7 research outputs found
Distinguishing Electronic and Vibronic Coherence in 2D Spectra by Their Temperature Dependence
Long-lived oscillations in 2D spectra of chlorophylls are at the heart of an ongoing debate. Their physical origin is either a multipigment effect, such as excitonic coherence, or localized vibrations. We show how relative phase differences of diagonal- and cross-peak oscillations can distinguish between electronic and vibrational (vibronic) effects. While direct discrimination between the two scenarios is obscured when peaks overlap, their sensitivity to temperature provides a stronger argument. We show that vibrational (vibronic) oscillations change relative phase with temperature, while electronic oscillations are only weakly dependent. This highlights that studies of relative phase difference as a function of temperature provide a clear and easily accessible method to distinguish between vibrational and electronic coherences
A Unified Picture of S* in Carotenoids
In
Ï-conjugated chain molecules such as carotenoids, coupling
between electronic and vibrational degrees of freedom is of central
importance. It governs both dynamic and static properties, such as
the time scales of excited state relaxation as well as absorption
spectra. In this work, we treat vibronic dynamics in carotenoids on
four electronic states (|S<sub>0</sub>â©, |S<sub>1</sub>â©,
|S<sub>2</sub>â©, and |S<sub>n</sub>â©) in a physically
rigorous framework. This model explains all features previously associated
with the intensely debated S* state. Besides successfully fitting
transient absorption data of a zeaxanthin homologue, this model also
accounts for previous results from global target analysis and chain
length-dependent studies. Additionally, we are able to incorporate
findings from pump-deplete-probe experiments, which were incompatible
to any pre-existing model. Thus, we present the first comprehensive
and unified interpretation of S*-related features, explaining them
by vibronic transitions on either S<sub>1</sub>, S<sub>0</sub>, or
both, depending on the chain length of the investigated carotenoid
System-Dependent Signatures of Electronic and Vibrational Coherences in Electronic Two-Dimensional Spectra
In this work, we examine vibrational coherence in a molecular monomer,
where time evolution of a nuclear wavepacket gives rise to oscillating
diagonal- and off-diagonal peaks in two-dimensional electronic spectra.
We find that the peaks oscillate out-of-phase, resulting in a cancellation
in the corresponding pumpâprobe spectra. Our results confirm
the unique disposition of two-dimensional electronic spectroscopy
(2D-ES) for the study of coherences. The oscillation pattern is in
excellent agreement with the diagrammatic analysis of the third-order
nonlinear response. We show how 2D-ES can be used to distinguish between
ground- and excited-state wavepackets. On the basis of our results,
we discuss coherences in coupled molecular aggregates involving both
electronic and nuclear degrees of freedom. We conclude that a general
distinguishing criterion based on the experimental data alone cannot
be devised
Vibronic and Vibrational Coherences in Two-Dimensional Electronic Spectra of Supramolecular JâAggregates
In J-aggregates of cyanine dyes,
closely packed molecules form
mesoscopic tubes with nanometer-diameter and micrometer-length. Their
efficient energy transfer pathways make them suitable candidates for
artificial light harvesting systems. This great potential calls for
an in-depth spectroscopic analysis of the underlying energy deactivation
network and coherence dynamics. We use two-dimensional electronic
spectroscopy with sub-10 fs laser pulses in combination with two-dimensional
decay-associated spectra analysis to describe the population flow
within the aggregate. Based on the analysis of Fourier-transform amplitude
maps, we distinguish between vibrational or vibronic coherence dynamics
as the origin of pronounced oscillations in our two-dimensional electronic
spectra
Photoinduced BâCl Bond Fission in Aldehyde-BCl<sub>3</sub> Complexes as a Mechanistic Scenario for CâH Bond Activation
In
concert with carbonyl compounds, Lewis acids have
been identified
as a versatile class of photocatalysts. Thus far, research has focused
on activation of the substrate, either by changing its photophysical
properties or by modifying its photochemistry. In this work, we expand
the established mode of action by demonstrating that UV photoexcitation
of a Lewis acidâbase complex can lead to homolytic cleavage
of a covalent bond in the Lewis acid. In a study on the complex of
benzaldehyde and the Lewis acid BCl3, we found evidence
for homolytic BâCl bond cleavage leading to formation of a
borylated ketyl radical and a free chlorine atom only hundreds of
femtoseconds after excitation. Both time-dependent density functional
theory and transient absorption experiments identify a benzaldehyde-BCl2 cation as the dominant species formed on the nanosecond time
scale. The experimentally validated BâCl bond homolysis was
synthetically exploited for a BCl3-mediated hydroalkylation
reaction of aromatic aldehydes (19 examples, 42â76% yield).
