Hydrogen‐Bonding Induced Alternate Stacking of Donor (D) and Acceptor (A) Chromophores and their Supramolecular Switching to Segregated States

This paper reports comprehensive studies on the mixed assembly of bis‐(trialkoxybenzamide)‐functionalized dialkoxynaphthalene (DAN) donors and naphthalene‐diimide (NDI) acceptors due the cooperative effects of hydrogen bonding, charge‐transfer (CT) interactions, and solvophobic effects. A series of DAN as well as NDI building blocks have been examined (wherein the relative distance between the two amide groups in a particular chromophore is the variable structural parameter) to understand the structure‐dependent variation in mode of supramolecular assembly and morphology (organogel, reverse vesicle, etc.) of the self‐assembled material. Interestingly, it was observed that when the amide functionalities are introduced to enhance the self‐assembly propensity, the mode of co‐assembly among the DAN and NDI chromophores no longer remained trivial and was dictated by a relatively stronger hydrogen‐bonding interaction instead of a weak CT interaction. Consequently, in a highly non‐polar solvent like methylcyclohexane (MCH), although kinetically controlled CT‐gelation was initially noticed, within a few hours the system sacrificed the CT‐interaction and switched over to the more stable self‐sorted gel to maximize the gain in enthalpy from the hydrogen‐bonding interaction. In contrast, in a relatively less non‐polar solvent such as tetrachloroethylene (TCE), in which the strength of hydrogen bonding is inherently weak, the contribution of the CT interaction also had to be accounted for along with hydrogen bonding leading to a stable CT‐state in the gel or solution phase. The stability and morphology of the CT complex and rate of supramolecular switching (from CT to segregated state) were found to be greatly influenced by subtle structural variation of the building blocks, solvent polarity, and the DAN/NDI ratio. For example, in a given D–A pair, by introducing just one methylene unit in the spacer segment of either of the building blocks a complete change in the mode of co‐assembly (CT state or segregated state) and the morphology (1D fiber to 2D reverse vesicle) was observed. The role of solvent polarity, structural variation, and D/A ratio on the nature of co‐assembly, morphology, and the unprecedented supramolecular‐switching phenomenon have been studied by detail spectroscopic and microscopic experiments in a gel as well as in the solution state and are well supported by DFT calculations.     

Alternating Donor–Acceptor Arrays from Hexa‐peri‐hexabenzocoronene and Benzothiadiazole: Synthesis,Optical Properties,and Self‐Assembly

Donor–acceptor (D–A) structures were obtained by alternating arrays of hexa‐peri‐hexabenzocoronene (HBC) and benzo[c][1,2,5]thiadiazole (BTZ). Optoelectronic investigations revealed a charge transfer due to strong push–pull interactions. 2 D wide‐angle X‐ray scattering (WAXS) data indicated an arrangement in liquid‐crystalline columnar assemblies, in which the π‐stacking distances and molecular orientation depend on the number of HBC units in the molecules.     

Highly Segregated Lamello‐Columnar Mesophase Organizations and Fast Charge Carrier Mobility in New Discotic Donor–Acceptor Triads

Four new donor–acceptor triads (D–A–D) based on discotic and arylene mesogens have been synthesized by using Sonogashira coupling and cyclization reactions. This family of triads consists of two side‐on pending triphenylene mesogens, acting as the electron‐donating groups (D), laterally connected through short lipophilic spacers to a central perylenediimide (PI), benzo[ghi]perylenediimide (BI), or coronenediimide (CI) molecular unit, respectively, playing the role of the electron acceptor (A). All D–A–D triads self‐organize to form a lamello‐columnar oblique mesophase, with a highly segregated donor–acceptor (D–A) heterojunction organization, consequent to efficient molecular self‐sorting. The structure consists in the regular alternation of two disrupted rows of triphenylene columns and a continuous row of diimine species. High‐resolution STM images demonstrate that PI‐TP2 forms stable 2D self‐assembly nanostructures with some various degrees of regularity, whereas the other triads do not self‐organize into ordered architectures. The electron‐transport mobility of CI‐TP2, measured by time‐of‐flight at 200 °C in the mesophase, is one order of magnitude higher than the hole mobility. By means of this specific molecular designing idea, we realized and demonstrated for the first time the so‐called p–n heterojunction at the molecular level in which the electron‐rich triphenylene columns act as the hole transient pathways, and the coronenediimide stacks form the electron‐transport channels.     

