10 research outputs found
Photophysical Study on the Effect of the External Potential on NiO-Based Photocathodes
In the present study, we investigate the effects of the
applied
external potential on a dye-sensitized NiO photocathode by time-resolved
photoluminescence and femtosecond transient absorption spectroscopy
under operating conditions. Instead of the anticipated acceleration
of photoinduced hole injection from dye into NiO at a more negative
applied potential, we observe that both hole injection and charge
recombination are slowed down. We cautiously assign this effect to
a variation in OHā ion concentration in the inner
Helmholtz plane of the electrochemical double layer with applied potential,
warranting further investigation for the realization of efficient
solar fuel devices
Bottom-Up Approach to Eumelanin Photoprotection: Emission Dynamics in Parallel Sets of Water-Soluble 5,6-Dihydroxyindole-Based Model Systems
The molecular mechanisms by which the black eumelanin
biopolymers
exert their photoprotective action on human skin and eyes are still
poorly understood, owing to critical insolubility and structural heterogeneity
issues hindering direct investigation of excitation and emission behavior.
Recently, we set up strategies to obtain water-soluble 5,6-dihydroxyindole
(DHI)-based polymers as useful models for disentangling intrinsic
photophysical properties of eumelanin components from aggregation
and scattering effects. Herein, we report the absorption properties
and ultrafast emission dynamics of two separate sets of DHI-based
monomerādimerāpolymer systems which were made water-soluble
by means of polyĀ(vinyl alcohol) or by galactosyl-thio substitution.
Data showed that dimerization and polymerization of DHI result in
long-lived excited states with profoundly altered properties relative
to the monomer and that glycosylation of DHI imparts monomer-like
behavior to oligomers and polymers, due to steric effects hindering
planar conformations and efficient interunit electron communication.
The potential of S-glycation as an effective tool to probe and control
emission characteristics of eumelanin-like polymers is disclosed
Excited-State Proton-Transfer Processes of DHICA Resolved: From Sub-Picoseconds to Nanoseconds
Excited-state proton transfer has
been hypothesized as a mechanism
for UV energy dissipation in eumelanin skin pigments. By using time-resolved
fluorescence spectroscopy, we show that the previously proposed, but
unresolved, excited-state intramolecular proton transfer (ESIPT) of
the eumelanin building block 5,6-dihydroxyindole-2-carboxylic acid
(DHICA) occurs with a time constant of 300 fs in aqueous solution
but completely stops in methanol. The previously disputed excited-state
proton transfer involving the 5- or 6-OH groups of the DHICA anion
is now found to occur from the 6-OH group to aqueous solvent with
a rate constant of 4.0 Ć 10<sup>8</sup> s<sup>ā1</sup>
Superior Photoprotective Motifs and Mechanisms in Eumelanins Uncovered
Human
pigmentation is a complex phenomenon commonly believed to
serve a photoprotective function through the generation and strategic
localization of black insoluble eumelanin biopolymers in sun exposed
areas of the body. Despite compelling biomedical relevance to skin
cancer and melanoma, eumelanin photoprotection is still an enigma:
What makes this pigment so efficient in dissipating the excess energy
brought by harmful UV-light as heat? Why has Nature selected 5,6-dihydroxyindole-2-carboxylic
acid (DHICA) as the major building block of the pigment instead of
the decarboxylated derivative (DHI)? By using pico- and femtosecond
fluorescence spectroscopy we demonstrate herein that the excited state
deactivation in DHICA oligomers is 3 orders of magnitude faster compared
to DHI oligomers. This drastic effect is attributed to their specific
structural patterns enabling multiple pathways of intra- and interunit
proton transfer. The discovery that DHICA-based scaffolds specifically
confer uniquely robust photoprotective properties to natural eumelanins
settles a fundamental gap in the biology of human pigmentation and
opens the doorway to attractive advances and applications
Impact of the Anchoring Ligand on Electron Injection and Recombination Dynamics at the Interface of Novel Asymmetric PushāPull Zinc Phthalocyanines and TiO<sub>2</sub>
Phthalocyanines
are promising photosensitizers for dye-sensitized
solar cells (DSSCs). A parameter that has been problematic for a long
time involves electron injection (EI) into the TiO<sub>2</sub>. The
development of pushāpull phthalocyanines shows great potential
to improve the ratio of EI to back electron transfer (BET). We have
studied the impact of the anchoring ligand on EI and BET using transient
absorption. The best performing derivative, which has a dicarboxylic
acid anchoring ligand (TT15, DSSC efficiency of 3.96%), shows the
fastest EI. The EI process occurs via an ultrafast component (ā¼700
fs for all derivatives) and a slower component (5.8 ps for TT15).
