Femtosecond Infrared Spectroscopy Resolving the Multiplicity of High-Spin Crossover States in Transition Metal Iron Complexes.

Tuning the photophysical properties of iron-based transition-metal complexes is crucial for their employment as photosensitizers in solar energy conversion. For the optimization of these new complexes, a detailed understanding of the excited-state deactivation paths is necessary. Here, we report femtosecond transient mid-IR spectroscopy data on a recently developed octahedral ligand-field enhancing [Fe(dqp)2]2+ (C1) complex with dqp = 2,6-diquinolylpyridine and prototypical [Fe(bpy)3]2+ (C0). By combining mid-IR spectroscopy with quantum chemical DFT calculations, we propose a method for disentangling the 5Q1 and 3T1 multiplicities of the long-lived metal-centered (MC) states, applicable to a variety of metal-organic iron complexes. Our results for C0 align well with the established assignment toward the 5Q1, validating our approach. For C1, we find that deactivation of the initially excited metal-to-ligand charge-transfer state leads to a population of a long-lived MC 5Q1 state. Analysis of transient changes in the mid-IR shows an ultrafast sub 200 fs rearrangement of ligand geometry for both complexes, accompanying the MLCT → MC deactivation. This confirms that the flexibility in the ligand sphere supports the stabilization of high spin states and plays a crucial role in the MLCT lifetime of metal-organic iron complexes.

Femtosecond anisotropy excitation spectroscopy to disentangle the Q x and Q y absorption in chlorophyll a.

Chlorophyll a (Chl a) belongs to the most important and most investigated molecules in the field of photosynthesis. The Q-band absorption is central for energy transfer in photosystems and the relative orientation of the Q y transitions of interacting chlorophylls governs the energy transfer. Chl a was well investigated, but a quantitative separation of Q x and Q y contributions to the Q-band of the Chl a absorption spectrum is still missing. We use femtosecond Vis-pump - IR-probe anisotropy excitation spectroscopy to disentangle the overlapping electronic Q x and Q y contributions quantitatively. In an anisotropy excitation spectrum we trace the dichroic ratio of a single vibration, i.e. the keto C[double bond, length as m-dash]O stretching vibration at 1690 cm-1, as a function of excitation wavelength. The change in dichroic ratio reflects altering Q y and Q x contributions. We identified Q x00 (0-0 transition of Q x ) and Q x01 transition at (636 ± 1) nm and (607 ± 2) nm, respectively, and the Q y01 and Q y02 at (650 ± 6) nm, and (619 ± 3) nm, respectively. We find that Q x absorption, contributes to 50% to 72% at 636 nm and 49% to 71% at 606 nm to the Chl a absorption at room temperature. The Q band was well modelled by a single vibronic progression for the Q x and Q y transition of (700 ± 100) cm-1, and the energy gap between Q x00 and Q y00 was found to be (820 ± 60) cm-1. This precise description of the hexa-coordinated Chl a absorption spectrum will foster more accurate calculations on energy transfer processes in photosystems, and advance the detailed understanding of the intricate interaction of chlorophyll molecules with the solvent.

Ultrafast Electron Transfer in a Self-Assembling Sulfonated Aluminum Corrole-Methylviologen Complex.

Photoinduced electron transfer systems can mimic certain features of natural photosynthetic reaction centers, which are crucial for solar energy production. Among other tetra-pyrroles, the versatile chemical and photophysical properties of corroles make them very promising donors applicable in donor-acceptor complexes. Here, we present a first comprehensive study of ultrafast photoinduced electron transfer in a self-assembling sulfonated aluminum corrole-methylviologen complex combining visible and mid-IR transient absorption spectroscopy. The noncovalent D-A association of the corrole-methylviologen complex has the great advantage that photoinduced charge separation becomes possible even though the back electron transfer (BET) rate is large. Initial forward electron transfer from corrole to methylviologen is observed on an ∼130 fs time scale. Subsequent back electron transfer takes place with τBET = (1.8 ± 0.5) ps, revealing very complex relaxation dynamics. Direct probing in the mid-IR allows us to unravel the back electron transfer and cooling dynamics/electronic reorganization. Upon tracing the dynamics of the methylviologen-radical marker band at 1640 cm-1 and the C═C stretching of corrole at around 1500 cm-1, we observe that large amounts of excess energy survive the back transfer, leading to the formation of hot ground state absorption. A closer examination of the signal after 300 ps, surviving the back transfer, exhibits a charge-separation yield of 10-15%.

Author Correction: Highly (001)-textured p-type WSe2 Thin Films as Efficient Large-Area Photocathodes for Solar Hydrogen Evolution.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Highly (001)-textured p-type WSe2 Thin Films as Efficient Large-Area Photocathodes for Solar Hydrogen Evolution.

Highly (001)-textured, photoactive WSe2 thin films have been prepared by an amorphous solid-liquid-crystalline solid process promoted by palladium. By increasing the thickness of the Pd promoter film (≥10 nm) the structure and texture of the WSe2 films can be improved significantly. However, these as-crystallized WSe2 films are only weakly photoactive in a 0.5 М H2SO4 electrolyte under AM 1.5 solar irradiation which we attribute to an inefficient photogenerated charge transfer across the WSe2/electrolyte interface via the prevailing van der Waals planes of the WSe2 crystallites. In this work photochemically deposited platinum on the p-type WSe2 photocathode is used for an efficient electron transfer thus inducing the hydrogen evolution reaction. Upon illuminating the WSe2 photocathodes in a Pt-ion containing electrolyte, the photogenerated electrons reduce Pt+ to Pt leading to the precipitation of Pt islands, preferentially at edge steps of the WSe2, i.e. at the grain boundaries of the WSe2 crystallites. The increasing amount of Pt islands at the grain boundaries linearly enhances the photocurrent density up to 2.5 mA cm-2 at 0 VRHE in sulfuric acid, the highest reported value up to now for WSe2 thin films.

Ultrafast Dynamics of Sb-Corroles: A Combined Vis-Pump Supercontinuum Probe and Broadband Fluorescence Up-Conversion Study.

Corroles are a developing class of tetrapyrrole-based molecules with significant chemical potential and relatively unexplored photophysical properties. We combined femtosecond broadband fluorescence up-conversion and fs broadband Vis-pump Vis-probe spectroscopy to comprehensively characterize the photoreaction of 5,10,15-tris-pentafluorophenyl-corrolato-antimony(V)-trans-difluoride (Sb-tpfc-F2). Upon fs Soret band excitation at ~400 nm, the energy relaxed almost completely to Q band electronic excited states with a time constant of 500 ± 100 fs; this is evident from the decay of Soret band fluorescence at around 430 nm and the rise time of Q band fluorescence, as well as from Q band stimulated emission signals at 600 and 650 nm with the same time constant. Relaxation processes on a time scale of 10 and 20 ps were observed in the fluorescence and absorption signals. Triplet formation showed a time constant of 400 ps, with an intersystem crossing yield from the Q band to the triplet manifold of between 95% and 99%. This efficient triplet formation is due to the spin-orbit coupling of the antimony ion.