LED-pump-X-ray-multiprobe crystallography for sub-second timescales

Abstract

The visualization of chemical processes that occur in the solid-state is key to the design of new functional materials. One of the challenges in these studies is to monitor the processes across a range of timescales in real-time. Here, we present a pump-multiprobe single-crystal X-ray diffraction (SCXRD) technique for studying photoexcited solid-state species with millisecond-to-minute lifetimes. We excite using pulsed LEDs and synchronise to a gated X-ray detector to collect 3D structures with sub-second time resolution while maximising photo-conversion and minimising beam damage. Our implementation provides complete control of the pump-multiprobe sequencing and can access a range of timescales using the same setup. Using LEDs allows variation of the intensity and pulse width and ensures uniform illumination of the crystal, spreading the energy load in time and space. We demonstrate our method by studying the variable-temperature kinetics of photo-activated linkage isomerism in [Pd(Bu₄dien)(NO₂)][BPh₄] single-crystals. We further show that our method extends to following indicative Bragg reflections with a continuous readout Timepix3 detector chip. Our approach is applicable to a range of physical and biological processes that occur on millisecond and slower timescales, which cannot be studied using existing techniques.

Fig. 1. Pump-probe SCXRD experiments using pulsed LEDs.

a, b A CAD drawing (a) and photograph (b) of the 3D-printed LED holder in situ on the diffractometer. c, d Illustrate the data-collection procedure. The sample is subjected to repeated excitation/decay cycles of duration $t_{\mathrm{cyc}}$ (c). The LEDs are activated for $t_{\mathrm{exc}}$ to generate a photo-stationary excited-state (ES) population (yellow) and then switched off for $t_{\mathrm{dec}}$ to allow the ES population to decay (blue). During each sequence the goniometer $\mathrm{\phi }$-axis is swept backward and forward through an angle range $△\mathrm{\phi }$ (d) and the detector is electronically gated to record a series of data frames over a time interval $t_{\mathrm{acq}}$ (e). The start and end points of the $\mathrm{\phi }$ sweep are incremented after each sequence, thereby obtaining a time series of complete $\mathrm{\phi }$ scans sampling the excitation/decay curve with a resolution $t_{\mathrm{acq}}$. The process is repeated to collect $\mathrm{\phi }$ scans with the other goniometer axes in different positions to build up a full single-crystal dataset.

Fig. 2. Time-resolved pump-multiprobe single-crystal diffraction measurements on 1.

The five plots show the time dependence of the ES population $\alpha \left(t\right)$ during successively shorter pump-multiprobe sequences where the temperature and excitation time are optimised to engineer similar kinetic behaviour. On each plot, the orange and blue shaded regions mark the excitation and decay parts of the sequence, respectively. The markers show the measured $\alpha \left(t\right)$, each refined from a full single-crystal X-ray data set averaged over the time $t_{\mathrm{acq}}$ indicated on each plot. The solid lines are fits of the data using numerical simulations with a two-process JMAK model with competing excitation and decay.

Fig. 3. Temperature dependence of the decay rate of the metastable isomer of 1.

The plot shows an Arrhenius analysis of decay rate constants k obtained from three sets of measurements, viz. low- and mid-temperature photocrystallographic decay measurements, performed using a lab source (blue) and at the Diamond Light Source synchrotron facility (DLS - red), and high-temperature time-resolved measurements performed with our pump-multiprobe strategy (yellow). The black line is a fit to the Arrhenius law with the fitting parameters as shown. The low-temperature and some of the mid-temperature data are taken from our previous work.

Fig. 4. Selected photo-difference maps illustrating the change in electron density in 1 during photoexcitation and decay at 265 K.

Fourier electron-density difference maps calculated between the fixed GS coordinates and the ES structure data (ellipsoids shown at 50% probability). Green and red contours show regions of density accumulation and depletion, respectively. a–d are from structures recorded during the LED illumination period $t_{\mathrm{exc}}$, while e–h are from structures recorded during the ES decay period $t_{\mathrm{dec}}$. All photo-difference maps are set to a consistent level of ± 0.8 e A⁻³ per contour for comparable images, with maximum and minimum electron density levels in each image as follows (in e A⁻³): a min = −0.901, max = 1.729; b min = −2.023, max = 2.864; c min = −2.511, max = 4.087; d min = −2.838, max = 5.249; e min = −2.230, max = 3.657; f min = −0.975, max = 1.682; g min = −0.670, max = 0.783; h min = −0.666, max = 1.095.

Fig. 5. Intensity of the (−2 1 0) reflection of 1 recorded using a Timepix3 detector chip.

a Intensity change during one cycle with $t_{\mathrm{exc}}$ = 2 s (LEDs on) and $t_{\mathrm{dec}}$ = 5 s (LEDs off) at $T$ = 286 K, averaged over eight repeats. b Intensity change over ten consecutive cycles with $t_{\mathrm{exc}}$ = 2 s and $t_{\mathrm{dec}}$ = 5 s at $T$ = 286 K. c Intensity change over ten consecutive cycles with $t_{\mathrm{exc}}$ = 8 s and $t_{\mathrm{dec}}$ = 24 s at $T$ = 280 K. In each case the photon events recorded by the detector are integrated into 50 ms time bins.