π–π Stacking Interaction of Metal Phenoxyl Radical Complexes

π–π stacking interaction is well-known to be one of the weak interactions. Its importance in the stabilization of protein structures and functionalization has been reported for various systems. We have focused on a single copper oxidase, galactose oxidase, which has the π–π stacking interaction of the alkylthio-substituted phenoxyl radical with the indole ring of the proximal tryptophan residue and catalyzes primary alcohol oxidation to give the corresponding aldehyde. This stacking interaction has been considered to stabilize the alkylthio-phenoxyl radical, but further details of the interaction are still unclear. In this review, we discuss the effect of the π–π stacking interaction of the alkylthio-substituted phenoxyl radical with an indole ring.     

π-Conjugated redox-active two-dimensional polymers as organic cathode materials

Redox-active two-dimensional polymers (RA-2DPs) are promising lithium battery organic cathode materials due to their regular porosities and high chemical stabilities. However, weak electrical conductivities inherent to the non-conjugated molecular motifs used thus far limit device performance and the practical relevance of these materials. We herein address this problem by developing a modular approach to construct π-conjugated RA-2DPs with a new polycyclic aromatic redox-active building block PDI-DA. Efficient imine-condensation between PDI-DA and two polyfunctional amine nodes followed by quantitative alkyl chain removal produced RA-2DPs TAPPy-PDI and TAPB-PDI as conjugated, porous, polycrystalline networks. In-plane conjugation and permanent porosity endow these materials with high electrical conductivity and high ion diffusion rates. As such, both RA-2DPs function as organic cathode materials with good rate performance and excellent cycling stability. Importantly, the improved design enables higher areal mass-loadings than were previously available, which drives a practical demonstration of TAPPy-PDI as the power source for a series of LED lights. Collectively, this investigation discloses viable synthetic methodologies and design principles for the realization of high-performance organic cathode materials.

Redox-active two-dimensional polymers (RA-2DPs) are promising lithium battery organic cathode materials due to their regular porosities and high chemical stabilities.     

Cooperation of σ–π and σ*–π* Conjugation in the UV/Vis and Fluorescence Spectra of 9,10-Disilylanthracene

In 1996, we reported that silyl groups of 9,10-disilylanthracenes significantly affect the UV/Vis and fluorescence spectra. Although the results indicate that the silyl groups have strong electronic effects on anthracene, the details of the mechanisms responsible for this have not yet been clarified. This article describes the analysis of the UV/Vis and fluorescence spectra of 9,10-bis(diisopropylsilyl)anthracene by theoretical calculations. This study reveals that π conjugation of anthracene is extended by cooperation of σ–π and σ*–π* conjugation between the silyl groups and anthracene. This effect increases the transition moment of the π–π* transition of anthracene. As a result, the molecular extinction coefficient of the ¹L_a band and the fluorescence quantum yield are increased.     

Band structure of quasi-1-dimensional polycondensed π-systems, 3 energy spectra of anti-aromatic polymers

The band structure of a class of quasi 1-dimensional (1-D) polymers with anti-aromatic structural units — anti-aromatic polymers (AAP) — has been investigated theoretically. The energy gap (EG) of the AAP is significantly smaller compared to the EG of polymers with aromatic building blocks having the same number of π-centers in the elementary units. The influence of the electron correlation in the calculation of the EG is investigated. As with the anti-aromatic molecules the Jahn-Teller distortion influences the geometric configuration of the AAP.     

