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.     

A Panchromatic Supramolecular Fullerene‐Based Donor–Acceptor Assembly Derived from a Peripherally Substituted Bodipy–Zinc Phthalocyanine Dyad

A panchromatic 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene –zinc phthalocyanine conjugate (Bodipy–ZnPc) 1 was synthesized starting from phthalocyanine aldehyde 4 , via dipyrromethane 3 and dipyrromethene 2 . Conjugate 1 represents the first example in which a Bodipy unit is tethered to the peripheral position of a phthalocyanine core. Electrochemical and optical measurements provided evidence for strong electronic interactions between the Bodipy and ZnPc constituents in the ground state of 1 . When conjugate 1 is subjected to photoexcitation in the spectral region corresponding to the Bodipy absorption, the strong fluorescence characteristic of the latter subunit is effectively quenched (i.e., ≥97 %). Excitation spectral analysis confirmed that the photoexcited Bodipy and the tethered ZnPc subunits interact and that intraconjugate singlet energy transfer occurs with an efficiency of ca. 25 %. Treatment of conjugate 1 with N‐pyridylfulleropyrrolidine ( 8 ), an electron‐acceptor system containing a nitrogen ligand, gives rise to the novel electron donor–acceptor hybrid 1 ? 8 through ligation to the ZnPc center. Irradiation of the resulting supramolecular ensemble within the visible range leads to a charge‐separated Bodipy–ZnPc^.+–C₆₀^.? radical‐ion‐pair state, through a sequence of excited‐state and charge transfers, characterized by a remarkably long lifetime of 39.9 ns in toluene.     

A New Golden Age for Donor–Acceptor Cyclopropanes

The effective use of ring strain has been applied to considerable advantage for the construction of complex systems. The focus here is directed towards cyclopropanes as building blocks for organic synthesis. Although thermodynamics should take the side of synthetic chemists, only a specific substitution pattern at the cyclopropane ring allows for particularly mild, efficient, and selective transformations. The required decrease in the activation barrier is achieved by the combined effects of vicinal electron‐donating and electron‐accepting moieties. This Review highlights the appropriate tools for successfully employing donor–acceptor cyclopropanes in ring‐opening reactions, cycloadditions, and rearrangements.     

A Stable Monomeric SiO2 Complex with Donor–Acceptor Ligands

Isolation of a monomeric SiO₂ compound 3 as a stable donor–acceptor complex with two different ligands —a σ‐donating ligand (pyridine, dimethylaminopyridine, N ‐heterocyclic carbene) and a donor–acceptor ligand (iminophosphorane)—is presented. The SiO₂ complex 3 is soluble in ordinary organic solvents and is stable at room temperature in solution and in the solid state. Of particular interest, 3 remains reactive and can be used as a stable and soluble unimolecular SiO₂ reagent.     

A Carbon Nano‐Onion–Ferrocene Donor–Acceptor System: Synthesis,Characterization and Properties

Don't cry! The attachment of ferrocene moieties on the surface of carbon nano‐onions influences the electrochemical properties of these moieties and the photophysical properties of the carbon nano‐onions (see figure). Quantum chemical calculations confirm that the spectral properties of carbon nano‐onions depend on their size and the degree of functionalisation.

     

Stimuli‐Responsive Reversible Fluorescence Switching in a Crystalline Donor–Acceptor Mixture Film: Mixed Stack Charge‐Transfer Emission versus Segregated Stack Monomer Emission

We report on a molecularly tailored 1:1 donor–acceptor (D‐A) charge‐transfer (CT) cocrystal that manifests strongly red‐shifted CT luminescence characteristics, as well as noteworthy reconfigurable self‐assembling behaviors. A loosely packed molecular organization is obtained as a consequence of the noncentrosymmetric chemical structure of molecule A1 , which gives rise to considerable free volume and weak intermolecular interactions. The stacking features of the CT complex result in an external stimuli‐responsive molecular stacking reorganization between the mixed and demixed phases of the D‐A pair. Accordingly, high‐contrast fluorescence switching (red?blue) is realized on the basis of the strong alternation of the electronic properties between the mixed and demixed phases. A combination of structural, spectroscopic, and computational studies reveal the underlying mechanism of this stimuli‐responsive behavior.     

Bright Solid‐State Emission of Disilane‐Bridged Donor–Acceptor–Donor and Acceptor–Donor–Acceptor Chromophores

The development of disilane‐bridged donor–acceptor–donor (D‐Si‐Si‐A‐Si‐Si‐D) and acceptor–donor–acceptor (A‐Si‐Si‐D‐Si‐Si‐A) compounds is described. Both types of compound showed strong emission (λ_em=ca. 500 and ca. 400 nm, respectively) in the solid state with high quantum yields (Φ: up to 0.85). Compound 4 exhibited aggregation‐induced emission enhancement in solution. X‐ray diffraction revealed that the crystal structures of 2 , 4 , and 12 had no intermolecular π–π interactions to suppress the nonradiative transition in the solid state.     

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.     

