Theoretical studies on a carbonaceous molecular bearing: association thermodynamics and dual-mode rolling dynamics

Stoichiometry validation of supramolecular complexes with a hydrocarbon cage host by van 't Hoff analyses

Defining chemical processes with equations is the first important step in characterizing equilibria for the assembly of supramolecular complexes, and the stoichiometry of the assembled components must be defined to generate the equation. Recently, this subject has attracted renewed interest, and statistical and/or information-theoretic measures were introduced to examine the validities of the equilibrium models used during curve fitting analyses of titration. The present study shows that these measures may not always be appropriate for credibility examinations and that further reformation of the protocols used to determine the overall stoichiometry is necessary. Hydrocarbon cage hosts and their chloroform complexes formed via weak CH-π hydrogen bonds were studied, which allowed us to introduce van 't Hoff analyses for effective validation of the stoichiometries of supramolecular complexes. This study shows that the stoichiometries of supramolecular complexes should be carefully examined by adopting multiple measures with different origins.

Nanoscale Saturn Systems Based on C60/70 Bucky Ball and a Newly Designed [4]Cyclopara-1,2-diphenylethylene Hoop: A Strategy for Fullerene Encapsulation Release and Selective Recognition for C70

A new carbonaceous nanohoop, [4]cyclopara-1,2-diphenylethylene ([4]CPDPE, composed by four 1,2-diphenylethylene units linked via the para of the phenyls), is designed together with two rational synthesis paths being proposed. The Saturn-like host-guest systems formed with the [4]CPDPE nanoring and fullerene C_60/70 are explored using density functional theory calculations. The results evidence that the geometry mutual matching between [4]CPDPE and C_60/70 is perfect, and the [4]CPDPE⊃C_60/70 complexes could be formed spontaneously with high binding energies. Thermodynamic calculation results show that it essentially prefers to selectively recognize C₇₀ over its smaller cousin C₆₀. More interestingly, the [4]CPDPE nanoring could present the regular ring cylinder and the saddle shapes via configuration transformation between its all-trans form and all-cis form, so as to theoretically realize the fullerene encapsulation and release under photoirradiation. Furthermore, the 2:1 interaction structure ([4]CPDPE₂⊃Dimer-C₆₀) and properties are investigated. Additionally, the ultraviolet-visible (UV-vis) spectra are simulated, and host-guest noncovalent interaction (NCI) regions are investigated based on the electron density and reduced density gradient (RDG), which may be helpful for a deep understanding of the present designed systems in future.

Concise Synthesis of Molecular Hyperboloids by Oligomeric Macrocyclization of Octagonal Molecules

Molecular hyperboloids were designed and synthesized. The synthesis was achieved by development of oligomeric macrocyclization of an octagonal molecule with a saddle shape. The saddle-shaped molecule, that is, [8]cyclo-meta-phenylene ([8]CMP), was decorated with two linkers for the oligomeric macrocyclization and was synthetically assembled by Ni-mediated Yamamoto coupling. Three congeners of the molecular hyperboloids (2mer-4mer) were obtained, and 2mer and 3mer were subjected to X-ray crystallographic analysis. Crystal structures revealed nanometer-sized hyperboloidal structures with 96π and 144π electrons, which also possessed nanopores on the curved molecular structures. Structures of [8]CMP cores of the molecular hyperboloids were compared with those of saddle-shaped phenine [8]circulene with a negative Gauss curvature to confirm their structural resemblance, which suggests further explorations of expanded networks of molecular hyperboloids.

Modular synthesis, host-guest complexation and solvation-controlled relaxation of nanohoops with donor-acceptor structures

Carbon nanohoops with donor-acceptor (D-A) structures are attractive electronic materials and biological fluorophores, but their synthesis is usually challenging. Moreover, the preparation of D-A nanohoop fluorophores exhibiting high fluorescence quantum yields beyond 500 nm remains a key challenge. This study presents a modular synthetic approach based on an efficient metal-free cyclocondensation reaction that readily produced nine congeners with D-A or donor-acceptor-donor' (D-A-D') structures, one of which is water-soluble. The tailored molecular design of nanohoops enabled a systematic and detailed study of their host-guest complexation with fullerene, optical properties, and charge transfer (CT) dynamics using X-ray crystallography, fluorescence titration, steady and ultrafast transient absorption spectroscopy, and theoretical calculations. The findings revealed intriguing physical properties associated with D-A motifs, such as tight binding with fullerene, moderate fluorescence quantum yields (37-67%) beyond 540 nm, and unique solvation-controlled CT relaxation of D-A-D' nanohoops, where two CT states (D-A and A-D') can be effectively tuned by solvation, resulting in dramatically changed relaxation pathways in different solvents.

