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.     

FeCl3‐Catalyzed Addition of 1,3‐Dicarbonyl Compounds to Aromatic Olefins

A direct, intermolecular addition of 1,3‐dicarbonyl compounds to styrenes in the presence of FeCl₃ as an inexpensive and disposable catalyst has been developed for the straightforward and practical synthesis of arylated diketones and ketoesters. The reactions proceed under mild conditions for most substrates (50–80 °C), and no strong acid or base is required. The synthetic value of the method is demonstrated by 15 examples, including the synthesis of the current pharmaceutical drug warfarin in one step and 42 % yield from commercially available substrates.     

Creating σ‐Holes through the Formation of Beryllium Bonds

Through the use of ab initio theoretical models based on MP2/aug‐cc‐pVDZ‐optimized geometries and CCSD(T)/aug‐cc‐pVTZ and CCSD(T)/aug‐c‐pVDZ total energies, it has been shown that the significant electron density rearrangements that follow the formation of a beryllium bond may lead to the appearance of a σ‐hole in systems that previously do not exhibit this feature, such as CH₃OF, NO₂F, NO₃F, and other fluorine‐containing systems. The creation of the σ‐hole is another manifestation of the bond activation–reinforcement (BAR) rule. The appearance of a σ‐hole on the F atoms of CH₃OF is due to the enhancement of the electronegativity of the O atom that participates in the beryllium bond. This atom recovers part of the charge transferred to Be by polarizing the valence density of the F into the bonding region. An analysis of the electron density shows that indeed this bond becomes reinforced, but the F atom becomes more electron deficient with the appearance of the σ‐hole. Importantly, similar effects are also observed even when the atom participating in the beryllium bond is not directly attached to the F atom, as in NO₂F, NO₃F, or NCF. Hence, whereas the isolated CH₃OF, NO₂F, and NO₃F are unable to yield F ??? Base halogen bonds, their complexes with BeX₂ derivatives are able to yield such bonds. Significant cooperative effects between the new halogen bond and the beryllium bond reinforce the strength of both noncovalent interactions.     

Luminescent Materials: Locking π‐Conjugated and Heterocyclic Ligands with Boron(III)

Multidisciplinary research on novel organic luminescent dyes is propelled by potential applications in plastic electronics and biomedical sciences. The construction of sophisticated fluorescent dyes around a tetrahedral boron(III) center is a particular approach that has fueled the creativity of chemists. Success in this enterprise has been readily achieved with simple synthetic protocols, the products of which display unusual spectroscopic behavior. This account is a critical review of recent advances in the field of boron(III) complexes (excluding BODIPYs and acetylacetonate boron complexes) involving species displaying similar coordination features, and we outline their potential development in several disciplines.     

Charge Transfer Complexes of Some Heterocyclic Aromatic Amines with π‐Organic Acceptors in Polar Solvents

A conductometric titration technique has been used to investigate the electron transfer activity of CT molecular complexes formed by arylazopyrimidine and naphthylazopyrimidine derivatives as donors and the organic π‐acceptors p‐nitroaniline, p‐chloroaniline, p‐bromoaniline, anthraquinone, picric acid, α‐nitroso‐β‐naphthol, p‐hydroxybenzaldehyde and maleic anhydride. The study was performed at different degree of temperature and in three different polar solvents namely N,N‐dimethylformamide (DMF), acetonitrile (ACN) and dimethylsulfoxide (DMSO). The stoichiometric ratios of these complexes were found to be 1:1. The dissociation constant (ασ_M) values of the formed complexes have been calculated, and the effects of solvents as well as types of electron donors on their conductance σ_p‐values have been examined.     

Study on the Cation-π Interactions between Ammonium Ion and Aromatic π Systems

The nature and strength of the cation-π interactions between NH4^＋ and toluene, p-cresol, or Me-indole were studied in terms of the topological properties of molecular charge density and binding energy decomposition. The results display that the diversity in the distribution pattern of bond and cage critical points reflects the profound influence of the number and nature of substituent on the electron density of the aromatic rings. On the other hand, the energy decomposition shows that dispersion and repulsive exchange forces play an important role in the organic cation （NH4^＋）-π interaction, although the electrostatic and induction forces dominate the interaction. In addition, it is intriguing that there is an excellent correlation between the electrostatic energy and ellipticity at the bond critical point of the aromatic π systems, which would be helpful to further understand the electrostatic interaction in the cation-π complexes.     

Quantum Control of Coherent ρρ‐Electron Dynamics in Chiral Aromatic Molecules

Control of mobile π‐electrons is one of the fundamental issues in the organic optoelectronics for designing the next generation ultrafast switching devices. The optimal control simulations of coherent π‐electron rotations in (P)‐2,2’‐biphenol, which is the typical nonplanar aromatic molecule with axial chirality, were performed by taking into account two types of the control targets: one is generation of the maximum π‐angular momentum, and the other is the maintaining of the generated unidirectional angular momentum during a setting time duration. The optimal control pulse for each target is designed. The analysis of the simulation results shows that the effective maintaining of the unidirectional angular momentum can be realized by applying 2π pulse to one of the electronic excited states forming the coherent electronic state. The 2π pulse prevents the reverse rotation of the π‐electrons by dumping the excited state population to the ground state and subsequently by pumping the population back to the excited state. The present results provide a theoretical basis for the designing next generation ultrafast switching devices made by organic aromatic molecules.     

