The Good Host: Formation of Discrete One-Dimensional Fullerene “Channels” in Well-Ordered Poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) Oligomers

Sequence-Defined Conjugated Oligomers in Donor-Acceptor Dyads

Conjugated macromolecules have a rich history in chemistry, owing to their chemical arrangements that intertwine physical and electronic properties. The continuing study and application of these systems, however, necessitates the development of atomically precise models that bridge the gap between molecules, polymers, and/or their blends. One class of conjugated polymers that have facilitated the advancement of structure-property relationships is discrete, precision oligomers that have remained an outstanding synthetic challenge with only a handful of reported examples. Here we show the first synthesis of molecular dyads featuring sequence-defined oligothiophene donors covalently linked a to small-molecule acceptor. These dyads serve as a platform for probing complex photophysical interactions involving sequence-defined oligomers. This assessment is facilitated through the unprecedented control of oligothiophene length- and sequence-dependent arrangement relative to the acceptor unit, made possible by the incorporation of hydroxyl-containing side chains at precise positions along the backbone through sequence-defined oligomerizations. We show that both the oligothiophene sequence and length play complementary roles in determining the transfer efficiency of photoexcited states. Overall, the work highlights the importance of the spatial arrangement of donor-acceptor systems that are commonly studied for a range of uses, including light harvesting and photocatalysis.

Mesoscopic Modeling of a Highly-Ordered Sanidic Polymer Mesophase and Comparison With Experimental Data

Board-shaped polymers form sanidic mesophases: assemblies of parallel lamellae of stacked polymer backbones separated by disordered side chains. Sanidics vary significantly with respect to polymer order inside their lamellae, making them “stepping stones” toward the crystalline state. Therefore, they are potentially interesting for studying crystallization and technological applications. Building on earlier mesoscopic models of the most disordered sanidics Σ_d, we focus on the other extreme, near-crystalline order, and develop a generic model that captures a highly ordered Σ_r mesophase. Polymers are described by generic hindered-rotation chains. Anisotropic nonbonded potentials, with strengths comparable to the thermal energy, mimic board-like monomer shapes. Lamellae equilibrated with Monte Carlo simulations, for a broad range of model parameters, have intralamellar order typical for Σ_r mesophases: periodically stacked polymers that are mutually registered along their backbones. Our mesophase shows registration on both monomer and chain levels. We calculate scattering patterns and compare with data published for highly ordered sanidic mesophases of two different polymers: polyesters and polypeptoids. Most of the generic structural features that were identified in these experiments are present in our model. However, our mesophase has correlations between chains located in different lamellae and is therefore closer to the crystalline state than the experimental samples.

syn/anti-Oligothienoacene Diimides with up to 10 Fused Rings

We report a facile and powerful strategy to prepare libraries of oligothienoacene diimides that include anti- and syn-isomers using palladium-catalyzed C-H activation and an unexpected 1,2-sulfur migration. Through this strategy, a series of oligothienoacene diimides containing 6, 8, and 10 fused rings were synthesized. The molecular geometry and extent of π-conjugation have dramatic effects on the electronic properties, degree of crystallinity, and charge-carrier transport properties. Notably, single-crystal microfibers of syn-3 c show electron mobilities up to 4.2 cm²  V^-1  s^-1 , illustrating the significant potential of these materials for organic electronic devices. Our work demonstrates the versatility of this strategy for the development of oligothienoacene diimide libraries, in particular complex and large syn-oligothienoacene diimides, which are difficult to prepare by present methods.

