Making Molecular Movies Time Resolved Xray Diffraction

1
Making Molecular Movies Time Resolved X-ray
Diffraction

• Paul Raithby
• University of Bath
• e-mail p.r.raithby_at_bath.ac.uk

2
Small Molecule Crystallography

• Advantages
• Provides a full 3D picture of a molecule.
• Provides valuable information about
  intermolecular interactions.
• Allows structure/property correlations to be made.

• Disadvantages
• Can only be used when single crystals are
  available.
• Only gives a solid state picture.
• Gives a time averaged (ground state) structure.

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Solid State Structural Processes

• Solid-gas reactions
• O. M. Yaghi, Science, 2003, 300, 1127.
• Solid-solid reactions
• D. Braga et.al., Chem. Commun. 2002, 2302.
• Isomerisation
• Spin crossovers
• J. A. K. Howard, et. al., Dalton Trans., 2004, 65
• Molecular activation within the crystal by
  external agents.

• Methods of Activation
• Photochemical activation
• Thermal activation
• Pressure activation
• Triboluminescence piezoelectric activation
• Magnetic activation

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Dynamic Diffraction Experiments

• Moffat and Wulff have shown that it is possible
  follow crystallographic transformations in
  protein systems using time resolved Laue
  diffraction techniques, but usually not down to
  atomic resolution. (M. Wulff, K. Moffat, et. al.,
  Science, 1998, 279, 1946 M. Wulff, P. A.
  Anfinrud, et. al., Science, 2003, 300, 1944)
• Chen has used time resolved EXAFS techniques to
  follow fast inorganic reactions in solution. (L.
  X. Chen, Angew. Chem., Int. Ed., 2004, 43, 2886 )

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Time Resolved Crystallography

• Bring the fourth dimension of TIME into the
  crystallographic experiment.
• Potential to make molecular movies.
• Single crystal single crystal transformations.
• Photo-activate crystals using laser irradiation
  (PHOTOCRYSTALLOGRAPHY).

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PHOTOCRYSTALLOGRAPHY

• The generation of a metastable or transient
  excited state species by the irradiation of a
  single crystal by a light source (laser).
• The structure determination of the excited
  state species by single crystal X-ray
  diffraction (laboratory or synchrotron
  radiation).
• P. Coppens, et. al., Dalton Trans., 1998, 865 P.
  Coppens, I. I. Vorontsov, T. Graber, M. Gembicky
  and A. Yu. Kovalesky, Acta Crystallogr., Sect. A,
  2005, 61, 162.

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Photocrystallography Processes Applications

• Digital light displays
• Fluorescent and phosphorescent screens
• Optical switches and shock-wave triggers

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A Knowledge of the Structure/Property Correlation
in Light-driven Processes

• The controlling light phenomena for the
  applications are all the result of electronic
  charge-transfer within a molecule or ionic array.
• Occur in various manifestations
• Light-induced fluorescence
• Light-induced phosphorescence
• Electroluminescence
• Triboluminescence
• Chemiluminescence
• Sono-luminescence
• J. M. Cole, Chem. Soc. Rev., 2004, 33, 501

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The First StepDont try to run before you can
walk!

• When considering photocrystallographic single
  crystal experiments, many inorganic systems form
  metastable intermediates at low temperatures,
  upon laser irradiation of the appropriate
  wavelength.
• These metastable states may be stable for hours
  once the crystal has been irradiated for a
  sufficient period, and if the low temperature is
  maintained.

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The Experimental Procedure for Determining the
Structure of a Metastable Crystalline Product

• From the UV absorption spectrum determine the
  appropriate wavelength for irradiation (not the
  absorption maximum!).
• Calculate the optical penetration depth from the
  absorption coefficient of the crystal, and pick a
  crystal of optimum size taking into account the
  intensity of the X-ray source as well as the
  light source.
• On the diffractometer cool the crystal to an
  appropriate temperature (30 100 K).
• Carry out a standard, accurate low temperature
  data collection (the ground state structure).

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U.V. Absorption Spectrum
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Metastable Structure Determination - Experimental
II

• Once the ground state structure has been
  obtained, irradiate the crystal until an
  appropriate percentage of molecules have been
  excited up into the metastable state (e.g. 20,
  at higher concentrations the crystal may
  explode).
• Switch off laser but maintain the low
  temperature.
• Collect a full set of accurate X-ray data using
  same experimental procedures as ground state
  (combined ground and metastable stable
  structures).
• Use combined data and refine using ground state
  coordinates see additional features
  corresponding to the metastable structure.
  Subtract ground state structure, leaving
  metastable structure.

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Experimental Arrangement
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Bruker CCD Diffractometer on Station 9.8
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with Helix Low Temperature Device and Laser in
Position
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Pond liner and the Laser
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Easy isnt it? Factors to Consider

• The crystal symmetry of the sample
• The detector coverage, sensitivity and readout
  time
• X-ray diffraction intensity
• X-ray wavelength

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A Couple of Metastable Examples
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Metastable Transition Metal Nitrosyl Complexes

• Sodium nitroprusside dihydrate Na2Fe(CN)5(NO).2H
  2O (SNP)
• Two light-induced metastable states, MS1 and MS2.
• MS2 decays at a lower temperature than MS1.
• MS1 and MS2 are linkage isomers of SNP.