One class classification as a practical approach for accelerating π–π co-crystal discovery

The implementation of machine learning models has brought major changes in the decision-making process for materials design. One matter of concern for the data-driven approaches is the lack of negative data from unsuccessful synthetic attempts, which might generate inherently imbalanced datasets. We propose the application of the one-class classification methodology as an effective tool for tackling these limitations on the materials design problems. This is a concept of learning based only on a well-defined class without counter examples. An extensive study on the different one-class classification algorithms is performed until the most appropriate workflow is identified for guiding the discovery of emerging materials belonging to a relatively small class, that being the weakly bound polyaromatic hydrocarbon co-crystals. The two-step approach presented in this study first trains the model using all the known molecular combinations that form this class of co-crystals extracted from the Cambridge Structural Database (1722 molecular combinations), followed by scoring possible yet unknown pairs from the ZINC15 database (21 736 possible molecular combinations). Focusing on the highest-ranking pairs predicted to have higher probability of forming co-crystals, materials discovery can be accelerated by reducing the vast molecular space and directing the synthetic efforts of chemists. Further on, using interpretability techniques a more detailed understanding of the molecular properties causing co-crystallization is sought after. The applicability of the current methodology is demonstrated with the discovery of two novel co-crystals, namely pyrene-6H-benzo[c]chromen-6-one (1) and pyrene-9,10-dicyanoanthracene (2).

Machine learning using one class classification on a database of existing co-crystals enables the identification of co-formers which are likely to form stable co-crystals, resulting in the synthesis of two co-crystals of polyaromatic hydrocarbons.     

The Different Story of π Bonds

We revisit “classical” issues in multiply bonded systems between main groups elements, namely the structural distortions that may occur at the multiple bonds and that lead, e.g., to trans-bent and bond-length alternated structures. The focus is on the role that orbital hybridization and electron correlation play in this context, here analyzed with the help of simple models for σ- and π-bonds, numerically exact solutions of Hubbard Hamiltonians and first principles (density functional theory) investigations of an extended set of systems.     

Rationalizing the diverse reactivity of [1.1.1]propellane through σ–π-delocalization

[1.1.1]Propellane is the ubiquitous precursor to bicyclo[1.1.1]pentanes (BCPs), motifs of high value in pharmaceutical and materials research. The classical Lewis representation of this molecule places an inter-bridgehead C–C bond along its central axis; ‘strain relief’-driven cleavage of this bond is commonly thought to enable reactions with nucleophiles, radicals and electrophiles. We propose that this broad reactivity profile instead derives from σ–π-delocalization of electron density in [1.1.1]propellane. Using ab initio and DFT calculations, we show that its reactions with anions and radicals are facilitated by increased delocalization of electron density over the propellane cage during addition, while reactions with cations involve charge transfer that relieves repulsion inside the cage. These results provide a unified framework to rationalize experimental observations of propellane reactivity, opening up opportunities for the exploration of new chemistry of [1.1.1]propellane and related strained systems that are useful building blocks in organic synthesis.

A unified framework that explains the reactivity of [1.1.1]propellane through electron delocalization.     

Halogenation of a twisted non-polar π-system as a tool to modulate phosphorescence at room temperature

Halogenation of a twisted three-fold symmetric hydrocarbon with F, Cl or Br leads to strong modulation of triplet–triplet annihilation and dual phosphorescence, one thermally activated and the other very persistent and visible by eye, with different relative contributions depending on the halide. The room temperature phosphorescence is highly unusual given the absence of lone-pair-contributing heteroatoms. The interplay between the spin–orbit coupling matrix elements and the spatial configuration of the triplet state induces efficient intersystem crossing and thus room temperature phosphorescence even without relying on heteroatomic electron lone pairs. A ninefold increase of the ISC rate after introduction of three bromine atoms is accompanied by a much higher 34-fold increase of phosphorescence rate.

Twisted π-systems investigation showed a very unusual HAE, influencing independently the ISC and the dual phosphorescence emission, one being very persistent at room temperature and visible by eye in powder.     

