Strong Enhancement of π‐Electron Donor/Acceptor Ability by Complementary DD/AA Hydrogen Bonding

π‐Conjugated organic materials possess a wide range of tunable optoelectronic properties which are dictated by their molecular structure and supramolecular arrangement. While many efforts have been put into tuning the molecular structure to achieve the desired properties, rational supramolecular control remains a challenge. Here, we report a novel series of supramolecular materials formed by the co‐assembly of weak π‐electron donor (indolo[2,3‐a]carbazole) and acceptor (aromatic o‐quinones) molecules via complementary hydrogen bonding. The resulting polarization creates a drastic perturbation of the molecular energy levels, causing strong charge transfer in the weak donor–acceptor pairs. This leads to a significant lowering (up to 1.5 eV) of the band gaps, intense absorption in the near‐IR region, very short π‐stacking distances (≥3.15 Å), and strong ESR signals in the co‐crystals. By varying the strength of the acceptor, the characteristics of the complexes can be tuned between intrinsic, gate‐, or light‐induced semiconductivity with a p‐type or ambipolar transport mechanism.     

Controlling the π‐Stacking Behavior of Pyrene Derivatives: Influence of H‐Bonding and Steric Effects in Different States of Aggregation

The performance of opto‐electronic devices built from low‐molecular‐weight dye molecules depends crucially on the stacking properties and the resulting coupling of the chromophoric systems. Herein we investigate the influence of H‐bonding amide and bulky substituents on the π‐stacking of pyrene‐containing small molecules in dilute solution, as supramolecular aggregates, and in the solid state. A set of four pyrene derivatives was synthesized in which benzene or 4‐tert‐butyl benzene was linked to the pyrene unit either through an ester or an amide. All four molecules form supramolecular H‐aggregates in THF solution at concentrations above 1×10^?4 mol L^?1. These aggregates were transferred on a solid support and crystallized. We investigate: the excimer formation rates within supramolecular aggregates; the formation of H‐bonds as well as the optical changes during the transition from the amorphous to the crystalline state; and the excimer to monomer fluorescence ratio in crystalline films at low temperatures. We reveal that in solution supramolecular aggregation depends predominantly on the pyrene chromophores. In the crystalline state, however, the pyrene stacking can be controlled gradually by H‐bonding and steric effects. These results are further confirmed by molecular modeling. This work bears fundamental information for tailoring the solid state of functional optoelectronic materials.     

Organic Photovoltaic Devices Based on a New Class of Oligothienylenevinylene Derivatives as Donor Materials

A new class of donor–acceptor‐containing oligothienylenevinylenes with a triphenylamine donor and a dicyanovinyl group as acceptor has been synthesized and characterized. By extending the oligothiophene backbone, both the optical bandgaps and the charge‐transport properties can be tuned. These oligothienylenevinylene derivatives show intense charge‐transfer absorption bands that cover the entire visible spectrum, with low optical bandgaps of approximately 1.64 eV. In addition, electrochemical studies reveal that these compounds possess relatively large ionization potentials of approximately 5.5 eV. On the basis of these newly developed dicyanovinyl‐substituted chromophores as donor materials and C₆₀ as acceptor material, bilayer organic photovoltaic devices have been fabricated, with the best device showing a high power conversion efficiency (PCE) of 2.0 %, with an open‐circuit voltage of 0.68 V and a fill factor of 0.60 after thermal annealing. The obvious morphology change with the formation of small domains in thin films and the reduction of series resistance are believed to be responsible for the dramatic performance improvement upon thermal annealing.     