Exquisite 1D Assemblies Arising from Rationally Designed Asymmetric Donor–Acceptor Architectures Exhibiting Aggregation‐Induced Emission as a Function of Auxiliary Acceptor Strength

One‐dimensional nanostructures with aggregation‐induced emission (AIE) properties have been fabricated to keep the pace with growing demand from optoelectronics applications. The compounds 2‐[4‐(4‐methylpiperazin‐1‐yl)benzylidene]malononitrile ( PM1 ), 2‐{4‐[4‐(pyridin‐2‐yl)piperazin‐1‐yl]‐benzylidene}malononitrile ( PM2 ), and 2‐{4‐[4‐(pyrimidin‐2‐yl)piperazin‐1‐yl]benzylidene}malononitrile ( PM3 ) have been designed and synthesized by melding piperazine and dicyanovinylene to investigate AIE in an asymmetric donor–acceptor (D–A) construct of A′–D–π–A‐ topology. The synthetic route has been simplified by using phenylpiperazine as a weak donor (D), dicyanovinylene as an acceptor (A), and pyridyl/pyrimidyl groups ( PM2/PM3 ) as auxiliary acceptors (A′). It has been established that A′ plays a vital role in triggering AIE in these compounds because the same D–A construct led to aggregation‐caused quenching upon replacing A′ with an electron‐donating ethyl group ( PM1 ). Moreover, the effect of restricted intramolecular rotation and twisted intramolecular charge transfer on the mechanism of AIE has also been investigated. Furthermore, it has been clearly shown that the optical disparities of these A′–D–π–A architectures are a direct consequence of comparative A′ strength. Single‐crystal X‐ray analyses provided justification for role of intermolecular interactions in aggregate morphology. Electrochemical and theoretical studies affirmed the effect of the A′ strength on the overall properties of the A′–D–π–A system.     

Revealing the Charge‐Transfer Interactions in Self‐Assembled Organic Cocrystals: Two‐Dimensional Photonic Applications

A new crystal of a charge‐transfer (CT) complex was prepared through supramolecular assembly and it has unique two‐dimensional (2D) morphology. The CT nature of the ground and excited states of this new Bpe‐TCNB cocrystal (BTC) were confirmed by electron spin resonance measurements, spectroscopic studies, and theoretical calculations, thus providing a comprehensive understanding of the CT interactions in organic donor–acceptor systems. And the lowest CT₁ excitons are responsible for the efficient photoluminescence (Φ_PL=19 %), which can actively propagate in individual 2D BTCs without anisotropy, thus implying that the optical waveguide property of the crystal is not related to the molecular stacking structure. This unique 2D CT cocrystal exhibits potential for use in functional photonic devices in the next‐generation optoelectronic communications.     

Versatile Self‐Complexing Compounds Based on Covalently Linked Donor–Acceptor Cyclophanes

A range of covalently linked donor–acceptor compounds which contain 1) a hydroquinone (HQ) unit, 2) a 1,5‐dioxynaphthalene (DNP) ring system, or 3) a tetrathiafulvalene (TTF) unit as the π‐donor, and 4) cyclobis(paraquat‐p‐phenylene) (CBPQT⁴⁺) as the π‐accepting tetracationic cyclophane were prepared and shown to operate as simple molecular machines. The π‐donating arms can be included inside the cyclophane in an intramolecular fashion by virtue of stabilizing noncovalent bonding interactions. What amounts to self‐complexing/decomplexing equilibria were shown to be highly temperature dependent when the π‐donating arm contains either an HQ or DNP moiety. The thermodynamic parameters associated with the equilibria have been unraveled by using variable‐temperature ¹H NMR spectroscopy. The negative ΔH° and ΔS° values account for the fact that the “uncomplexed” conformation becomes the dominant species, since the entropy gain associated with the decomplexation process overcomes the enthalpy loss resulting from the breaking of the donor–acceptor interactions. The arm's in‐and‐out movements with respect to the linked cyclophanes can be arrested by installing a bulky substituent at the end of the arm. In the case of compounds carrying a DNP ring system in their side arm, two diastereoisomeric, self‐complexing conformations are observed below 272 K in hexadeuterioacetone. By contrast, control over the TTF‐containing arm's movement is more or less ineffective through the thermally sensitive equilibrium although it can be realized by chemical and electrochemical ways as a result of TTF's excellent redox properties. Such self‐complexing compounds could find applications as thermo‐ and electroswitches. In addition, the thermochromism associated with the arm's movement could lead to some of the compounds finding uses as imaging and sensing materials.     

