Excited state molecular structure determination in disordered media using laser pump/X-ray probe time-domain X-ray absorption spectroscopy

Abstract

Advances in X-ray technologies provide opportunities for solving structures of photoexcited state molecules with short lifetimes. Using X-ray pulses from a modern synchrotron source, the structure of a metal-to-ligand-charge-transfer (MLCT) excited state of Cu1(dmp)2+ (dmp = 2,9-dimethyl-1,10-phenanthroline) was investigated by laser pump/X-ray probe X-ray absorption fine structure (LPXP-XAFS) in fluid solution at room temperature on a nanosecond time scale. The experimental requirements for such pump probe XAFS are described in terms of technical challenges: (1) conversion of optimal excited state population, (2) synchronization of the pump laser pulse and probe X-ray pulse, and (3) timing of the detection. Using a laser pump pulse for the photoexcitation, a photoluminescent MLCT excited state of Cu1(dmp)2(BArF), (dmp = 2,9-dimethyl-1,10-phenanthroline, BArF = tetrakis(3,5-bis(trifluoromethylphenyl)borate) with a lifetime of 98 +/- 5 ns was created. Probing the structure of this state at its optimal concentration using an X-ray pulse cluster with a total duration of 14.2 ns revealed that (1) a CuII center was generated via a whole charge transfer; (2) the copper in the MLCT state bound an additional ligand to form a penta-coordinate complex with a likely trigonal bipyramidal geometry; and (3) the average Cu-N bond length increases in the MLCT excited state by 0.07 A. In contrast to previously reported literature, the photoluminescence of this pentacoordinate MLCT state was not quenched upon ligation with the fifth ligand. On the basis of experimental results, we propose that the absorptive and emissive states have distinct geometries. The results represent X-ray characterization of a molecular excited state in fluid solution on a nanosecond time scale.

Tuberculosis and other bacterial co-infection in Cambodia: a single center retrospective cross-sectional study

“…While the lack of diagnostic microbiology facilities has limited study of LRI etiologies in Cambodia, available data suggest that Streptococcus pneumoniae , Haemophilus influenzae , Pseudomonas aeruginosa , Klebsiella pneumoniae , and Burkholderia pseudomallei are the most common etiologies of community-acquired pneumonia (CAP) in Cambodia and its neighbors [2]. Mycobacterium tuberculosis infection (TB) is also a major cause of respiratory disease in Cambodia, and the incidence of TB in 2017 (326 per 100,000 population) was among the highest in the world [3].…”

Photochemical Processes Revealed by X-ray Transient Absorption Spectroscopy

“…The 1–2 kHz pump–probe repetition rates seemed to be optimal then. Under such conditions, a spectrum for a sample with about a 1 mM metal center concentration could take several tens of hours and still have a low signal-to-noise ratio. − ,, In the last couple of years, this situation has been significantly improved by new insertion devices at the APS, such as inline dual undulators, enabling an increase of X-ray pulse photon flux by a factor of 10–100. Such a photon flux increase coupled with the use of now available high repetition rate pump lasers (e.g., from 10 kHz and even higher up to hundreds of kHz in Beamline 7ID), focused X-ray beams, and new detection systems has enabled efficient data acquisition and much higher signal-to-noise ratios in XTA spectra.…”

Applications of X-Ray Transient Absorption Spectroscopy in Photocatalysis for Hydrogen Generation

“…Differences in XANES spectra for [Cu(I)(dmp) 2 ] þ within 200 ps of the laser excitation in both solvents [84] are clearly seen (Figure 7.9A). After subtracting the contribution from the remaining ground state, the resulting MLCT spectra in both solvents resemble the ground state of [Cu(II)(dmp) 2 ] 2 þ (Figure 7.9B).…”

“…Our general approach is combining optical transient absorption and emission spectroscopy with the XTA studies, as well as quantum mechanical calculations to acquire a comprehensive understanding of the reaction and to provide feedback for synthetic chemistry in catalyst designs. The optical spectroscopy characterizes dynamics of excited states in terms of reaction rates, energetics, and coherence for energy/electron transfer, intersystem crossing, internal and overall rotation, as well as the fraction and stability of the excited state molecules created by the same laser system used in the XTA experiments [84][85][86][87][88][89][90].…”

“…We studied [Cu(I)(dmp) 2 ] þ (dmp, 2,9-dimethyl-1,10-phenanthroline) using both ultrafast transient optical spectroscopy and XTA [86]. The MLCT state dynamics of [Cu(I)(dmp) 2 ] þ fit a triexponential function, with two solvent-independent fast time constants of $0.5 ps and 15 ps, and one solvent-dependent slow time constant of 2 ns in acetonitrile (coordinating solvent), and 100 ns in toluene (noncoordinating solvent) [84,85,123]. The ultrafast transient optical spectroscopy and the DFT (density functional theory) results revealed two parallel excited-state pathways (Figure 7.8) [123,124]: (i) from the Franck-Condon (F-C) state to a flattened geometry then to the thermally equilibrated triplet MLCT state; and (ii) from the F-C state to a triplet MLCT state then flattening to reach thermal equilibrium [125].…”