Design and Sensing Properties of a Self‐Assembled Supramolecular Oligomer

Supramolecular polymers are a class of macromolecules stabilized by weak non‐covalent interactions. These self‐assembled aggregates typically undergo stimuli‐induced reversible assembly and disassembly. They thus hold great promise as so‐called functional materials. In this work, we present the design, synthesis, and responsive behavior of a short supramolecular oligomeric system based on two hetero‐complementary subunits. These “monomers” consist of a tetrathiafulvalene‐functionalized calix[4]pyrrole (TTF‐C[4]P) and a glycol diester‐linked bis‐2,5,7‐trinitrodicyanomethylenefluorene‐4‐carboxylate (TNDCF), respectively. We show that when mixed in organic solvents, such as CHCl₃, CH₂ClCH₂Cl, and methylcyclohexane, supramolecular aggregation takes place to produce short oligomers stabilized by hydrogen bonding and donor–acceptor charge‐transfer (CT) interactions. The self‐associated materials were characterized by ¹H NMR and UV/Vis/NIR absorption spectroscopy, as well as by concentration‐ and temperature‐dependent absorption spectroscopy and dynamic light scattering (DLS) analyses of both the monomeric and oligomerized species. The self‐associated system produced from TTF‐C[4]P and TNDCF exhibits a concentration‐dependent aggregation behavior typical of supramolecular polymers. Further support for the proposed self‐assembly came from theoretical calculations. The fluorescence emitting properties of TNDCF are quenched under conditions that promote the formation of supramolecular aggregates containing TTF‐C[4]P and TNDCF. This quenching effect has been utilized as a probe for the detection of substrates in the form of anions (i.e., chloride) and nitroaromatic explosives (i.e., 1,3,5‐trinitrobenzene). Specifically, the addition of these substrates to mixtures of TTF‐C[4]P and TNDCF produced a fluorescence “turn‐on” response.     

Unusually Short Chalcogen Bonds Involving Organoselenium: Insights into the Se–N Bond Cleavage Mechanism of the Antioxidant Ebselen and Analogues

Structural studies on the polymorphs of the organoselenium antioxidant ebselen and its derivative show the potential of organic selenium to form unusually short Se???O chalcogen bonds that lead to conserved supramolecular recognition units. Se???O interactions observed in these polymorphs are the shortest such chalcogen bonds known for organoselenium compounds. The FTIR spectral evolution characteristics of this interaction from solution state to solid crystalline state further validates the robustness of this class of supramolecular recognition units. The strength and electronic nature of the Se???O chalcogen bonds were explored using high‐resolution X‐ray charge density analysis and atons‐in‐molecules (AIM) theoretical analysis. A charge density study unravels the strong electrostatic nature of Se???O chalcogen bonding and soft‐metal‐like behavior of organoselenium. An analysis of the charge density around Se?N and Se?C covalent bonds in conjunction with the Se???O chalcogen bonding modes in ebselen and its analogues provides insights into the mechanism of drug action in this class of organoselenium antioxidants. The potential role of the intermolecular Se???O chalcogen bonding in forming the intermediate supramolecular assembly that leads to the bond cleavage mechanism has been proposed in terms of electron density topological parameters in a series of molecular complexes of ebselen with reactive oxygen species (ROS).     

Supramolecular elastomers: Switchable mechanical properties and tuning photohealing with changes in supramolecular interactions

To study light‐triggered self‐healing in supramolecular materials, we synthesized supramolecular thermoplastic elastomers with mechanical properties that were reversibly modulated with temperature. By changing the supramolecular architecture, we created polymers with different temperature responses. Detailed characterization of the hydrogen‐bonding material revealed dramatically different temperature and mechanical stress response due to two different stable states with changes in the hydrogen bonding interactions. A semi‐crystalline state showed no response to oscillatory shear deformations while the melt state behaved as a typical energy dissipative material with a clear crossover between storage and loss moduli. Comparison studies on heat generation after light excitation revealed no differences in photo‐thermal conversion when an Fe(II)‐phenanthroline chromophore was either physically blended into the H‐bonding polymer or covalently attached to the supramolecular network. These materials showed healing of scratches with light‐irradiation, as long as the overlap of material absorbance and laser excitation was sufficient. Differences in the efficiency and rate of photohealing were observed, depending on the type of supramolecular interaction, and these were attributed to the differences in the thermal response of the materials' moduli. Such results provide insight into how materials can be designed with chromophores and supramolecular bonding interactions to tune the light‐healing efficiency of the materials. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1003–1011     

Influence of Intermolecular N?H⋅⋅⋅π Interactions on Molecular Packing and Field‐Effect Performance of Organic Semiconductors

Charge transport in organic semiconductors is strongly dependent on their molecular packing modes in the solid state. Therefore, understanding the relationship between molecular packing and charge transport is imperative, both experimentally and theoretically. However, so far, the fundamental effects of solid‐state packing and molecular interactions (e.g. N? H ??? π) on charge transport need further elucidation. Herein, indolo[3,2‐b]carbazole (ICZ) and a derivative thereof are used as examples to approach this scientific target. An interesting insight obtained thereby is that N? H ??? π interactions among ICZ molecules facilitate charge transport for higher mobility. Subtle changes in the of N? H ??? π interactions can significantly influence both the molecular packing and the charge‐transport properties. Therefore, a method for exploiting intermolecular N? H ??? π interactions would yield novel molecular systems with designable characteristics.     

