Supramolekulare Anordnungen mit Charge-Transfer-Wechselwirkungen zwischen Donor- und Akzeptor-Chromophoren

Macrocycle-Based Crystalline Supramolecular Assemblies Built with Intermolecular Charge-Transfer Interactions

Synthetic macrocycles have served as principal tools for supramolecular chemistry, have greatly extended the scope of organic charge transfer (CT) complexes, and have proved to be of great practical value in the solid state during the past few years. In this Minireview, we summarize the research progress on the macrocycle-based crystalline supramolecular assemblies primarily driven by intermolecular CT interactions (a.k.a. macrocycle-based crystalline CT assemblies, MCCAs for short), which are classified by their donor-acceptor (D-A) constituent elements, including simplex macrocyclic hosts, heterogeneous macrocyclic hosts, and host-guest D-A pairs. Particular attention will be focused on their diverse functions and applications, as well as the underlying CT mechanisms from the perspective of crystal engineering. Finally, the remaining challenges and prospects are outlined.

Induced CPL-Active Materials Based on Chiral Supramolecular Co-Assemblies

Circularly polarized luminescence (CPL) has attracted much interest due to its potential applications on chiral photonic techniques and optoelectronic materials science. As known, dissymmetry factor (g_em ) of CPL is one essential factor for evaluating the features of CPL-active materials. Much attention has focused on how to increase the g_em value, which is one of the most important issues for CPL practical applications. Recently, more and more works have demonstrated that chiral supramolecular could provide the significant strategy to improve the g_em value through the orderly helical superstructure of chiral building blocks. Normally, this kind of chiral supramolecular assembly process can be accompanied by chirality transfer and induction mechanism, which can promote the amplification effect on the induced CPL of achiral dyes. In this review, we fully summarized recent advances on the induced CPL-active materials of chiral supramolecular co-assemblies, their applications in circularly polarized organic light-emitting diodes (CP-OLEDs) and current challenges.

The Story of the Little Blue Box: A Tribute to Siegfried Hünig

The tetracationic cyclophane, cyclobis(paraquat-p-phenylene), also known as the little blue box, constitutes a modular receptor that has facilitated the discovery of many host-guest complexes and mechanically interlocked molecules during the past 35 years. Its versatility in binding small π-donors in its tetracationic state, as well as forming trisradical tricationic complexes with viologen radical cations in its doubly reduced bisradical dicationic state, renders it valuable for the construction of various stimuli-responsive materials. Since the first reports in 1988, the little blue box has been featured in over 500 publications in the literature. All this research activity would not have been possible without the seminal contributions carried out by Siegfried Hünig, who not only pioneered the syntheses of viologen-containing cyclophanes, but also revealed their rich redox chemistry in addition to their ability to undergo intramolecular π-dimerization. This Review describes how his pioneering research led to the design and synthesis of the little blue box, and how this redox-active host evolved into the key component of molecular shuttles, switches, and machines.

Direct Participation of Solvent Molecules in the Formation of Supramolecular Polymers

This article reports supramolecular polymerization of two bis-amide functionalized naphthalene-diimide (NDI) building blocks (NDI-L and NDI-C) in two solvents, namely n-heptane (Hep) and methylcyclohexane (MCH). NDI-L and NDI-C differ only by the peripheral hydrocarbon wedges, consisting of linear C7 chains or cyclic methylcyclohexane rings, respectively. UV/Vis and FTIR spectroscopy studies reveal distinct internal order and H-bonding pattern for NDI-L and NDI-C aggregates irrespective of the solvent system, indicating the dominant role of the intrinsic packing parameters of the individual building block, possibly influenced by the peripheral steric crowding. However, NDI-L produces a significantly stronger gel in Hep compared to MCH as evident from the rheological and thermal properties. In contrast, NDI-C exhibits a clear preference for MCH, producing gel with moderate strength but in Hep it fails to produce 1D morphology or gelation. All-atom molecular dynamics (MD) simulation studies corroborate with the experimental observation and provide the rationale for the observed solvent-shape effect by revealing a quantitative estimate regarding the thermodynamics of self-assembly in these four combinations. Such clear-cut shape-matching effect (between the peripheral hydrocarbon wedge and the solvent system) unambiguously support a direct participation of the solvent molecules during supramolecular polymerization and presence of a closely-adhered solvent shell around the supramolecular polymers, similar to the first layer of water molecules around the protein surface. Solvent induced CD experiments support this hypothesis as induced CD band was observed only from a chiral co-solvent of matching shape. This is reconfirmed by the higher de-solvation temperature of the shape-matching NDI/solvent system combination compared to the shape mis-match combination in variable temperature UV/Vis experiments, revealing transformation to a different aggregate at higher temperatures rather than disassembly to the monomer for all four combinations.

