Intramolecular Excimer Formation between 3,6-Di(thiophen-2-yl)pyrrolo[3,4‑<i>c</i>]pyrrole-1,4(2<i>H</i>,5<i>H</i>)‑dione Chromophoric Groups Linked by a Flexible Alkyl Spacer

Bichromophoric molecules containing two 3,6-di­(thiophen-2-yl)­pyrrolo­[3,4-<i>c</i>]­pyrrole-1,4­(2<i>H</i>,5<i>H</i>)-dione (DPP) moieties linked via aliphatic spacers of different length are synthesized. Optical absorption spectroscopy indicates that the molecules adopt an extended conformation in solution. Fluorescence spectroscopy shows that photons are emitted from the locally excited singlet state in an extended conformation. In sufficiently polar solvents, quenching of fluorescence occurs and fluorescence quantum yield (Φ_F) and fluorescence lifetime (τ_F) measurements indicate formation of an intramolecular excimer as the quenching mechanism. The redox potentials of the molecules and the solvent polarity dependence of the quenching are consistent with significant charge-transfer character of the excimer state. Photoinduced absorption measurements show enhanced intersystem crossing to the triplet state in polar solvents. Results indicate that in donor–acceptor π-conjugated materials involving the DPP moiety, excimer-like interchain polaron pair excited states could play an important role in the photophysics because of their close proximity in energy to the lowest singlet excited state

Amplifying Chiroptical Properties of Conjugated Polymer Thin-Film Using an Achiral Additive

Chiral conjugated polymers bearing enantiopure side chains offer the possibility to harness the effect of chirality in organic electronic devices. However, its use is hampered by the low degree of circular polarization in absorption (<i>g</i>_abs) in most of the conjugated polymer thin-films studied. Here we demonstrate a versatile method to significantly increase the <i>g</i>_abs by using a few weight percentages of a commercially available achiral long-chain alcohol as an additive. This additive enhances the chiroptical properties in both absorption and emission by ca. 5–10 times in the thin-films. We envisage that the alcohol additive acts as a plasticizer which enhances the long-range chiral liquid crystalline ordering of the polymer chains, thereby amplifying the chiroptical properties in the thin-film. The application of this methodology to various conjugated polymers has been demonstrated

Synthesis and Optical Properties of Pyrrolo[3,2‑<i>b</i>]pyrrole-2,5(1<i>H</i>,4<i>H</i>)‑dione (iDPP)-Based Molecules

We describe the synthesis and photophysical properties of a series of derivatives of pyrrolo­[3,2-<i>b</i>]­pyrrole-2,5­(1<i>H</i>,4<i>H</i>)-dione-3,6-diyl (iDPP) linked to two oligothiophenes of variable length (<i>n</i>T). The iso-DPP-oligothiophenes (iDPP<i>n</i>Ts) differ from the common pyrrolo­[3,4-<i>c</i>]­pyrrole-1,4­(2<i>H</i>,5<i>H</i>)-dione-3,6-diyl-oligothiophene analogues (DPP<i>n</i>Ts) by a different orientation of the two lactam rings in the bicyclic iDPP unit compared to DPP. In contrast to the highly fluorescent DPP<i>n</i>Ts, the new isomeric iDPP<i>n</i>Ts exhibit only very weak fluorescence. We demonstrate with the help of quantum-chemical calculations that this can be attributed to a different symmetry of the lowest excited state in iDPP<i>n</i>T (<i>A</i> in <i>C</i>₂ symmetry) compared to DPP<i>n</i>Ts (<i>B</i>) and the corresponding loss in oscillator strength of the lowest energy transition. Upon extending the oligothiophene moiety in the iDPP<i>n</i>Ts molecules, the charge transfer character of the lowest <i>A</i> excited state becomes more pronounced. This tends to preclude high fluorescence quantum yields even in extended iDPP<i>n</i>Ts systems

