Steady-state and pseudo-steady-state photocrystallographic studies on linkage isomers of [Ni(Et4dien)(η2-O,ON)(η1-NO2)]: identification of a new linkage isomer

Methods in molecular photocrystallography

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.

Pressure-induced single-crystal-to-single-crystal nitrite ligand isomerisation accompanied by a piezochromic effect

High-pressure structural study of a piezochromic crystal of a rare di-exo-nitrito nickel(II) complex supported by computational analysis is presented. The examined system is not photoswitchable, however, in the 0-6.2 GPa pressure range the crystal undergoes two phase-transitions accompanied by a colour change and the nitrite ligand isomerisation, which is unique.

Interrelations between Linkage Isomers of an Efficient Square-planar Nickel(II) Nitrite Photoswitch in the Solid State

An efficient nitrite nickel(II) photoswitch, with the 1-phenyl-2-hydroxyimino-3-[(2'-dimethylamino)ethyl]imino-1-propanone moiety used as the ancillary ligand, is reported. In the ground-state ('dark') crystal structure, the studied compound exists predominantly as the nitro-(η1 -N(O)2 ) isomer, however, traces of the exo- and endo-nitrito-(η1 -ONO) forms are detected both at 100 K (4-5 % each) and under ambient conditions (~9 % each). When excited with the 405-530 nm LED light, the nitro-to-nitrito isomerization takes place. The total conversion exceeds 90 %. The exo-nitrito linkage isomer constitutes the dominant photo-generated form, whereas the relative population of both nitrito species depends on temperature. The reaction is fully reversible and reproducible. The photo-products are stable up to 200 K. The system constitutes a good model case for the reaction mechanism studies. Thus, experimental and theoretical investigations on the photo-isomerism were conducted and are presented in detail. Eventually, the nitro→exo-nitrito→endo-nitrito reaction pathway is proposed.

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.

Photocrystallography of [Ru(bpy)2(dmso)2]2+ reveals an O-bonded metastable state

We report the first instance of observing the phototriggered isomerization of dmso ligands on a bis sulfoxide complex, [Ru(bpy)₂(dmso)₂], in the crystalline solid state. The solid-state UV-vis spectrum of the crystal demonstrates an increase in optical density around 550 nm after irradiation, which is consistent with the solution isomerization results. Digital images of the crystal before and after irradiation display a notable color change (pale orange to red) and cleavage occurs along planes, (1̄01) and (100), during irradiation. Single crystal X-ray diffraction data also confirms that isomerization is occurring throughout the lattice and a structure that contains a mix of the S,S and O,O/S,O isomer was attained from a crystal irradiated ex situ. In situ irradiation XRD studies reveal that the percentage of the O-bonded isomer increases as a function of 405 nm exposure time.

Photogeneration of Several Linkage Isomers and Investigation of Forward and Backward Nitro-Nitrito Isomerization Processes in a Palladium Complex

Photoinduced linkage isomers (PLIs) of the nitro-ligand were generated and comprehensively characterized in a square planar unit [Pd(NH₃)₃NO₂]⁺ of the complex salts [Pd(NH₃)₄][Pd(NH₃)₃NO₂][MOx₃]·yH₂O (M = Cr (Cr), Rh (Rh), Co (Co), Ox = oxalate). Structural (XRD) and spectroscopic (IR, UV-vis) investigations at 10 and 150 K allowed determining the structures of several photoinduced linkage isomers, endo-ONO (PLI1, 2) and exo-ONO (PLI3, 4) isomers generated by irradiation with 365 nm from the initial NO₂ (GS), along with the assignment of the infrared (IR) bands to each structural isomer. Based on a combination of these methods, the photo- and thermally induced interplay of PLIs was investigated. Irradiation in the temperature range of 10-80 K induces the formation of both endo- and exo-ONO isomers, while increasing the temperature up to 150 K results in the formation of only endo-ONO isomers. The structural arrangement of the endo-ONO and exo-ONO PLI is strongly influenced by intermolecular interactions due to the partial occupation of a neighboring site by water molecules. The investigation of thermal dynamics of PLIs revealed that the thermal decay of the exo-ONO isomer occurs via two steps exo-ONO → endo-ONO → NO₂. The kinetic parameters (E_a, k₀) of both decay processes were determined together with the characteristic decay temperatures (T_d) by IR spectroscopy. According to the photoinduced dynamics measured by IR spectroscopy, the mechanism of PLI formation in [Pd(NH₃)₃NO₂]⁺ could be described as NO₂ → endo-ONO → exo-ONO.