The role of halogen bonding in improving OFET performance of a naphthalenediimide derivative

A naphthalenediimide derivative (1) was designed and synthesized as a halogen bonding (XB) donor that is capable of forming complementary XBs with 2,2-dipyridine or 2,2-bipyrimidine acceptor. The XB interactions in the complexes (1/2,2-dipyridine and 1/2,2-bipyrimidine) significantly improved their OFET performance relative to the devices based on pure 1, with the average electron mobility increased more than doubled.     

Photosynthetic Antenna–Reaction Center Mimicry by Using Boron Dipyrromethene Sensitizers

Various molecular and supramolecular systems have been synthesized and characterized recently to mimic the functions of photosynthesis, in which solar energy conversion is achieved. Artificial photosynthesis consists of light‐harvesting and charge‐separation processes together with catalytic units of water oxidation and reduction. Among the organic molecules, derivatives of BF₂‐chelated dipyrromethene (BODIPY), “porphyrin’s little sister”, have been widely used in constructing these artificial photosynthetic models due to their unique properties. In these photosynthetic models, BODIPYs act as not only excellent antenna molecules, but also as electron‐donor and ‐acceptor molecules in both the covalently linked molecular and supramolecular systems formed by axial coordination, hydrogen bonding, or crown ether complexation. The relationships between the structures and photochemical reactivities of these novel molecular and supramolecular systems are discussed in relation to the efficiency of charge separation and charge recombination. Femto‐ and nanosecond transient absorption and photoelectrochemical techniques have been employed in these studies to give clear evidence for the occurrence of energy‐ and electron‐transfer reactions and to determine their rates and efficiencies.     

Stack the Bowls: Tailoring the Electronic Structure of Corannulene‐Integrated Crystalline Materials

We report the first examples of purely organic donor–acceptor materials with integrated π‐bowls (πBs) that combine not only crystallinity and high surface areas but also exhibit tunable electronic properties, resulting in a four‐orders‐of‐magnitude conductivity enhancement in comparison with the parent framework. In addition to the first report of alkyne–azide cycloaddition utilized for corannulene immobilization in the solid state, we also probed the charge transfer rate within the Marcus theory as a function of mutual πB orientation for the first time, as well as shed light on the density of states near the Fermi edge. These studies could foreshadow new avenues for πB utilization for the development of optoelectronic devices or a route for highly efficient porous electrodes.     

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.     

Experimental and Computational Probes of the Nature of Halogen Bonding: Complexes of Bromine‐Containing Molecules with Bromide Anions

The nature of halogen bonding is examined via experimental and computational characterizations of a series of associates between electrophilic bromocarbons R? Br (R? Br=CBr₃F, CBr₃NO₂, CBr₃COCBr₃, CBr₃CONH₂, CBr₃CN, etc.) and bromide anions. The [R? Br, Br^?] complexes show intense absorption bands in the 200–350 nm range which follow the same Mulliken correlation as those observed for the charge‐transfer associates of bromide anions with common organic π‐acceptors. For a wide range of the associates, intermolecular R? Br???Br^? separations decrease and intramolecular C? Br bond lengths increase proportionally to the Br^?→R? Br charge transfer; and the energies of R? Br???Br^? bonds are correlated with the linear combination of orbital (charge‐transfer) and electrostatic interactions. On the whole, spectral, structural and thermodynamic characteristics of the [R? Br, Br^?] complexes indicate that besides electrostatics, the orbital (charge‐transfer) interactions play a vital role in the R? Br???Br^? halogen bonding. This indicates that in addition to controlling the geometries of supramolecular assemblies, halogen bonding leads to electronic coupling between interacting species, and thus affects reactivity of halogenated molecules, as well as conducting and magnetic properties of their solid‐state materials.     

