5 research outputs found
Multifaceted Deactivation Dynamics of Fe(II) <i>N</i>‑Heterocyclic Carbene Photosensitizers
Excited state dynamics
of three iron(II) carbene complexes that
serve as prototype Earth-abundant photosensitizers were investigated
by ultrafast optical spectroscopy. Significant differences in the
dynamics between the investigated complexes down to femtosecond time
scales are used to characterize fundamental differences in the depopulation
of triplet metal-to-ligand charge-transfer (3MLCT) excited
states in the presence of energetically accessible triplet metal-centered
(3MC) states. Novel insights into the full deactivation
cascades of the investigated complexes include evidence of the need
to revise the deactivation model for a prominent iron carbene prototype
complex, a refined understanding of complex 3MC dynamics,
and a quantitative discrimination between activated and barrierless
deactivation steps along the 3MLCT → 3MC → 1GS path. Overall, the study provides an improved
understanding of photophysical limitations and opportunities for the
use of iron(II)-based photosensitizers in photochemical applications
Fe<sup>II</sup> Hexa <i>N</i>‑Heterocyclic Carbene Complex with a 528 ps Metal-to-Ligand Charge-Transfer Excited-State Lifetime
The iron carbene complex [Fe<sup>II</sup>(btz)<sub>3</sub>](PF<sub>6</sub>)<sub>2</sub> (where btz
= 3,3′-dimethyl-1,1′-bis(<i>p</i>-tolyl)-4,4′-bis(1,2,3-triazol-5-ylidene))
has been
synthesized, isolated, and characterized as a low-spin ferrous complex.
It exhibits strong metal-to-ligand charge transfer (MLCT) absorption
bands throughout the visible spectrum, and excitation of these bands
gives rise to a <sup>3</sup>MLCT state with a 528 ps excited-state
lifetime in CH<sub>3</sub>CN solution that is more than one order
of magnitude longer compared with the MLCT lifetime of any previously
reported Fe<sup>II</sup> complex. The low potential of the [Fe(btz)<sub>3</sub>]<sup>3+</sup>/[Fe(btz)<sub>3</sub>]<sup>2+</sup> redox couple
makes the <sup>3</sup>MLCT state of [Fe<sup>II</sup>(btz)<sub>3</sub>]<sup>2+</sup> a potent photoreductant that can be generated by light
absorption throughout the visible spectrum. Taken together with our
recent results on the [Fe<sup>III</sup>(btz)<sub>3</sub>]<sup>3+</sup> form of this complex, these results show that the Fe<sup>II</sup> and Fe<sup>III</sup> oxidation states of the same Fe(btz)<sub>3</sub> complex feature long-lived MLCT and LMCT states, respectively, demonstrating
the versatility of iron <i>N-</i>heterocyclic carbene complexes
as promising light-harvesters for a broad range of oxidizing and reducing
conditions
Shedding Light on the Nature of Photoinduced States Formed in a Hydrogen-Generating Supramolecular RuPt Photocatalyst by Ultrafast Spectroscopy
Photoinduced
electronic and structural changes of a hydrogen-generating
supramolecular RuPt photocatalyst are studied by a combination of
time-resolved photoluminescence, optical transient absorption, and
X-ray absorption spectroscopy. This work uses the element specificity
of X-ray techniques to focus on the interplay between the photophysical
and -chemical processes and the associated time scales at the catalytic
Pt moiety. We observe very fast (<30 ps) photoreduction of the
Pt catalytic site, followed by an ∼600 ps step into a strongly
oxidized Pt center. The latter process is likely induced by oxidative
addition of reactive iodine species. The oxidized Pt species is long-lived
and fully recovers to the original ground state complex on a >10
μs
time scale. However, the photosensitizing Ru moiety is fully restored
on a much shorter ∼300 ns time scale. This reaction scheme
implies that we may withdraw two electrons from a catalyst that is
activated by a single photon
Ultrafast Time-Resolved X‑ray Absorption Spectroscopy of Ferrioxalate Photolysis with a Laser Plasma X‑ray Source and Microcalorimeter Array
The detailed pathways of photoactivity
on ultrafast time scales are a topic of contemporary interest. Using
a tabletop apparatus based on a laser plasma X-ray source and an array
of cryogenic microcalorimeter X-ray detectors, we measured a transient
X-ray absorption spectrum during the ferrioxalate photoreduction reaction.
With these high-efficiency detectors, we observe the Fe K edge move
to lower energies and the amplitude of the extended X-ray absorption
fine structure reduce, consistent with a photoreduction mechanism
in which electron transfer precedes disassociation. These results
are compared to previously published transient X-ray absorption measurements
on the same reaction and found to be consistent with the results from
Ogi et al. and inconsistent with the results of Chen et al. (Ogi, Y.; et al. Struct.
Dyn. 2015, 2, 034901; Chen, J.; Zhang, H.; Tomov, I. V.; Ding, X.; Rentzepis, P. M. Chem.
Phys. Lett. 2007, 437, 50–55). We provide quantitative
limits on the Fe–O bond length change. Finally, we review potential
improvements to our measurement technique, highlighting the future
potential of tabletop X-ray science using microcalorimeter sensors
Toward Highlighting the Ultrafast Electron Transfer Dynamics at the Optically Dark Sites of Photocatalysts
Building a detailed understanding
of the structure–function
relationship is a crucial step in the optimization of molecular photocatalysts
employed in water splitting schemes. The optically dark nature of
their active sites usually prevents a complete mapping of the photoinduced
dynamics. In this work, transient X-ray absorption spectroscopy highlights
the electronic and geometric changes that affect such a center in
a bimetallic model complex. Upon selective excitation of the ruthenium
chromophore, the cobalt moiety is reduced through intramolecular electron
transfer and undergoes a spin flip accompanied by an average bond
elongation of 0.20 ± 0.03 Å. The analysis is supported by
simulations based on density functional theory structures (B3LYP*/TZVP)
and FEFF 9.0 multiple scattering calculations. More generally, these
results exemplify the large potential of the technique for tracking
elusive intermediates that impart unique functionalities in photochemical
devices