Dual XH–π Interaction of Hexafluoroisopropanol with Arenes

The dual XH (OH and CH) hydrogen-bond-donating property of 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) and the strong dual XH–π interaction with arenes were firstly disclosed by theoretical studies. Here, the high accuracy post-Hartree–Fock methods, CCSD(T)/CBS, reveal the interaction energy of HFIP/benzene complex (−7.22 kcal/mol) and the contribution of the electronic correlation energy in the total interaction energy. Strong orbital interaction between HFIP and benzene was found by using the DFT method in this work to disclose the dual XH–π intermolecular orbital interaction of HFIP with benzene-forming bonding and antibonding orbitals resulting from the orbital symmetry of HFIP. The density of states and charge decomposition analyses were used to investigate the orbital interactions. Isopropanol (IP), an analogue of HFIP, and chloroform (CHCl₃) were studied to compare them with the classical OH–π, and non-classical CH–π interactions. In addition, the influence of the aggregating effect of HFIP, and the numbers of substituted methyl groups in benzene rings were also studied. The interaction energies of HFIP with the selected 24 common organic compounds were calculated to understand the role of HFIP as solvent or additive in organic transformation in a more detailed manner. A single-crystal X-ray diffraction study of hexafluoroisopropyl benzoate further disclosed and confirmed that the CH of HFIP shows the non-classical hydrogen-bond-donating behavior.     

Synthesis and clustering of supramolecular “graft” polymers

The synthesis and melt rheology of supramolecular poly(isobutylene) polymers bearing statistically distributed hydrogen‐bonding moieties is reported, aiming at understanding the formation of the underlying supramolecular networks for self‐healing polymers. Two different hydrogen bonds were incorporated into a poly(isobutylene) (PIB) copolymer, one based on a (weak) pyridinium/pyridine interaction, the other based on a (stronger) 2,6‐diaminotriazine/thymine interaction. A direct copolymerization based on living cationic polymerization of isobutene and the comonomers 1 , 2 , and 4 in amounts of 1 mol % lead to the copolymers PIB‐ 1 , PIB‐ 2 , and PIB‐ 4 with a content of ～1 mol % of comonomer and molecular weights ranging from ～2000 to 19,000 g mol^?1 (M_w/M_n ～ 1.2–1.5). Subsequent azide/alkyne “click” chemistry enabled the attachment of 2,6‐diaminotriazine‐ and thymine‐moieties to yield the copolymers PIB‐ 5 , PIB‐ 6 , and PIB‐ 7 . Proof of the statistical incorporation of ～1 mol % of hydrogen‐bonding moieties was achieved by ¹H NMR spectroscopy and matrix‐assisted laser desorption ionization measurements. The true presence of a supramolecular network in PIB‐ 1 (pyridinium/pyridine interaction) as well as with 1/1 blends of PIBs interacting via the 2,6‐diaminotriazine/thymine interaction (PIB‐ 5 /PIB‐ 6 ) was proven via the increasing plateau modulus with increasing molecular weights (5.5k, 9.9k, 12.4k, 16k, and 19k). Dynamics of the hydrogen bonds in the melt state was investigated by determining the effective cluster lifetime ( τ ) observing a clear difference in the (weaker) pyridinium/pyridine interaction ( τ ～ 1 s) to the 2,6‐ (stronger) diamintriazine/thymine interaction ( τ ～ 100 s). The so‐generated materials will be useful as a basis for self‐healing polymers, as dynamics plays a major role in such polymers. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Organosilicon polymers with alternating σ- and π-conjugated systems

A series of organosilicon polymers containing polysilane and diethynylaryl units along the polymer backbone were synthesized and examined with respect to their optical absorptions. The results indicate that delocalization takes place through the σ–π conjugated system. Lengthening of π-conjugation leads to lower excitation energies while nearly identical UV–vis spectra are observed with increased Si–Si chain length. Introducing a thiophene unit into the π-system instead of a benzene unit leads to a bathochromic shift reflecting greater σ–π delocalization. The polymers undergo photodegradation, probably via cleavage of the Si–Si bonds, and thermal crosslinking by reaction at the C≡C triple bonds. When doped with iodine, these polymers become semiconducting with conductivity of the order of 10^?4 S cm^?1.     

