Microsecond-resolved XAFS of the triplet excited state of Pt2(P2O5H2)4–4

Abstract

Little is known about the excited-state structures of most inorganic compounds. Time-resolved resonance Raman and time-resolved infrared spectroscopies can provide only indirect structural information for short-lived excited species in solution at room temperature. Time-resolved X-ray diffraction has the potential to give more direct information, but no excited-state structures have yet been reported; picosecond gas-phase electron diffraction has been proposed recently, but not yet demonstrated. Here we report a technique that combines the X-ray absorption fine-structure (XAFS) method with rapid-flow laser spectroscopy to measure structural changes in a solution-phase excited-state transition-metal complex with microsecond resolution. We find that the triplet excited state of Pt2(P2O5H2)4(4-), with a lifetime of about 4 microseconds, undergoes a contraction in the Pt-Pt distance of 0.52 +/- 0.13 A relative to the ground state. We anticipate that time-resolved XAFS will have broad applications in chemistry and biology.

X-ray Direct Observation of Reactions and Labile Species on the Basis of Crystal Design

“…30,31 In 1993, Thiel et al reported 0.52 « 0.13 ¡ contraction in the PtPt distance in a liquid phase by EXAFS. 32 Other diffraction studies reached basically similar results indicating the contraction of PtPt distance in the excited state. In 1980, X-ray diffraction studies of a ground state K 4 [Pt 2 (pop) 4 ] crystal were first carried out by Sadler et al, who found the PtPt distance to be 2.925 ¡.…”

Synchrotron ultrafast techniques for photoactive transition metal complexes

“…Initially, the time resolution of these studies was limited to the ms-ns range [139][140][141][142], except for one groundbreaking case [143].Theoretical and experimental critical issues had to be considered to extend the feasibility of time-resolved XAS characterization to the sub-ns time domain.…”

Excited-state structure by time-resolved X-ray diffraction

“…A second study, based on Raman data and using the empirical Badger's rule, which relates a change in stretching frequency with a change in bond length, leads to a value of 2.81 A Ê (Stein et al, 1983). EXAFS results have been used to derive a contraction of 0.52 (13) A Ê of the P planes along the PtÐPt axis but are based on an assumed PtÐPt excited-state distance of 2.75 A Ê (Thiel et al, 1993). Theoretical DFT (density functional theory) calculations of the excited-state structure, to be published elsewhere (Novozhilova & Coppens, 2001), con®rm the shortening of the PtÐPt bond, though the theoretical value is quite dependent on the functionals used in the calculation.…”

“…The results indicate the feasibility of the approach, and point the way for the further re®nement of the methods employed. triplet state has been previously investigated by spectroscopic techniques (Rice & Gray, 1983;Stiegman et al, 1987) and by EXAFS (Thiel et al, 1993). The excited state is split into 3 E u and 3 A 1u levels, with a spacing of only 42 cm À1 (Fig.…”