20 research outputs found
Charge-Transfer Interactions in a Multichromophoric Hexaarylbenzene Containing Pyrene and Triarylamines
Two different hexaarylbenzenes with three pyrene and
three triarylamine
substituents in different positions (trigonal symmetric and asymmetric
arrangement) were synthesized, and their charge-transfer states were
investigated by optical spectroscopy. In these multichromophoric systems
triarylamine acts as the electron donor and pyrene as the electron
acceptor. A reference chromophore with only one donor–acceptor
pair was also investigated. All these chromophores form charge-transfer
states upon photoexcitation which relax with a moderate fluorescence
quantum yield to the ground state. The compounds do not differ significantly
concerning most of their fluorescence properties, which shows that
the fluorescent charge-transfer state is very similar in all chromophores.
This observation indicates symmetry breaking for the symmetric chromophore
within fluorescence lifetime of several tens of ns. This interpretation
was substantiated by fluorescence excitation anisotropy measurements
in a sucrose octaacetate matrix
Long-Lived Singlet and Triplet Charge Separated States in Small Cyclophane-Bridged Triarylamine–Naphthalene Diimide Dyads
Two different dyads containing a triarylamine (TAA) donor
and a
naphthalene-1,8:4,5-bisÂ(dicarboximide) (NDI) acceptor bridged by either
a [2.2]- or a [3.3]Âparacyclophane (CP) were synthesized. These dyads
show a high population of long-lived charge separated (CS) singlet
and triplet states. The lifetimes of these different spin states vary
only by 1 order of magnitude. This unique situation is a consequence
of both a large electronic coupling <i>V</i> and a large
exchange coupling 2<i>J</i>. The population of the different
CS spin states and therefore the charge recombination (CR) and intersystem
crossing (ISC) kinetics were monitored by standard ns-transient absorption
spectroscopy. Together with fs-transient absorption spectroscopy supported
by electrochemistry, steady state fluorescence and steady state absorption
spectroscopy a detailed model of the photoinduced processes was derived
Exciton Coupling Enhancement in the Relaxed Excited State
The steady-state
and photoinduced dynamical optical properties
of two squaraine-bodipy dye conjugates are the focus of this work.
While the squared absorption transition moments of the dye conjugates
can be traced back in an additive way to the constituents of the conjugates,
this is not possible for the squared fluorescence transition moments.
We suggest an enhancement of electronic coupling in the relaxed excited
state to be responsible for this observation. Transient absorption
and fluorescence upconversion experiments with femtosecond-time resolution
give insight into the relaxation phenomena of the dye conjugates,
in particular concerning the relaxation within the exciton manifold
Complete Monitoring of Coherent and Incoherent Spin Flip Domains in the Recombination of Charge-Separated States of Donor-Iridium Complex-Acceptor Triads
The spin chemistry of photoinduced
charge-separated (CS) states
of three triads comprising one or two triarylamine donors, a cyclometalated
iridium complex sensitizer and a naphthalene diimide (NDI) acceptor,
was investigated by transient absorption spectroscopy in the ns−μs
time regime. Strong magnetic-field effects (MFE) were observed for
two triads with a phenylene bridge between iridium complex sensitizer
and NDI acceptor. For these triads, the lifetimes of the CS states
increased from 0.6 μs at zero field to 40 μs at about
2 T. Substituting the phenylene by a biphenyl bridge causes the lifetime
of the CS state at zero field to increase by more than 2 orders of
magnitude (τ = 79 μs) and the MFE to disappear almost
completely. The kinetic MFE was analyzed in the framework of a generalized
Hayashi–Nagakura scheme describing coherent (S, T<sub>0</sub> ↔ T<sub>±</sub>) as well as incoherent (S, T<sub>0</sub> ⇌ T<sub>±</sub>) processes by a single rate constant <i>k</i><sub>±</sub>. The magnetic-field dependence of <i>k</i><sub>±</sub> of the triads with phenylene bridge spans
2 orders of magnitude and exhibits a biphasic behavior characterized
by a superposition of two Lorentzians. This biphasic MFE is observed
for the first time and is clearly attributable to the coherent (<i>B</i> < 10 mT) and incoherent (10 mT < <i>B</i> < 2 T) domains of spin motion induced by isotropic and anisotropic
hyperfine coupling. The parameters of both domains are well understood
in terms of the structural properties of the two triads, including
the effect of electron hopping in the triad with two donor moieties.
