Time-resolved synchrotron diffraction and theoretical studies of very short-lived photo-induced molecular species

Abstract

Definitive experimental results on the geometry of fleeting species are at the time of writing still limited to monochromatic data collection, but methods for modifications of the polychromatic Laue data to increase their accuracy and their suitability for pump-probe experiments have been implemented and are reviewed. In the monochromatic experiments summarized, excited-state conversion percentages are small when neat crystals are used, but are higher when photoactive species are embedded in an inert framework in supramolecular crystals. With polychromatic techniques and increasing source brightness, smaller samples down to tenths of a micrometre or less can be used, increasing homogeneity of exposure and the fractional population of the excited species. Experiments described include a series of transition metal complexes and a fully organic example involving excimer formation. In the final section, experimental findings are compared with those from theoretical calculations on the isolated species. Qualitative agreement is generally obtained, but the theoretical results are strongly dependent on the details of the calculation, indicating the need for further systematic analysis.

Figure 1

Average population as a function of X-ray probe duration.

Figure 2

Experimental spectrum (blue) versus wavelength normalization (λ) curve.

Figure 3

A single-pulse diffraction pattern for a (CuCl)₄ cubane complex.

Figure 4

Spot shape as a function of the delay time between the pump and probe pulses. The weaker spot on the left expands less than the spot on the right.

Figure 5

Pixel intensities (left) and results of the seed-skewness analysis of the spot shape (right).

Figure 6

Transmission for a 10 µm crystal as a function of the extinction coefficient ∊ (in m² mol⁻¹) and the molarity of the crystal.

Figure 7

Contraction of the Rh—Rh distance in the excited triplet state of Rh₂(1,8-diisocyano-p-menthane)₄ ²⁺ (Coppens et al., 2004 ▶).

Figure 8

Left: the {[3,5-(CF₃)₂pyrazolate]Cu}₃ trimer. Right: packing of the molecules in the crystal. Distances shown are between atoms.

Figure 9

Excited-state geometry of one of the independent molecules (orange) superimposed on the ground state of the complex (Cu: green; C: black; P: purple; N, blue). The change in rocking distortion and the displacement of aromatic rings from their ground-state planes on excitation are evident. [Reprinted with permission from Vorontsov et al. (2009 ▶). Copyright 2009 American Chemical Society.]

Figure 10

Left: the THPE molecule. Right: the [Cu(NH₃)₂]₂ ⁺⁺ cation embedded in an anionic framework of [H₂THPE]⁻ (Zheng et al., 2005 ▶). Copyright Wiley-VCH Verlag GmbH & Co. KGA. Reproduced with permission.

Figure 11

Bond shortening and molecular rotation of the [Cu(NH₃)₂]₂ ⁺⁺ ion on excitation. Filled lines: ground state. Open broken lines: excited state.

Figure 12

Three-dimensional view of HECR-2xanthone-6MeOH containing dimeric xanthone (Zheng & Coppens, 2005b ▶). Copyright Wiley-VCH Verlag GmbH & Co. KGA. Reproduced with permission.

Figure 13

Change in the xanthone dimer on excimer formation. (a) Full lines: ground state; broken lines: excited state. (b) Sideways view. Note the relative offset of the molecular planes. [Coppens et al. (2006 ▶) – Reproduced by permission of the Royal Society of Chemistry.]

Figure 14

Summary of theoretical and experimental results for Pt—Pt and Pt—P bond-length changes upon (3dσ → 4pσ) excitation, and TD-DFT and experimental excitation energies. [Reprinted with permission from Novozhilova et al. (2003 ▶). Copyright 2003 American Chemical Society.]

Figure 15

Rh—Rh potential energy curves of the ground state (top) and the first triplet state (bottom) of [Rh₂(1,8-diisocyano-p-menthane)₄]²⁺ BP86/SV(P). Calculated energy differences between the theoretical-optimized and experimental geometries are 0.37 and 7.3 kJ mol⁻¹ for the ground and triplet states, respectively.

Figure 16

Frontier molecular orbitals of the dinuclear Rh complexes. Isosurface values at 0.03 a.u. for the LUMO, 0.02 a.u. for the HOMO.

Figure 17

Top view of the Cu pyrazolate trimer, indicating the interplanar Cu—Cu distance (pointed out by the blue arrow) which is shortened on excitation from 4.020 (1) to 3.46 (1) Å.

Figure 18

The HOMO seen from above (left) with the bond-forming Cu atoms [Cu(1)] in the center, and the LUMO, seen sideways, of the {[3,5-(CF₃)₂Pz]Cu}₃ trimer. Surfaces are at an isodensity value of ±0.03 a.u. (Vorontsov et al., 2005 ▶).

Figure 19

Schematic representation of the distortions of the [Cu^I(dmp)(diphos)]⁺ complex (molecule 1), with the distortions as defined by Dobson et al. (1984 ▶) for Cu(dmp)₂ complexes. The θ_x and θ_y angles describe the rocking and wagging distortions, and θ_z the flattening distortion. The coordinate system is chosen such that triangle N—Cu—N lies in the XZ plane. The unit vector ξ bisects the P—Cu—P angle; the unit vector η is perpendicular to the P—Cu—P plane. [Reprinted with permission from Vorontsov et al. (2009 ▶). Copyright 2009 American Chemical Society.]