Alternating Tetrafluorobenzene and Thiophene Units by Direct Arylation for Organic Electronics

Direct arylation represents an attractive alternative to the conventional cross-coupling methods because of its step-economic and eco-friendly advantages. A set of simple D-A oligomeric molecules (F-3, F-5, and F-7) by integrating thiophene (T) and tetrafluorobenzene (F4B) as alternating units through a direct arylation strategy is presented to obtain high-performance charge-transporting materials. Single-crystal analysis revealed their herringbone packing arrangements driven by intensive C-H⋅⋅⋅π interactions. An excellent hole-transporting efficiency based on single-crystalline micro-plates/ribbons was witnessed, and larger π-conjugation and D-A constitution gave higher mobilities. Consequently, an average mobility of 1.31 cm²  V^-1  s^-1 and a maximum mobility of 2.44 cm²  V^-1  s^-1 for F-7 were achieved, providing an effective way to obtain high-performance materials by designing simple D-A oligomeric systems.

High-Resolution Vibronic Spectra of Molecules on Molybdenum Disulfide Allow for Rotamer Identification

Tunneling spectroscopy is an important tool for the chemical identification of single molecules on surfaces. Here, we show that oligothiophene-based large organic molecules which only differ by single bond orientations can be distinguished by their vibronic fingerprint. These molecules were deposited on a monolayer of the transition metal dichalcogenide molybdenum disulfide (MoS₂) on top of a Au(111) substrate. MoS₂ features an electronic band gap for efficient decoupling of the molecular states. Furthermore, it exhibits a small electron-phonon coupling strength. Both of these material properties allow for the resolution of vibronic states in the range of the limit set by temperature broadening in our scanning tunneling microscope at 4.6 K. Using DFT calculations of the molecule in gas phase provides all details for an accurate simulation of the vibronic spectra of both rotamers.

A new CdII coordination polymer with a self-penetrating architecture induced by the molecular conformation of a rigid bithiophene ligand

The design and synthesis of coordination polymers with a self-penetrating architecture has attracted much interest not only due to their interesting structures but also due to their potential applications. 5,5'-Bis(pyridin-4-yl)-2,2'-bithiophene (bpbp), as a conjugated bithiophene ligand, can exhibit trans and cis conformations and this can lead to the construction of a self-penetrating architecture. In addition, the semi-rigid ancillary ligand 4,4'-oxybis(benzoic acid) (H₂oba) can adopt different coordination modes, resulting in coordination polymers with high-dimensional skeletons. A new Cd^II coordination polymer based on mixed ligands, namely poly[diaquapentakis[μ-5,5'-bis(pyridin-4-yl)-2,2'-bithiophene-κ²N:N']bis(nitrato-κ²O,O')tetrakis(μ₃-4,4'-oxydibenzoato)-κ¹⁰O:O,O':O'',O''';κ⁶O:O':O''-pentacadmium(II)], [Cd₅(C₁₄H₁₄O₅)₄(NO₃)₂(C₁₈H₁₂N₂S₂)₅(H₂O)₂]_n, (I), has been synthesized under solvothermal conditions and characterized by single-crystal X-ray diffraction, IR spectroscopy and elemental analysis. Single-crystal X-ray diffraction indicates that there are three crystallographically independent Cd^II cations, three bpbp ligands, two deprotonated oba^2- ligands, one nitrate ligand and one coordinated water molecule in the asymmetric unit. One Cd^II centre is seven-coordinated, exhibiting a distorted {CdN₂O₅} pentagonal bipyramidal geometry, while the other two Cd centres are both six-coordinated, showing slightly distorted {CdN₂O₄} octahedral geometries. The most interesting feature is the co-existence of trans and cis conformations in a single net, allowing structural interpenetration via self-threading and yet the expected self-penetrating structure was obtained. Topological analysis shows that the whole three-dimensional framework can be classified as a 3-nodal (4,6,6)-c net with Schläfli symbol {6¹³.8²}₂{6⁶}, which is a new topology. Furthermore, the luminescence properties of (I) were examined in the solid state at room temperature.