Chirality memory of α-methylene-π-allyl iridium species

Chirality is one of the most important types of steric information in nature. In addition to central chirality, axial chirality has been catching more and more attention from scientists. However, although much attention has recently been paid to the creation of axial chirality and the chirality transfer of allenes, no study has been disclosed as to the memory of such an axial chirality. The reason is very obvious: the chiral information is stored over three carbon atoms. Here, the first example of the memory of chirality (MOC) of allenes has been recorded, which was realized via an optically active alkylidene-π-allyl iridium intermediate, leading to a highly stereoselective electrophilic allenylation with amines. Specifically, we have established the transition metal-mediated highly stereoselective 2,3-allenylation of amines by using optically active 2,3-allenyl carbonates under the catalysis of a nonchiral iridium(iii) complex. This method is compatible with sterically bulky and small substituents on both amines and 2,3-allenyl carbonates and furnishes the desired optically active products with a high efficiency of chirality transfer. Further mechanistic experiments reveal that the isomerization of the optically active alkylidene-π-allyl iridium intermediate is very slow.

Chirality is one of the most important types of steric information in nature.     

Electrical conductivity of π-conjugated polymers with nitro substituents

π-Conjugated polymers bearing nitro substituent(s), e.g., poly(aryleneethynylene) (PAE) type polymers and poly(4,8-dinitroanthraquinone-1,5-diyl) P(4,8-NO₂-1,5-AQ), show semiconducting properties with electrical conductivities of an order of 10⁻⁷ to 10⁻⁶ S · cm⁻¹ at room temperature without special oxidation and reduction of the polymer. P(4,8-NO₂-1,5-AQ) shows a large shift of phase in alternating current (ac) measurements and a unique magnetism at low temperature.     

A new type of noncovalent surface–π stacking interaction occurring on peroxide-modified titania nanosheets driven by vertical π-state polarization

Noncovalent π stacking of aromatic molecules is a universal form of noncovalent interactions normally occurring on planar structures (such as aromatic molecules and graphene) based on sp²-hybridized atoms. Here we reveal a new type of noncovalent surface–π stacking unusually occurring between aromatic groups and peroxide-modified titania (PMT) nanosheets, which can drive versatile aromatic adsorptions. We experimentally explore the underlying electronic-level origin by probing the perturbed changes of unoccupied Ti 3d states with near-edge X-ray absorption fine structures (NEXAFS), and find that aromatic groups can vertically attract π electrons in the surface peroxo-Ti states and increase their delocalization regions. Our discovery updates the concept of noncovalent π-stacking interactions by extending the substrates from carbon-based structures to a transition metal oxide, and presents an approach to exploit the surface chemistry of nanomaterials based on noncovalent interactions.

A new type of noncovalent surface–π stacking interaction occurring on a transition metal oxide, titania, is reported, which is different from the traditional forms on sp²-hybridized planar structures like graphene.     

Complementary,Cooperative Ditopic Halogen Bonding and Electron Donor-Acceptor π-π Complexation in the Formation of Cocrystals

This study expands and combines concepts from two of our earlier studies. One study reported the complementary halogen bonding and π-π charge transfer complexation observed between isomeric electron rich 4-N,N-dimethylaminophenylethynylpyridines and the electron poor halogen bond donor, 1-(3,5-dinitrophenylethynyl)-2,3,5,6-tetrafluoro-4-iodobenzene while the second study elaborated the ditopic halogen bonding of activated pyrimidines. Leveraging our understanding on the combination of these non-covalent interactions, we describe cocrystallization featuring ditopic halogen bonding and π-stacking. Specifically, red cocrystals are formed between the ditopic electron poor halogen bond donor 1-(3,5-dinitrophenylethynyl)-2,4,6-triflouro-3,5-diiodobenzene and each of electron rich pyrimidines 2- and 5-(4-N,N-dimethyl-aminophenylethynyl)pyrimidine. The X-ray single crystal structures of these cocrystals are described in terms of halogen bonding and electron donor-acceptor π-complexation. Computations confirm that the donor-acceptor π-stacking interactions are consistently stronger than the halogen bonding interactions and that there is cooperativity between π-stacking and halogen bonding in the crystals.     

