Relative substituent position on the strength of π-π stacking interactions

It was observed that the relative position of the arene substituents has a profound influence on the strength of π-π stacking in the 9-benzyl-substituted triptycene system. A new series of model compounds (3a-i) capable of revealing quantitatively π-π stacking interactions was studied. This series of compounds (3a-i) has an ortho-substituted methyl group in one of the two interacting arenes and the syn/anti ratios were determined and compared to a series previously studied compounds (4a-i) that have a para methyl group on the corresponding arene. A greater than 50% increase in the strength of π-π stacking interactions was observed with the methyl group in the ortho position comparing to that in the para position. No difference in π-π stacking interactions was observed when the other aromatic ring was a pentafluorobenzoate group.     

A simple chiral recognition system to investigate substituent effects on π-π interactions

We have used a simple molecular recognition system to study substituent effects in aromatic interactions. A series of substituted benzoylleucine diethyl amides with aromatic rings of varying electronic character were crystallized. All of the substituted dimers organized into homochiral dimers in the solid state but with pronounced differences in regard to the orientation of the aromatic rings with respect to each other. However, no homochiral dimerization was observed in the unsubstituted case.     

Interplay between halogen bonds and π-π stacking interactions: CSD search and theoretical study

According to our survey of the Cambridge Structural Database (CSD), a great number of crystal structures, in which halogen bonds and aromatic stacking interactions are present and play an important role in crystal packing, have been extracted. In this work, ab initio calculations at the MP2 level of theory were performed to investigate the mutual influence between halogen bonds and π-π stacking interactions. Different energetic effects are observed in the studied complexes where the two kinds of noncovalent interactions coexist, which can be rationalized by the direction of charge transfer for the two interactions. These effects have been analyzed in detail in terms of the structural, energetic, and charge transfer properties of the complexes. In addition, the quantum theory of atoms in molecules (QTAIM) was also employed to characterize the interactions and to examine the strengthening or weakening of the interactions, depending on the variations of electron density on the bond and cage critical points. Finally, certain crystal structures retrieved from the CSD have been selected to provide experimental evidence of the combination of the two interactions.     

pH-responsive assembly and disassembly of a supramolecular cryptand-based pseudorotaxane driven by π-π stacking interaction

Driven by π-π stacking interaction, a supramolecular cryptand-based [2]pseudorotaxane was formed and its formation was demonstrated to be pH-responsive.     

Efficient conducting channels formed by the π-π stacking in single [2,2]paracyclophane molecules

The electronic transport properties of single [2,2]paracyclophane molecules directly connected to gold and platinum electrodes have been investigated both theoretically and experimentally by using first-principles quantum transport simulations and break-junction experiments. For comparison, investigations on [3,3]- and [4,4]-paracyclophanes have also been performed. Our calculations show that the strength of the π-π interaction in paracyclophanes is critically dependent on the inter-ring distance. In contrast to [4,4]paracyclophane in which the π-π interaction is very weak due to the large inter-ring distance, the π-π interaction in [2,2]- and [3,3]-paracyclophanes is rather strong and dominates the electronic transport properties. In particular, for the asymmetric Au-[2,2]paracyclophane-Au junction in which the [2,2]paracyclophane molecule is connected to each gold electrode through a Au adatom and the two Au adatoms are attached in η(1)-fashion to two carbon atoms in the benzene backbones connecting with different ethylene groups, the transmission coefficient at the Fermi level is calculated to be 1.0 × 10(-2), in excellent agreement with experiments. When the gold electrodes are replaced by platinum, the calculated transmission coefficient at the Fermi level of the symmetric Pt-[2,2]paracyclophane-Pt junction with one Pt adatom used as the linker group is increased to 0.83, demonstrating that the π-π stacking in [2,2]paracyclophane is efficient for electron transport when the molecule-electrode interfaces are electronically transparent. This is confirmed by our preliminary experimental studies on the Pt-[2,2]paracyclophane-Pt junctions, for which the low-bias junction conductance has reached 0.40 ± 0.02 G(0) (G(0) is the conductance quantum). These findings are helpful for the design of molecular electronic devices incorporating π-π stacking molecular systems.     

Coexistence of two conformational isomeric chains in a zinc(II) phosphonate induced by π···π stacking interactions

Hydrothermal reaction of zinc acetate with diethyl [(phenylsulfonyl)methyl]phosphonate as well as 1,10-phenanthroline (phen) afforded a novel zinc(II) phosphonate with the formula of [Zn₄(PhSO₂CH₂PO₃)₄(phen)₂(H₂O)₂]·2H₂O. Such compound features two conformational isomeric 1D chains which are regulated by two different π···π stacking interactions. In addition, it exhibits broad blue fluorescent emission band at 387 nm.     

