A–π–D–π–A Electron‐Donating Small Molecules for Solution‐Processed Organic Solar Cells: A Review

Organic solar cells based on semiconducting polymers and small molecules have attracted considerable attention in the last two decades. Moreover, the power conversion efficiencies for solution‐processed solar cells containing A–π–D–π–A‐type small molecules and fullerenes have reached 11%. However, the method for designing high‐performance, photovoltaic small molecules still remains unclear. In this review, recent studies on A–π–D–π–A electron‐donating small molecules for organic solar cells are introduced. Moreover, the relationships between molecular properties and device performances are summarized, from which inspiration for the future design of high performance organic solar cells may be obtained.     

The Influence of Oxygen Atoms on Conformation and π–π Stacking of Ladder‐Type Donor‐Based Polymers and Their Photovoltaic Properties

A novel ladder‐type donor pyran‐bridged indacenodithiophene (IDTP) is developed by introducing two oxygen atoms into indacenodithiophene unit. IDTP possesses a twisted backbone and leads to facially asymmetric arrangement of side chains, resulting in enhanced local π–π stacking of according polymer poly[(5,5,11,11‐tetrakis(4‐octylphenyl)‐5,11‐dihydrothieno[2′,3′:5,6]pyrano[3,4‐g]thieno[3,2‐c]isochromene)‐alt‐4,7‐(5‐fluoro‐2,1,3‐benzothiadiazole)] (PIDTP)‐FBT, which shows extended absorption range. Moreover, oxygen atoms render deeper highest occupied molecular orbital (HOMO) levels of poly[indacenodithiophene‐alt‐4,7‐(5‐fluoro‐2,1,3‐benzothiadiazole)] (PIDTP)‐FBT compared with PIDT‐FBT, therefore bringing a higher open‐circuit voltage (V _oc).     

σ–π Continuum in Indole–Palladium(II) Complexes

The intrinsic features of (hetero‐arene)–metal interactions have been elusive mainly because the systematic structure analysis of non‐anchored hetero‐arene–metal complexes has been hampered by their labile nature. We report successful isolation and systematic structure analysis of a series of non‐anchored indole–palladium(II) complexes. It was revealed that there is a σ–π continuum for the indole–metal interaction, while it has been thought that the dominant coordination mode of indole to a metal center is the Wheland‐intermediate‐type σ‐mode in light of the seemingly strong electron‐donating ability of indole. Several factors which affect the σ‐ or π‐character of indole–metal interactions are discussed.     

Photophysical properties of σ–π‐conjugated alternating oligothienylene–oligosilylene polymers

UV‐visible absorption and fluorescence properties of three series of σ–π‐conjugated polymers (copolymers of alternative oligothienylene and oligosilylene units) have been studied in dioxane solution. The energies of the absorption maximum, fluorescence maximum, and the 0–0 transition are found to be linearly dependent on the reciprocal of the number of thiophene rings in the repeating unit of the polymer chain, but almost independent of the silicon atom number. The σ–π‐conjugation in the polymers results in red shift in the absorption and fluorescence maxima, higher fluorescence quantum yields, and longer fluorescence lifetimes of the polymers, with respect to their corresponding analogous α‐oligothiophenes. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1873–1880, 1999     

B=N Units as Part of Extended π‐Conjugated Oligomers and Polymers

The replacement of C=C units by their isoelectronic and isosteric B=N units (BN/CC isosterism) in π‐conjugated organic compounds, as a strategy to produce novel organic–inorganic hybrid materials, has recently been successfully transferred to π‐conjugated polymers. This Concept provides an overview of the recent advances in this quickly evolving field, with a focus on synthesis, photophysical and electrochemical properties of the new polymers and related oligomers, as well as possible future applications in organic electronics and optoelectronics.     

D–π–A polysulfones for blue electroluminescence

Donor–π–acceptor type fluorene‐based copolymers with a sulfone unit were designed and synthesized for application in efficient pure‐blue light emitting. The electroluminescence behaviors of these copolymers were investigated by fabricating light‐emitting diodes and electrochemical cell devices. The former device little functioned but the latter worked well. The electrochemical cell devices having a configuration of ITO/PEDOT:PSS/copolymer:ionic liquid/Al exhibited purplish blue electroluminescence with an emission maximum at 434 nm (CIE coordinates (x, y) = (0.17, 0.10)) measured at 7 V. The initial positive scan of the D–π–A polysulfone based light emitting electrochemical cell with a sweep rate of 0.1 V s^?1 afforded a maximum luminance of 1080 cd m^?2 with a current efficiency of 1.96 cd A^?1 at an operating voltage of 12.5 V. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3454–3461     

