fold for 6-fluoroindole), enables quantitative in situ actinometry, achieves ICH Q1B-level UV dosages with online NMR monitoring, and supports multicolor, console-controlled kinetics studies of photoswitches, all without probe modifications or fiber optics. The approach is compatible with standard probes (including cryoprobes), preserves magnetic field homogeneity, and facilitates conventional sample handling. Future work could focus on industrial fabrication of standardized etched tubes and lightheads for reproducibility and ease of adoption, integration with automated sample changers and control software, exploration of higher-power or novel light sources (e.g., laser diodes, deep-UV LEDs), extension to different tube diameters and benchtop/low-field instruments, and application to a broader range of photochemical, biochemical, and materials systems.

• Current implementations require heavy-walled tubes and reduce sample volume to ~170–200 µL for 5 mm tubes, comparable to 3 mm tubes.
• Etching patterns are manually fabricated, leading to variability; industrial machining would improve consistency. Components (lighthead housings, custom MPCBs) are not yet commercially standardized, necessitating in-house builds.
• Transparent or semi-transparent caps are required to couple light into the tube rim; cap material must tolerate solvents and potential local heating.
• High-power LEDs (e.g., 10 W UV) may require auxiliary cooling via compressed gas; careful thermal management is needed.
• Light intensity within the sample may decay radially for strongly absorbing samples; UV transmission is better with quartz than borosilicate, affecting performance at shorter wavelengths.
• Optimization of etching density/position is needed to achieve desired Z-uniformity and intensity for specific applications.