Structural and Spectral Studies on the Ni(II) Complexes of 1,5‐Diazacyclooctane (DACO) Bearing Heterocyclic Pendants: Formation of a Two‐dimensional Network via Hydrogen Bonds and π‐π Stacking Interactions

A penta‐coordinated Ni(II) complex with a 1,5‐diazacyclooctane (DACO) ligand functionalized by two imidazole donor pendants, [NiL¹Cl] (ClO₄) H₂O (1) (where L¹ = 1,5‐bis (imidazol‐4‐ylmethyl)‐l,5‐diazacyclooctane) has been synthesized and characterized by X‐ray diffraction, infrared spectra, elemental analyses, conductance, thermal analyses and UV‐Vis techniques. Complex 1 crystallizes in triclinic crystal system, P‐l space group with a = 0.74782(7), b = 1.15082 (10), c = 1.23781(11) nm, α = 82.090(2), β = 73.011(2), γ = 83.462(2)°, V = 1.00603(16) nm³, M, = 486.00, Z = 2, D_c = 1.604 g/cm³, final R = 0.0435, and wR = 0.1244. The structures of 1 and its related complexes show that in all the three mononuclear complexes, each Ni(II) center is penta‐coordinated with a near regular square pyramid (RSP) to distorted square‐pyramidal (DSP) coordination environment due to the boat/chair configuration of DACO ring in these complexes, and the degree of distortion increases with the augment of the size of the heterocyclic pendants. In addition, the most striking feature of complex 1 resides in the formation of a two‐dimensional network structure through hydrogen bonds and stabilized by π‐π stacking. The solution behaviors of the Ni(II) complexes are also discussed in detail.     

Activation of Aromatic C−C Bonds of 2,2’‐Bipyridine Ligands

4,4’‐Disubstituted‐2,2′‐bipyridine ligands coordinated to Mo^II and Re^I cationic fragments become dearomatized by an intramolecular nucleophilic attack from a deprotonated N‐alkylimidazole ligand in cis disposition. The subsequent protonation of these neutral complexes takes place on a pyridine carbon atom rather than at nitrogen, weakening an aromatic C?C bond and affording a dihydropyridyl moiety. Computational calculations allowed for the rationalization of the formation of the experimentally obtained products over other plausible alternatives.     

Dissymmetrical U‐Shaped π‐Stacked Supramolecular Assemblies by Using a Dinuclear CuI Clip with Organophosphorus Ligands and Monotopic Fully π‐Conjugated Ligands

Reactions between the U‐shaped binuclear Cu^I complex A that bears short metal–metal distances and the cyano‐capped monotopic π‐conjugated ligands 1 – 5 that carry gradually bulkier polyaromatic terminal fragments lead to the formation of π‐stacked supramolecular assemblies 6 – 10 , respectively, in yields of 50–80 %. These derivatives have been characterized by multinuclear NMR spectroscopic analysis and X‐ray diffraction studies. Their solid‐state structures show the selective formation of U‐shaped supramolecular assemblies in which two monotopic π‐conjugated systems present large ( 6 , 7 , and 9 ) or medium ( 8 and 10 ) intramolecular π overlap, thus revealing π–π interactions. These assemblies self‐organize into head‐to‐tail π‐stacked dimers that in turn self‐assemble to afford infinite columnar π stacks. The nature, extent, and complexity of the intermolecular contacts within the head‐to‐tail π‐stacked dimer depend on the nature of the terminal polyaromatic fragment carried by the cyano‐capped monotopic ligand, but it does not alter the result of the self‐assembling process. These results demonstrate that the dinuclear molecular clip A that bears short metal–metal distances allows selective supramolecular assembly processes driven by the formation of intra‐ and intermolecular short π–π interactions in the resulting self‐assembled structures; thus, demonstrating that their shape is not only dictated by the symmetry of the building blocks. This approach opens perspectives toward the formation of extended π‐stacked columns based on dissymmetrical and functional π‐conjugated systems.     

Aromatic Ring‐Fused Carborane‐Based Luminescent π‐Conjugated Polymers

A series of π‐conjugated polymers linked by benzocarborane (1,2‐(buta‐1′,3′‐diene‐1′,4′‐diyl)‐1,2‐dicarbadodecaborane) were synthesized via Sonogashira–Hagihara polycondensation reaction. The opened molecular structure of diiodo monomer containing benzocarborane resulted in fast polymerization and high molecular weights. The obtained polymers were fully characterized by ¹H, ¹³C, and ¹¹B NMR spectroscopies. UV‐vis absorption and photoluminescence studies revealed the acceptor‐profile of benzocarborane. Unlike the polymers linked by o‐carborane, these polymers exhibited strong luminescence in the solution state, presumably because the inductive effect of carborane is dominant, rather than cage‐π interactions.