Optoelectronic Properties of Dicyanofluorene‐Based n‐Type Polymers

Three new donor–acceptor‐type copolymers ( P1 , P2 , P3 ) consisting of dicyanofluorene as acceptor and various donor moieties were designed and synthesized. Optoelectronic properties were studied in detail by means of UV‐visible absorption and fluorescence spectroscopy, cyclic voltammetry, space‐charge‐limited current (SCLC), flash‐photolysis time‐resolved microwave conductivity (FP‐TRMC), and density functional theory (DFT). All polymers showed strong absorption in the UV‐visible region and the absorption maximum undergoes redshift with an increasing number of thiophene units in the polymer backbone. SCLC analysis showed that the electron mobilities of the polymers in the bulk state were 1 to 2 orders higher than that of the corresponding hole mobilities, which indicated the n‐type nature of the materials. By using FP‐TRMC, the intrapolymer charge‐carrier mobility was assessed and compared with the interpolymer mobility obtained by SCLC. The polymers exhibited good electron‐accepting properties sufficiently high enough to oxidize the excited states of regioregular poly(3‐hexylthiophene) (P3HT (donor)), as evident from the FP‐TRMC analysis. The P3 polymer exhibited the highest FP‐TRMC transients in the pristine form as well as when blended with P3HT. Use of these polymers as n‐type materials in all‐polymer organic solar cells was also explored in combination with P3HT. In accordance with the TRMC results, P3 exhibited superior electron‐transport and photovoltaic properties to the other two polymers, which is explained by the distribution of the energy levels of the polymers by using DFT calculations.     

Synthesis and Crystal Structure of a [Mo8O26]4– Cluster Derivative with 4‐MePyH+. First β‐Octamolybdate Derivative with π–π Stacking

Dioxobis(pyridine‐2‐thiolate‐N, S)molybdenum(VI) (MoO₂(Py‐S)₂), reacts with of 4‐methylpyridine (4‐MePy) in acetonitrile, by slow diffusion, to afford the title compound. This has been characterized by elemental analysis, IR and ¹H NMR spectroscopy. The X‐ray single crystal structure of the complex is described. Structural studies reveal that the molecular structure consists of a β‐Mo₈O₂₆ polyanion with eight MoO₆ distorted edge‐shared octahedra with short terminal Mo–O bonds (1.692–1.714 Å), bonds of intermediate length (1.887–1.999 Å) and long bonds (2.150–2.473 Å). Two different types of hydrogen bonds have been found: N–H···O (2.800–3.075 Å) and C–H···O (3.095–3.316 Å). The presence of π–π stacking interactions and strong hydrogen bonds are presumably responsible for the special disposition of the pyridinic rings around the polyanion cluster.     

Energetic N‐Nitramino/N‐Oxyl‐Functionalized Pyrazoles with Versatile π–π Stacking: Structure–Property Relationships of High‐Performance Energetic Materials

N‐Nitramino/N‐oxyl functionalization strategies were employed to investigate structure–property relationships of energetic materials. Based on single‐crystal diffraction data, π–π stacking of pyrazole backbones can be tailored effectively by energetic functionalities, thereby resulting in diversified energetic compounds. Among them, hydroxylammonium 4‐amino‐3,5‐dinitro‐1H‐pyrazol‐1‐olate and dipotassium N,N′‐(3,5‐dinitro‐1H‐pyrazol‐1,4‐diyl)dinitramidate, with unique face‐to‐face π–π stacking, can be potentially used as a high‐performance explosive and an energetic oxidizer, respectively.     

Stable Actinide π Complexes of a Neutral 1,4‐Diborabenzene

The π coordination of arene and anionic heteroarene ligands is a ubiquitous bonding motif in the organometallic chemistry of d‐block and f‐block elements. By contrast, related π interactions of neutral heteroarenes including neutral bora‐π‐aromatics are less prevalent particularly for the f‐block, due to less effective metal‐to‐ligand backbonding. In fact, π complexes with neutral heteroarene ligands are essentially unknown for the actinides. We have now overcome these limitations by exploiting the exceptionally strong π donor capabilities of a neutral 1,4‐diborabenzene. A series of remarkably robust, π‐coordinated thorium(IV) and uranium(IV) half‐sandwich complexes were synthesized by simply combining the bora‐π‐aromatic with ThCl₄(dme)₂ or UCl₄, representing the first examples of actinide complexes with a neutral boracycle as sandwich‐type ligand. Experimental and computational studies showed that the strong actinide–heteroarene interactions are predominately electrostatic in nature with distinct ligand‐to‐metal π donation and without significant π/δ backbonding contributions.     

The peptide NCbz‐Val‐Tyr‐OMe and aromatic π–π interactions

The peptide N‐benzyloxycarbonyl‐L‐valyl‐L‐tyrosine methyl ester or NCbz‐Val‐Tyr‐OMe (where NCbz is N‐benzyloxycarbonyl and OMe indicates the methyl ester), C₂₃H₂₈N₂O₆, has an extended backbone conformation. The aromatic rings of the Tyr residue and the NCbz group are involved in various attractive intra‐ and intermolecular aromatic π–π interactions which stabilize the conformation and packing in the crystal structure, in addition to N—H...O and O—H...O hydrogen bonds. The aromatic π–π interactions include parallel‐displaced, perpendicular T‐shaped, perpendicular L‐shaped and inclined orientations.     

