Raising the (metastable) bar: 100% photo-switching in [Pd(Bu4dien)(η1-O2)]+ approaches ambient temperature

100% nitro–nitrito linkage isomerism is reported in single-crystals of the Pd(II)–nitrite system [Pd(Bu4dien)(η1-O2)]BPh4 (Bu4dien = N,N,N′,N′-tetrabutyldiethylenetriamine, BPh4 = tetraphenylborate). Complete conversion to a metastable endo-nitrito-(η1-NO) isomer is achieved after just 15 min irradiation with 400 nm LED light. The system is entirely metastable below 240 K, while pseudo-steady-state photocrystallographic experiments confirm that the excited state isomer is retained, at substantial conversion levels, under continuous illumination until 260 K. These results show promise for new linkage isomer systems, based on heavier transition metal centres, which display full functionality under near-ambient conditions

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

Two crystal systems: [Pd(Et4dien)(NO2)]OTf [1] and [Pt(Et4dien)(NO2)]OTf [2] (Et4dien = N,N,N′,N′-tetraethyldiethylene-triamine, OTf = trifluoromethanesulfonate) are investigated by steady-state photocrystallographic methods. Both structures contain intermolecular hydrogen bonds to the ground state nitro-(η1-NO2) isomer, which are previously shown to limit the achievable level of nitro → nitrito photo-conversion. Irradiation at 100 K induces a mixture of endo-ONO and exo-ONO isomers in 1 and 2, with overall incomplete photo-activation. In contrast, irradiation at higher temperatures leads to much higher conversion, with 100% excitation in 1 at 150 K. The results show that the detrimental effects of hydrogen bonding on the photo-reaction are overcome at higher temperature, adding a new dimension of control to the isomerisation process

Understanding solid-state photoswitching in [ReIJOMe2-bpy)IJCO)3IJη1-NO2)] crystals via in situ photocrystallography

Single-crystal-to-single-crystal nitro → nitrito isomerism is reported for the novel rhenium(I)-bipyridine complex [Re(OMe2-bpy)(CO)3(η1-NO2)], achieving a maximum conversion of 66% to a photoinduced nitrito-(η1-[O with combining low line]NO) isomer under continuous illumination. The 3D X-ray structure of the photoinduced isomer is determined by steady-state and psuedo-steady-state photocrystallographic methods, providing insight into the structural changes required to accommodate photoswitching. Photocrystallographic kinetic studies follow the progress of photoswitching with time, determining a reaction rate constant of k = 0.38(2) min−1 at 150 K. Linkage isomerism is fully-reversible on warming, and pseudo-steady-state experiments confirm that the photoexcited state is retained, at measurable occupancy, up to a temperature of 240 K. These results confirm the validity of combining photoactive linkage isomer and Re(I) photocatalyst chemistries, and the detailed determination of the photoexcited state structure will facilitate the future design of new photoactive Re(I) crystals for a range of applications

Exploring the influence of polymorphism and chromophore co-ligands on linkage isomer photoswitching in [Pd(bpy4dca)(NO2)2]

The polymorphic Pd(II)–nitrite complex [Pd(bpy4dca)(NO2)2] (1) (bpy4dca = 2,2′-bipyridine-4,4′-dicarboxylic acid methyl ester) is shown to undergo photoinduced nitro → nitrito linkage isomer switching in two crystal forms, to varying excited state population levels. Detailed photocrystallographic kinetic studies, structural analyses of the ground and photoexcited states and density functional theory calculations all combine to explain the unusually high maximum excited state population of 80% in 1, where other linkage isomer complexes containing strong chromophore co-ligands have traditionally been challenging to excite. Comparison of the photo-response in crystals for forms I and II reveals that, while the local crystal packing environment has a role in controlling the maximum photostationary population that can be achieved, the rate of isomerisation is comparable across different nitrite ligand environments. Our results reinforce the hypothesis that a complex combination of steric, electronic and kinetic factors govern the progress of linkage isomer switching in the solid-state and highlight the need for better understanding of the structural dynamics involved in isomer switching at the molecular level

Photocrystallographic Studies on Transition Metal Nitrito Metastable Linkage Isomers: Manipulating the Metastable State

Conspectus The design of solid-state materials whose properties and functions can be manipulated in a controlled manner by the application of light is an important objective in modern materials chemistry. When the material changes property or function, it is helpful if a simple measurable response, such as a change in color, can be detected. Potential applications for such materials are wide ranging, from data storage to smart windows. With the growing emphasis on solid-state materials that have two or more accessible energy states and which exhibit bistability, attention has turned to transition metal complexes that contain ambidentate ligands that can switch between linkage isomeric forms when activated by light. Suitable ligands that show promise in this area include nitrosyls, nitro groups, and coordinated sulfur dioxide molecules, each of which can coordinate to a metal center in more than one bonding mode. A nitrosyl normally coordinates through its N atom (η1-NO) but when photoactivated can undergo isomerism and coordinate through its O atom (η1-ON). At a molecular level, converting between these two configurations can act as an “on/off” switch. The analysis of such materials has been aided by the development of photocrystallographic techniques, which allow the full three-dimensional structure of a single crystal of a complex, under photoactivation, to be determined, when it is in either a metastable or short-lived excited state. The technique effectively brings the dimension of “time” to the crystallographic experiment and brings us closer to being able to watch solid-state processes occur in real time. In this Account, we highlight the advances made in photocrystallography for studying solid-state, photoactivated linkage isomerism and describe the factors that favor the switching process and which allow complete switching between isomers. We demonstrate that control of temperature is key to achieving either a metastable state or an excited state with a specific lifetime. We draw our conclusions from published work on the formation of photoactivated metastable states for nitrosyl and sulfur dioxide complexes and from our own work on photoactivated switching between nitro and nitrito groups. We show that efficient switching between isomers is dependent on the wavelength of light used, on the temperature at which the experiment is carried out, on the flexibility of the crystal lattice, and on both the electronic and steric environment of the ambidentate ligand undergoing isomerism. We have designed and prepared a number of nitro/nitrito isomeric metal complexes that undergo reversible 100% conversion between the two forms at temperatures close to room temperature. Through our fine control over the generation of the metastable states, it should be possible to effectively “dial up” a suitable temperature to give a metastable or an excited state with a desired lifetime