Chloride‐Anion‐Templated Synthesis of a Strapped‐Porphyrin‐Containing Catenane Host System

The synthesis, structure and anion‐recognition properties of a new strapped‐porphyrin‐containing [2]catenane anion host system are described. The assembly of the catenane is directed by discrete chloride anion templation acting in synergy with secondary aromatic donor–acceptor and coordinative pyridine–zinc interactions. The [2]catenane incorporates a three‐dimensional, hydrogen‐bond‐donating anion‐binding pocket; solid‐state structural analysis of the catenane?chloride complex reveals that the chloride anion is encapsulated within the catenane’s interlocked binding cavity through six convergent CH????Cl and NH???Cl hydrogen‐bonding interactions and solution‐phase ¹H NMR titration experiments demonstrate that this complementary hydrogen‐bonding arrangement facilitates the selective recognition of chloride over larger halide anions in DMSO solution.     

Uncovering the Intramolecular Emission and Tuning the Nonlinear Optical Properties of Organic Materials by Cocrystallization

The spectroscopic and photophysical properties of organic materials in the solid‐state are widely accepted as a result of their molecular packing structure and intermolecular interactions, such as J‐ and H‐aggregation, charge‐transfer (CT), excimer and exciplex. However, in this work, we show that Spe‐F₄DIB cocrystals (SFCs) surprisingly retain the energy levels of photoluminescence (PL) states of Spe crystals, despite a significantly altered molecular packing structure after cocrystallization. In comparison, Npe‐F₄DIB cocrystals (NFCs) with new spectroscopic states display different spectra and photophysical behaviors as compared with those of individual component crystals. These may be related to the molecular configuration in crystals, and we propose Spe as an “intramolecular emissive” material, thus providing a new viewpoint on light‐emitting species of organic chromophores. Moreover, the nonlinear optical (NLO) properties of Npe and Spe are firstly demonstrated and modulated by cocrystallization. The established “molecule‐packing‐property” relationship helps to rationally control the optical properties of organic materials through cocrystallization.     

Charge carrier mobilities in organic semiconductor crystals based on the spectral overlap

The prediction of substance‐related charge‐transport properties is important for the tayloring of new materials for organic devices, such as organic solar cells. Assuming a hopping process, the Marcus theory is frequently used to model charge transport. Here another approach, which is already widely used for exciton transport, is adapted to charge transport. It is based on the spectral overlap of the vibrational donor and acceptor spectra. As the Marcus theory it is derived from Fermi's Golden rule, however, it contains less approximations, as the molecular vibrations are treated quantum mechanically. In contrast, the Marcus theory reduces all vibrational degrees of freedom to one and treats its influence classically. The approach is tested on different acenes and predicts most of the experimentally available hole mobilities in these materials within a factor of 2. This represents a significant improvement to values obtained from Marcus theory which is qualitatively correct but frequently overestimates the mobilities by factors up to 10. Furthermore, the charge‐transport properties of two derivatives of perylene bisimide are investigated. © 2016 Wiley Periodicals, Inc.     

High Degree of Polymerization in a Fullerene‐Containing Supramolecular Polymer

Supramolecular polymers based on dispersion forces typically show lower molecular weights (MW) than those based on hydrogen bonding or metal–ligand coordination. We present the synthesis and self‐assembling properties of a monomer featuring two complementary units, a C₆₀ derivative and an exTTF‐based macrocycle, that interact mainly through π–π, charge‐transfer, and van der Waals interactions. Thanks to the preorganization in the host part, a remarkable log K_a=5.1±0.5 in CHCl₃ at room temperature is determined for the host–guest couple. In accordance with the large binding constant, the monomer self‐assembles in the gas phase, in solution, and in the solid state to form linear supramolecular polymers with a very high degree of polymerization. A MW above 150 kDa has been found experimentally in solution, while in the solid state the monomer forms extraordinarily long, straight, and uniform fibers with lengths reaching several microns.     

