Picosecond excited state dynamics and excitation transport in solution and on surfaces

Time correlated single photon counting was employed in studies of excitation transport (ET) depolarization in 2- and 3-dimensional disordered systems. Reduced concentrations ranged up to ~3 in ethylene glycol solutions of DODCI (3-dimensional system) and ~5 in Langmuir-Blodgett monolayers containing octadecyl rhodamine B (2-dimensional system). Discrepancies between experiment and current ET theories (3-body Gochanour-Andersen-Fayer and/or 2-particle Huber) are small when artifacts due to reabsorption, dimerization (and concomitant trapping), solvent reorganization, and molecular reorientation are minimized or correctly modeled. Discrepancies persist in glycerol solutions of DODCI; they are attributed to orientational correlation between chromophores. The orientational correlation apparently arises from 3-dimensional liquid glycerol structure and extends to ~R[subscript]0;An optical shot noise limited detection scheme was developed which enabled pump-probe transient absorption studies of submonolayer rhodamine 640 adsorbed on fused silica and ZnO. The ground state recovery dynamics on silica are coverage dependent due to excitation trapping by dye aggregates. In contrast, the recovery on ZnO is significantly faster and essentially coverage independent. This provides strong evidence for efficient dye → semiconductor nonradiative excitation decay. ftn[superscript]1DOE Report IS-T-1291. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy

Structural dynamics probed by X-ray pulses from synchrotrons and XFELs

This review focuses on how short X-ray pulses from synchrotrons and XFELs can be used to track light-induced structural changes in molecular complexes and proteins via the pump–probe method. The upgrade of the European Synchrotron Radiation Facility to a diffraction-limited storage ring, based on the seven-bend achromat lattice, and how it might boost future pump–probe experiments are described. We discuss some of the first X-ray experiments to achieve 100 ps time resolution, including the dissociation and in-cage recombination of diatomic molecules, as probed by wide-angle X-ray scattering, and the 3D filming of ligand transport in myoglobin, as probed by Laue diffraction. Finally, the use of femtosecond XFEL pulses to investigate primary chemical reactions, bond breakage and bond formation, isomerisation and electron transfer are discussed

Picosecond excited state dynamics and excitation transport in solution and on surfaces

Time correlated single photon counting was employed in studies of excitation transport (ET) depolarization in 2- and 3-dimensional disordered systems. Reduced concentrations ranged up to ~3 in ethylene glycol solutions of DODCI (3-dimensional system) and ~5 in Langmuir-Blodgett monolayers containing octadecyl rhodamine B (2-dimensional system). Discrepancies between experiment and current ET theories (3-body Gochanour-Andersen-Fayer and/or 2-particle Huber) are small when artifacts due to reabsorption, dimerization (and concomitant trapping), solvent reorganization, and molecular reorientation are minimized or correctly modeled. Discrepancies persist in glycerol solutions of DODCI; they are attributed to orientational correlation between chromophores. The orientational correlation apparently arises from 3-dimensional liquid glycerol structure and extends to ~R[subscript]0;An optical shot noise limited detection scheme was developed which enabled pump-probe transient absorption studies of submonolayer rhodamine 640 adsorbed on fused silica and ZnO. The ground state recovery dynamics on silica are coverage dependent due to excitation trapping by dye aggregates. In contrast, the recovery on ZnO is significantly faster and essentially coverage independent. This provides strong evidence for efficient dye → semiconductor nonradiative excitation decay. ftn[superscript]1DOE Report IS-T-1291. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.</p

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Watching a signaling protein function: What has been learned over four decades of time-resolved studies of photoactive yellow protein

Photoactive yellow protein (PYP) is a signaling protein whose internal p-coumaric acid chromophore undergoes reversible, light-induced trans-to-cis isomerization, which triggers a sequence of structural changes that ultimately lead to a signaling state. Since its discovery nearly 40 years ago, PYP has attracted much interest and has become one of the most extensively studied proteins found in nature. The method of time-resolved crystallography, pioneered by Keith Moffat, has successfully characterized intermediates in the PYP photocycle at near atomic resolution over 12 decades of time down to the sub-picosecond time scale, allowing one to stitch together a movie and literally watch a protein as it functions. But how close to reality is this movie? To address this question, results from numerous complementary time-resolved techniques including x-ray crystallography, x-ray scattering, and spectroscopy are discussed. Emerging from spectroscopic studies is a general consensus that three time constants are required to model the excited state relaxation, with a highly strained ground-state cis intermediate formed in less than 2.4 ps. Persistent strain drives the sequence of structural transitions that ultimately produce the signaling state. Crystal packing forces produce a restoring force that slows somewhat the rates of interconversion between the intermediates. Moreover, the solvent composition surrounding PYP can influence the number and structures of intermediates as well as the rates at which they interconvert. When chloride is present, the PYP photocycle in a crystal closely tracks that in solution, which suggests the epic movie of the PYP photocycle is indeed based in reality