Solid-state photochemistry of molecular photo-switchable species: the role of photocrystallographic techniques

Abstract

Over the last 30 years, the single-crystal photocrystallographic technique has been developed to determine the three-dimensional crystal and molecular structures of metastable species which have been generated in the crystal photochemically. Transition-metal complexes that have been investigated using this methodology include complexes that contain nitrosyl, dinitrogen, sulfur dioxide and nitrite ligands, all of which form new linkage isomers in the solid state when photoactivated by light of the appropriate wavelength. Both steric and electronic factors determine the level of the conversion from the ground-state structure to the metastable isomeric structure, and both the `reaction cavity' size and the nature of the intermolecular interactions are shown to be among the key factors that influence the percentage conversion.

The impact of hydrogen bonding on 100% photo-switching in solid-state nitro–nitrito linkage isomers

“…There has been a prior assumption that, for each crystal system, there is a fixed overall level of achievable MS occupation, bound either by the steric properties of the crystal or the electronic properties of the active molecule. 4,8,32,33 This study challenges that supposition, and the observation of temperature-regulated photo-activation adds a new dimension of control for single-crystal-to-single-crystal linkage isomer transformations, which may be exploited in future device technologies. Importantly, in terms of crystal engineering, the inclusion of hydrogen bond donor groups need not be avoided, a factor that will assist in the targeted design of new linkage isomer systems aimed towards specific photoswitching applications.…”

Monitoring photo-induced population dynamics in metastable linkage isomer crystals: a crystallographic kinetic study of [Pd(Bu4dien)NO2]BPh4

“…The first observation is easily appreciated from the Arrhenius equation. The second observation is more interesting, as it reinforces the point that the ''metastable limit'' -which has historically been used as a figure-of-merit to compare the stability of the photo-excited state in linkage isomer systems 3,8,17,20 -is dependent on the timescale of experiment method in use. The third observation is perhaps also intuitive, as it indicates that a cooperative switching process will lead to faster conversion rates.…”

“…This in turn suggests that new approaches to design cooperative linkage isomer systems may produce photo-switches with faster kinetics, whereas strategies aimed at reducing the activation barrier may yield diminishing returns, particularly with respect to the already-small apparent barrier to excitation. Conversely, however, the most successful route to high photoconversion reported to-date has been the engineering of large reaction cavities in which the isomerisation can take place with little steric constraint, 3,14,20 and this strategy favours the photo-active species being well separated from one another, ruling out cooperativity by design. Fig.…”

“…Photo-switchable linkage isomers are a class of metastable molecular materials that have been extensively studied using photocrystallographic methods. 3 Photocrystallography allows the full three-dimensional structures of photo-excited species to be determined by in situ X-ray diffraction, 4 and the technique is particularly advantageous for the study of molecular systems, for which structural information can be obtained to better than atomic resolution. The earliest photocrystallographic studies with linkage isomers were carried out on metal-nitrosyl complexes, including seminal work by Coppens et al on sodium nitroprusside.…”

In situlaser irradiation setup for a Bruker three-circle goniometer

“…Such studies provide ample information about photochemical processes in the solid state, be it about light-induced chemical transformations (Chaudhary et al, 2017), structures of exited states (Raithby, 2015;Hatcher & Raithby, 2013) or spin-crossover phase transitions (Lé tard et al, 2012). To the best of our knowledge in the vast majority of such studies, especially those related to timeresolved crystallography, synchrotron sources were used (Fullagar et al, 2000).…”