Optoelectronic properties of diketopyrrolopyrrole homopolymers compared to donor-acceptor copolymers

Diketopyrrolopyrrole (DPP) is a component of a large number of materials used for optoelectronic applications. As it is exclusively used in combination with aromatic donors, the properties of its homopolymers are unknown. Because donor-acceptor character has been shown for other systems to reduce bandwidths, DPP homopolymers should have even larger conduction bands and better n-type conductivity than the thiophene-flanked systems, which have exceptional n-type conductivity and ambipolar character. Therefore, a theoretical study was carried out to elucidate the properties of the unknown DPP homopolymer. Calculations were done with density functional theory and with the complete active space self-consistent field method plus n-electron valence state perturbation theory for the dynamic correlation. Poly-DPP is predicted to have radical character and an extremely wide low-lying conduction band. If it were possible to produce this material, it should have unprecedented n-type conductivity and might be a synthetic metal. A comparison with various unknown donor-acceptor systems containing vinyl groups and thienyl rings with a higher concentration of DPP than the known copolymers reveals how donor-acceptor substitution reduces bandwidths and decreases electron affinities.

Spin density encodes intramolecular singlet exciton fission in pentacene dimers

The formation of two triplet excitons at the cost of one photon via singlet exciton fission in organic semiconductors can potentially enhance the photocurrent in photovoltaic devices. However, the role of spin density distribution in driving this photophysical process has been unclear until now. Here we present the significance of electronic spin density distribution in facilitating efficient intramolecular singlet exciton fission (iSEF) in π-bridged pentacene dimers. We synthetically modulate the spin density distribution in a series of pentacene dimers using phenyl-, thienyl- and selenyl- flanked diketopyrrolopyrrole (DPP) derivatives as π-bridges. Using femtosecond transient absorption spectroscopy, we find that efficient iSEF is only observed for the phenyl-derivative in ~2.4 ps while absent in the other two dimers. Electronic structure calculations reveal that phenyl-DPP bridge localizes α- and β-spin densities on distinct terminal pentacenes. Upon photoexcitation, a spin exchange mechanism enables iSEF from a singlet state which has an innate triplet pair character.

Air-Stable n-channel Diketopyrrolopyrrole-Diketopyrrolopyrrole Oligomers for High Performance Ambipolar Organic Transistors

n-channel organic semiconductors are prone to oxidation upon exposed to ambient conditions. Herein, we report design and synthesis of diketopyrrolopyrrole (DPP)-based oligomers for ambipolar organic thin-film transistors (OFETs) with excellent air and bias stability at ambient conditions. The cyclic voltammetry measurements reveal exceptional electrochemical stability during the redox cycle of oligomers. Structural properties including aggregation, crystallinity, and morphology in thin film were investigated by UV-visible spectroscopy, atomic force microscopy (AFM), thin-film X-ray diffraction (XRD), and grazing incidence small-angle X-ray scattering (GISAXS) measurements. AFM reveals morphological changes induced by different processing conditions whereas GISAXS measurements show an increase in the population of face-on oriented crystallites in films subjected to a combination of solvent and thermal treatments. These measurements also highlight the significance of chalcogen atom from sulfur to selenium on the photophysical, optical, electronic, and solid-state properties of DPP-DPP oligomers. Charge carrier mobilities of the oligomers were investigated by fabricating top-gate bottom-contact (TG-BC) thin-film transistors by annealing the thin films under various conditions. Combined solvent and thermal annealing of DPP-DPP oligomer thin films results in consistent electron mobilities as high as ∼0.2 cm(2) V(-1) s(-1) with an on/off ratio exceeding 10(4). Field-effect behavior was retained for up to ∼4 weeks, which illustrates remarkable air and bias stability. This work paves the way toward the development of n-channel DPP-DPP-based oligomers exhibiting retention of field-effect behavior with superior stability at ambient conditions.

Use of side-chain for rational design of n-type diketopyrrolopyrrole-based conjugated polymers: what did we find out?

