Reversible conversion between excited-states is key to many photophysical phenomena. We studied the equilibrium between LE and CT states by time-resolved and temperature-dependent fluorescence, fs- and ns-transient absorption, and LR-TDDFT calculations.
While spin-orbit coupling does not play a decisive role in the photophysics of unsubstituted perylene diimides (PDI), this changes dramatically when two phenylselenyl or phenyltelluryl substituents were attached to the PDI bay positions. In the series of PhO-, PhS-, PhSe-, and PhTe-substituted PDIs we observed strongly decreasing fluorescence quantum yield as a consequence of strongly increasing intersystem crossing (ISC) rate, measured by transient absorption spectroscopy with fs- and ns-time resolution as well as by broadband fluorescence upconversion. Time-dependent density functional calculations suggest increasing spin-orbit coupling due to the internal heavy-atom effect as the reason for fast ISC. In case of the selenium PDI derivative we found significant singlet oxygen sensitization via the PDI triplet state. The corresponding radical anions of the chalcogen substituted PDIs were also prepared and investigated by optical and EPR spectroscopy. Here, the increasing SOC results in an increase of the g-tensor anisotropy, and of the isotropic g-value in solution, albeit quasirelativistic density functional calculations show only a relatively small fraction of the spin density to be located on the chalcogen atom.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
A series of copper(I) complexes bearing a cyclic (amino)(aryl)carbene (CAArC) ligand with various complex geometries have been investigated in great detail with regard to their structural, electronic, and photophysical properties. Comparison of [CuX(CAArC)] (X = Br (1), Cbz (2), acac (3), Ph2acac (4), Cp (5), and Cp* (6)) with known CuI complexes bearing cyclic (amino)(alkyl), monoamido, or diamido carbenes (CAAC, MAC, or DAC, respectively) as chromophore ligands reveals that the expanded π-system of the CAArC leads to relatively low energy absorption maxima between 350 and 550 nm in THF with high absorption coefficients of 5–15 × 103 M–1 cm–1 for 1–6. Furthermore, 1–5 show intense deep red to near-IR emission involving their triplet excited states in the solid state and in PMMA films with λemmax = 621–784 nm. Linear [Cu(Cbz)(DippCAArC)] (2) has been found to be an exceptional deep red (λmax = 621 nm, ϕ = 0.32, τav = 366 ns) thermally activated delayed fluorescence (TADF) emitter with a radiative rate constant kr of ca. 9 × 105 s–1, exceeding those of commercially employed IrIII- or PtII-based emitters. Time-resolved transient absorption and fluorescence upconversion experiments complemented by quantum chemical calculations employing Kohn–Sham density functional theory and multireference configuration interaction methods as well as temperature-dependent steady-state and time-resolved luminescence studies provide a detailed picture of the excited-state dynamics of 2. To demonstrate the potential applicability of this new class of low-energy emitters in future photonic applications, such as nonclassical light sources for quantum communication or quantum cryptography, we have successfully conducted single-molecule photon-correlation experiments of 2, showing distinct antibunching as required for single-photon emitters.
Herein we describe a helically chiral push–pull molecule named 9,10-dimethoxy-[7]helicene diimide, displaying fluorescence and CPL over nearly the entire visible spectrum dependent on solvent polarity along with high dissymmetry factors.
A rigidly bridged squaraine dimer serves as a model compound to study exciton interactions between two chromophores without interfering with conformational or other stereochemical isomers. We describe the synthesis as well as steady state and fs- and ps-time-resolved optical spectroscopic data. The spectra are interpreted using a vibronic coupling model, which considers a single vibrational mode that produces a shallow excited state surface with two minima. These two minima cause symmetry breaking of the excited state, which leads to a partial localization of excitation. The localization of the wave function causes a reduced fluorescence transition moment, although both the absorption and the emission spectra display exchange narrowing typical of excitonically coupled chromophores.
We study the optically induced charge-transfer dynamics in donor–acceptor oligomers of different chain lengths. The combination of systematic synthesis, electrochemical measurements, and ultrafast transient absorption spectroscopy allows us to determine the charge-transfer properties and dynamics in donor–acceptor systems of confined lengths. Calculations within Marcus and Jortner electron-transfer theory explain the different charge-recombination times. For compounds in which complete charge separation can occur we deduce fast equilibration between different charge-transfer configurations prior to charge recombination. Thus, monoexponential charge-recombination kinetics are observed, as only the smallest-barrier configuration leads to relaxation to the ground state. The systematic oligomer length variation along with time-resolved spectroscopy allows us to determine how far apart charges can be separated in multichromophore donor–acceptor systems. Such information is relevant for understanding on a microscopic level the charge carrier mobility in materials for organic electronics and photovoltaics.
Abstract Zwei formtreue Sternmesogene mit Oligo(phenylenethenylen)‐Armen und einem Phthalocyanin‐Kern wurden erfolgreich synthetisiert. Dabei stellt eines ( 2 ) Freiraum bereit, und das andere ( 3 ) ist mit vier Fullerenen, die mittels eines Abstandhalters verbunden sind, sterisch überfrachtet. Im Gegensatz zu einem kleineren diskotischen Derivat ( 1 ), bildet Mesogen 2 eine kolumnare flüssigkristalline (LC) Phase aus, die keine π‐Stapelung zeigt und nur zum Teil orientiert werden kann, während 3 kein LC ist. Die 1:1‐Mischung von 2 und 3 generiert einen außergewöhnlichen, orientierbaren kolumnaren LC mit deutlicher π‐Stapelung und einer Fulleren‐Quadrupelhelix. Dies ist die Folge eines beispiellosen Klick‐Mechanismus, der einem Kugelarretierungs‐Mechanismus ähnelt. Die C 60 ‐Einheiten verbinden dabei auch unterschiedliche Kolumnen. Dieser Prozess wird durch Nanosegregation und Raumfüllung der Lücken durch die Fullerene angetrieben. Photophysikalische Untersuchungen bestätigen das Vorhandensein eines Lichtsammelsystems, wobei sowohl in Lösung als auch im Festkörper ladungsgetrennte Zustände generiert wurden, was solche hochgeordneten Materialien attraktiv für das Studium in zukünftigen photovoltaischen Zellen macht.
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