RESUMEN
The ultra-bright femtosecond X-ray pulses provided by X-ray Free Electron Lasers (XFELs) open capabilities for studying the structure and dynamics of a wide variety of biological and inorganic systems beyond what is possible at synchrotron sources. Although the structure and chemistry at the catalytic sites have been studied intensively in both biological and inorganic systems, a full understanding of the atomic-scale chemistry requires new approaches beyond the steady state X-ray crystallography and X-ray spectroscopy at cryogenic temperatures. Following the dynamic changes in the geometric and electronic structure at ambient conditions, while overcoming X-ray damage to the redox active catalytic center, is key for deriving reaction mechanisms. Such studies become possible by using the intense and ultra-short femtosecond X-ray pulses from an XFEL, where sample is probed before it is damaged. We have developed methodology for simultaneously collecting X-ray diffraction data and X-ray emission spectra, using an energy dispersive spectrometer, at ambient conditions, and used this approach to study the room temperature structure and intermediate states of the photosynthetic water oxidizing metallo-protein, photosystem II. Moreover, we have also used this setup to simultaneously collect the X-ray emission spectra from multiple metals to follow the ultrafast dynamics of light-induced charge transfer between multiple metal sites. A Mn-Ti containing system was studied at an XFEL to demonstrate the efficacy and potential of this method.
Asunto(s)
Cristalografía por Rayos X , Electrones , Rayos Láser , Catálisis , Rayos XRESUMEN
Non-coherent sensitized red-to-green upconversion has been achieved utilizing platinum(II) tetraphenyltetrabenzoporphyrin (PtTPTBP) as the triplet sensitizer and a nearly quantitatively fluorescent meso-(2,6-dichloropyridyl)-substituted boron dipyrromethene (Cl2PyBODIPY) chromophore (Φ = 0.99 in toluene) as the energy acceptor/annihilator in deoxygenated toluene. Dynamic Stern-Volmer analysis revealed that PtTPTBP phosphorescence as quenched by Cl2PyBODIPY occurs with a KSV of 108,000 M(-1), yielding a triplet-triplet energy transfer rate constant of 2.3 × 10(9) M(-1) s(-1). Using a non-coherent red light-emitting diode excitation source centered at 626 nm, the incident power dependence responsible for generating singlet BODIPY fluorescence in the green was shown to traverse quadratic to linear regimes, the latter being achieved near 60 mW cm(-2). These data were consistent with a photochemical upconversion mechanism being responsible for generating singlet fluorescence from the Cl2PyBODIPY chromophores through sensitized triplet-triplet annihilation (TTA). Integrated delayed fluorescence transients were utilized to reveal the TTA efficiency for the Cl2PyBODIPY chromophore and saturated near 46%, representing the lower limit for the TTA process. Kinetic modelling of the delayed fluorescence transient produced from 1.5 mJ laser pulses (λex = 615 nm) revealed a maximum limiting TTA efficiency of 64% for this upconverting composition, implying that this is indeed an extremely relevant acceptor/annihilator composition for photochemical upconversion.
RESUMEN
A terminal and unsupported chromium(III) hydroxide is reported. The terminal hydroxide is used to synthesize the first example of a heterobimetallic Ti-O-Cr compound containing an unsupported µ-oxo bridge. The heterobimetallic complex exhibits a new absorbance at 288 nm (4.32 eV), which is assigned to a metal-to-metal charge transfer (MMCT) transition.