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Metastable States in the Iron Nitrosyl Complex
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Iron Nitrosyl Isomerism in SNP
Coppens et. al., JACS, 1997, 119, 2669
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Isomerism in SO2 Complexes
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Metastable Structure of a SO2-containing
Ruthenium Complex

• Trans-Ru(NH3)4(SO2)ClCl
• IR spectrum of KBr pellets of the complex at 195K
  upon irradiation with 365 nm light showed two new
  IR bands at 1165 and 940 cm-1 in addition to
  those at 1255 and 1110 cm-1 in the non-irradiated
  sample.
• Metastable structure determination.

Coppens, Cole, et. al., Inorg. Chem., 2003, 42,
140.
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Photo-isomerisation study on RuII(NH3)4Cl(SO2)
Cl
O
355nm
O
S
Ru S Ru
O
O-
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A Second Metastable Isomer of Ru(NH3)4(H2O)(SO2)
2
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?1-SO2 bonding mode at 13K MS1
Cole, Raithby, et. al., Chem. Commun., 2006, 2448.
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Ru(NH3)4(H2O)(SO2)2 - GS, MS2, MS1 and whole
picture
(a)
(b)
(d)
(c)
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Complexes with Short-lived Excited States

• Many interesting chemical species have lifetimes
  in the ns - ?s range.
• Many fast processes, such as electron transfer
  between molecules, are of crucial importance in
  chemistry and biology.
• Can no longer establish a steady state
  concentration by long laser irradiation.
• Instead, establish instantaneous non-equilibrium
  concentrations and probe before significant decay
  occurs.
• This can be achieved by synchronizing pulsed
  laser irradiation with the inherent time
  structure of synchrotron radiation.

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Station 9.8 at the Daresbury Laboratory
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Excited (left) and ground (right) states of
co-crystal 2,2-dihydrobenzophenone
4,13-diaza-18-crown-6
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Experimental Set-up Depends on Lifetime of Sample
to be Studied

• All experiments at 30 K as this usually lengthens
  the lifetime of the excited state species.
• For samples with lifetimes in range ?s
  milliseconds, a pulsed laser with a mechanical
  chopper to interrupt the synchrotron X-ray beam.
  X-rays should hit the sample just after it has
  been excited excited by the laser.
• For samples with lifetimes in ns - ?s range, the
  time structure of the synchrotron is used.
  Particles move round the synchrotron ring in
  bunches with a very ordered time structure. The
  laser pulse is synchronised to hit the sample
  just before (ns) the X-ray pulse.

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Stroboscopic method in which a pulsed laserbeam
is synchronized with probing X-ray pulses
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The Mechanical Chopper
P. Coppens, et al., J. Appl. Cryst., 1998, 31,
128 Cole Husheer, unpublished results
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The Experiment

• The repetition rate of the X-ray pulses is
  synchronised with that of the pulsed laser and
  the timing of the opening of the chopper is
  matched to the shutter opening of the laser so
  that both the optical and X-ray pulses are either
  on or off at a given instant.
• The on periods correspond to the structure of
  the X photo-excited state together with 100-X
  of non excited ground state, while the off
  periods relate to the ground state.

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The First Time Resolved Experiment

• The Pt2(pop)44- tetra-anion has a triplet
  excited state lifetime of 50 ?s at 17 K.
• A stroboscopic pump-probe technique used where
  short, chopped X-ray pulses (synchrotron
  radiation) are synchronised with pulses of an
  exciting laser beam.

Coppens, et. al., J. Am. Chem. Soc., 2003, 125,
1079.
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The ESRF, Grenoble
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Luminescent Rhenium Carbene Complex
Re(HNCH2CH2NH)(2,2-bipy)(CO)3Br

• 3MLCT state 3d(Re) ? ?(diimine).
• Excited state should exhibit reduced
  Re-C(carbene) double bond character.
• Lifetime 230 ns in dichloromethane at room
  temperature.
• Laser wavelength, 400 nm, power 50?J.

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Structure of the Rhenium Carbene Complex
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Crystals of the Rhenium Carbene Complex Before,
During and After Irradiation
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The Future Pulsed X-rays using the Time
structure of a Synchrotron

• For crystals with ps-ms lifetimes, the temporal
  structure of a synchrotron can be used to provide
  the X-ray pulses.
• Electrons are accelerated around a synchrotron
  ring in discreet bunches.
• Accelerator physics can be used to harness these
  bunches individually to provide pulsed X-rays on
  a ps-ms timescale, or bunched together to provide
  a train of pulses.

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The diamond Synchrotron
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Acknowledgements

• Dr Jacqui Cole, Shamus Husheer, Dr Katherine
  Bowes (Cambridge)
• Dr Hazel Sparkes, Dr Andy Johnson, Dr Olivia
  Koentjoro and Teresa Savarese (Bath)
• Professor Philip Coppens (SUNY Buffalo, USA)
• Drs Simon Teat, John Warren and Graham
  Bushnell-Wye (Daresbury Laboratory)
• Drs Michael Wulff, Friedrich Schotte and Anton
  Plech (ESRF)
• Drs Tony Parker and Pavel Matousek (Central Laser
  Facility, RAL)
• Funding Royal Society, CLRC Daresbury
  Laboratory, University of Bath.