The tropylium ion annelated with bicyclo[2.1.1]hex-2-ene: Stabilization due to σ-π conjugation versus destabilization due to mills-nixon type π-bond localization

In the tropylium ion annelated with bicyclo[2.1.1]hex-2-ene, stabilization due to the σ-π conjugation of tropylium p-orbitals with the highly strained bicyclic σ-framework was shown to be more effective than destabilization due to the Mills-Nixon type π-bond localization.     

Control of intramolecular π-π stacking interaction in cationic iridium complexes via fluorination of pendant phenyl rings

Intramolecular π-π stacking interaction in one kind of phosphorescent cationic iridium complexes has been controlled through fluorination of the pendant phenyl rings on the ancillary ligands. Two blue-green-emitting cationic iridium complexes, [Ir(ppy)(2)(F2phpzpy)]PF(6) (2) and [Ir(ppy)(2)(F5phpzpy)]PF(6) (3), with the pendant phenyl rings on the ancillary ligands substituted with two and five fluorine atoms, respectively, have been synthesized and compared to the parent complex, [Ir(ppy)(2)(phpzpy)]PF(6) (1). Here Hppy is 2-phenylpyridine, F2phpzpy is 2-(1-(3,5-difluorophenyl)-1H-pyrazol-3-yl)pyridine, F5phpzpy is 2-(1-pentafluorophenyl-1H-pyrazol-3-yl)-pyridine, and phpzpy is 2-(1-phenyl-1H-pyrazol-3-yl)pyridine. Single crystal structures reveal that the pendant phenyl rings on the ancillary ligands stack to the phenyl rings of the ppy ligands, with dihedral angles of 21°, 18°, and 5.0° between least-squares planes for complexes 1, 2, and 3, respectively, and centroid-centroid distances of 3.75, 3.65, and 3.52 ? for complexes 1, 2, and 3, respectively, indicating progressively reinforced intramolecular π-π stacking interactions from complexes 1 to 2 and 3. Compared to complex 1, complex 3 with a significantly reinforced intramolecular face-to-face π-π stacking interaction exhibits a significantly enhanced (by 1 order of magnitude) photoluminescent efficiency in solution. Theoretical calculations reveal that in complex 3 it is unfavorable in energy for the pentafluorophenyl ring to swing by a large degree and the intramolecular π-π stacking interaction remains on the lowest triplet state.     

The use of a molecular balance derived from 5,5′-bipyrazole to calculate π−π stacking interactions

Theoretical calculations have been carried out on 3,3′-dimethyl-1,1′-diphenyl-5,5′-bi-1H-pyrazole to evaluate its usefulness as a molecular balance to measure π−π stacking. After removing the methyl groups (1,1′-diphenyl-5,5′-bi-1H-pyrazole), the N-phenyl groups have been replaced by a series of aromatic rings, and the energy difference between the syn and anti forms is discussed in terms of π−π stacking and dipole-dipole interactions.     

Oriented poly(3-hexylthiophene) nanofibril with the π-π stacking growth direction by solvent directional evaporation

In this article, the uniaxial alignment of poly(3-hexylthiophene) (P3HT) nanofibrils with a π-π stacking growth direction in which P3HT chains adopt a flat-on conformation was obtained by solvent directional evaporation using a glass cover slide and a poly(dimethylsiloxane) (PDMS) sheet to press the P3HT film in a carbon disulfide (CS(2)) atmosphere. By controlling the CS(2) vapor pressure during the film-forming process, we got a well-oriented P3HT film whose order parameter reached as high as 0.97. The orientation of the film was induced by the crystallization nucleation of P3HT and the directional evaporation of the solvent. Under a CS(2) vapor atmosphere, P3HT crystals preferred to adopt the form II modification, which started by nucleation. Owing to the solvent directional evaporation from the center to the margin, P3HT at the center of the sample would precipitate first to induce nucleation. Then the peripheral P3HT would directly diffuse, precipitate, and then adhere to the nucleus to form the uniaxial alignment of P3HT nanofibrils along the direction of solvent evaporation. Furthermore, in the P3HT nanofibrils, the π-π stacking direction of P3HT lamellae was parallel to the crystal growth direction, which would provide an effective path for charge transport.     