The ps component is considerably slower for the other derivatives
studied. Also BET depends on the anchoring ligand and is the slowest
for TT15. This knowledge is essential for the optimization of the
EI/BET ratio and the efficiency of a phthalocyanine-based DSSC
Directionality of Ultrafast Electron Transfer in a Hydrogen Evolving RuāPd-Based Photocatalyst
Directionality
of electron transfer and long-lived charge separation are of key importance
for efficient photocatalytic water splitting. Knowledge of the processes
that follow photoexcitation is essential for the optimization of supramolecular
assembly designs in order to improve the efficiency of photocatalytic
hydrogen generation. Photoinduced intramolecular electron transfer
processes within the hydrogen-evolving photocatalyst [RuĀ(bpy)<sub>2</sub>(tpy)ĀPdĀ(CH<sub>3</sub>CN)ĀCl]<sup>2+</sup> (<b>RuPd</b>; bpy = bipyridine, tpy = 2,2ā²:5ā²,2ā³-terpyridine)
have been studied by resonance Raman, femtosecond transient absorption,
and time-resolved photoluminescence spectroscopies. Comparison of
the photophysical properties of <b>RuPd</b> with those of the
mononuclear precursor [(bpy)<sub>2</sub>RuĀ(tpy)]<sup>2+</sup> (<b>Ru</b>) enables establishment of a photophysical model ranging
from the femtosecond to the submicrosecond domain. Optical excitation
of <b>Ru</b> and <b>RuPd</b> populates both bpy- and tpy-based <sup>1</sup>MLCT (metal-to-ligand charge transfer) singlet states, from
where intersystem crossing (ISC) into corresponding <sup>3</sup>MLCT
triplet states occurs. Electron density localized on the peripheral
bpy ligands can subsequently flow to the tpy bridging ligand by interligand
electron transfer, which process occurs with a time constant of 32.5
(Ā±1.5) ps for <b>RuPd</b>. Not all electron density undergoes
this process, most likely due to a competing loss channel on the bpy
ligand caused by vibrational relaxation occurring at a time scale
of 9.1 (Ā±0.4) ps. The relaxed <sup>3</sup>MLCT<sub>bpy</sub> and <sup>3</sup>MLCT<sub>tpy</sub> states have excited state lifetimes of
400 (Ā±1) ns and 88 (Ā±1) ns, respectively. Electron transfer
from the tpy ligand to Pd may take place on a ā¼100 ns time
scale, but it is also possible that the final relaxed excited state
is delocalized over the tpy ligand and the Pd center. The insight
that optical excitation populates both the peripheral bpy ligands
and the bridging tpy ligand, and that part of the electron density
subsequently flows from the former to the latter, is important for
the realization of efficient photocatalytic hydrogen generation. The
next step is to make the interligand electron transfer process faster,
by functionalizing the peripheral ligands with electron-donating moieties,
and adapting the nature of the bridging ligand and the catalytic metal
center
Disentangling Hot Carrier Decay and the Nature of Lowā<i>n</i> to Highā<i>n</i> Transfer Processes in Quasi-Two-Dimensional Layered Perovskites
Quasi-two-dimensional
(2D) metal halide perovskites (MHPs) are
promising photovoltaic (PV) materials because of their impressive
optical and optoelectronic properties and improved stability compared
to their 3D counterparts. The presence of domains with varying numbers
of inorganic layers between the organic spacers (n-phases), each with different bandgaps, makes the photoinduced carrier
dynamics in films of these materials complex and intriguing. Existing
interpretations of the ultrafast femto- or picosecond spectroscopy
data have been inconsistent, most of them focusing either on exciton/charge
transfer from low-n to high-n phases
or on hot carrier cooling, but not combined. Here, we present a comprehensive
study of the carrier dynamics in the DionāJacobson type (PDMA)(MA)(nā1)PbnI(3n+1) (PDMA = 1,4-phenylenedimethylammonium,
MA = methylammonium) perovskite, stoichiometrically prepared as āØnā© = 5. Within the film, a coexistence of various n-phases is observed instead of solely the n = 5 phase, resulting in an interesting energy landscape for the