The kinetic model also accounts for the reduction of the MFE on reducing
the rate constant of charge recombination in the triad with the biphenyl
bridge
<i>J</i>‑Resonance Line Shape of Magnetic Field-Affected Reaction Yield Spectrum from Charge Recombination in a Linked Donor–Acceptor Dyad
Magnetic
field effects (MFEs) allow detailed insight into spin
conversion processes of radical pairs that are formed, for example,
in all charge separation processes, and are supposed to play the key
role in avian navigation. In this work, the MFE of charge recombination
in the charge-separated state of a rigid donor–bridge–acceptor
dyad was analyzed by a classical and a quantum theoretical model and
represents a paradigm case of understanding spin chemistry with unprecedented
detail. The MFE is represented by magnetic field-affected reaction
yield (MARY) spectra that exhibit a sharp resonance, resulting from
S/T level crossing as the Zeeman splitting equals twice the exchange
interaction. Although in the classical kinetic model, the spin conversion
processes between the four singlet and triplet substates are shown
for the first time to obey an identical generalized energy dependence,
quantum theory proves that the MARY resonance line is composed of
relaxation, coherent hyperfine induced spin mixing, and S/T dephasing
contributions
Coupled Oscillators for Tuning Fluorescence Properties of Squaraine Dyes
Combining a squaraine (S) and a BODIPY
(B) chromophore in a heterodimer (SB) and two heterotrimers (BSB and
SBS) by alkyne bridges leads to the formation of coupled oscillators
whose fluorescence properties are superior compared to the parent
squaraine chromophore. The lowest energy absorption and emission properties
of these superchromophores are mainly governed by the squaraine part
and are shifted by more than 1000 cm<sup>–1</sup> to the red
by excitonic interaction between the squaraine and the BODIPY dye.
Employing polarization-dependent transient absorption and fluorescence
upconversion measurements, we could prove that the lowest energy absorption
in SB and BSB is caused by a single excitonic state but by two for
SBS. Despite the spectral red-shift of their lowest absorption band,
the fluorescence quantum yields increase for SB and BSB compared to
the parent squaraine chromophore SQA. This is caused by intensity
borrowing from the BODIPY states, which increases the squared transition
moments of the lowest energy band dramatically by 29% for SB and 63%
for BSB compared to SQA. Thereby, exciton coupling leads to a substantial
enhancement of fluorescence quantum yield by 26% for SB and by 46%
for BSB and shifts the emission from the red into the near-infrared.
In this way, the BODIPY-squaraine conjugates combine the best properties
of each class of dye. Thus, exciton coupling in heterodimers and -trimers
is a valuable alternative to tuning fluorescence properties by, e.g.,
attaching substituents to chromophores
Green-to-Red Electrochromic Fe(II) Metallo-Supramolecular Polyelectrolytes Self-Assembled from Fluorescent 2,6-Bis(2-pyridyl)pyrimidine Bithiophene
The
structure and properties of metallo-supramolecular polyelectrolytes
(MEPEs) self-assembled from rigid 2,6-bisÂ(2-pyridyl)Âpyrimidine and
the metal ions Fe<sup>II</sup> and Co<sup>II</sup> are presented.
While <b>FeL1-MEPE</b> (<b>L1</b> = 1,4-bisÂ[2,6-bisÂ(2-pyridyl)Âpyrimidin-4-yl]Âbenzene)
is deep blue, <b>FeL2-</b> and <b>CoL2-MEPE</b> (<b>L2</b> = 5,5′-bisÂ[2,6-bisÂ(2-pyridyl)Âpyrimidin-4-yl]-2,2′-bithiophene)
are intense green and red in color, respectively. These novel MEPEs
display a high extinction coefficient and solvatochromism. Ligand <b>L2</b> shows a high absolute fluorescence quantum yield (Φ<sub>f</sub> = 82%). Viscosity and static light-scattering measurements
reveal that the molar masses of these MEPEs are in the range of 1
× 10<sup>8</sup> g/mol under the current experimental conditions.
In water, <b>FeL1-MEPE</b> forms a viscous gel at 20 °C
(<i>c</i> = 8 mM). Thin films of high optical quality are
fabricated by dip coating on transparent conducting indium tin oxide
(ITO) glass substrate. Optical, electrochemical, and electrochromic
properties of the obtained MEPEs are presented. Green to red and blue
to colorless electrochromism is observed for <b>FeL2-MEPE</b> and <b>FeL1-MEPE</b>, respectively. The results show that
the electrochromic properties are affected by the ligand topology.