Target-oriented design of helical nanotube molecules for rolled incommensurate bilayers

Incommensurate double-wall carbon nanotubes give rise to unique stereochemistry originating from twisted stacks of hexagon arrays. However, atomic-level studies on such unique systems have rarely been performed, even though syntheses of molecular segments of carbon nanotubes have been extensively explored. The design of cylindrical molecules with chirality, particularly, in pairs provides synthetic challenges, because relationships between diameters specified with chiral indices and structures of arylene panels have not been investigated in a systematic manner. Here we show that a molecular version of incommensurate double-wall carbon nanotubes can be designed through the development of an atlas for the top-down design of cylindrical molecules. A large-bore cylindrical molecule with a diameter of 1.77 nm was synthesized using a readily available pigment and encapsulated a small-bore cylindrical molecule with a diameter of 1.04 nm. The large- and small-bore molecules possessed helicity in atomic arrangements, and their coaxial assembly proceeded in nonstereoselective manner to give both heterohelical and homohelical combinations.

Activation of positive cooperativity by size-mismatch assembly via inclination of guests in a single-site receptor

A halogenated bowl-shaped guest, corannulene, was encapsulated in a cylindrical host, [4]cyclochrysenylene, to form a bowl-in-tube complex, which mimicked supramolecular complexes between bowl guests and carbon nanotubes. As was the case with carbon nanotubes, the cylindrical space of [4]cyclochrysenylene trapped multiple corannulene molecules in an array, and 1:2 complexes were commonly obtained with the corannulene guests with various halogen substituents (F, Cl, Br and I). Careful statistical analyses of isothermal titration calorimetry titration data succeeded in revealing the stoichiometry, and the molecular structures of the 1:2 complexes were further clarified by X-ray crystallographic analyses. Two fluorinated corannulene guests were stacked perpendicular to the cylinder axis, while two chlorinated guests were stacked with inclined orientations. The structural difference resulted in a large difference in the cooperativity of the two-stage association in solution: fluorinated corannulene guests showed negative cooperativity for the 1:2 complexation, and the other, larger halogenated corannulene guests showed positive cooperativity.

A hybrid molecular peapod of sp2- and sp3-nanocarbons enabling ultrafast terahertz rotations

The internal hollow space of carbon nanotubes provides a unique nanometre-sized space to capture various molecular entities. The inner space circumfused by sp²-carbon networks can also encapsulate diamondoid molecules to afford sp²/sp³-hybrid nanocarbon peapods that have recently emerged as unique nanostructures. In this study, the sp²/sp³-hybrid peapods have been mimicked by adopting a cylindrical molecule and the smallest diamondoid, i.e., adamantane, to demonstrate the existence of ultrafast rotational motion. The solid-state rotational frequency is measured by NMR spectroscopy to record 1.06 THz that is, to the best of our knowledge, the largest value recorded for solid-state rotations of molecules. Theoretical calculations reveal that multivalent CH-π hydrogen bonds anchored the diamondoid guest on the π-wall of the cylindrical host. The weak hydrogen bonds are prone not only to cleave but also to regenerate at the interfaces, which give freedom to the guest for ultrafast isotropic rotations in the inertial regime.

Mechanical motions in hybrid sp²/sp³ -hybrid nanocarbon peapods might lead to promising materials applications, but have been insufficiently explored. Here the authors demonstrate that a diamondoid molecule trapped inside a carbonaceous cylinder undergoes solid-state rotations at terahertz frequencies.