It was found that hydrocarbons undergo addition to the CO
double bond via a radical pathway. The photogenerated chlorine radical
abstracts a hydrogen atom from the alkane, and the resulting carbon-centered
radical either recombines with the borylated ketyl radical or adds
to the ground-state aldehyde-BCl3 complex, releasing a
chlorine atom. The existence of a radical chain was corroborated by
quantum yield measurements and by theory. The photolytic mechanism
described here is based on electron transfer between a bound chlorine
and an aromatic Ï-system on the substrate. Thereby, it avoids
the use of redox-active transition metals
Photoinduced BâCl Bond Fission in Aldehyde-BCl<sub>3</sub> Complexes as a Mechanistic Scenario for CâH Bond Activation
In
concert with carbonyl compounds, Lewis acids have
been identified
as a versatile class of photocatalysts. Thus far, research has focused
on activation of the substrate, either by changing its photophysical
properties or by modifying its photochemistry. In this work, we expand
the established mode of action by demonstrating that UV photoexcitation
of a Lewis acidâbase complex can lead to homolytic cleavage
of a covalent bond in the Lewis acid. In a study on the complex of
benzaldehyde and the Lewis acid BCl3, we found evidence
for homolytic BâCl bond cleavage leading to formation of a
borylated ketyl radical and a free chlorine atom only hundreds of
femtoseconds after excitation. Both time-dependent density functional
theory and transient absorption experiments identify a benzaldehyde-BCl2 cation as the dominant species formed on the nanosecond time
scale. The experimentally validated BâCl bond homolysis was
synthetically exploited for a BCl3-mediated hydroalkylation
reaction of aromatic aldehydes (19 examples, 42â76% yield).
It was found that hydrocarbons undergo addition to the CO
double bond via a radical pathway. The photogenerated chlorine radical
abstracts a hydrogen atom from the alkane, and the resulting carbon-centered
radical either recombines with the borylated ketyl radical or adds
to the ground-state aldehyde-BCl3 complex, releasing a
chlorine atom. The existence of a radical chain was corroborated by
quantum yield measurements and by theory. The photolytic mechanism
described here is based on electron transfer between a bound chlorine
and an aromatic Ï-system on the substrate. Thereby, it avoids
the use of redox-active transition metals
Photoinduced BâCl Bond Fission in Aldehyde-BCl<sub>3</sub> Complexes as a Mechanistic Scenario for CâH Bond Activation
In
concert with carbonyl compounds, Lewis acids have
been identified
as a versatile class of photocatalysts. Thus far, research has focused
on activation of the substrate, either by changing its photophysical
properties or by modifying its photochemistry. In this work, we expand
the established mode of action by demonstrating that UV photoexcitation
of a Lewis acidâbase complex can lead to homolytic cleavage
of a covalent bond in the Lewis acid. In a study on the complex of
benzaldehyde and the Lewis acid BCl3, we found evidence
for homolytic BâCl bond cleavage leading to formation of a
borylated ketyl radical and a free chlorine atom only hundreds of
femtoseconds after excitation. Both time-dependent density functional
theory and transient absorption experiments identify a benzaldehyde-BCl2 cation as the dominant species formed on the nanosecond time
scale. The experimentally validated BâCl bond homolysis was
synthetically exploited for a BCl3-mediated hydroalkylation
reaction of aromatic aldehydes (19 examples, 42â76% yield).
It was found that hydrocarbons undergo addition to the CO
double bond via a radical pathway. The photogenerated chlorine radical
abstracts a hydrogen atom from the alkane, and the resulting carbon-centered
radical either recombines with the borylated ketyl radical or adds
to the ground-state aldehyde-BCl3 complex, releasing a
chlorine atom. The existence of a radical chain was corroborated by
quantum yield measurements and by theory. The photolytic mechanism
described here is based on electron transfer between a bound chlorine
and an aromatic Ï-system on the substrate. Thereby, it avoids
the use of redox-active transition metals