Impact of morphology on polaron delocalization in a semicrystalline conjugated polymer

We investigate the delocalization of holes in the semicrystalline conjugated polymer poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT) by directly measuring the hyperfine coupling between photogenerated polarons and bound nuclear spins using electron nuclear double resonance spectroscopy. An extrapolation of the corresponding oligomer spectra reveals that charges tend to delocalize over 4.0-4.8 nm with delocalization strongly dependent on molecular order and crystallinity of the PBTTT polymer thin films. Density functional theory calculations of hyperfine couplings confirm that long-range corrected functionals appropriately describe the change in coupling strength with increasing oligomer size and agree well with the experimentally measured polymer limit. Our discussion presents general guidelines illustrating the various pitfalls and opportunities when deducing polaron localization lengths from hyperfine coupling spectra of conjugated polymers.

Dimeric Porphyrin Small Molecules for Efficient Organic Solar Cells with High Photoelectron Response in the Near-Infrared Region

Small molecules (SMs) with elongated backbones are promising for achieving a higher photovoltaic performance. Herein, a dimeric porphyrin small molecule, ZnP2-DPP, consisting of two porphyrin units linked with an ethynylene as the core and two diketopyrrolopyrrole (DPP) units as the arms is designed and synthesized as an electron donor for solution-processed bulk-heterojunction (BHJ) organic solar cells (OSCs). A significantly enhanced power conversion efficiency of 8.45% with an impressive short-circuit current density (J_sc) up to 19.65 mA cm^-2 is achieved for the BHJ OSCs based on ZnP2-DPP under AM 1.5G irradiation (100 mW cm^-2) compared to that for the OSCs based on the dimeric porphyrin linked with bis-ethynylenes reported previously. Furthermore, the devices show broad photoelectron responses up to 1000 nm with high near-infrared external quantum efficiency up to 66% at 780 nm. This is the first study reporting SM OSCs displaying such a large J_sc of about 20 mA cm^-2 simultaneously with a considerably high and deep photoelectron response of up to 1000 nm.

Donor-Acceptor Conjugated Macrocycles: Synthesis and Host-Guest Coassembly with Fullerene toward Photovoltaic Application

Electron-rich (donor) and electron-deficient (acceptor) units to construct donor-acceptor (D-A) conjugated macrocycles were investigated to elucidate their interactions with electron-deficient fullerene. Triphenylamine and 4,7-bisthienyl-2,1,3-benzothiadiazole were alternately linked through acetylene, as the donor and acceptor units, respectively, for pentagonal 3B2A and hexagonal 4B2A macrocycles. As detected by scanning tunneling microscopy, both D-A macrocycles were found to form an interesting concentration-controlled nanoporous monolayer on highly oriented pyrolytic graphite, which could effectively capture fullerene. Significantly, the fullerene filling was cavity-size-dependent with only one C₇₀ or PC₇₁BM molecule accommodated by 3B2A, while two were accommodated by 4B2A. Density functional theory calculations were also utilized to gain insight into the host-guest systems and indicted that the S···π contact is responsible for stabilizing these host-guest systems. Owing to the ellipsoidal shape of C₇₀, C₇₀ molecules are standing or lying in molecular cavities depending on the energy optimization. For the 3B2A/PC₇₁BM blended film, PC₇₁BM was intercalated into the cavity formed by the macrocycle 3B2A and provided excellent power conversion efficiency despite the broad band gap (2.1 eV) of 3B2A. This study of D-A macrocycles incorporating fullerene provides insights into the interaction mechanism and electronic structure in the host-guest complexes. More importantly, this is a representative example using D-A macrocycles as a donor to match with the spherical fullerene acceptor for photovoltaic applications, which offer a good approach to achieve molecular scale p-n junctions for substantially enhanced efficiencies of organic solar cells through replacing linear polymer donors by cyclic conjugated oligomers.

Acceptor End-Capped Oligomeric Conjugated Molecules with Broadened Absorption and Enhanced Extinction Coefficients for High-Efficiency Organic Solar Cells

Acceptor end-capping of oligomeric conjugated molecules is found to be an effective strategy for simultaneous spectral broadening, extinction coefficient enhancement, and energy level optimization, resulting in profoundly enhanced power conversion efficiencies (of 9.25% and 8.91%) compared to the original oligomers. This strategy is effective in overcoming the absorption disadvantage of oligomers and small molecules due to conjugation limitation.