     

Elucidation of π‐Conjugation Modes in Diarene‐Fused 1,2‐Dihydro‐1,2‐diborin Dianions

A series of diarene‐fused 1,2‐dihydro‐1,2‐diborins were prepared as a new B? B‐bond‐embedded polycyclic π‐electron system. The reduction of these compounds with metals produced their corresponding dianions, the π‐conjugation modes of which varied from 6π‐conjugation within the central 1,2‐diborin skeleton to 14π peripheral conjugation over the tricyclic skeleton, depending on the nature of the reduced biaryl framework. Moreover, the countercation to the dianions had a significant effect on the absorption spectra, with a dramatic color change from yellow to deep blue, depending on the distance between the tricyclic dianion skeleton and the countercation.     

π‐Hole Bonds: Boron and Aluminum Lewis Acid Centers

MP2/aug‐cc‐pVTZ calculations were performed on complexes of boron and aluminum trihydrides and trihalides with hydrogen cyanide (ZH₃‐NCH and ZX₃‐NCH; Z=B, Al; X=F, Cl). The complexes are linked through the B???N and Al???N interactions, which are named as triel bonds and which are classified as π‐hole bonds. It was found that they possess numerous characteristics of typical covalent bonds, since they are ruled mainly by processes of the electron charge shift from the Lewis base to the Lewis acid unit. Other configurations of the ZH₃‐NCH and ZX₃‐NCH complexes linked by the dihydrogen, hydrogen, and halogen bonds were found. However, these interactions are much weaker than the corresponding π‐hole bonds. The quantum theory of atoms in molecules and the natural bond orbital approaches were applied to characterize the complexes and interactions analyzed. The crystal structures of triel trihydrides and triel trihalides were also analyzed for comparison with the results of calculations.     

Proline-Based P,N Ligands in Palladium-Catalyzed Asymmetric π-Allyl Additions

Excellent yields and at least 95 % ee can be achieved for the addition of dimethyl malonate to cycloalkenyl acetates by using a palladium complex of the new phosphanyldihydrooxazole ligand L as a catalyst (see scheme). The ligand L can be synthesized from commercially available trans 4-hydroxy-L -proline in four steps. BOC=tert-butoxycarbonyl.     

Synthesis and properties of through‐space conjugated polymers based on π–π stacked 1,3‐biarylpropane tethering units

Novel skipped‐π polymers in which the π‐components are connected with 2‐substituted trimethylene tethering units exhibit bathochromically shifted, broadened ultraviolet absorption with a unique lower‐energy absorption band and a largely red‐shifted fluorescent emission. These results suggest that through‐space π–π interactions owing to a stair‐like stacking substructure in these polymers extend the π‐conjugation of the components in the ground and excited states. As the photophysical properties of the polymers observed both in a solution and in a dried film are similar to those of the J‐aggregates of π‐molecules, these polymers may be considered as pseudo J‐stacking (or J‐like‐stacking) polymers. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3412–3419     

A‐π‐D‐π‐A type Small Molecules Using Ethynylene Linkages for Organic Solar Cells with High Open‐circuit Voltages

Introducing ethynylene linkages in a conjugated molecule can deepen the HOMO level, decrease the steric constraints and better delocalize the π electrons and so on, which are beneficial for organic solar cells. Furthermore, the typical method of introducing acetylene linkages by Sonogashira reactions can avoid the usage of toxic stannyl intermediates and potentially dangerous lithiation reactions. In this study, two simple small molecules BEDPP and NEDPP are designed and synthesized, in which two diketopyrrolopyrrole units are symmetrically connected to benzene and naphthalene cores, respectively, via acetylene linkages. And the BHJ (Bulk Heterojunction) solar cells based on BEDPP and NEDPP without using solvent additive and without any post‐treatment for the active layers provide us power conversion efficiencies of 1.48% and 2.31% with remarkably high open circuit voltages up to 0.90 and 0.98 V, respectively.     

Magnetic Circular Dichroism of the Long‐Wavelength π‐π* Transitions in Nitrogen‐Containing Heteroconjugated Aromatic Compounds and in Selected Ruthenium Complexes

The long‐wavelength magnetic circular dichroism (MCD) spectra of some nitrogen‐containing heteroconjugated aromatic compounds have been recorded and interpreted on the basis of quantum‐chemical model calculations. In particular, the dependence of the MCD bands on the positions of the N‐atoms inside the aromatic rings and on substituents has been investigated. Some of the compounds considered form Ru²⁺ complexes. The influence of complexation on the long‐wavelength π‐π* bands of the ligands has also been recorded and discussed.