motion of excitons and charge carriers. We disentangle hot carrier
cooling from exciton transfer between low-n and high-n phases using ultrafast time-resolved photoluminescence
and transient absorption spectroscopy. Photophysical modeling by target
analysis shows that carrier cooling occurring on a subpicosecond time
scale is followed by exciton transfer from low-n into
high-n phases in ca. 35 ps when the film is excited
by 532 or 490 nm light. Carriers in the high-n phase
are much longer lived and decay in a ns time window. Overall, our
results provide a comprehensive understanding of the photophysics
of this material, which helps to optimize quasi-2D MHP materials for
a new generation of PV devices
Ultrafast Anisotropy Decay Reveals Structure and Energy Transfer in Supramolecular Aggregates
Chlorosomes from green bacteria perform the most efficient
light
capture and energy transfer, as observed among natural light-harvesting
antennae. Hence, their unique functional properties inspire developments
in artificial light-harvesting and molecular optoelectronics. We examine
two distinct organizations of the molecular building blocks as proposed
in the literature, demonstrating how these organizations alter light
capture and energy transfer, which can serve as a mechanism that the
bacteria utilize to adapt to changes in light conditions. Spectral
simulations of polarization-resolved two-dimensional electronic spectra
unravel how changes in the helicity of chlorosomal aggregates alter
energy transfer. We show that ultrafast anisotropy decay presents
a spectral signature that reveals contrasting energy pathways in different
chlorosomes
Photon Energy-Dependent Ultrafast Exciton Transfer in Chlorosomes of Chlorobium tepidum and the Role of Supramolecular Dynamics
The antenna complex
of green sulfur bacteria, the chlorosome, is
one of the most efficient supramolecular systems for efficient long-range
exciton transfer in nature. Femtosecond transient absorption experiments
provide new insight into how vibrationally induced quantum overlap
between exciton states supports highly efficient long-range exciton
transfer in the chlorosome of Chlorobium tepidum. Our work shows that excitation energy is delocalized over the chlorosome
in <1 ps at room temperature. The following exciton transfer to
the baseplate occurs in ā¼3 to 5 ps, in line with earlier work
also performed at room temperature, but significantly faster than
at the cryogenic temperatures used in previous studies. This difference
can be attributed to the increased vibrational motion at room temperature.
We observe a so far unknown impact of the excitation photon energy
on the efficiency of this process. This dependency can be assigned
to distinct optical domains due to structural disorder, combined with
an exciton trapping channel competing with exciton transfer toward
the baseplate. An oscillatory transient signal damped in <1 ps
has the highest intensity in the case of the most efficient exciton
transfer to the baseplate. These results agree well with an earlier
computational finding of exciton transfer driven by low-frequency
rotational motion of molecules in the chlorosome. Such an exciton
transfer process belongs to the quantum coherent regime, for which
the FoĢrster theory for intermolecular exciton transfer does
not apply. Our work hence strongly indicates that structural flexibility
is important for efficient long-range exciton transfer in chlorosomes
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 ā¼600 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
Ī¼s
time scale. However, the photosensitizing Ru moiety is fully restored
on a much shorter ā¼300 ns time scale. This reaction scheme
implies that we may withdraw two electrons from a catalyst that is
activated by a single photon