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Abstract: Photoresponsive materials are an important contemporary research area with applications in, for example, energy and catalysis. Mechanistic information on solid-state photochemical reactions has traditionally come from spectroscopy and modelling, with crystallography limited to snapshots of endpoints and long-lived intermediates. Recent advances in X-ray sources and detectors have made it possible to follow solid-state reactions in situ with dynamic single-crystal X-ray diffraction (SCXRD) methods, allowing a full set of atomic positions to be determined over the course of the reaction. These experiments provide valuable structural information that can be used to interpret spectroscopic measurements and to inform materials design and optimisation.
Solid-state linkage isomers, where small-molecule ligands such as NO, NO2−, N2 and SO2 show photo-induced changes in binding to a transition metal centre, have played a leading role in the development of dynamic SCXRD methodology, since the movement of whole atoms and the predictable temperature dependence of the excited-state lifetimes make them ideal test systems. The field of “photocrystallography”, pioneered by Coppens in the late 1990s, has developed alongside advances in instrumentation and computing and can now provide the 3D structures of species with lifetimes down to femtoseconds.
In this chapter, we will review the development of photocrystallography experiments against linkage isomer systems, from the early identification of metastable species under continuous illumination, through measuring kinetics at low temperature, to recent experiments studying species with sub-second lifetimes. We will discuss the advances in X-ray sources and instrumentation that have made dynamic SCXRD experiments possible, and we will highlight the role of kinetic modelling and complementary spectroscopy in designing experiments. Finally, we will discuss possible directions for future development and identify some of the outstanding challenges that remain to be addressed.
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Oct 2020
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[18193]
Open Access
Abstract: Seven multi-component molecular crystals containing O–H⋯O/O+–H⋯O− and N+–H⋯O− short strong hydrogen bonds (SSHBs) have been engineered by combining substituted organic acids with hydrogen bond acceptor molecules N,N-dimethylurea and isonicotinamide. In these materials, the shortest of the SSHBs are formed in the N,N-dimethylurea set for the ortho/para nitro-substituted organic acids whilst a twisted molecular approach favours the shorter SSHBs N+–H⋯O− in the isonicotinamide set. Temperature dependent proton migration behaviour has been explored in these systems using single crystal synchrotron X-ray diffraction (SCSXRD). By using a protocol which considers a combination of structural information when assessing the hydrogen atom (H-atom) behaviour, including refined H-atom positions alongside heavy atom geometry and Fourier difference maps, temperature dependent proton migration is indicated in two complexes (2: N,N-dimethylurea 2,4-dinitrobenzoic acid 1:1 and 5: isonicotinamide phthalic acid 2:1). We also implement Hirshfeld atom refinement for further confidence in this observation; this highlights the importance of having corroborating trends when applying the SCSXRD technique in these studies. Further insights into the SSHB donor–acceptor distance limit for temperature dependent proton migration are also revealed. For the O–H⋯O/O+–H⋯O− SSHBs, the systems here support the previously proposed maximum limit of 2.45 Å whilst for the charge assisted N+–H⋯O− SSHBs, a limit in the region of 2.55 Å may be suggested.
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Aug 2019
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[8803]
Open Access
Abstract: Nine new molecular complexes of the proton sponge 1,8-bis(dimethylamino)naphthalene (DMAN) with substituted benzoic acid co-formers have been engineered with varying component stoichiometries (1[thin space (1/6-em)]:[thin space (1/6-em)]1, 1[thin space (1/6-em)]:[thin space (1/6-em)]2 or 1[thin space (1/6-em)]:[thin space (1/6-em)]3). These complexes are all ionic in nature, following proton transfer between the acid co-former and DMAN; the extracted proton is held by DMAN in all instances in an intramolecular [N–H⋯N]+ hydrogen bond. A number of structural features are common to all complexes and are found to be tunable in a predictable way using systematic acid co-former substitution. These features include charge-assisted hydrogen bonds formed between acid co-formers in hydrogen bonding motifs consistent with complex stoichiometry, and weak hydrogen bonds which facilitate the crystal packing of DMAN and acid co-former components into a regular motif. Possible crystal structure tuning by co-former substitution can aid the rational design of such materials, offering the potential to target solid-state properties that may be influenced by these interactions
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May 2018
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[13015]
Abstract: The uptake of gaseous iodine into the crystalline sponge material [(ZnI2)3(tpt)2]·0.7triphenylene·0.3PhNO2·0.7C6H12 1 (tpt = 2,4,6-tris(4-pyridyl)-1,3,5-triazine) has been monitored by dynamic X-ray diffraction and thermogravimetric analysis. The X-ray analyses have enabled the location, quantity, uptake rate, and subsequent chemistry of the iodine upon inclusion into the pores to be determined. An uptake of 6.0 wt % (0.43 I2 per formula unit) was observed crystallographically over a period of 90 min before crystal degradation occurred. The included iodine molecules interact with the iodine atoms of the ZnI2 nodes at three different sites, forming coordinated I3– ions. The results contrast to recent observations on [(ZnI2)3(tpt)2] without the triphenylene guests which show the presence of I42– ions with low quantities of absorbed iodine.
