Methods in molecular photocrystallography

Abstract

Over the last three decades, the technology that makes it possible to follow chemical processes in the solid state in real time has grown enormously. These studies have important implications for the design of new functional materials for applications in optoelectronics and sensors. Light-matter interactions are of particular importance, and photocrystallography has proved to be an important tool for studying these interactions. In this technique, the three-dimensional structures of light-activated molecules, in their excited states, are determined using single-crystal X-ray crystallography. With advances in the design of high-power lasers, pulsed LEDs and time-gated X-ray detectors, the increased availability of synchrotron facilities, and most recently, the development of XFELs, it is now possible to determine the structures of molecules with lifetimes ranging from minutes down to picoseconds, within a single crystal, using the photocrystallographic technique. This review discusses the procedures for conducting successful photocrystallographic studies and outlines the different methodologies that have been developed to study structures with specific lifetime ranges. The complexity of the methods required increases considerably as the lifetime of the excited state shortens. The discussion is supported by examples of successful photocrystallographic studies across a range of timescales and emphasises the importance of the use of complementary analytical techniques in order to understand the solid-state processes fully.

Keywords: LEDs; XFELs; absorption spectra; excited states; lasers; lifetimes; metastable molecules; photocrystallography; pump-multiprobe experiments; pump-probe experiments; single-crystal X-ray diffraction; synchrotrons; time-resolution.

Figure 1

Structural diagram of the nitrosyl linkage isomers found in Na[Fe(CN)₅(NO)] in the ground state and upon photoexcitation: (a) [Fe(CN)₅(η¹-NO)]⁻, (b) [Fe(CN)₅(η¹-ON)]⁻ and (c) [Fe(CN)₅(η²-NO)]⁻.

Figure 2

The photocrystallographic photolysis and redimerization of the cis-dimer of nitroso­benzene.

Figure 3

The reversible photoconversion of [Ni(dppe)(η¹-NO₂)Cl] into [Ni(dppe)(η¹-ONO)Cl].

Figure 4

Photoactivated linkage isomerism in [RuCl(py)₄(NO)](PF₆)₂·0.5H₂O, showing the two metastable forms.

Figure 5

The η¹-SO₂ to η¹-OSO photoisomerization in trans-[Ru(SO₂)(NH₃)₄(3-bromo­pyridine)](tosyl­ate)₂.

Figure 6

The light-induced transformation of [Co(Me-dpt)(NO₂)₃] between the nitro and nitrito forms.

Figure 7

The core of the [Pt₂(pop)₄]⁴⁻ tetra­anion. The Pt⋯Pt separation reduces by 0.28 (9) Å upon photoactivation.

Figure 8

The mol­ecular structure of [Ag₂Cu₂(2-di­phenyl­phosphino-3-methyl­in­dole)₄].

Figure 9

The mol­ecular structure of [Cu₄(PhCO₂)₄].

Figure 10

The timescales of dynamic processes that occur in chemistry.

Figure 11

An Oxford Diffraction Gemini A Ultra diffractometer equipped with an Oxford Cryostream crystal-cooling device and a ring of LEDs to illuminate the crystal. [Reproduced from Brayshaw et al. (2010 ▸) with permission from the Inter­national Union of Crystallography.]

Figure 12

The electron-density difference map drawn through the ground state Ru1/S1/O1/O2 plane calculated using the rigid-body ground state structure and the X-ray data set recorded after photoactivation. The highest residual electron density in the map (green) shows the positions of the two disordered com­ponents of the (η¹-SO₂) linkage isomer. [Reproduced from Bowes et al. (2006 ▸) with permission from the Royal Society of Chemistry.]

Figure 13

Schematic of a typical (a) ‘steady state’ and a (b) ‘pseudo-steady-state’ experiment. In part (a), the sample is illuminated continuously, and the excited state population builds to a steady-state value. This equilibrium population, marked by a dashed black line, is then measured after a pre-determined equilibration time. In part (b), the sample is illuminated with a pulsed laser or LED, resulting in the excited state population oscillating about an equilibrium value. If the measurement time is slower than the pulse frequency, the average population, again denoted by the dashed black line, is measured. [Reproduced from Hatcher et al. (2020 ▸) with permission.]

Figure 14

(a) In a pump–probe experiment, a single X-ray probe pulse is timed to measure the excited state population at a specific time delay Δt after excitation by the pump light source. The com­plete experiment is repeated for each Δt to be measured. (b) In the pump–multiprobe method, a series of probe pulses are generated after each pulse to measure multiple time delays in a single experiment. [Reproduced from Hatcher et al. (2020 ▸) with permission.]

Figure 15

A schematic diagram of a synchrotron showing the various com­ponents.