π–π Noncovalent Interaction Involving 1,2,4- and 1,3,4-Oxadiazole Systems: The Combined Experimental,Theoretical, and Database Study

A series of N-pyridyl ureas bearing 1,2,4- (1a, 2a, and 3a) and 1,3,4-oxadiazole moiety (1b, 2b, 3b) was prepared and characterized by HRMS, ¹H and ¹³C NMR spectroscopy, as well as X-ray diffraction. The inspection of the crystal structures of (1–3)a,b and the Hirshfeld surface analysis made possible the recognition of the (oxadiazole)···(pyridine) and (oxadiazole)···(oxadiazole) interactions. The presence of these interactions was confirmed theoretically by DFT calculations, including NCI analysis for experimentally determined crystal structures as well as QTAIM analysis for optimized equilibrium structures. The preformed database survey allowed the verification of additional examples of relevant (oxadiazole)···π interactions both in Cambridge Structural Database and in Protein Data Bank, including the cocrystal of commercial anti-HIV drug Raltegravir.     

Elastic Organic Crystals of a Fluorescent π‐Conjugated Molecule

An elastic organic crystal of a π‐conjugated molecule has been fabricated. A large fluorescent single crystal of 1,4‐bis[2‐(4‐methylthienyl)]‐2,3,5,6‐tetrafluorobenzene (over 1 cm long) exhibited a fibril lamella morphology based on slip‐stacked molecular wires, and it was found to be a remarkably elastic crystalline material. The straight crystal was capable of bending more than 180° under applied stress and then quickly reverted to its original shape upon relaxation. In addition, the fluorescence quantum yield of the crystal was about twice that of the compound in THF solution. Mechanical bending–relaxation resulted in reversible change of the morphology and fluorescence. This research offers a more general approach to flexible crystals as a promising new family of organic semiconducting materials.     

Sulfonium cations as versatile strongly π-acidic ligands

More than a century old, sulfonium cations are still intriguing species in the landscape of organic chemistry. On one hand they have found broad applications in organic synthesis and materials science, but on the other hand, while isoelectronic to the ubiquitous tertiary phosphine ligands, their own coordination chemistry has been neglected for the last three decades. Here we report the synthesis and full characterization of the first Rh(i) and Pt(ii) complexes of sulfonium. Moreover, for the first time, coordination of an aromatic sulfonium has been established. A thorough computational analysis of the exceptionally short S–Rh bonds obtained attests to the strongly π-accepting nature of sulfonium cations and places them among the best π-acceptor ligands available today. Our calculations also show that embedding within a pincer framework enhances their π-acidity even further. Therefore, in addition to the stability and modularity that these frameworks offer, our pincer complexes might open the way for sulfonium cations to become powerful tools in π-acid catalysis.

Back to the scene: while isolobal to the ubiquitous tertiary phosphines, sulfonium cations as ligands were neglected for decades. This work revives the coordination chemistry of these species showing their potential as ligands for π-acid catalysis.     

Controllable assembly of a three‐dimensional metal–organic supramolecular framework including π–π stacking interactions

The mixed‐ligand metal–organic complex poly[(μ₃‐phthalato)[μ₂‐3‐(pyridin‐2‐yl)‐1H‐pyrazol‐1‐ido]dicopper(II)], [Cu₂(C₈H₄O₄)(C₈H₆N₃)₂]_n, has been synthesized by the reaction of copper(II) acetate with 2‐(1H‐pyrazol‐3‐yl)pyridine (HL) and phthalic acid. The binuclear chelating–bridging L units are further linked by bridging phthalate ligands into a two‐dimensional network parallel to the (010) plane. The two‐dimensional networks are extended into a three‐dimensional supramolecular architecture viaπ–π stacking interactions.