Two-photon, visible light water splitting at a molecular ruthenium complex

Water splitting to give molecular oxygen and hydrogen or the corresponding protons and electrons is a fundamental four-electron redox process, which forms the basis of photosynthesis and is a promising approach to convert solar into chemical energy. Artificial water splitting systems have struggled with orchestrating the kinetically complex absorption of four photons as well as the difficult utilization of visible light. Based on a detailed kinetic, spectroscopic and computational study of Milstein's ruthenium complex, we report a new mechanistic paradigm for water splitting, which requires only two photons and offers a new method to extend the range of usable wavelengths far into the visible region. We show that two-photon water splitting is enabled by absorption of the first, shorter wavelength photon, which produces an intermediate capable of absorbing the second, longer wavelength photon (up to 630 nm). The second absorption then causes O–O bond formation and liberation of O2. Theoretical modelling shows that two-photon water splitting can be used to achieve a maximum solar-to-hydrogen efficiency of 18.8%, which could be increased further to 28.6% through photochemical instead of thermal H2 release. It is therefore possible to exceed the maximum efficiency of dual absorber systems while only requiring a single catalyst. Due to the lower kinetic complexity, intrinsic utilization of a wide wavelength range and high-performance potential, we believe that this mechanism will inspire the development of a new class of water splitting systems that go beyond the reaction blueprint of photosynthesis

Size dependent exciton dynamics in one-dimensional perylene bisimide aggregates

The size dependent exciton dynamics of one-dimensional aggregates of substituted perylene bisimides are studied by ultrafast transient absorption spectroscopy and kinetic Monte-Carlo simulations in dependence on the temperature and the excitation density. For low temperatures the aggregates can be treated as infinite chains and the dynamics is dominated by diffusion driven exciton-exciton annihilation. With increasing temperature the aggregates decompose into small fragments consisting of very few monomers. This scenario is also supported by the time dependent anisotropy deduced from polarization dependent experiments

Site-Selective Real-Time Observation of Bimolecular Electron Transfer in a Photocatalytic System Using L-Edge X-Ray Absorption Spectroscopy

Time-resolved X-ray absorption spectroscopy has been utilized to monitor the bimolecular electron transfer in a photocatalytic water splitting system. This has been possible by uniting the local probe and element specific character of X-ray transitions with insights from high-level ab initio calculations. The specific target has been a heteroleptic [IrIII (ppy)2 (bpy)]+ photosensitizer, in combination with triethylamine as a sacrificial reductant and Fe3(CO)12 as a water reduction catalyst. The relevant molecular transitions have been characterized via high-resolution Ir L-edge X-ray absorption spectroscopy on the picosecond time scale and restricted active space self-consistent field calculations. The presented methods and results will enhance our understanding of functionally relevant bimolecular electron transfer reactions and thus will pave the road to rational optimization of photocatalytic performance

Efficient Photocatalytic Water Reduction Using In Situ Generated Knölker's Iron Complexes

Inâ situ generated ironâ based Knölker complexes were found to be efficient catalysts in a fully nonâ noble metal Cuâ Fe photocatalytic water reduction system. These mononuclear iron catalysts were able to generate hydrogen up to 15 times faster than previously reported [Fe3(CO)12]. A reductive quenching mechanism was shown to operate by fluorescence experiments.Photo finish: Inâ situ generated ironâ based Knölker complexes are efficient catalysts in a fully nonâ noble metal Cuâ Fe photocatalytic water reduction system. These mononuclear iron catalysts generate hydrogen up to 15 times faster than previously reported [Fe3(CO)12]. A reductive quenching mechanism is shown to operate by fluorescence experiments. CuPS=copper(I) photosensitizer; SR=sacrificial reductant.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137356/1/cctc201600186.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137356/2/cctc201600186_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137356/3/cctc201600186-sup-0001-misc_information.pd

Widely tunable sub-30 fs ultraviolet pulses by chirped sum frequency mixing

A novel scheme for the generation of UV pulses in the 295 - 450 nm range is presented. Sum frequency mixing of the chirped visible pulses from a noncollinear optical parametric amplifier with deliberately chirped pulses from the Ti:sapphire amplifier ensures efficient energy conversion and easy tunability. Pulse energies as high as 5.5 J at 295 nm, and >2 J in most of the tuning range are obtained with highly symmetric and smooth spectra. They are compressed to sub-30 fs throughout the entire tuning range (20 fs at 348 nm) with a newly designed prism compressor.publishe

Switch of dimensionality of exciton diffusion in aggregates

The ultrafast exciton dynamics in J-aggregates of a perylene bisimide dye is investigated for temperatures down to 77 K revealing at low temperatures a decrease of the exciton mobility and a change in the dimensionality

Photodynamics of Fe complexes: Variation with number of NHC functions

Ultrafast spectroscopy on a series of Fe(II) complexes finds an increase of the 3MLCT lifetime with increasing number of N-heterocyclic carbene (NHC) donor functions revealing a promising route for the design of Fe photosensitizers