1D Coordination π–d Conjugated Polymers with Distinct Structures Defined by the Choice of the Transition Metal: Towards a New Class of Antiaromatic Macrocycles

Recently π–d conjugated coordination polymers have received a lot of attention owing to their unique material properties, although synthesis of long and defect‐free polymers remains challenging. Herein we introduce a novel on‐surface synthesis of coordination polymers with quinoidal ligands under ultra‐high vacuum conditions, which enables formation of flexible coordination polymers with lengths up to hundreds of nanometers. Moreover, this procedure allows the incorporation of different transition‐metal atoms with four‐ or two‐fold coordination. Remarkably, the two‐fold coordination mode revealed the formation of wires constituted by (electronically) independent 12‐membered antiaromatic macrocycles linked together through two C?C single bonds.     

Synthesis and optical properties of σ–π conjugated organic–inorganic hybrid gels

Organic–inorganic hybrid gels containing Si‐vinylene units have been synthesized by a hydrosilylation reaction of tri‐ or tetra‐ethynyl aryl compounds, 1,3,5‐triethynylbenzene (TEB), 3,3′,5,5′‐tetraethynylbiphenyl (TEBP), or tetrakis(4‐ethynylphenyl)methane (TEPM), and bisdimethylsilyl compounds, 1,1,3,3‐tetramethyldisiloxane (TMDS) or 1,4‐bisdimetylsilylbenzene (BDMSB), in toluene. Network structure of the resulting gels was quantitatively characterized by a scanning microscopic light scattering. The reactions yielded the gels having homogeneous network structure of 1.5–2.9 nm mesh size under the monomer concentrations that were relatively higher than the critical gelation concentration. The gels obtained from TEB showed broad absorption in the range from 340 to 370 nm, and emission in the range from 440 to 490 nm. The TEB–BDMSB gels showed remarkable red shift of the emission in comparison with that of the corresponding reaction solutions derived from the network formed by σ–π conjugation. The TEPM–TMDS, BDMSB gels exited by 280 nm showed not only the emission peak at around 360 nm derived from TEPM, but the broad peak at around 420 nm, which should be derived from interaction between phenyl groups of TEPM in the gels. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1360–1368     

Incorporation of 2,6‐Connected Azulene Units into the Backbone of Conjugated Polymers: Towards High‐Performance Organic Optoelectronic Materials

Azulene is a promising candidate for constructing optoelectronic materials. An effective strategy is presented to obtain high‐performance conjugated polymers by incorporating 2,6‐connected azulene units into the polymeric backbone, and two conjugated copolymers P(TBAzDI‐TPD) and P(TBAzDI‐TFB) were designed and synthesized based on this strategy. They are the first two examples for 2,6‐connected azulene‐based conjugated polymers and exhibit unipolar n‐type transistor performance with an electron mobility of up to 0.42 cm² V^?1 s^?1, which is among the highest values for n‐type polymeric semiconductors in bottom‐gate top‐contact organic field‐effect transistors. Preliminary all‐polymer solar cell devices with P(TBAzDI‐TPD) as the electron acceptor and PTB7‐Th as the electron donor display a power conversion efficiency of 1.82 %.     

Remote Control of Anion–π Catalysis on Fullerene‐Centered Catalytic Triads

The design, synthesis and evaluation of catalytic triads composed of a central C₆₀ fullerene with an amine base on one side and polarizability enhancers on the other side are reported. According to an enolate addition benchmark reaction, fullerene–fullerene–amine triads display the highest selectivity in anion–π catalysis observed so far, whereas NDI–fullerene–amine triads are not much better than fullerene–amine controls (NDI=naphthalenediimide). These large differences in activity are in conflict with the small differences in intrinsic π acidity, that is, LUMO energy levels and π holes on the central fullerene. However, they are in agreement with the high polarizability of fullerene–fullerene–amine triads. Activation and deactivation of the fullerene‐centered triads by intercalators and computational data on anion binding further indicate that for functional relevance, intrinsic π acidity is less important than induced π acidity, that is, the size of the oriented macrodipole of polarizable π systems that emerges only in response to the interaction with anions and anionic transition states. The resulting transformation is thus self‐induced, the anionic intermediates and transition states create their own anion–π catalyst.     

A Double Cation–π‐Driven Strategy Enabling Two‐Dimensional Supramolecular Polymers as Efficient Catalyst Carriers

The cation–π interaction is a strong non‐covalent interaction that can be used to prepare high‐strength, stable supramolecular materials. However, because the molecular plane of a cation‐containing group and that of aromatic structure are usually perpendicular when forming a cation–π complex, it is difficult to exploit the cation–π interaction to prepare a 2D self‐assembly in which the molecular plane of all the building blocks are parallel. Herein, a double cation–π‐driven strategy is proposed to overcome this difficulty and have prepared 2D self‐assemblies with long‐range ordered molecular hollow hexagons. The double cation–π interaction makes the 2D self‐assemblies stable. The 2D self‐assemblies are to be an effective carrier that can eliminate metal‐nanoparticle aggregation. Such 2D assembly/palladium nanoparticle hybrids are shown to exhibit recyclability and superior catalytic activity for a model reaction.