Time-resolved single-crystal X-ray crystallography

In this chapter the development of time-resolved crystallography is traced from its beginnings more than 30 years ago. The importance of being able to “watch” chemical processes as they occur rather than just being limited to three-dimensional pictures of the reactant and final product is emphasised, and time-resolved crystallography provides the opportunity to bring the dimension of time into the crystallographic experiment. The technique has evolved in time with developments in technology: synchrotron radiation, cryoscopic techniques, tuneable lasers, increased computing power and vastly improved X-ray detectors. The shorter the lifetime of the species being studied, the more complex is the experiment. The chapter focusses on the results of solid-state reactions that are activated by light, since this process does not require the addition of a reagent to the crystalline material and the single-crystalline nature of the solid may be preserved. Because of this photoactivation, time-resolved crystallography is often described as “photocrystallography”. The initial photocrystallographic studies were carried out on molecular complexes that either underwent irreversible photoactivated processes where the conversion took hours or days. Structural snapshots were taken during the process. Materials that achieved a metastable state under photoactivation and the excited (metastable) state had a long enough lifetime for the data from the crystal to be collected and the structure solved. For systems with shorter lifetimes, the first time-resolved results were obtained for macromolecular structures, where pulsed lasers were used to pump up the short lifetime excited state species and their structures were probed by using synchronised X-ray pulses from a high-intensity source. Developments in molecular crystallography soon followed, initially with monochromatic X-ray radiation, and pump-probe techniques were used to establish the structures of photoactivated molecules with lifetimes in the micro- to millisecond range. For molecules with even shorter lifetimes in the sub-microsecond range, Laue diffraction methods (rather than using monochromatic radiation) were employed to speed up the data collections and reduce crystal damage. Future developments in time-resolved crystallography are likely to involve the use of XFELs to complete “single-shot” time-resolved diffraction studies that are already proving successful in the macromolecular crystallographic field.</p

The intrinsic 3[dσ*pσ] emission of binuclear gold(I) complexes with two bridging diphosphane ligands lies in the near UV; emissions in the visible region are due to exciplexes

A near-UV triplet emission from [Au2(dcpm)2](ClO4)2 has been discovered. Studies on the spectroscopic properties of the complexes [Au2(dcpm)2]Y2 (Y=ClO4 -, PF6 -, CF3SO3 -, [Au(CN)2]-, Cl-, and I- ; dcpm = bis(dicyclohexylphosphanyl)methane) support the assignment of the high-energy emissions at 360-368 nm to the 3[dσ*pσ] excited state, adducts of which exhibit exciplex emissions in the visible region with solvent or counterions (see schematic diagram).link_to_subscribed_fulltex

Resonance Raman intensity analysis investigation of metal-metal bonded transitions: An examination of the1A2u ← 1A1g (5dσ* → 6pσ) transition of Pt2(P2O5H2)4 4-

A preliminary resonance Raman intensity analysis study of the 1A2u ← 1A1g (5dσ* → 6pσ) transition of (n-Bu4N)4[Pt2(P2O5H 2)4] in acetonitrile solution at room temperature is reported. The absolute resonance Raman and absorption intensities were simultaneously simulated using wavepacket calculations and a simple model. The best fit parameters indicate that the Pt - Pt bond length changes by about 0.225 Å in the initially excited 1A2u state relative to the ground state. This is in good agreement with previous studies on the vibronically structured absorption and emission spectra of low-temperature crystalline (n-Bu4N)4[Pt2(P2O5H 2)4] which suggested that the Pt - Pt bond length changes by about 0.21 Å in the 1,3A2u states. The resonance Raman intensity analysis demonstrated here can be generally applied to metal-metal bonded electronic transitions for compounds and sample conditions (such as room temperature liquids for many samples) which do not exhibit any vibronic structure. Copyright © 1999 John Wiley & Sons, Ltd.link_to_subscribed_fulltex

Electronic spectra and photophysics of platinum(II) complexes with α-diimine ligands. Mixed complexes with halide ligands