Uncovering the role of non-covalent interactions in solid-state photoswitches by non-spherical structure refinements with NoSpherA2

We present a charge density study of two linkage isomer photoswitches, [Pd(Bu4dien)(NO2)]BPh4·THF (1) and [Ni(Et4dien)(NO2)2] (2) using Hirshfeld Atom Refinement (HAR) methods implemented via the NoSpherA2 interface in Olex2. HAR is used to explore the electron density distribution in the photoswitchable molecules of 1 and 2, to gain an in-depth understanding of key bonding features and their influence on the single-crystal-to-single-crystal reaction. HAR analysis is also combined with ab initio calculations to explore the non-covalent interactions that influence physical properties of the photoswitches, such as the stability of the excited state nitrito-(η1-ONO) isomer. This insight can be fed back into the crystal engineering process to develop new and improved photoswitches that can be optimised towards specific applications

#4 - Endosomal Proteins in Neurodevelopmental Disorders

Rett Syndrome is a neurodevelopmental disorder that primarily affects females and is detected at 6-18 months of age. Rett Syndrome results from a mutations in the methyl-CpG binding protein 2 gene (MECP2) which is found on the X chromosome. Its mutation results in impairment in cognitive, sensory, emotional, motor and autonomic functions. Schizophrenia is also a neurodevelopmental disorder with onset in adulthood, however there is no single genetic cause. Endosomal proteins have been implicated in both disorders through GWAS studies and animal models, suggesting a common molecular mechanism shared between these neurodevelopmental disorders. Previous research in our lab has demonstrated a disruption in endosomal trafficking in animal models of both disorders. This study explores the levels and localization of endosomal proteins in coronal brain sections for mice models for these neurodevelopmental disorders. Using immunohistochemistry, we will examine protein levels in the hippocampus and cortex for endosomal trafficking markers. We will also use whole-brain derived synaptosomes and western blots to examine the subcellular levels of these endosomal markers. Our data demonstrate a reduction of endosomal proteins in the hippocampus of mouse models of neurodevelopmental disorders. Future studies will include investigations into the affected cargo being mis-trafficked in these disorders

Phase Behavior and Substitution Limit of Mixed Cesium-Formamidinium Lead TriIodide Perovskites

The mixed cation lead iodide perovskite photovoltaics show improved stability following site substitution of cesium ions (Cs+) onto the formamidinium cation sites (FA+) of (CH(NH2)2PbI3 (FAPbI3) and increased resistance to formation of the undesirable ∂-phase. The structural phase behavior of Cs0.1FA0.9PbI3 has been investigated by neutron powder diffraction (NPD), complemented by single crystal and power X-ray diffraction and photoluminescence spectroscopy. The Cs-substitution limit has been determined to be less than 15%, and the cubic α-phase, Cs0.1FA0.9PbI3, is shown to be synthesizable in bulk and stable at 300 K. On cooling the cubic Cs0.1FA0.9PbI3, a slow, second-order cubic to tetragonal transition is observed close to 290 K, with variable temperature NPD indicating the presence of the tetragonal β-phase, adopting the space group P4/mbm between 290 and 180 K. An orthorhombic phase or twinned tetragonal phase is formed below 180 K, and the temperature for further transition to a disordered state is lowered to 125 K compared to that seen in phase pure α-FAPbI3 (140 K). These results demonstrate the importance of understanding the effect of cation site substitution on structure–property relationships in perovskite materials

Instrument-model refinement in normalized reciprocal-vector space for X-ray Laue diffraction

A simple yet efficient instrument-model refinement method for X-ray diffraction data is presented and discussed. The method is based on least-squares minimization of differences between respective normalized (i.e. unit length) reciprocal vectors computed for adjacent frames. The approach was primarily designed to work with synchrotron X-ray Laue diffraction data collected for small-molecule single-crystal samples. The method has been shown to work well on both simulated and experimental data. Tests performed on simulated data sets for small-molecule and protein crystals confirmed the validity of the proposed instrument-model refinement approach. Finally, examination of data sets collected at both BioCARS 14-ID-B (Advanced Photon Source) and ID09 (European Synchrotron Radiation Facility) beamlines indicated that the approach is capable of retrieving goniometer parameters (e.g. detector distance or primary X-ray beam centre) reliably, even when their initial estimates are rather inaccurate. Keywords: data processing; Laue diffraction; instrument models; refinement; X-ray diffraction