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Apr 2018
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[8567, 9734, 13698]
Open Access
Abstract: 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.
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Jan 2018
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I24-Microfocus Macromolecular Crystallography
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M. J.
Bryant
,
J. M.
Skelton
,
L. E.
Hatcher
,
C.
Stubbs
,
E.
Madrid
,
A. R.
Pallipurath
,
L. H.
Thomas
,
Ch. H.
Woodall
,
J.
Christensen
,
S.
Fuertes
,
T. P.
Robinson
,
C. M.
Beavers
,
S. J.
Teat
,
M.
Warren
,
F.
Pradaux-Caggiano
,
A.
Walsh
,
F.
Marken
,
D. R.
Carbery
,
S. C.
Parker
,
N. B.
Mckeown
,
R.
Malpass-Evans
,
M.
Carta
,
P. R.
Raithby
Open Access
Abstract: Selective, robust and cost-effective chemical sensors for detecting small volatile-organic compounds (VOCs) have widespread applications in industry, healthcare and environmental monitoring. Here we design a Pt(II) pincer-type material with selective absorptive and emissive responses to methanol and water. The yellow anhydrous form converts reversibly on a subsecond timescale to a red hydrate in the presence of parts-per-thousand levels of atmospheric water vapour. Exposure to methanol induces a similarly-rapid and reversible colour change to a blue methanol solvate. Stable smart coatings on glass demonstrate robust switching over 104 cycles, and flexible microporous polymer membranes incorporating microcrystals of the complex show identical vapochromic behaviour. The rapid vapochromic response can be rationalised from the crystal structure, and in combination with quantum-chemical modelling, we provide a complete microscopic picture of the switching mechanism. We discuss how this multiscale design approach can be used to obtain new compounds with tailored VOC selectivity and spectral responses.
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Dec 2017
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I19-Small Molecule Single Crystal Diffraction
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Christopher H.
Woodall
,
Jeppe
Christensen
,
Jonathan M.
Skelton
,
Lauren E.
Hatcher
,
Andrew
Parlett
,
Paul R.
Raithby
,
Aron
Walsh
,
Stephen C.
Parker
,
Christine M.
Beavers
,
Simon J.
Teat
,
Mourad
Intissar
,
Christian
Reber
,
David R.
Allan
Open Access
Abstract: We report a molecular crystal that exhibits four successive phase transitions under hydrostatic pressure, driven by aurophilic interactions, with the ground-state structure re-emerging at high pressure. The effect of pressure on two polytypes of tris(μ2-3,5-diisopropyl-1,2,4-triazolato-κ2N1:N2)trigold(I) (denoted Form-I and Form-II) has been analysed using luminescence spectroscopy, single-crystal X-ray diffraction and first-principles computation. A unique phase behaviour was observed in Form-I, with a complex sequence of phase transitions between 1 and 3.5 GPa. The ambient C2/c mother cell transforms to a P21/n phase above 1 GPa, followed by a P21/a phase above 2 GPa and a large-volume C2/c supercell at 2.70 GPa, with the previously observed P21/n phase then reappearing at higher pressure. The observation of crystallographically identical low- and high-pressure P21/n phases makes this a rare example of a re-entrant phase transformation. The phase behaviour has been characterized using detailed crystallographic theory and modelling, and rationalized in terms of molecular structural distortions. The dramatic changes in conformation are correlated with shifts of the luminescence maxima, from a band maximum at 14040 cm−1 at 2.40 GPa, decreasing steeply to 13550 cm−1 at 3 GPa. A similar study of Form-II displays more conventional crystallographic behaviour, indicating that the complex behaviour observed in Form-I is likely to be a direct consequence of the differences in crystal packing between the two polytypes.