Sequence Effects in Conjugated Donor–Acceptor Trimers and Polymers

To investigate the sequence effect on donor–acceptor conjugated oligomers and polymers, the trimeric isomers PBP and BPP , comprising dialkoxy phenylene vinylene ( P ), benzothiadiazole vinylene ( B ), and alkyl endgroups with terminal olefins, are synthesized. Sequence effects are evident in the optical/electrochemical properties and thermal properties. Absorption maxima for PBP and BPP differ by 41 nm and the electrochemical band gaps by 0.1 V. The molar emission intensity is five times greater in PBP than BPP . Both trimers are crystalline and the melting points differ by 17 °C. The PBP and BPP trimers are used as macromonomers in an acyclic diene metathesis polymerization to give PolyPBP and PolyBPP . The optical and electrochemical properties are similar to those of their trimer precursors—sequence effects are still evident. These results suggest that sequence is a tunable variable for electronic materials and that the polymerization of oligomeric sequences is a useful approach to introducing sequence into polymers.

     

Stereospecific 1,3‐Aminobromination of Donor–Acceptor Cyclopropanes

Sn(OTf)₂‐catalyzed 1,3‐aminobromination of donor–acceptor cyclopropanes with various sulfonyl amides or electron‐poor anilines and N ‐bromosuccinimide is reported. These experimentally straightforward reactions occurred with complete regio‐ and stereospecificity (for anilines) to give γ‐aminated α‐brominated malonic diesters in good to excellent yields (up to 98 %). These compounds served as valuable substrates for subsequent reactions to provide substituted azetidines and γ‐lactams in high yields.     

Uncovering the Neglected Similarities of Arynes and Donor–Acceptor Cyclopropanes

Arynes and donor–acceptor (D–A) cyclopropanes are two classes of strained systems having the potential for numerous applications in organic synthesis. The last two decades have witnessed a renaissance of interest in the chemistry of these species primarily because of the mild and robust methods for their generation or activation. Commonly, arynes as easily polarizable systems result in 1,2‐disubstitution, whereas D‐A cyclopropanes as polarized systems lead to 1,3‐bisfunctionalization thereby showing striking similarities. Transformations with 1,2‐ and 1,3‐dipoles afford cyclic structures. With arynes, emerging four‐membered rings as intermediates might react further, whereas the analogous five‐membered rings obtained from D–A cyclopropanes are most often the final products. However, there are a few cases where these intermediates behave surprisingly differently. This Minireview highlights the parallels in reactivity between arynes and D–A cyclopropanes thereby shedding light on the neglected similarities of these two reactive species.     

A Periodic Mesoporous Organosilica‐Based Donor–Acceptor System for Photocatalytic Hydrogen Evolution

A new solid‐sate donor–acceptor system based on periodic mesoporous organosilica (PMO) has been constructed. Viologen (Vio) was covalently attached to the framework of a biphenyl (Bp)‐bridged PMO. The diffuse reflectance spectrum showed the formation of charge‐transfer (CT) complexes of Bp in the framework with Vio in the mesochannels. The transient absorption spectra upon excitation of the CT complexes displayed two absorption bands due to radical cations of Bp and Vio species, which indicated electron transfer from Bp to Vio. The absorption bands slowly decayed with a half‐decay period of approximately 10 μs but maintained the spectral shape, thereby suggesting persistent charge separation followed by recombination. To utilize the charge separation for photocatalysis, Vio–Bp–PMO was loaded with platinum and its photocatalytic performance was tested. The catalyst successfully evolved hydrogen with excitation of the CT complexes in the presence of a sacrificial agent. In contrast, reference catalysts without either Bp–PMO or Vio gave no or little hydrogen generation, respectively. In addition, a homogeneous solution system of Bp molecules, methylviologen, and colloidal platinum also evolved no hydrogen, possibly due to a weaker electron‐donating feature of molecular Bp than that of densely packed Bp in Bp–PMO. These results indicated that densely packed Bp and Vio are essential for hydrogen evolution in this system and demonstrated the potential of PMO as the basis for donor–acceptor systems suitable for photocatalysis.     

Dual Upconverted and Downconverted Circularly Polarized Luminescence in Donor–Acceptor Assemblies

Through mimicking both the chiral and energy transfer in an artificial self‐assembled system, not only was chiral transfer realized but also a dual upconverted and downconverted energy transfer system was created that emit circularly polarized luminescence. The individual chiral π‐gelator can self‐assemble into a nanofiber exhibiting supramolecular chirality and circularly polarized luminescence (CPL). In the presence of an achiral sensitizer Pd^II octaethylporphyrin derivative, both chirality transfer from chiral gelator to achiral sensitizer and triplet‐triplet energy transfer from excited sensitizer to chiral gelator could be realized. Upconverted CPL could be observed through a triplet–triplet annihilation photon upconversion (TTA‐UC), while downconverted CPL could be obtained from chirality‐transfer‐induced emission of the achiral sensitizer. The interplay between chiral energy acceptor and achiral sensitizer promoted the communication of chiral and excited energy information.     

Lewis Acid Catalyzed Enantioselective Desymmetrization of Donor–Acceptor meso‐Diaminocyclopropanes

The first Lewis acid catalyzed enantioselective ring‐opening desymmetrization of a donor–acceptor meso‐diaminocyclopropane is reported. The copper(II)‐catalyzed Friedel–Crafts alkylation of indoles and one pyrrole with an unprecedented meso‐diaminocyclopropane delivered enantioenriched, diastereomerically pure urea products, which are structurally related to natural and synthetic bioactive compounds. The development of a new ligand through the investigation of an underexplored subclass of bis(oxazoline) ligands was essential for achieving high enantioselectivities.