Solvents incubated π-π stacking in hole transport layer for perovskite-silicon 2-terminal tandem solar cells with 27.21% efficiency

Room temperature sputtered inorganic nickel oxide(NiOx) is one of the most promising hole transport layers(HTL) for perovskite-sillion 2-terminal tandem solar cells with the aid of ultrathin and compact organic layers to passivate the surface defects. In this study, the aromatic solvent with different substituent groups was used to regulate the conformation of poly[bis(4-phenyl)(2,4,6-trimethylphenyl)am ine](PTAA) layer. As a result, the single-junction perovskite solar cell(PSC) gained a power ...     

Strong phenyl-perfluorophenyl π-π stacking and C-H?F-C hydrogen bonding interactions in the crystals of the corresponding aromatic aldimines

The X-ray diffraction analysis of two co-crystals, 1·2 (aldimines 1 and 2) and 3·4 (aldimines 3 and 4), reveals that there are strong phenyl-perfluorophenyl π-π stacking and intermolecular hydrogen bonding interactions. The new perfluoroaryl-aryl face-to-face interaction of the crystalline aldimines provides a design motif for a new class of self-assembling system.     

Constructing single-chain magnets by supramolecular π-π stacking and spin canting: a case study on manganese(III) corroles

A single‐chain magnet (SCM) was constructed from manganese(III) 5,10,15‐tris(pentafluorophenyl)corrole complex [Mn^III(tpfc)] through supramolecular π–π stacking without bridging ligands. In the crystal structures, [Mn(tpfc)] molecules crystallized from different solvents, such as methanol, ethyl acetate, and ethanol, exhibit different molecular orientations and intermolecular π–π interaction or weak Mn ??? O interaction to form a supramolecular one‐dimensional motif or dimer. These three complexes show very different magnetic behaviors at low temperature. Methanol solvate 1 shows obvious frequency dependence of out‐of‐phase alternating‐current magnetic susceptibility below 2 K and a magnetization hysteresis loop with a coercive field of 400 Oe at 0.5 K. It is the first example of spin‐canted supramolecular single‐chain magnet due to weak π–π stacking interaction. By fitting the susceptibility data χ_MT (20–300 K) of 1 with the spin Hamiltonian expression ${\overrightarrow{H}}A single-chain magnet (SCM) was constructed from manganese(III) 5,10,15-tris(pentafluorophenyl)corrole complex [Mn(III) (tpfc)] through supramolecular π-π stacking without bridging ligands. In the crystal structures, [Mn(tpfc)] molecules crystallized from different solvents, such as methanol, ethyl acetate, and ethanol, exhibit different molecular orientations and intermolecular π-π interaction or weak Mn???O interaction to form a supramolecular one-dimensional motif or dimer. These three complexes show very different magnetic behaviors at low temperature. Methanol solvate 1 shows obvious frequency dependence of out-of-phase alternating-current magnetic susceptibility below 2?K and a magnetization hysteresis loop with a coercive field of 400?Oe at 0.5?K. It is the first example of spin-canted supramolecular single-chain magnet due to weak π-π stacking interaction. By fitting the susceptibility data χ(M) T (20-300?K) of 1 with the spin Hamiltonian expression H = -2J Σ(i=1)(n-1) S(Ai) S(Ai+1) + D Σ(i) S((iZ)(2)), the intrachain magnetic coupling parameter transmitted by π-π interaction of -0.31?cm(-1) and zero field splitting parameter D of -2.59?cm(-1) are obtained. Ethyl acetate solvate 2 behaves as an antiferromagnetic chain without ordering or slow magnetic relaxation down to 0.5?K. The magnetic susceptibility data χ(M) T (20-300?K) of 2 was fitted by assuming the spin Hamiltonian H = -2JΣ(i=1)(n-1) S(Ai) S(Ai+1), and the intrachain antiferromagnetic coupling constant of -0.07?cm(-1) is much weaker than that of 1. Ethanol solvate 3 with a dimer motif shows field-induced single-molecule magnet like behavior below 2.5?K. The exchange coupling constant J within the dimer propagated by π-π interaction is -0.14?cm(-1) by fitting the susceptibility data χ(M) T (20-300?K) with the spin Hamiltonian H = -2J S(A) S(B) + β(S((A)g(A)) + S((B)g(B)))H. The present studies open a new way to construct SCMs from anisotropic magnetic single-ion units through weak intermolecular interactions in the absence of bridging ligands.     

