Facile Synthesis and Chain-Length Dependence of the Optical and Structural Properties of Diketopyrrolopyrrole-Based Oligomers.

Here, we report the synthesis, optical properties, and solid-state packing of monodisperse oligomers of diketopyrrolopyrrole (DPP) up to five repeating units. The optical properties of DPP oligomers in solution and the solid state were investigated by a combination of steady-state and transient spectroscopy. Transient absorption spectroscopy and time-correlated single photon counting (TCSPC) measurements show that the fluorescence lifetime decreases with an increase in the oligomer size from monomer to trimer, thereby reaching saturation for pentameric DPP oligomers. The solid-state packing and crystallinity were probed by using advanced techniques, which included grazing incidence small-angle X-ray scattering (GISAXS) and X-ray diffraction (XRD) to elucidate the structure-property trend. Collectively, our chain-length dependent studies establish the fundamental correlation between the structure and property and provide a comprehensive understanding of the solid-state properties in DPP-DPP based conjugated systems.

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 ß-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.

Author Correction: Ultrafast bridge planarization in donor-π-acceptor copolymers drives intramolecular charge transfer.

The original version of this Article omitted the following from the Acknowledgements:"Satish Patil thanks Department of Science and Technology, New Delhi, India for a Swarnajayanti fellowship."This has now been corrected in both the PDF and HTML versions of the Article.

Correlating Charge Transport with Structure in Deconstructed Diketopyrrolopyrrole Oligomers: A Case Study of a Monomer in Field-Effect Transistors.

Copolymers based on diketopyrrolopyrrole (DPP) cores have attracted a lot of attention because of their high p-type as well as n-type carrier mobilities in organic field-effect transistors (FETs) and high power conversion efficiencies in solar cell structures. We report the structural and charge transport properties of n-dialkyl side-chain-substituted thiophene DPP end-capped with a phenyl group (Ph-TDPP-Ph) monomer in FETs which were fabricated by vacuum deposition and solvent coating. Grazing-incidence X-ray diffraction (GIXRD) from bottom-gate, bottom-contact FET architectures was measured with and without biasing. Ph-TDPP-Ph reveals a polymorphic structure with π-conjugated stacking direction oriented in-plane. The unit cell comprises either one monomer with a = 20.89 Å, b = 13.02 Å, c = 5.85 Å, α = 101.4°, ß = 90.6°, and γ = 94.7° for one phase (TR1) or two monomers with a = 24.92 Å, b = 25.59 Å, c = 5.42 Å, α = 80.3°, ß = 83.5°, and γ = 111.8° for the second phase (TR2). The TR2 phase thus signals a shift from a coplanar to herringbone orientation of the molecules. The device performance is sensitive to the ratio of the two triclinic phases found in the film. Some of the best FET performances with p-type carrier mobilities of 0.1 cm2/V s and an on/off ratio of 106 are for films that comprise mainly the TR1 phase. GIXRD from in operando FETs demonstrates the crystalline stability of Ph-TDPP-Ph.

Ultrafast bridge planarization in donor-π-acceptor copolymers drives intramolecular charge transfer.

Donor-π-acceptor conjugated polymers form the material basis for high power conversion efficiencies in organic solar cells. Large dipole moment change upon photoexcitation via intramolecular charge transfer in donor-π-acceptor backbone is conjectured to facilitate efficient charge-carrier generation. However, the primary structural changes that drive ultrafast charge transfer step have remained elusive thereby limiting a rational structure-function correlation for such copolymers. Here we use structure-sensitive femtosecond stimulated Raman spectroscopy to demonstrate that π-bridge torsion forms the primary reaction coordinate for intramolecular charge transfer in donor-π-acceptor copolymers. Resonance-selective Raman snapshots of exciton relaxation reveal rich vibrational dynamics of the bridge modes associated with backbone planarization within 400 fs, leading to hot intramolecular charge transfer state formation while subsequent cooling dynamics of backbone-centric modes probe the charge transfer relaxation. Our work establishes a phenomenological gating role of bridge torsions in determining the fundamental timescale and energy of photogenerated carriers, and therefore opens up dynamics-based guidelines for fabricating energy-efficient organic photovoltaics.

Is the Chemical Strategy for Imbuing "Polyene" Character in Diketopyrrolopyrrole-Based Chromophores Sufficient for Singlet Fission?

In this work, we have rationally designed and synthesized a novel thiophene-diketopyrrolopyrrole (TDPP)-vinyl-based dimer. We have investigated the optical and electronic properties and have probed the photophysical dynamics using transient absorption to investigate the possibility of singlet exciton fission. These revealed extremely rapid decay to the ground state (<50 ps), which we confirm is due to intramolecular excitonic processes rather than large-scale conformational change enabled by the vinyl linker. In all cases, the main excited state appears to be "dark", suggesting rapid internal conversion into a dark 2Ag-type singlet state. We found no evidence of triplet formation in TDPP-V-TDPP under direct photoexcitation. This may be a consequence of significant singlet stabilization in the dimer, bringing it below the energy needed to form two triplets. Our studies on this model compound set valuable lessons for design of novel triplet-forming materials and highlight the need for more broadly applicable design principles.

A Deep-Blue Electroluminescent Device Based on a Coumarin Derivative.

The design and synthesis is reported of 7-(9H-carbazol-9-yl)-4-methylcoumarin (Cz-Cm), comprising a carbazole donor moiety and a 4-methylcoumarin acceptor unit, for use in a blue organic light-emitting diode. A detailed solid state, theoretical and spectroscopic study was performed to understand the structure-property relationships. The material exhibits deep-blue emission and high photoluminescence quantum yield both in solution and in a doped matrix. A deep-blue electroluminescence emission at 430 nm, a maximum brightness of 292 cd m-2 and an external quantum efficiency of 0.4 % was achieved with a device configured as follows: ITO/NPD (30 nm)/TCTA (20 nm)/CzSi(10 nm)/10 wt % Cz-Cm:DPEPO (10 nm)/TPBI (30 nm)/LiF (1 nm)/Al [ITO=indium tin oxide, NPD=N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine, TCTA=tris(4-carbazoyl-9-ylphenyl)amine, CzSi=9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole, DPEPO=bis[2-(diphenylphosphino)phenyl]ether oxide, TPBI=1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene].

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.