Photoinduced Electron Transfer within a Zinc Porphyrin–Cyclobis(paraquat‐p‐phenylene) Donor–Acceptor Dyad

Understanding the mechanism of efficient photoinduced electron‐transfer processes is essential for developing molecular systems for artificial photosynthesis. Towards this goal, we describe the synthesis of a donor–acceptor dyad comprising a zinc porphyrin donor and a tetracationic cyclobis(paraquat‐p‐phenylene) (CBPQT⁴⁺) acceptor. The X‐ray crystal structure of the dyad reveals the formation of a dimeric motif through the intermolecular coordination between the triazole nitrogen and the central Zn metal of two adjacent units of the dyad. Photoinduced electron transfer within the dyad in MeCN was investigated by femtosecond and nanosecond transient absorption spectroscopy, as well as by transient EPR spectroscopy. Photoexcitation of the dyad produced a weakly coupled ZnP^+.–CBPQT^3+. spin‐correlated radical‐ion pair having a τ=146 ns lifetime and a spin–spin exchange interaction of only 0.23 mT. The long radical‐ion‐pair lifetime results from weak donor–acceptor electronic coupling as a consequence of having nine bonds between the donor and the acceptor, and the reduction in reorganization energy for electron transfer caused by charge dispersal over both paraquat units within CBPQT^3+..     

Imidazole as a Donor/Acceptor Unit in Charge‐Transfer Chromophores with Extended π‐Linkers

Eleven new, stable, push–pull systems that feature 4,5‐bis[4‐(N,N‐dimethylamino)phenyl]imidazole and 4,5‐dicyanoimidazole as the donor and acceptor moieties and the systematically extended and varied π‐linker were prepared and investigated. Evaluation of the measured UV/Vis spectra, electrochemical data (cyclic voltammetry (CV), rotating‐disc voltammetry (RDV), and polarography) and calculated β and γ polarizabilities showed efficient charge transfer (CT) in biimidazole‐type chromophores. Push–pull system 27 , which features a planar thiophene‐derived π‐linker, was revealed to be the most efficient chromophore within the studied series. This chromophore possessed the most bathochromically shifted CT band, the lowest electrochemical gap, and highest β and γ polarizabilities. The CT transition was most significantly affected by structural features such as π‐linker length, planarity, conjugating arrangement, and the presence of olefinic/acetylenic or 1,4‐phenylene/thiophene subunits in the π‐linker.     

Organic Donor–Acceptor Assemblies form Coaxial p–n Heterojunctions with High Photoconductivity

The formation of coaxial p–n heterojunctions by mesoscale alignment of self‐sorted donor and acceptor molecules, important to achieve high photocurrent generation in organic semiconductor‐based assemblies, remains a challenging topic. Herein, we show that mixing a p‐type π gelator (TTV) with an n‐type semiconductor (PBI) results in the formation of self‐sorted fibers which are coaxially aligned to form interfacial p–n heterojunctions. UV/Vis absorption spectroscopy, powder X‐ray diffraction studies, atomic force microscopy, and Kelvin‐probe force microscopy revealed an initial self‐sorting at the molecular level and a subsequent mesoscale self‐assembly of the resulted supramolecular fibers leading to coaxially aligned p–n heterojunctions. A flash photolysis time‐resolved microwave conductivity (FP‐TRMC) study revealed a 12‐fold enhancement in the anisotropic photoconductivity of TTV/PBI coaxial fibers when compared to the individual assemblies of the donor/acceptor molecules.     

Targeting π‐Conjugated Multiple Donor–Acceptor Motifs Exemplified by Tetrathiafulvalene‐Linked Quinoxalines and Tetrabenz[bc,ef,hi,uv]ovalenes: Synthesis,Spectroscopic, Electrochemical,and Theoretical Characterization