A Hierarchical (Macro)molecular Assembly Assisted by Donor-Acceptor Charge-Transfer Interactions Exhibiting Room-Temperature Ferroelectricity

This communication reveals co-assembly of an electron-deficient naphthalene-diimide (NDI)-appended polyurethane (P1) and electron-rich pyrene (Py), forming an organogel with prominent room-temperature ferroelectricity. In a non-polar medium, intra-chain hydrogen-bonding among the urethane groups of P1 produces a folded structure with an array of the NDIs in the periphery, which intercalate Py by charge-transfer (CT)-interaction. Such CT-complexation enables slow crystallization of the peripheral hydrocarbons, causing gelation with nanotubular morphology, in which the wall consists of the alternating NDI-Py stack. Such D-A assembly exhibits ferroelectricity (saturation polarization P_s ≈0.8 μC cm^-2 and coercive field E_c ≈8 kV cm^-1 at 500 V and 10 Hz frequency) with Curie temperature (T_c ) of ≈350 K, which can be related to the disassembly of the CT-complex. In the absence of Py, P1 forms spherical aggregates, showing dielectric behaviour.

Peptide Backbone Directed Self‐Assembly of Merocyanine Oligomers into Duplex Structures

The pseudopeptide backbone provided by N‐(2‐aminoethyl)‐glycine oligomers with attached nucleobases has been widely utilized in peptide nucleic acids (PNAs) as DNA mimics. Here we demonstrate the suitability of this backbone for the formation of structurally defined dye stacks. Toward this goal a series of peptide merocyanine (PMC) dye oligomers connected to a N‐(2‐aminoethyl)‐glycine backbone were prepared through peptide synthesis. Our concentration‐, temperature‐ and solvent‐dependent UV/Vis absorption studies show that under the control of dipole–dipole interactions, smaller‐sized oligomers consisting of one, two or three dyes self‐assemble into defined duplex structures containing two up to six chromophores. In contrast, upon further extension of the oligomer, the chosen peptide backbone cannot direct the formation of a defined duplex architecture anymore due to intramolecular aggregation between the dyes. For all aggregate species a moderate aggregation‐induced emission enhancement is observed.

Responsive Interfacial Assemblies Based on Charge-Transfer Interactions

Charge transfer (CT) interactions have been widely used to construct supramolecular systems, such as functional nanostructures and gels. However, to date, there is no report on the generation of CT complexes at the liquid-liquid interface. Here, by using an electron-deficient acceptor dissolved in water and an electron-rich donor dissolved in oil, we present the in situ formation and assembly of CT complex surfactants (CTCSs) at the oil-water interface. With time, CTCSs can assemble into higher-order nanofilms with exceptional mechanical properties, allowing the stabilization of liquids and offering the possibility to structure liquids into nonequilibrium shapes. Moreover, due to the redox-responsiveness of the electron-deficient acceptor, the association and dissociation of CTCSs can be reversibly manipulated in a redox process, leading to the switchable assembly and disassembly of the resultant constructs.

The Unusual Photochromic and Hydrochromic Switching Behavior of Cellulose-Embedded 1,8-Naphthalimide-Viologen Derivatives in the Solid-State

Stimuli-responsive chromic materials such as photochromics, hydrochromics, thermochromics, and electrochromics have a long history of capturing the attention of scientists due to their potential industrial applications and novelty in popular culture. However, hybrid chromic materials that combine two or more stimuli-triggered color changing properties are not so well known. Herein, we report a design strategy that has led to a series of emissive 1,8-naphthalimide-viologen dyads which exhibit unusual dual photochromic and hydrochromic switching behavior in the solid-state when embedded in a cellulose matrix. This behavior manifests as reversible solid state fluorescence hydrochromism upon changes in atmospheric relative humidity (RH), and reversible solid state photochromism upon generation of a cellulose-stabilized viologen radical cation. In this design strategy, the bipyridinium unit serves as both a water-sensitive receptor for the hydrochromic fluorophore-receptor system, and a photochromic group, capable of eliciting its own visible colorimetric response, generating a fluorescence quenching radical cation with prolonged exposure to ultraviolet (UV) light. These dyes can be inkjet-printed onto cellulose paper or drop-cast as cellulose powder-based films and can be unidirectionally cycled between three different states which can be characteristically visualized under UV light or visible light. The material's photochromism, hydrochromism, and underlying mechanism of action was investigated using computational analysis, dynamic vapor sorption/desorption isotherms, electron paramagnetic resonance spectroscopy, and variable humidity UV-Vis adsorption and fluorescence spectroscopies.