Tubular supramolecular alternating copolymers fabricated by cyclic peptide–polymer conjugates

Supramolecular copolymers are an emerging class of materials, which bring together different properties and functionalities of multiple components via noncovalent interactions. While it is widely acknowledged that the repeating unit sequence plays an essential role on the performance of these materials, mastering and tuning the supramolecular copolymer sequence is still an open challenge. To date, only statistical supramolecular copolymers have been reported using cyclic peptide–polymer conjugates as building blocks. To enrich the diversity of tubular supramolecular copolymers, we report here a strategy of controlling their sequences by introducing an extra complementary noncovalent interaction. Hence, two conjugates bearing one electron donor and one electron acceptor, respectively, are designed. The two conjugates can individually assemble into tubular supramolecular homopolymers driven by the multiple hydrogen bonding interactions between cyclic peptides. However, the complementary charge transfer interaction between the electron donor and acceptor makes each conjugate more favorable for complexing with its counterpart, resulting in an alternating sequence of the supramolecular copolymer. Following the same principle, more functional supramolecular alternating copolymers are expected to be designed and constructed via other complementary noncovalent interactions (electrostatic interactions, metal coordination interactions, and host–guest interactions, etc.).

Tubular supramolecular alternating copolymers using cyclic peptide–polymer conjugates are synthesized by the introduction of an extra complementary noncovalent interaction.     

Self‐recognition in a flexible bis(pyrrole) Schiff base derivative: formation of a one‐dimensional hydrogen‐bonded polymer

The title Schiff base compound, N,N′‐bis­(pyrrol‐2‐yl­methyl­ene)­propane‐1,2‐di­amine, C₁₃H₁₆N₄, forms an interesting supramolecular structure (a one‐dimensional ladder‐like polymer) in the solid state that is based on the existence of complementary intermolecular N—H⋯N=C hydrogen bonds between the monomer units. The polymer axis is collinear with the c axis of the orthorhombic unit cell. Quantum‐chemical AM1 calculations clearly indicate that self‐recognition in this system by hydrogen bonding is favoured on electrostatic grounds, since the partial atomic charge on the H atom of the pyrrole NH group (0.274 e) complements the partial atomic charge of the N atom of the C=N group (−0.239 e) on a neighbouring mol­ecule.     

Concurring Chalcogen- and Halogen-Bonding Interactions in Supramolecular Polymers for Crystal Engineering Applications

The engineering of crystalline molecular solids through the simultaneous combination of distinctive non-covalent interactions is an important field of research, as it could allow chemist to prepare materials depicting multi-responsive properties. It is in this context that, pushed by a will to expand the chemical space of chalcogen-bonding interactions, a concept is put forward for which chalcogen- and halogen-bonding interactions can be used simultaneously to engineer multicomponent co-crystals. Through the rational design of crystallizable molecules, chalcogenazolo pyridine scaffold (CGP) modules were prepared that, bearing either a halogen-bond acceptor or donor at the 2-position, can interact with suitable complementary molecular modules undergoing formation of supramolecular polymers at the solid state. The recognition reliability of the CGP moiety to form chalcogen-bonded dimers allows the formation of heteromolecular supramolecular polymers through halogen-bonding interactions, as confirmed by single-crystal X-ray diffraction analysis.     

Herringbone and Helical Self‐Assembly of π‐Conjugated Molecules in the Solid State through CH/π Hydrogen Bonds

Self‐organization of organic molecules through weak noncovalent forces such as CH/π interactions and creation of large hierarchical supramolecular structures in the solid state are at the very early stage of research. The present study reports direct evidence for CH/π interaction driven hierarchical self‐assembly in π‐conjugated molecules based on custom‐designed oligophenylenevinylenes (OPVs) whose structures differ only in the number of carbon atoms in the tails. Single‐crystal X‐ray structures were resolved for these OPV synthons and the existence of long‐range multiple‐arm CH/π interactions was revealed in the crystal lattices. Alignment of these π‐conjugated OPVs in the solid state was found to be crucial in producing either right‐handed herringbone packing in the crystal or left‐handed helices in the liquid‐crystalline mesophase. Pitch‐ and roll‐angle displacements of OPV chromophores were determined to trace the effect of the molecular inclination on the ordering of hierarchical structures. Furthermore, circular dichroism studies on the OPVs were carried out in the aligned helical structures to prove the existence of molecular self‐assembly. Thus, the present strategy opens up new approaches in supramolecular chemistry based on weak CH/π hydrogen bonding, more specifically in π‐conjugated materials.