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Sep 2016
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I19-Small Molecule Single Crystal Diffraction
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Abstract: The methods that have been developed to determine the three-dimensional crystal and molecular structures while they are in metastable or short-lived photoactivated states are described. The structural science of photocrystallography has developed over the last two decades because of the use of synchrotron radiation, coupled with advances in cryogenics, computer hardware and software, and laser technology. Initial studies have been carried out on metastable linkage isomers and LIESST-generated metastable structures and, more recently, by using the synchronisation of laser pulses with X-ray pulses, it has been possible to determine the structures of complexes with microsecond lifetimes. In the future X-ray Laue techniques and one-shot XFEL studies applied to molecular systems promise to make the study of sub-microsecond species a reality.
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Oct 2014
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I19-Small Molecule Single Crystal Diffraction
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Abstract: The X-ray scattering process occurs on the time scale of about 10-18 seconds; the complete data collection is in the order of hours at synchrotron sources and consequently gives a time-averaged structure of the crystalline material. Previously on beamline I19 at Diamond Light Source we have used a method which involves mechanically chopping the X-ray beam to produce a pulsed source. The pulsed X-ray beam can then be used to probe the crystal a short period after the sample has been photo-activated by a laser beam.
This method can be repeated changing the period between the laser (pump) and X-ray pulse (probe) until the entire time series is obtained. Beamline I19 in collaboration with the Dynamic Structural Sciences Consortium at the Research Complex at Harwell have designed a novel strategy to collect an entire time-series (zero to 100 ms) in one data collection utilising the fast image collection time
of the Pilatus detector. The 300K Pilatus detector has a readout out time of 2.7 ms and can be gated down to 200 ns. This means that we can use this gating (instead of the mechanical chopper) to obtain single crystal time-resolved structures. This technique shortens the data collection time and as the entire series is obtained from one crystal during the same data collection, this reduces decay and
scaling issues.
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Aug 2014
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Lauren E.
Hatcher
,
Edward J.
Bigos
,
Mathew J.
Bryant
,
Emily M.
Maccready
,
Thomas P.
Robinson
,
Lucy K.
Saunders
,
Lynne H.
Thomas
,
Christine M.
Beavers
,
Simon J.
Teat
,
Jeppe
Christensen
,
Paul R.
Raithby
Abstract: The known complex [Ni(medpt)(η1-NO2)(η2-ONO)] 1 (medpt = 3,3′-diamino-N-methyldipropylamine) crystallises in the monoclinic space group P21/m with 1.5 molecules in the asymmetric unit with two different η1-NO2 ligand environments in the crystal structure. At 298 K the molecule (A) sitting in a general crystallographic site displays a mixture of isomers, 78% of the η1-NO2 isomer and 22% of an endo-nitrito–(η1-ONO) form. The molecule (B) sitting on a crystallographic mirror plane adopts the η1-NO2 isomeric form exclusively. However, a variable temperature crystallographic study showed that the two isomers were in equilibrium and upon cooling to 150 K the η1-ONO isomer converted completely to the η1-NO2 isomer, so that both independent molecules in the asymmetric unit were 100% in the η1-NO2 form. A kinetic analysis of the equilibrium afforded values of ΔH = −9.6 (±0.4) kJ mol−1, ΔS = −21.5 (±1.8) J K−1 mol−1 and EA = −1.6 (±0.05) kJ mol−1. Photoirradiation of single crystals of 1 with 400 nm light, at 100 K, resulted in partial isomerisation of the η1-NO2 isomer to the metastable η1-ONO isomer, with 89% for molecule (A), and 32% for molecule (B). The crystallographic space group also reduced in symmetry to P21 with Z′ = 3. The metastable state existed up to a temperature of 150 K above which temperature it reverted to the ground state. An analysis of the crystal packing in the ground and metastable states suggests that hydrogen bonding is responsible for the difference in the conversion between molecules (A) and (B).
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Jun 2014
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