Electron mobility of diketopyrrolopyrrole (DPP) based alternating copolymers of >2 cm²V⁻¹s⁻¹requires the combination of strong intramolecular conjugation along the polymer backbone with optimized intermolecular interactions that can be tuned with suitable side-chains.

Does the donor–acceptor concept work for designing synthetic metals?

Homopolymers of quinoxaline (QX), benzothiadiazole (BT), benzobisthiadiazole (BBT), thienopyrazine (TP), thienothiadiazole (TT), and thienopyrazinothiadiazole (TTP) and copolymers of these acceptors with thiophene (TH) and pyrrole (PY) were investigated with density functional theory. Theoretical band-gap predictions reproduce experimental data well. For all but six copolymers, band-gap reductions with respect to either homopolymer are obtained. Four of the acceptors, BBT, TP, TT, and TTP, give rise to copolymers with band gaps that are smaller than that of polyacetylene. BBT and TTP copolymers with PY in 1:2 stoichiometry are predicted to be synthetic metals. Band-gap reductions result from upshifts of HOMO energies and much smaller upshifts of LUMO values. The smallest band gaps are predicted with TTP, since changes in LUMO energies upon copolymerization are particularly small. The consequence of the small interactions between LUMO levels of donor and acceptor are vanishingly small conduction bandwidths.

Pyramidalized Double Bonds Containing Endoperoxide Linkages: Photooxygenation of Dimethyl cis-3,8-Dihydroheptalene-3,8-dicarboxylate

Diels-Alder cycloaddition utilizing singlet oxygen as the dienophile with dimethyl cis-3,8-dihydroheptalene-3,8-dicarboxylate (5) has been investigated, and monoaddition product 7 has been isolated. The addition of a second singlet oxygen to the cycloheptatriene unit in 7 gave syn-bis(norcaradiene) bis(endoperoxide) 4. (1)H NMR spectral studies and theoretical calculations indicate the increased pyramidalization in syn-4 compared with carbon analogue. The increased pyramidalization results from hyperconjugation between the central pi-bond and the four adjacent C-O bonds and by rehybridization at C3, C4, C5, and C6. Furthermore, the increased reactivity for syn-4, which is probably arising from further folding of the central double bond, is also in agreement with theoretical calculations.

Theoretical Analysis of Substituent Effects on Building Blocks of Conducting Polymers: 3,4'-Substituted Bithiophenes

Substituents are widely used to modify the properties of conducting polymers. To study substituent effects on energy levels and energy gaps systematically, CH(3)-, OH-, NH(2)-, CN-, and CCH-substituted bithiophenes were examined with density functional theory and NBO analysis. Total charges and pi-electron densities were analyzed separately to examine pi- and sigma-effects. Second-order perturbation theory was used to quantify conjugation in terms of orbital interactions. NBO orbital energies were employed to investigate the effect of alternating donor-acceptor substitution. Substituents in 3- and 4-positions shift HOMO and LUMO levels in parallel and hardly influence HOMO-LUMO gaps. For level shifting the pi-donating and pi-accepting abilities are most important; electronegativity mainly influences the sigma-orbitals and is less crucial in determining energy gaps. Alternating donor-acceptor substitution leads to HOMO and LUMO energies that are average between those of the parent systems and has little effect on energy gaps.

[Pharmacokinetics and pharmacodynamics of theophylline in geriatric patients with multiple diseases]

The pharmacokinetics of theophylline (200 mg i.v.) were determined in 20 geriatric patients with multiple diseases. Serum concentrations were fitted to an open 2-compartment model. Percent changes in venous pCO2 and pO2 were used as parameters of the pharmacodynamic action. Total clearance was decreased and elimination half-life of theophylline was found to be prolonged in the elderly patients compared with data of a study with young healthy volunteers. The stronger the pharmacodynamic action of theophylline (percent change of venous pCO2 after 30 min), the faster was its elimination and the lower were measured concentrations after 2, 6, and 12 h.