Emission properties have been studied for a series of compounds of the formula (L2)PtCl2, where L2 is N,N,N′,N′-tetramethylethylenediamine, 2,2′-bipyridine (bpy), 4,4′-Me2bpy, 5,5′-Me2bpy, 4,4′-(t-Bu)2bpy, 3,3′-(CH3OCO)2bpy, and 1,10-phenanthroline, and also for the compound Pt(bpy)I2. Most of them exhibit orange to red luminescence from a triplet ligand-field (3LF) state, both as solids and in glassy solution. These emissions are very broad (fwhm 2300-3400 cm-1 at 10 K) and structureless and are strongly Stokes-shifted from absorption. The two exceptions are the solid "red" form of Pt(bpy)Cl2, which exhibits a relatively narrow (fwhm 1050 cm-1 at 10 K), vibronically structured (Δν ∼ 1500 cm-1) red emission, and Pt(3,3′-(CH3OCO)2bpy)Cl2, which exhibits a broad (fwhm 2500 cm-1 at 10 K) but structured (Δν ∼ 1300 cm-1) orange emission. Both of these emissions are assigned to triplet metal-to-ligand charge-transfer (3MLCT) excited states. For the former compound, a linear-chain structure has destabilized a dσ*(dz2) level, yielding a dσ* → π*(bpy) state as the lowest energy excited state, while for the latter, the strongly electron-withdrawing substituents have stabilized a bpy π* level, yielding a dxz,yz → π*(bpy) state as the lowest energy excited state. The relative energies of the various types of excited states, including ligand 3ππ* states, are discussed in detail. The crystal structures of Pt(5,5′-Me2bpy)Cl2 (monoclinic Cc, Z = 4, a = 13.413(7) Å, b = 9.063(4) Å, c = 12.261(9) Å, β = 121.71(6)°) and Pt(3,3′-(CH3OCO)2bpy)Cl2 (triclinic P1, Z = 2, a = 7.288(2) Å, b = 9.932(3) Å, c = 11.881(5) Å, α = 98.04(3)°, β = 103.56(3)°, γ = 106.54(3)°) are reported. © 1993 American Chemical Society.link_to_subscribed_fulltex

Binuclear platinum(III) complexes. Preparation, structure, and dσ → dσ* spectrum of [Bu4N]2[Pt2(P2O5H2)4(CH3CN)2]

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Resonance raman investigation of the Au(I) - Au(I) interaction of the 1[dσ*pσ] Excited state of Au2(dcpm)2(ClO4)2 (dcpm = bis(dicyclohexylphosphine)methane)

We present a resonance Raman investigation of the lowest energy dipole- allowed absorption band of [Au2(dcpm)2](ClO4)2 (dcpm = bis(dicyclohexylphosphine)methane). The resonance Raman spectra provide the first experimental proof of the 5dσ*→6pσ electronic transition in dinuclear gold(I)-phosphine compounds. A resonance Raman intensity analysis of the spectra allows estimation of the structural changes of the [dσ*pσ] excited states relative to the ground state.link_to_subscribed_fulltex

Luminescent μ-ethynediyl and μ-butadiynediyl binuclear gold(I) complexes: Observation of 3(ππ*) emissions from bridging C n 2- units

The synthesis and X-ray structural and spectroscopic characterization for LAuC≡CAuL·4CHCl 3 and LAuC≡C-C≡CAuL·2CH 2Cl 2 (1·4CHCl 3 and 2·2CH 2Cl 2, respectively; L = PCy 3, tricyclohexylphosphine) are reported. The bridging C n 2- units are structurally characterized as acetylene or diacetylene units, with C≡C distances of 1.19(1) and 1.199(8) Å for 1·4CHCl 3 and 2·2CH 2Cl 2, respectively. An important consequence of bonding to Au(I) for the C n 2- moieties is that the lowest-energy electronic excited states, which are essentially acetylenic 3(ππ*) in nature, acquire sufficient allowedness via Au spin-orbit coupling to appear prominently in both electronic absorption and emission spectra. The origin lines for both complexes are well-defined and are observed at 331 and 413 nm for 1 and 2, respectively. Sharp vibronic progressions corresponding to v(C≡C) are observed in both emission and absorption spectra. The acetylenic 3(ππ*) excited state of 2 has a long lifetime (τ 0 = 10.8 μs) in dichloromethane at room temperature and is a powerful reductant (E°[Au 2 +/Au 2*] ≤ -1.85 V vs SSCE).link_to_subscribed_fulltex

Electronic and resonance Raman spectra of [Au2(CS3)2]2-. Spectroscopic properties of a 'short' Au(I)-Au(I) bond

The anion [Au 2(CS 3) 2] 2- has an unusually short Au-Au distance (2.80 Å) for a binuclear Au(I) complex. We report detailed Raman studies of the (n)Bu 4N + salt of this complex, including FT-Raman of the solid and UV/vis resonance Raman of dimethyl sulfoxide solutions. All five totally symmetric vibrations of the anion have been located and assigned. A band at Δv = 125 cm -1 is assigned to v(Au 2). The visible-region electronic absorption bands (384 (ε 30680) and 472 nm (ε 610 M -1 cm -1)) are attributable to CS 3 2- localized transitions, as confirmed by the dominance of v(sym)(C-S(exo)) (Δv = 951 cm -1) in RR spectra measured in this region. An absorption band at 314 nm (22 250 M -1 cm -1) is assigned as the metal-metal 1(dσ* → pσ) transition, largely because v(sym)(C-S(exo)) is not strongly enhanced in RR involving this band. Observation of the expected strong resonance enhancement of v(Au 2) was precluded as a result of masking by intense solvent Rayleigh scattering in the UV.link_to_subscribed_fulltex