Externally Regulated Specific Molecular Recognition Driven Pathway Selectivity in Supramolecular Polymerization

This article reveals 4-dimethylaminopyridine (DMAP) regulated pathway selectivity in the supramolecular polymerization of a naphthalene-diimide derivative (NDI-1), appended with a carboxylic acid group. In decane, NDI-1 produces ill-defined aggregate (Agg-1) due to different H-bonding motifs of the -COOH group. With one mole equivalent DMAP, the NDI-1/DMAP complex introduces new nucleation condition and exhibits a cooperative supramolecular polymerization producing J-aggregated fibrillar nanostructure (Agg-2). With 10 % DMAP and fast cooling (10 K/min), similar nucleation and open chain H-bonding with the free monomer in an anti-parallel arrangement produces identical J-aggregate (Agg-2a). With 2.5 % DMAP and slow cooling (1 K/min), a distinct nucleation and supramolecular polymerization pathway emerge leading to the thermodynamically controlled Agg-3 with face-to-face stacking and 2D-morphology. Slow cooling with 5-10 % DMAP produces a mixture of Agg-2a and Agg-3. Computational modelling studies provide valuable insights into the internal order and the pathway complexity.

Stepwise Adsorption of Alkoxy-Pyrene Derivatives onto a Lamellar, Non-Porous Naphthalenediimide-Template on HOPG

The development of new strategies for the preparation of multicomponent supramolecular assemblies is a major challenge on the road to complex functional molecular systems. Here we present the use of a non-porous self-assembled monolayer from uC₃₃ -NDI-uC₃₃ , a naphthalenediimide symmetrically functionalized with unsaturated 33 carbon-atom-chains, to prepare bicomponent supramolecular surface systems with a series of alkoxy-pyrene (PyrOR) derivatives at the liquid/HOPG interface. While previous attempts at directly depositing many of these PyrOR units at the liquid/HOPG interface failed, the multicomponent approach through the uC₃₃ -NDI-uC₃₃ template enabled control over molecular interactions and facilitated adsorption. The PyrOR deposition restructured the initial uC₃₃ -NDI-uC₃₃ monolayer, causing an expansion in two dimensions to accommodate the guests. As far as we know, this represents the first example of a non-porous or non-metal complex-bearing monolayer that allows the stepwise formation of multicomponent supramolecular architectures on surfaces.

Supramolecular Photocatalysis by Utilizing the Host-Guest Charge-Transfer Interaction: Visible-Light-Induced Generation of Triplet Anthracenes for [4+2] Cycloaddition Reactions

Supramolecular photocatalysis via charge-transfer excitation of a host-guest complex was developed by use of the macrocyclic boronic ester [2+2]_BTH-F containing highly electron-deficient difluorobenzothiadiazole moieties. In the presence of a catalytic amount of [2+2]_BTH-F , the triplet excited state of anthracene was generated from the charge-transfer excited state of anthracene@[2+2]_BTH-F by visible-light irradiation, and cycloaddition of the excited anthracene with several dienes and alkenes proceeded in a [4+2] manner in high yields.

Anion-Coordination-Assisted Assembly of Supramolecular Charge-Transfer Complexes Based on Tris(urea) Ligands

Charge-transfer (CT) complexes, formed by noncovalent bonding between electron-rich (donor, D) and electron-deficient (acceptor, A) molecules (or moieties) have attracted considerable attention due to their fascinating structures and potential applications. Herein, we demonstrate that anion coordination is a promising strategy to promote CT complex formation between anion-binding, electron-rich tris(urea) donor ligands (D) and electron-deficient viologen cation acceptors (A), which form co-crystals featuring infinite ⋅⋅⋅DADA⋅⋅⋅ or discrete (circular DADA or three-decker DAD) π-stacking interactions. These CT complexes were studied by X-ray diffraction, UV/Vis spectroscopy, electric conductivity measurements, charge displacement curve (CDC) calculations, and DFT computations.

Supramolecular Nanowires from an Acceptor-Donor-Acceptor Conjugated Chromophore

Oligothiophene derivatives have been extensively studied as p-type semiconducting materials in organic electronics applications. This work reports the synthesis, self-assembly and photophysical properties of acceptor-donor-acceptor (A-D-A)-type oligothiophene derivatives by end-group engineering of quaterthiophene (QT) with naphthalene monoimide (NMI) chromophores that are further connected to a trialkoxy benzamide wedge. Conjugation to the NMI units reduces the HOMO-LUMO gap significantly, and consequently the absorption spectrum exhibits a bathochromic shift of about 50 nm compared with QT. Furthermore, extended H-bonding interactions among the amido groups of the peripheral wedges produce entangled fibrillar nanostructures and gelation in hydrocarbon solvents such as methylcyclohexane, wherein the A-D-A chromophore exhibits typical H-aggregation. On the contrary, the fact that the same chromophore, lacking only the amido units, does not produce gels or H-aggregates indicates strong impact of H-bonding on the self-assembly. Computational studies revealed the electronic properties of the chromophore and predicted the geometry of a dimer in the H-aggregate that reasonably matches with the experimental results. Bulk electrical conductivity measurements determined an excellent conductivity of 2.3×10^-2  S cm^-1 for the H-aggregated system (OT-1), which is two orders of magnitude higher than that of the same chromophore lacking the amido groups (OT-2).