Real-space visualization of conformation-independent oligothiophene electronic structure

We present scanning tunneling microscopy and spectroscopy (STM/STS) investigations of the electronic structures of different alkyl-substituted oligothiophenes on the Au(111) surface. STM imaging showed that on Au(111), oligothiophenes adopted distinct straight and bent conformations. By combining STS maps with STM images, we visualize, in real space, particle-in-a-box-like oligothiophene molecular orbitals. We demonstrate that different planar conformers with significant geometrical distortions of oligothiophene backbones surprisingly exhibit very similar electronic structures, indicating a low degree of conformation-induced electronic disorder. The agreement of these results with gas-phase density functional theory calculations implies that the oligothiophene interaction with the Au(111) surface is generally insensitive to molecular conformation.

Oligothiophene wires: impact of torsional conformation on the electronic structure

Different torsional conformations of alkyl-substituted oligothiophenes show nearly identical progressions of particle-in-a-box-like electronic orbitals.

Controlled Directional Crystallization of Oligothiophenes Using Zone Annealing of Preseeded Thin Films

We demonstrate a simple route to directionally grow crystals of oligothiophenes, based on 2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene with degrees of polymerization of 2 (BTTT-2) and 4 (BTTT-4) via zone annealing (ZA) of preseeded films. ZA of spun-cast films of BTTT-2 does not yield highly aligned crystals. However, if the film is oven-annealed briefly prior to ZA, highly aligned crystals that are millimeters in length can be grown, whose length depends on the velocity of the ZA front. The precrystallized region provides existing nuclei that promote crystal growth and limit nucleation of new crystals in the melted region. Aligned crystals of BTTT-2 can be obtained even when the moving velocity for ZA is an order of magnitude greater than the crystal growth rate. The relative nucleation rate to the crystallization rate for BTTT-4 is greater than that for BTTT-2, which decreases the length over which BTTT-4 can be aligned to ∼500 μm for the conditions examined. The temperature gradient and moving velocity of ZA enable control of the length of the aligned crystalline structure at the macroscale.

A solution-processed high performance organic solar cell using a small molecule with the thieno[3,2-b]thiophene central unit

A solution processed acceptor-donor-acceptor (A-D-A) small molecule with thieno[3,2-b]thiophene as the central building block and 2-(1,1-dicyanomethylene)-rhodanine as the terminal unit, DRCN8TT, was designed and synthesized. The optimized power conversion efficiency (PCE) of 8.11% was achieved, which is much higher than that of its analogue molecule DRCN8T. The improved performance was ascribed to the morphology which consisted of small, highly crystalline domains that were nearly commensurate with the exiton diffusion length.

Enhanced mobility and effective control of threshold voltage in P3HT-based field-effect transistors via inclusion of oligothiophenes

Improved organic field-effect transistor (OFET) performance through a polymer-oligomer semiconductor blend approach is demonstrated. Incorporation of 2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene (BTTT) into poly(3-hexylthiophene) (P3HT) thin films leads to approximately a 5-fold increase in charge carrier mobility, a 10-fold increase in current on-off ratio, and concomitantly, a decreased threshold voltage to as low as 1.7 V in comparison to single component thin films. The blend approach required no pre- and/or post treatments, and processing was conducted under ambient conditions. The correlation of crystallinity, surface morphology and photophysical properties of the blend thin films was systematically investigated via X-ray diffraction, atomic force microscopy and optical absorption measurements respectively, as a function of blend composition. The dependence of thin-film morphology on the blend composition is illustrated for the P3HT:BTTT system. The blend approach provides an alternative avenue to combine the advantageous properties of conjugated polymers and oligomers for optimized semiconductor performance.