Cooperative Photochemical Reaction Kinetics in Organic Molecular Crystals

Photoreactive molecular crystals have been intensively investigated as next-generation functional materials. Changes in physicochemical properties are usually interpreted in terms of static pre- and post-reaction molecular structures and packings determined by X-ray structure analysis. However, to elucidate the dynamic properties, it is necessary to understand the dynamic nature of photochemical kinetics in crystals. Reaction dynamics in the crystal phase can be dramatically different from those in dilute solution because the local molecular environment evolves as the surrounding reactant molecules are transformed into products. In this Review article, we summarize multiple examples of photochemical reactions in the crystalline phase that do not follow classical kinetic behavior. We also discuss different theoretical methods that can be used to describe this behavior. This Review article should help provide a foundation for future workers to understand and analyze photochemical reaction kinetics in crystals.

An optically reversible room-temperature solid-state cobalt(III) photoswitch based on nitro-to-nitrito linkage isomerism

A simple trinitro cobalt complex [Co(3,3'-diamino-N-methylpropanediamine)(NO₂)₃] was proven to be photoswitchable at room temperature as the Pca2₁ polymorph with the maximum nitro-to-nitrito conversion reaching ca. 55%. Solid-state IR, UV-vis and XRD indicate that the transformation can be triggered optically in both ways via 470 nm and 570-660 nm LED light, respectively.

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

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.

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...

Quasi-aromatic Möbius chelates of Cadmium(II) nitrite and/or nitrate

We report the design, structural, spectroscopic and computational characterizations of the two new quasi-aromatic Möbius chelate coordination compounds fabricated from Cd(NO3)2·4H2O and a bulky helical organic ligand derived from benzildihydrazone...

Photocrystallographic and spectroscopic studies of a model (N,N,O)-donor square-planar nickel(II) nitro complex: in search of high-conversion and stable photoswitchable materials

A new, cheap, easy-to-synthesize and air-stable photoswitchable nickel(II) complex, QTNiNO2, is reported. The metal centre in QTNiNO2 is coordinated by a nitro group and a [2-methyl-8-amino-quinoline]-1-tetralone ligand. The compound crystallizes in the tetragonal space group I41/a with one complex molecule comprising the asymmetric unit, and the crystals are stable under ambient conditions. Irradiation of the solid-state form of QTNiNO2 with 530-660 nm LED light at 160 K converts the ambidentate nitro moiety fully to the nitrito linkage isomer which is stable up to around 230 K, as indicated by IR spectroscopy measurements. The structures of all species present in the examined crystals and their thermal stability were confirmed via X-ray multi-temperature and photocrystallographic experiments. The impact of temperature on the (photo)isomerization reaction taking place in a single crystal was additionally investigated. The experimental results are supported by computational analyses of crystal packing and intermolecular interactions that influence the isomerization process studied.

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

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.

First heterobimetallic AgI-CoIII coordination compound with both bridging and terminal -NO2 coordination modes: synthesis, characterization, structural and computational studies of (PPh3)2AgI-(μ-κ2O,O':κN-NO2)-CoIII(DMGH)2(κN-NO2)

An unusual heterobimetallic bis(triphenylphosphane)(NO₂)Ag^I-Co^III(dimethylglyoximate)(NO₂) coordination compound with both bridging and terminal -NO₂ (nitro) coordination modes has been isolated and characterized from the reaction of [CoCl(DMGH)₂(PPh₃)] (DMGH₂ is dimethylglyoxime or N,N'-dihydroxybutane-2,3-diimine) with excess AgNO₂. In the title compound, namely bis(dimethylglyoximato-1κ²O,O')(μ-nitro-1κN:2κ²O,O')(nitro-1κN)bis(triphenylphosphane-2κP)cobalt(III)silver(I), [AgCo(C₄H₇N₂O₂)₂(NO₂)₂(C₁₈H₁₅P)₂], one of the ambidentate -NO₂ ligands, in a bridging mode, chelates the Ag^I atom in an isobidentate κ²O,O'-manner and its N atom is coordinated to the Co^III atom. The other -NO₂ ligand is terminally κN-coordinated to the Co^III atom. The structure has been fully characterized by X-ray crystallography and spectroscopic methods. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) have been used to study the ground-state electronic structure and elucidate the origin of the electronic transitions, respectively.

