Violene/cyanine hybrids as electrochromics part 2: tetrakis(4-dimethylaminophenyl)ethene and its derivatives

The general structure of violene/cyanine hybrids (see below) is exemplified by tetrakis(4-dimethylaminophenyl)ethene 1(RED) its vinylogue 2(RED) and its diazavinylogue 3(RED). As judged from their cyclic voltammograms and spectroelectrograms, oxidation occurs perfectly reversible by loss of two electrons creating closed shell systems 1-3(OX)+2 with strong bathochromic shifts (Michlers hydrol blue moieties). ESR spectra indicate only minor amounts of radical cations. At much higher potentials by another reversible loss of two electrons (-->1-3(OX)+4) the long wavelengths absorptions are replaced by shorter ones. In system 4, containing two 4-dimethylaminophenyl units only, the violene character is better preserved since oxidation occurs stepwise by single electron transfer up to 4(OX)+4. These results are backed by theoretical calculations for 1-4, demonstrating the strong geometrical differences between the various oxidation levels. Besides, new types of cyclic structures for 1-4(OX)+4 are indicated by these calculations: For systems 1-3 cyclic structures for tetracations have been found to be more stable by 3-20 kcalmol(-1) than acyclic structures, whereas for system 4 the acyclic structure is more stable by about 22 kcalmol(-1). The redox behavior of systems 1-4 is of general importance for electrochromic systems.     

Complementary Hydrogen Bonding Modulates Electronic Properties and Controls Self‐Assembly of Donor/Acceptor Semiconductors

A comprehensive investigation of the complementary H‐bonding‐mediated self‐assembly between dipyrrolo[2,3‐b:3′,2′‐e]pyridine (P2P) electron donors and naphthalenediimide/perylenediimide (NDI/PDI) acceptors is reported. The synthesis of parent P2P and several aryl‐substituted derivatives is described, along with their optical, redox, and single‐crystal packing characteristics. The dual functionality of heteroatoms in the P2P/NDI(PDI) assembly, which act as proton donors/acceptors and also contribute to π‐conjugation, leads to H‐bonding‐induced perturbation of electronic levels. Concentration‐dependent NMR and UV/Vis spectroscopic studies revealed a cooperative effect of H‐bonding and π–π stacking interactions. This H‐bonding‐mediated co‐assembly of donor (D) and acceptor (A) components leads to a new charge‐transfer (CT) absorption that can be controlled throughout the visible range. The electronic interactions between D and A were further investigated by time‐dependent DFT, which provided insights into the nature of the CT transition. Electropolymerization of difuryl‐P2P afforded the first conjugated polymer incorporating H‐bonding recognition units in its main chain.     

Pressure-induced order transition in nanodot-forming diblock copolymers at the air/water interface

Understanding and controlling the processes in block copolymer (BC) monolayers at the air/water interface during surface area compression is a key issue for producing ultrathin films of predetermined morphology with well-defined order and known dimensions. Langmuir isotherms of nanodot-forming BC monolayers generally display a plateau indicative of a 2D phase transition, which has been the subject of various interpretations in the literature. Here, based on investigations of Langmuir-Blodgett and Langmuir-Schaefer nanodot films of PS-P4VP mixed with 3-n-pentadecylphenol (PDP), we show by atomic force microscopy (AFM) that it involves a change in nanodot packing order (from quasi-hexagonal to quasi-square), argued to be a general phenomenon for nanodot BC monolayers. It is accompanied by system-specific conformational changes (as discussed in previous literature), which, in the present case, implicate PDP alkyl chain ordering, as deduced previously from in situ infrared data and indirectly supported here by AFM imaging.     

A Molecular Necklace: Threading β‐Cyclodextrins onto Polymers Derived from Bile Acids

A molecular necklace of polypseudorotaxanes was prepared by threading β‐cyclodextrins (β‐CD) onto biodegradable and thermoresponsive polyurethanes derived from bile acids. These polyurethanes were synthesized via a simple step condensation of bile acid‐based dicarbonate with poly(ethylene glycol)‐diamine. The β‐CD rings slide onto the poly(ethylene glycol) segments and selectively recognize the bile acid units of the polyurethane chains, whereas the poly(ethylene glycol) segments remain crystalline with a lower crystallinity. This bio‐compound‐derived molecular necklace can be visualized by scanning tunneling microscopy. The polypseudorotaxanes show thermosensitivity in water and the phase transition temperature may be fine‐tuned by varying the molar ratios of β‐CD to the bile acid units. Such an interesting necklace model of polypseudorotaxane constructed from natural compounds may lead to the further exploration of their applications, such as as an enzyme model, due to their biological nature.     