Comparable charge transport property based on S…S interactions with that of π-π stacking in a bis-fused tetrathiafulvalene compound

Compact molecular packing with short π-π stacking and large π-overlap in organic semiconductors is desirable for efficient charge transport and high carrier mobility.Thus charge transport anisotropy along different directions is commonly observed in organic semiconductors.Interestingly,in this article,we found that comparable charge transport property were achieved based on the single crystals of a bis-fused tetrathiafulvalene derivative(EM-TTP) compound along two interaction directions,that is,the multiple strong S…S intermolecular interactions and the π-π stacking direction,with the measured electrical conductivity and hole mobility of 0.4 S cm~(-1),0.94 cm~2 V~(-1) s~(-1) and 0.2 S cm~(-1),0.65 cm2 V~(-1) s~(-1),respectively.This finding provides us a new molecular design concept for developing novel organic semiconductors with isotropic charge transport property through the synergistic effect of multiple intermolecular interactions(such as S…S interactions) and π-π stacking.     

New mercury(II) and silver(I) complexes containing NHC metallacrown ethers with the π-π stacking interactions

The oligoether-linked bis-benzimidazolium salt 1,1′-[1,2-ethanediylbis(oxy-1,2-ethanediyl)]bis[(3-^secbutyl)benzimidazolium-1-yl]iodide (H₂L¹ · I₂), 1,1′-[1,2-ethanediylbis(oxy-1,2-ethanediyl)]bis[(3-ethyl)benzimidazolium-1-yl]iodide (H₂L² · I₂) and 1,1′-[1,2-ethanediylbis(oxy-1,2-ethanediyl)]bis[(3-^secbutyl)benzimidazolium-1-yl]hexafluorophosphate (H₂L¹ · (PF₆)₂) and their three new mercury(II) and silver(I) complexes containing NHC metallacrown ethers, HgL¹ · (Hg₂ · I₆) (1), HgL² · I₂ (2) and AgL¹ · PF₆ (3) were prepared and characterized. In the packing diagrams of H₂L² · I₂, 1, 2 and 3 benzimidazole ring head-to-tail π-π stacking interactions are observed.     

Combination of π-π stacking and electrostatic repulsion between carboxylic carbon nanoparticles and fluorescent oligonucleotides for rapid and sensitive detection of thrombin

On the basis of simultaneous electrostatic repulsion and π-π stacking interactions of carboxylic carbon nanoparticles (cCNPs) with single-stranded DNA, a noncovalent assembly of cCNPs and fluorescent aptamers is reported for rapid, sensitive, and selective detection of thrombin.     

New N-heterocyclic carbene silver(I) and mercury(II) 2-D supramolecular layers by the π-π stacking interactions

The precursor 1-(9-anthracenylmethyl)-3-alkylbenzimidazolium chlorides (1a, alkyl = C₄H₉, 1b, alkyl = C₆H₁₃) and their three new NHC silver(I) and mercury(II) complexes {[1-(9-anthracylmethyl)-3-alkylbimy]MCl}₂ (2a, alkyl = C₄H₉, M = Ag; 2b, alkyl = C₆H₁₃, M = Ag; 3a, alkyl = C₄H₉, M = Hg; bimy = benzimidazol-2-ylidene) have been prepared and characterized. The crystal structures of 2a, 2b and 3a showed that 2-D supramolecular layers are formed by both benzimidazole ring head to tail π-π stacking interactions and anthracene ring face-to-face π-π stacking interactions.     

Zinc paddlewheel dimers containing a strong π···π stacking supramolecular synthon: designed single-crystal to single-crystal phase changes and gas/solid guest exchange