Optical Properties of Oligothiophene Substituted Diketopyrrolopyrrole Derivatives in the Solid Phase: Joint J- and H‑Type Aggregation

Photophysical properties of diketopyrrolopyrrole derivatives substituted with oligothiophenes are investigated. All compounds are found to be fluorescent both in solution and in the solid phase. At low temperature in the solid, fluorescence originates from excimer-like excited states. Comparison of absorption and fluorescence excitation spectra taken under matrix isolated conditions and on solid films show the presence of both J- and H-type absorption bands in the solid phase. Quantum-chemical calculations, including exciton–phonon coupling to account for deviations from the Born–Oppenheimer approximation, are performed to simulate the band shape of the lowest absorption band in the molecular solid. The joint presence of J- and H-bands is explained by the presence of two molecules in the unit cell. The Davydov splitting is substantial for molecules with linear alkyl substituents on the nitrogen atom (on the order of 0.2 eV) but can be reduced to almost zero by introducing branching at the β-carbon of the alkyl side chain

Pathway Complexity in the Enantioselective Self-Assembly of Functional Carbonyl-Bridged Triarylamine Trisamides

Functional supramolecular systems like carbonyl-bridged triarylamine (CBT) trisamides are known for their long-range energy transport at room temperature. Understanding the complex self-assembly processes of this system allows for control over generated structures using controlled supramolecular polymerization. Here, we present two novel CBT trisamides with (<i>S</i>)- or (<i>R</i>)-chiral side chains which show a two-pathway self-assembly behavior in solution. Depending on the thermal profile during the self-assembly process, two different stable states are obtained under otherwise identical conditions. A kinetically trapped state <b>A</b> is reached upon cooling to 7 °C, via a proposed isodesmic process. In addition, there is a thermodynamically stable state <b>B</b> at 7 °C that is induced by first undercooling to −5 °C, via a nucleation-elongation mechanism. In both cases, helical supramolecular aggregates comprising H-aggregated CBTs are formed. Additionally, controlled supramolecular polymerization was achieved by mixing the two different states (<b>A</b> and <b>B</b>) from the same enantiomer, leading to a conversion of the kinetically trapped state to the thermodynamically stable state. This process is highly enantioselective, as no conversion is observed if the two states consist of opposite enantiomers. We thus show the importance and opportunities emerging from understanding the pathway complexity of functional supramolecular systems

Bis(arylimidazole) Iridium Picolinate Emitters and Preferential Dipole Orientation in Films

The straightforward synthesis and photophysical properties of a new series of heteroleptic iridium­(III) bis­(2-arylimidazole) picolinate complexes are reported. Each complex has been characterized by nuclear magnetic resonance, UV–vis, cyclic voltammetry, and photoluminescent angle dependency, and the emissive properties of each are described. The preferred orientation of transition dipoles in emitter/host thin films indicated more preferred orientation than homoleptic complex Ir­(ppy)₃

High Circular Polarization of Electroluminescence Achieved <i>via</i> Self-Assembly of a Light-Emitting Chiral Conjugated Polymer into Multidomain Cholesteric Films

We demonstrate a facile route to obtain high and broad-band circular polarization of electroluminescence in single-layer polymer OLEDs. As a light-emitting material we use a donor–acceptor polyfluorene with enantiomerically pure chiral side-chains. We show that upon thermal annealing the polymer self-assembles into a multidomain cholesteric film. By varying the thickness of the polymer emitting layer, we achieve high levels of circular polarization of electroluminescence (up to 40% excess of right-handed polarization), which are the highest reported for polymer OLEDs not using chiral dopants or alignment layers. Mueller matrix ellipsometry shows strong optical anisotropies in the film, indicating that the circular polarization of luminescence arises mainly after the photon has been generated, through selective scattering and birefringence correlated in the direction of the initial linear polarization of the photon. Our work demonstrates that chirally substituted conjugated polymers can combine photonic and semiconducting properties in advanced optoelectronic devices