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

We present a detailed kinetic study of photo-induced solid state linkage isomerism in the compound [Pd(Bu₄dien)NO₂]BPh₄ (Bu₄dien = N,N,N',N'-tetrabutyldiethylenetriamine) using in situ photocrystallographic techniques. We explore the key variables that influence the photoconversion and develop a detailed kinetic model for the excitation and decay processes and the temperature dependence of the conversion rates. We show that by varying the temperature the lifetime of the excited state can be varied over orders of magnitude, making these systems ideal test cases for the development of new time-resolved X-ray diffraction methods. The kinetic model is used to build a numerical-simulation tool, which we use to explore the practicalities of pump-probe single-crystal diffraction experiments with minute and second time-resolution.

Understanding solid-state photoswitching in [Re(OMe2-bpy)(CO)3(η1-NO2)] crystals via in situ photocrystallography

Single-crystal-to-single-crystal linkage isomerism is determined in a photoswitchable Re(i)-bpy crystal, shedding new light on the photoactive properties of potential Re(i)-photocatalysts.

An investigation into the unusual linkage isomerization and nitrite reduction activity of a novel tris(2-pyridyl) copper complex

The copper-containing nitrite reductases (CuNIRs) are a class of enzymes that mediate the reduction of nitrite to nitric oxide in biological systems. Metal-ligand complexes that reproduce the salient features of the active site of CuNIRs are therefore of fundamental interest, both for elucidating the possible mode of action of the enzymes and for developing biomimetic catalysts for nitrite reduction. Herein, we describe the synthesis and characterization of a new tris(2-pyridyl) copper complex ([Cu1(NO2)2]) that binds two molecules of nitrite, and displays all three of the common binding modes for [Formula: see text], with one nitrite bound in an asymmetric quasi-bidentate κ2-ONO manner and the other bound in a monodentate fashion with a linkage isomerism between the κ1-ONO and κ1-NO2 binding modes. We use density functional theory to help rationalize the presence of all three of these linkage isomers in one compound, before assessing the redox activity of [Cu1(NO2)2]. These latter studies show that the complex is not a competent nitrite reduction electrocatalyst in non-aqueous solvent, even in the presence of additional proton donors, a finding which may have implications for the design of biomimetic catalysts for nitrite reduction.

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

Temperature-regulated control of photo-induced linkage isomer switching engineered through intermolecular hydrogen bonding to the nitro-(η¹-NO₂) group.

Raising the (metastable) bar: 100% photo-switching in [Pd(Bu4dien)(η1-N̲O2)]+ approaches ambient temperature

Rapid, fully-reversible photo-switching is observed under near-ambient conditions for the first time in crystals of a novel palladium–nitrite linkage isomer.

Photocrystallography and IR spectroscopy of light-induced linkage NO isomers in [RuBr(NO)2(PCyp3)2]BF4

One single photo-induced linkage NO isomer (PLI) is detected and characterized in the dinitrosyl pentacoordinated compound [RuBr(NO)2(PCyp3)2]BF4 by a combination of photocrystallographic and IR analysis. In the ground state, the molecule adopts a trigonal–bipyramidal structure with the two NO ligands almost linear with angles Ru—N1—O1 = 168.92 (16), Ru—N2—O2 = 166.64 (16)°, and exactly equal distances of Ru—N = 1.7838 (17) and O—N = 1.158 (2) Å. After light irradiation of 405 nm at T = 10 K, the angle of Ru—N2—O2 changes to 114.2 (6)° by rotation of the O atom towards the Br ligand with increased distances of Ru—N2 = 1.992 (6) and N2—O2 = 1.184 (8) Å, forming a bent κN bonded configuration. Using IR spectroscopy, the optimal wavelength and maximum population of 39 (1)% of the PLI is determined. In the ground state (GS), the two symmetric νs(NO) and asymmetric νas(NO) vibrations are measured at 1820 and 1778 cm−1, respectively. Upon photo-irradiation, the detection of only one new vibrational ν(NO) stretching band at 1655 cm−1, assigned to the antisymmetric coupled vibration mode and shifted to lower wavenumbers by −123 cm−1, supports the photocrystallographic result. These experimental results are supported by additional DFT calculations, which reproduce the structural parameters and vibrational properties of both the ground state and the photo-induced linkage isomer well. Especially the experimentally characterized molecular structure of the PLI state corresponds to an energy minimum in the calculations; the stabilization of the bent κN bonded configuration of the PLI state originates from specific intramolecular orbital overlap.