Design of Furan-Based Acceptors for Organic Photovoltaics

We explore a series of furan-based non-fullerene acceptors and report their optoelectronic properties, solid-state packing, photodegradation mechanism and application in photovoltaic devices. Incorporating furan building blocks leads to the expected enhanced backbone planarity, reduced band gap and red-shifted absorption of these acceptors. Still, their position in the molecule is critical for stability and device performance. We found that the photodegradation of these acceptors originates from two distinct pathways: electrocyclic photoisomerization and Diels–Alder cycloaddition of singlet oxygen. These mechanisms are of general significance to most non-fullerene acceptors, and the photostability depends strongly on the molecular structure. Placement of furans next to the acceptor termini leads to better photostability, well-balanced hole/electron transport, and significantly improved device performance. Methylfuran as the linker offers the best photostability and power conversion efficiency (>14 %), outperforming all furan-based acceptors reported to date and all indacenodithiophene-based acceptors. Our findings show the possibility of photostable furan-based alternatives to the currently omnipresent thiophene-based photovoltaic materials.     

Stabilizing and Activating Nitrogen Catenates

The remarkably stable catenated hexa‐nitrogen chain in bis(benzotriazene‐4‐one) is structurally, theoretically, and spectroscopically characterized to illustrate the durability of the central N?N bond in this hexaazo chain. The reactions of this species illustrate the potential of these nitrogen catenates for the preparation of other condensed heterocycles, such as bispyrazolones, by thermal nitrogen exclusion or by trapping the single ring‐opened Dimroth intermediates. In these latter reactions, 2‐naphtholate anion condenses with bis(benzotriazene‐4‐one) to trap and retain a zwitterionic diazonium intermediate as an isolated diazo product, whereas transition metals ring effect ring‐extrusion of dinitrogen from the Dimroth intermediate to generate chelating σ‐aryls. The catenated nitrogen species can be stabilized by incorporating strong formal sp²?sp² N?N σ bonds with orthogonal orientations. Extending these stabilization and activation principles may allow these types of nitrogen catenates to be useful synthons for other polyaza species.     

E versus Z Diazeniumdiolation of Acetoacetate-Derived Carbanions

Nitric oxide adds to methyl acetoacetate in the presence of KOH in methanol at room temperature to form potassium acetylsydnonate N-oxide (K1) with an (E)-diazeniumdiolation and potassium acetate diazenium diolate (K(2)2) from a (Z)-diazeniumdiolation. A study of the reaction with LiOH, NaOH, and NMe(4)OH and with ethyl acetate substrate reveals that the temperature of the reaction greatly influences the nitric oxide reactivity. At 23 °C, nitric oxide adds to give both E and Z products, whereas at -5 °C the gas reacts almost exclusively to give Z addition. The (Z)-diazeniumdiolation products, namely, the alkali metal and NMe(4)(+) salts of methyl and ethylbutenoate-2-diazeniumdiolate-3-hydroxylate (3(2-) and 4(2-)), are isolated in good yields. The alkali metal salts are not amenable for recrystallization because of their ready decomposition in aqueous solutions. However, [NMe(4)](2)[MeC(O)C(N(2)O(2))CO(2)Me] is readily recrystallized from a methanol/acetonitrile solvent mixture. The crystals are unambiguously characterized by X-ray crystallography. NMR spectra for all of the 3(2-) and 4(2-) salts reveal the presence of two isomers in aq solutions. But the structure of the NMe(4)(+) salt contains only one of the isomers. Our attempts to cyclize the isolated and purified butenoatediazeniumdiolates from the (Z)-diazeniumdiolation to the E-containing sydnonate products were unsuccessful. TGA/DSC data for all of the products demonstrate the thermal instability of the salts at high temperatures. The salts decompose exothermally possibly with the release of N(2)O among other gases.     

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.     

Reaction of 2‐acyl‐6‐methylbenzo[b]furan‐3‐acetic acids derivatives with hydrazine

Reaction of 2‐acyl‐6‐methylbenzo[b]furan‐3‐acetic acids and their derivatives such as amides and esters with hydrazine does not give expected 1‐alkyl‐5H‐benzofuro[2,3‐e]diazepin‐4‐ones ones but results in 2‐amino‐7‐methyl‐2H‐benzo[4,5]furo[2,3‐c]pyridin‐3‐ones or (3‐R‐6‐methylbenzo[b]furan‐2‐yl)alkyl ketone azines.