An efficient synthetic approach to a symmetrically functionalized tetrathiafulvalene (TTF) derivative with two diamine moieties, 2‐[5,6‐diamino‐4,7‐bis(4‐pentylphenoxy)‐1,3‐benzodithiol‐2‐ylidene]‐4,7‐bis(4‐pentylphenoxy)‐1,3‐benzodithiole‐5,6‐diamine ( 2 ), is reported. The subsequent Schiff‐base reactions of 2 afford large π‐conjugated multiple donor–acceptor (D–A) arrays, for example, the triad 2‐[4,9‐bis(4‐pentylphenoxy)‐1,3‐dithiolo[4,5‐g]quinoxalin‐2‐ylidene]‐4,9‐bis(4‐pentylphenoxy)‐1,3‐dithiolo[4,5‐g]quinoxaline ( 8 ) and the corresponding tetrabenz[bc,ef,hi,uv]ovalene‐fused pentad 1 , in good yields and high purity. The novel redox‐active nanographene 1 is so far the largest known TTF‐functionalized polycyclic aromatic hydrocarbon (PAH) with a well‐resolved ¹H NMR spectrum. The electrochemically highly amphoteric pentad 1 and triad 8 exhibit various electronically excited charge‐transfer states in different oxidation states, thus leading to intense optical intramolecular charge‐transfer (ICT) absorbances over a wide spectral range. The chemical and electrochemical oxidations of 1 result in an unprecedented TTF^?+ radical cation dimerization, thereby leading to the formation of [ 1 ^?+]₂ at room temperature in solution due to the stabilizing effect, which arises from strong π–π interactions. Moreover, ICT fluorescence is observed with large solvent‐dependent Stokes shifts and quantum efficiencies of 0.05 for 1 and 0.035 for 8 in dichloromethane.     

Multistimuli‐Responsive Luminescence Switching of Pyrazine Derivative Based Donor–Acceptor–Donor Luminophores

Three symmetrical donor–acceptor–donor (D–A–D) luminophores ( C1 , C2 , and C3 ) with pyrazine derivatives as electron‐withdrawing groups have been developed for multistimuli‐responsive luminescence switching. For comparison, reference compounds R1 and R2 without the pyrazine moiety have also been synthesized. Intramolecular charge transfer (ICT) interactions can be found for all D–A–D luminophores owing to the electron‐withdrawing properties of the two imine nitrogen atoms in the pyrazine ring and the electron‐donating properties of the other two amine nitrogen atoms in the two triphenylamine units. Moreover, luminophores C1 , C2 , and C3 exhibit “on–off–on” luminescence switching properties in mixtures of water/tetrahydrofuran with increasing water content, which is different from the “on–off” switching for typical aggregation‐caused quenching (ACQ) materials and “off–on” switching for traditional aggregation‐induced emission (AIE) materials. Additionally, upon grinding the pristine samples, luminophores C1 , C2 , and C3 display bathochromically shifted photoluminescence maxima that can be recovered by either solvent fuming or thermal annealing treatments. The piezofluorochromic (PFC) properties are more pronounced than those for reference compounds R1 and R2 , which indicates that D–A molecules have the ability to amplify the PFC effect by tuning the ICT interactions upon tiny structural changes under pressure. Furthermore, the target luminophores demonstrate acid‐responsive photoluminescence spectra that can be recovered in either basic or ambient environments. These results suggest that D–A complexes are potential candidates for multistimuli‐responsive luminescence switching because their ICT profiles can be facilely tuned with tiny external stimuli.     

Luminescence Properties of C‐Diazaborolyl‐ortho‐Carboranes as Donor–Acceptor Systems

Seven derivatives of 1,2‐dicarbadodecaborane (ortho‐carborane, 1,2‐C₂B₁₀H₁₂) with a 1,3‐diethyl‐ or 1,3‐diphenyl‐1,3,2‐benzodiazaborolyl group on one cage carbon atom were synthesized and structurally characterized. Six of these compounds showed remarkable low‐energy fluorescence emissions with large Stokes shifts of 15100–20260 cm^?1 and quantum yields (Φ_F) of up to 65 % in the solid state. The low‐energy fluorescence emission, which was assigned to a charge‐transfer (CT) transition between the cage and the heterocyclic unit, depended on the orientation (torsion angle, ψ) of the diazaborolyl group with respect to the cage C? C bond. In cyclohexane, two compounds exhibited very weak dual fluorescence emissions with Stokes shifts of 15660–18090 cm^?1 for the CT bands and 1960–5540 cm^?1 for the high‐energy bands, which were assigned to local transitions within the benzodiazaborole units (local excitation, LE), whereas four compounds showed only CT bands with Φ_F values between 8–32 %. Two distinct excited singlet‐state (S₁) geometries, denoted S₁(LE) and S₁(CT), were observed computationally for the benzodiazaborolyl‐ortho‐carboranes, the population of which depended on their orientation (ψ). TD‐DFT calculations on these excited state geometries were in accord with their CT and LE emissions. These C‐diazaborolyl‐ortho‐carboranes were viewed as donor–acceptor systems with the diazaborolyl group as the donor and the ortho‐carboranyl group as the acceptor.     