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.

Mechanistic Insights into the Self-Assembly of an Acid-Sensitive Photoresponsive Supramolecular Polymer

The supramolecular polymerization of an acid‐sensitive pyridyl‐based ligand (L₁) bearing a photoresponsive azobenzene moiety was elucidated by mechanistic studies. Addition of trifluoroacetic acid (TFA) led to the transformation of the antiparallel H‐bonded fibers of L₁ in methylcyclohexane into superhelical braid‐like fibers stabilized by H‐bonding of parallel‐stacked monomer units. Interestingly, L₁ dimers held together by unconventional pyridine–TFA N⋅⋅⋅H⋅⋅⋅O bridges represent the main structural elements of the assembly. UV‐light irradiation caused a strain‐driven disassembly and subsequent aggregate reconstruction, which ultimately led to short fibers. The results allowed to understand the mechanism of mutual influence of acid and light stimuli on supramolecular polymerization processes, thus opening up new possibilities to design advanced stimuli‐triggered supramolecular systems.

Controlled Formation of a Main Chain Supramolecular Polymer Based on Metal-Ligand Interactions and a Thiol-Ene Click Reaction

Supramolecular polymers with multiple functionalities and hierarchical structures have received considerable attention and become a hot research topic over the past years. Herein, a main-chain supramolecular polymer has been successfully fabricated by using metal-ligand interactions and a thiol-ene click reaction. ¹ H NMR, UV/Vis, DOSY, and viscosity measurements were carried out to investigate the molecular recognition and the process of supramolecular polymerization. From the study, the orthogonality between thiol-ene click reactions and the terpyridine-metal ions complexation behavior was testified, and supramolecular polymeric assemblies could be constructed by a one pot method. In the meantime, due to the incorporation of metal-ligand interactions, the supramolecular polymer shows stimuli-responsive properties toward chemical stimuli. Hence, this work could provide a methodology for developing supramolecular polymers as smart materials.

Tuning the Mesomorphism and Redox Response of Anionic-Ligand-Based Mixed-Valent Nickel(II) Complexes by Alkyl-Substituted Quaternary Ammonium Cations

The combination of the redox-active mesogenic anion [Ni^II (Bdt)(BdtSQ)]^- (Bdt=1,2-benzenedithiolato; BdtSQ=1,2-dithia-semi-benzoquinonato) with alkyl-substituted ammonium cations afforded a series of redox-active ionic complexes of the type [NR₄ ][Ni^II (Bdt)(BdtSQ)] [R=nC₁₆ H₃₃ (NC16₄ Ni) and C8,10 (NC8,10₄ Ni); C8,10=6-octylhexadecyl] or [NMe₂ R₂ ][Ni^II (Bdt)(BdtSQ)] [R=nC₁₆ H₃₃ (NMe₂ C16₂ Ni) and C8,10 (NMe₂ C8,10₂ Ni)]. X-ray crystallographic analyses of NMe₂ C16₂ Ni and NC16₄ Ni revealed the formation of cation-dependent integrated ionic layers separated by interdigittated alkyl chains. Complexes NMe₂ C16₂ Ni and NC16₄ Ni commonly form crystalline phases at room temperature, whereas complexes NMe₂ C8,10₂ Ni and NC8,10₄ Ni, which contain branched alkyl chains, form a metastable mesophase and an amorphous phase at the same temperature, respectively. Furthermore, complexes NMe₂ C16₂ Ni, NMe₂ C8,10₂ Ni, and NC16₄ Ni commonly form a smectic A phase (SmA) at 375, 317, and 342 K, respectively. For the four complexes, well-defined cyclic voltammetry responses, derived from ligand-based oxidation and reduction, were observed in solution and the condensed phases, that is, upon casting these complexes on an indium-doped tin oxide working electrode. The present study demonstrates the tunability of the mesomorphism of ionic molecular assemblies composed of alkyl-substituted quaternary ammonium cations, while maintaining the well-defined redox responses of the anions even in the condensed phases.