20 research outputs found

    Charge-Transfer Interactions in a Multichromophoric Hexaarylbenzene Containing Pyrene and Triarylamines

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    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

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    Two different dyads containing a triarylamine (TAA) donor and a naphthalene-1,8:4,5-bis­(dicarboximide) (NDI) acceptor bridged by either a [2.2]- or a [3.3]­paracyclophane (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

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    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

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    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

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    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

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    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

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    The structure and properties of metallo-supramolecular polyelectrolytes (MEPEs) self-assembled from rigid 2,6-bis­(2-pyridyl)­pyrimidine 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-bis­[2,6-bis­(2-pyridyl)­pyrimidin-4-yl]­benzene) is deep blue, <b>FeL2-</b> and <b>CoL2-MEPE</b> (<b>L2</b> = 5,5′-bis­[2,6-bis­(2-pyridyl)­pyrimidin-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

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    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

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    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

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    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
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