Structure–Property Relationships in Click‐Derived Donor–Triazole–Acceptor Materials

To shed light on intramolecular charge‐transfer phenomena in 1,2,3‐triazole‐linked materials, a series of 1,2,3‐triazole‐linked push–pull chromophores were prepared and studied experimentally and computationally. Investigated modifications include variation of donor and/or acceptor strength and linker moiety as well as regioisomers. Photophysical characterization of intramolecular charge‐transfer features revealed ambipolar behavior of the triazole linker, depending on the substitution position. Furthermore, non‐centrosymmetric materials were subjected to second‐harmonic generation measurements, which revealed the high nonlinear optical activity of this class of materials.     

Charge‐Transfer Interactions in Tris‐Donor–Tris‐Acceptor Hexaarylbenzene Redox Chromophores

Symmetric‐ and asymmetric hexaarylbenzenes (HABs), each substituted with three electron‐donor triarylamine redox centers and three electron‐acceptor triarylborane redox centers, were synthesized by cobalt‐catalyzed cyclotrimerization, thereby forming compounds with six‐ and four donor–acceptor interactions, respectively. The electrochemical‐ and photophysical properties of these systems were investigated by cyclovoltammetry (CV), as well as by absorption‐ and fluorescence spectroscopy, and compared to a HAB that only contained one neighboring donor–acceptor pair. CV measurements of the asymmetric HAB show three oxidation peaks and three reduction peaks, whose peak‐separation is greatly influenced by the conducting salt, owing to ion‐pairing and shielding effects. Consequently, the peak‐separations cannot be interpreted in terms of the electronic couplings in the generated mixed‐valence species. Transient‐absorption spectra, fluorescence‐solvatochromism, and absorption spectra show that charge‐transfer states from the amine‐ to the boron centers are generated after optical excitation. The electronic donor–acceptor interactions are weak because the charge transfer has to occur predominantly through space. Moreover, the excitation energy of the localized excited charge‐transfer states can be redistributed between the aryl substituents of these multidimensional chromophores within the fluorescence lifetime (about 60 ns). This result was confirmed by steady‐state fluorescence‐anisotropy measurements, which further indicated symmetry‐breaking in the superficially symmetric HAB. Adding fluoride ions causes the boron centers to lose their accepting ability owing to complexation. Consequently, the charge‐transfer character in the donor–acceptor chromophores vanishes, as observed in both the absorption‐ and fluorescence spectra. However, the ability of the boron center as a fluoride sensor is strongly influenced by the moisture content of the solvent, possibly owing to the formation of hydrogen‐bonding interactions between water molecules and the fluoride anions.     

Energy Transfer at the Single‐Molecule Level: Synthesis of a Donor–Acceptor Dyad from Perylene and Terrylene Diimides

In 2004, we reported single‐pair fluorescence resonance energy transfer (spFRET), based on a perylene diimide (PDI) and terrylene diimide (TDI) dyad ( 1 ) that was bridged by a rigid substituted para‐terphenyl spacer. Since then, several further single‐molecule‐level investigations on this specific compound have been performed. Herein, we focus on the synthesis of this dyad and the different approaches that can be employed. An optimized reaction pathway was chosen, considering the solubilities, reactivities, and accessibilities of the building blocks for each individual reaction whilst still using established synthetic techniques, including imidization, Suzuki coupling, and cyclization reactions. The key differentiating consideration in this approach to the synthesis of dyad 1 is the introduction of functional groups in a nonsymmetrical manner onto either the perylene diimide or the terrylene diimide by using imidization reactions. Combined with well‐defined purification conditions, this modified approach allows dyad 1 to be obtained in reasonable quantities in good yield.     

Photoinduced Charge Separation in a Donor–Spacer–Acceptor Dyad with N‐Annulated Perylene Donor and Methylviologen Acceptor

The first donor–acceptor species in which a strongly emissive N‐annulated perylene dye is connected to a methylviologen electron acceptor unit via its macrocyclic nitrogen atom, is prepared by a stepwise, modular procedure. The absorption spectra, redox behavior, spectroelectrochemistry and photophysical properties of this dyad and of its model species are investigated, also by pump–probe fs transient absorption spectroscopy. Photoinduced oxidative electron transfer from the excited state of the dyad, centered on the N‐annulated perylene subunit, to the appended methyviologen electron acceptor takes place in a few ps. The charge‐separated species recombines in 19 ps. Our results indicate that N‐annulated perylene can be connected to functional units by taking advantage of the macrocyclic nitrogen, an option never used until now, without losing their properties, so opening the way to new designing approaches.