The Fe-MEPEs show a reversible redox behavior of the Fe<sup>II</sup>/Fe<sup>III</sup> couple at 0.86 and 0.82 V versus Fc<sup>+</sup>/Fc and display an excellent redox cycle stability under switching
conditions. <b>FeL2-MEPE</b> in its oxidized state exhibits
a broad absorption band covering the near-IR region (ca. 1500 nm)
due to the ligand-to-metal charge transfer transition originating
due to charge delocalization in the bithiophene spacer
Solvent Controlled Energy Transfer Processes in Triarylamine-Triazole Based Dendrimers
Fluorescence upconversion measurements
of three different dendrimers <b>G1</b><b>–G3</b> based on triarylamines connected
by triazole linkers show a strong and fast initial decay of fluorescence
anisotropy for <i>t</i> < 2 ps followed by anisotropy
decay on a much longer time scale (10–100 ps). At the same
time, a pronounced solvent relaxation takes place. Comparison of the
decay data in different solvents revealed that the initial decay of
fluorescence anisotropy is governed by a competition of solvent relaxation
and incoherent hopping of energy between the different dendrimer branches.
Thus, it is decisive to discriminate between energy transfer processes
in the Franck–Condon state or in the solvent relaxed state.
We demonstrate that even for charge transfer chromophores, where a
large Stokes shift leads to very weak spectral overlap of donor fluorescence
and acceptor absorption, rapid homotransfer is possible if there is
sufficient spectral overlap with the time zero fluorescence spectrum
Singlet–Singlet Exciton Annihilation in an Exciton-Coupled Squaraine-Squaraine Copolymer: A Model toward Hetero-J-Aggregates
Low-band-gap polymers with broad
spectral absorption are highly
sought after for application in organic photovoltaic cells and other
optoelectronic devices. Thus, a conjugated copolymer based on two
different indolenine squaraine dyes SQA and SQB was synthesized by
Suzuki coupling, and its steady-state and time-resolved optical properties
were investigated in detail. In CHCl<sub>3</sub> the copolymer [SQA-SQB]<sub><i>n</i></sub> shows a strongly broadened and red-shifted
absorption compared to that of its monomers, which was explained by
exciton coupling of localized transition moments. The theoretical
background of exciton coupling theory for copolymers was worked out
in detail. In toluene, [SQA-SQB]<sub><i>n</i></sub> displays
a spectral narrowing of the lowest excitation band which resembles
the exchange narrowing effect found in cyanine J-aggregates. In this
way [SQA-SQB]<sub><i>n</i></sub> behaves like a one-dimensional
covalently bound hetero-J-aggregate. The photoinduced dynamics of
the copolymer was investigated by transient absorption pump–probe
spectroscopy with femtosecond resolution. Because of the unusually
high exciton diffusion constant, singlet–singlet annihilation
is the rate-limiting step for deactivation of the copolymer in solution
at high laser fluencies. This is unlike the situation for many conjugated
polymers in the solid state, where diffusion-limited annihilation
is usually found. Thus, the [SQA-SQB]<sub><i>n</i></sub> copolymer is a unique model system which combines the excitonic
features of J-aggregates with the chemical robustness of a polymer
Annihilation dynamics of molecular excitons measured at a single perturbative excitation energy
Exciton–exciton annihilation (EEA) is a ubiquitous phenomenon, which may limit the efficiency of photovoltaic devices. Conventional methods of determining EEA time scales rely on measuring the intensity dependence of third-order signals. In this work, we directly extract the annihilation rate of molecular excitons in a covalently joined molecular trimer without the need to perform and analyze intensity dependent data by employing fifth-order coherent optical spectroscopy signals emitted into ±2k⃗1 ∓ 2k⃗2 + k⃗3 phase matching directions. Measured two-dimensional line shapes and their time traces are analyzed in the framework of the many-body version of the Frenkel exciton model, extended to incorporate annihilation dynamics. Combining double-sided Feynman diagrams with explicit simulations of the fifth-order response, we identify a single peak as a direct reporter of EEA. We retrieve an annihilation time of 30 fs for the investigated squaraine trimer