Electronic excitations in molecular solids: bridging theory and experiment

As the spatial and temporal resolution accessible to experiment and theory converge, computational chemistry is an increasingly powerful tool for modelling and interpreting spectroscopic data. However, the study of molecular processes, in particular those related to electronic excitations (e.g. photochemistry), frequently pushes quantum-chemical techniques to their limit. The disparity in the level of theory accessible to periodic and molecular calculations presents a significant challenge when modelling molecular crystals, since accurate calculations require a high level of theory to describe the molecular species, but must also take into account the influence of the crystalline environment on their properties. In this article, we briefly review the different classes of quantum-chemical techniques, and present an overview of methods that account for environmental influences with varying levels of approximation. Using a combination of solid-state and molecular calculations, we quantitatively evaluate the performance of implicit-solvent models for the [Ni(Et4dien)(η2-O,ON)(η1-NO2)] linkage-isomer system as a test case. We focus particularly on the accurate reproduction of the energetics of the isomerisation, and on predicting spectroscopic properties to compare with experimental results. This work illustrates how the synergy between periodic and molecular calculations can be exploited for the study of molecular crystals, and forms a basis for the investigation of more challenging phenomena, such as excited-state dynamics, and for further methodological developments.

Linkage Isomerization via Geminate Cage or Bimolecular Mechanisms: Time-Resolved Investigations of an Organometallic Photochrome

The extent of the photoinitiated linkage isomerization of dicarbonyl(3-cyanomethylpyridine-κN)(η(5)-methylcyclopentadienyl)manganese (4) to dicarbonyl(3-cyano-κN-methylpyridine)(η(5)-methylcyclopentadienyl)manganese (5) was examined by time-resolved infrared spectroscopy on picosecond to microsecond time scales in room temperature isooctane to determine the extent the isomerization occurs as a geminate cage rearrangement. We previously reported that a substantial part of the conversion between 4 and 5 must be a bimolecular reaction between a solvent coordinated dicarbonyl(η(5)-methylcyclopentadienyl)manganese (3) and uncoordinated 3-cyanomethylpyridine. For the purpose of designing a molecular device, it would be desirable for the photoisomerization to occur in a geminate cage reaction, because the faster the isomerization, the less opportunity for side reactions to occur. In this study, assignments of transients are identified by comparison with transients observed for model reactions. Within 100 μs after photolysis of 4 in isooctane, no 5 is observed. Instead, the solvent coordinated 3 is observed within 25 ps after irradiation. The formation of 5 is observed only in the presence of 9 mM 3-cyanomethylpyridine but not until 10-50 μs after irradiation of 4. Within the limits of detection, these results indicate the conversion of 4 to 5 occurs exclusively via a bimolecular reaction of 3-cyanomethylpyridine with solvent coordinated 3 and not a geminate cage reaction between 3-cyanomethylpyridine and the dicarbonyl(η(5)-methylcyclopentadienyl)manganese fragment.

Perspective: On the relevance of slower-than-femtosecond time scales in chemical structural-dynamics studies

A number of examples illustrate structural-dynamics studies of picosecond and slower photo-induced processes. They include molecular rearrangements and excitations. The information that can be obtained from such studies is discussed. The results are complementary to the information obtained from femtosecond studies. The point is made that all pertinent time scales should be covered to obtain comprehensive insight in dynamic processes of chemical and biological importance.

Reversible single-crystal-to-single-crystal photoisomerization of a silver(i) macropolyhedral borane

A reversible conversion of [trans-B₂₀H₁₈]²⁻ to [iso-B₂₀H₁₈]²⁻ and linkage isomerism occur in a single crystal of {Ag₂(PPh₃)₆[B₂₀H₁₈]} (the positions of [Ag(PPh₃)₃]⁺ are shown in red and blue).

Energetics, thermal isomerisation and photochemistry of the linkage-isomer system [Ni(Et4dien)(η2-O,ON)(η1-NO2)]

Isomerisation of an NO₂ ligand coordinated to Ni in a molecular crystal is explored using a range of quantum chemical techniques.

A facile single crystal to single crystal transition with significant structural contraction on desolvation

Ladder network, [Pb(ebdc)(MeOH)]₂·H₂O, undergoes a facile SCSC transformation to yield [Pb(ebdc)] with a 20% contraction of the crystallographic b axis.

Thermal and photochemical control of nitro–nitrito linkage isomerism in single-crystals of [Ni(medpt)(NO2)(η2-ONO)]

[Ni(medpt)(NO₂)(η²-ONO)] displays an equilibrium between the η¹-NO₂ and the η¹-ONO linkage isomers between 150–298 K; upon photoactivation at 100 K the percentage of the η¹-ONO isomer increases.