The ligand 4-(1,8-naphthalimido)benzoate, L(C4)(-), containing a linear link between the strong π···π stacking 1,8-naphthalimide supramolecular synthon and the carboxylate donor group, reacts with Zn(O(2)CCH(3))(2)(H(2)O)(2) in the presence of dimethylsulfoxide (DMSO) to yield [Zn(2)(L(C4))(4)(DMSO)(2)]·2(CH(2)Cl(2)). This compound contains the "paddlewheel" Zn(2)(O(2)CR)(4) secondary building unit (SBU) that organizes the rigid phenylene and naphthalimide rings of the carboxylate ligands in a square arrangement. The supramolecular architecture is dominated by π···π stacking interactions between naphthalimide rings of one dimer with four adjacent dimers, essentially at right angles, forming an open three-dimensional network structure. Two symmetry equivalent networks of this type interpenetrate generating overall a densely packed three-dimensional, 2-fold interpenetrated architecture in which the CH(2)Cl(2) solvate molecules are trapped in isolated pockets. Upon cooling, single crystals of [Zn(2)(L(C4))(4)(DMSO)(2)]·2(CH(2)Cl(2)) undergo two distinct crystallographic phase transitions, as characterized by X-ray diffraction at different temperatures, without loss of crystallinity. These two new phases have supramolecular structures very similar to the room temperature structure, but changes in the ordering of the CH(2)Cl(2) solvate cause shifting of the naphthalimide rings and a lowering of the symmetry. Crystals of [Zn(2)(L(C4))(4)(DMSO)(2)]·2(CH(2)Cl(2)) undergo a single-crystal to single-crystal gas/solid guest exchange upon exposure to atmospheric moisture, or faster if placed under vacuum or heated under dry gas to 100 °C, followed by atmospheric moisture, to yield [Zn(2)(L(C4))(4)(DMSO)(2)]·3.9(H(2)O). The molecular and supramolecular structures of this new compound are very similar to the dichloromethane adduct, with now the water molecules encapsulated into the framework. The remarkable feature of both the phase changes and exchange of solvates is that this robust network is not porous; local distortions (ring slippage and tilting changes) of the π···π stacking interactions of the naphthalimide rings that organize these structures allow these changes to take place without the loss of crystallinity. The complexes [Zn(2)(L(C4))(4)(DMSO)(2)]·2(CH(2)Cl(2)) and [Zn(2)(L(C4))(4)(DMSO)(2)]·3.9(H(2)O) show green emission in the solid state.     

Copper(II) carboxylate dimers prepared from ligands designed to form a robust π···π stacking synthon: supramolecular structures and molecular properties

The reactions of bifunctional carboxylate ligands (1,8-naphthalimido)propanoate, (L(C2)(-)), (1,8-naphthalimido)ethanoate, (L(C1)(-)), and (1,8-naphthalimido)benzoate, (L(C4)(-)) with Cu(2)(O(2)CCH(3))(4)(H(2)O)(2) in methanol or ethanol at room temperature lead to the formation of novel dimeric [Cu(2)(L(C2))(4)(MeOH)(2)] (1), [Cu(2)(L(C1))(4)(MeOH)(2)]·2(CH(2)Cl(2)) (2), [Cu(2)(L(C4))(4)(EtOH)(2)]·2(CH(2)Cl(2)) (3) complexes. When the reaction of L(C1)(-) with Cu(2)(O(2)CCH(3))(4)(H(2)O)(2) was carried out at -20 °C in the presence of pyridine, [Cu(2)(L(C1))(4)(py)(4)]·2(CH(2)Cl(2)) (4) was produced. At the core of complexes 1-3 lies the square Cu(2)(O(2)CR)(4) "paddlewheel" secondary building unit, where the two copper centers have a nearly square pyramidal geometry with methanol or ethanol occupying the axial coordination sites. Complex 4 contains a different type of dimeric core generated by two κ(1)-bridging carboxylate ligands. Additionally, two terminal carboxylates and four trans situated pyridine molecules complete the coordination environment of the five-coordinate copper(II) centers. In all four compounds, robust π···π stacking interactions of the naphthalimide rings organize the dimeric units into two-dimensional sheets. These two-dimensional networks are organized into a three-dimensional architecture by two different noncovalent interactions: strong π···π stacking of the naphthalimide rings (also the pyridine rings for 4) in 1, 3, and 4, and intermolecular hydrogen bonding of the coordinated methanol or ethanol molecules in 1-3. Magnetic measurements show that the copper ions in the paddlewheel complexes 1-3 are strongly antiferromagnetically coupled with -J values ranging from 255 to 325 cm(-1), whereas the copper ions in 4 are only weakly antiferromagnetically coupled. Typical values of the zero-field splitting parameter D were found from EPR studies of 1-3and the related known complexes [Cu(2)(L(C2))(4)(py)(2)]·2(CH(2)Cl(2))·(CH(3)OH), [Cu(2)(L(C3))(4)(py)(2)]·2(CH(2)Cl(2)) and [Cu(2)(L(C3))(4)(bipy)]·(CH(3)OH)(2)·(CH(2)Cl(2))(3.37) (L(C3)(-) = (1,8-naphthalimido)butanoate)), while its abnormal magnitude in [Cu(2)(L(C2))(4)(bipy)] was qualitatively rationalized by structural analysis and DFT calculations.