[10]CPP-Based Inclusion Complexes of Charged Fulleropyrrolidines. Effect of the Charge Location on the Photoinduced Electron Transfer

A number of non-covalently bound donor-acceptor dyads, consisting of C60 as the electron acceptor and cycloparaphenylene (CPP) as the electron donor, have been reported. A hypsochromic shift of the charge transfer (CT) band in polar medium has been found in [10]CPP⊃Li+ @C60 . To explore this anomalous effect, we study inclusion complexes [10]CPP⊃Li+ @C60 -MP, [10]CPP⊃C60 -MPH+ , and [10]CPP⊃C60 -PPyMe+ formed by fulleropyrrolidine derivatives and [10]CPP using the DFT/TDDFT approach. We show that the introduction of a positively charged fragment into fullerene stabilizes CT states that become the lowest-lying excited states. These charge-separated states can be generated by the decay of locally excited states on a nanosecond to picosecond time scale. The distance of the charged fragment to the center of the fullerenic cage and its accessibility to the solvent determine the strength of the hypsochromic shift.

Synthesis, Structures, and Properties of Highly Strained Cyclophenylene-Ethynylenes with Axial and Helical Chirality

Single and double cyclophenylene-ethynylenes (CPEs) with axial and helical chirality have been synthesized by the Sonogashira cross-coupling of di- and tetraethynyl biphenyls with a U-shaped prearomatic diiodoparaphenylene followed by reductive aromatization. X-ray crystallographic analyses and DFT calculations revealed that the CPEs possess highly twisted bent structures. Bend angles on the edge of the paraphenylene units were close to the value of [5]cycloparaphenylene (CPP)-the smallest CPP to date. The double and single CPEs possessed stable chirality despite flexible biphenyl structures because of the high strain in the diethynyl-paraphenylene moiety. In both the single and double CPEs, orbital interactions along the biphenyl axis were observed by DFT calculations in LUMO and LUMO+2 of the single CPE and LUMO+1 of the double CPE, which likely cause lowering of these orbital energies. Concerning chiroptical properties: boosting of the g_abs value was observed in the biphenyl-based double CPE, as well as the binaphthyl-based single CPE, compared to the biphenyl-based single CPE.

Enantioselective Synthesis of Planar Chiral Zigzag-Type Cyclophenylene Belts by Rhodium-Catalyzed Alkyne Cyclotrimerization

Planar chiral zigzag-type [8]- and [12]cyclophenylene (CP) belts have been synthesized in good yields with high ee values of 98% and 83%, respectively, by the rhodium-catalyzed enantioselective intramolecular sequential cyclotrimerizations of the corresponding cyclic polyynes. The observed high enantioselectivity arises from the regioselective formation of a rhodacycle intermediate from an unsymmetric triyne unit. The X-ray crystal structural analysis of the racemic planar chiral zigzag-type [8]CP belt revealed that the uneven molecules mesh with each other to form a one-dimensional columnar packing structure, in which one column contains single enantiomers, giving two types of chiral columns [(S)- and (R)-form columns] arranged alternately. The ring strain of the zigzag-type [8]CP belt was smaller than that of the armchair-type [8]CPP belt despite its smaller ring size, due to the presence of the strain-relieving m-terphenyl moieties. The effect of the number of the benzene rings of the zigzag-type CP belts on absorption and emission peaks was small due to interruption of π-conjugation at the m-phenylene moieties. However, the bending effect on the absolute fluorescence quantum yield as well as absorption and emission peaks was significant. Concerning chiroptical properties, the modest anisotropy dissymmetry factors of ECD and CPL were observed in the [8]CP belt.

The Supramolecular Chemistry of Cycloparaphenylenes and Their Analogs

Cycloparaphenylenes (CPPs) and their analogs have recently attracted much attention due to their aesthetical structures and optoelectronic properties with radial π-conjugation systems. The past 10 years have witnessed a remarkable advancement in CPPs research, from synthetic methodology to optoelectronic investigations. In this present minireview, we highlight the supramolecular chemistry of CPPs and their analogs, mainly focusing on the size-selective encapsulation of fullerenes, endohedral metallofullerenes, and small molecules by these hoop-shaped macrocycles. We will also discuss the assembly of molecular bearings using some belt-persistent tubular cycloarylene molecules and fullerenes, photoinduced electron transfer properties in supramolecular systems containing carbon nanohoop hosts and fullerene guests, as well as the shape recognition properties for structure self-sorting by using dumbbell-shaped dimer of [60]fullerene ligand. Besides, the supramolecular complexes with guest molecules other than fullerenes, such as CPPs themselves, iodine, pyridinium cations, and bowl-shaped corannulene, are also discussed.

The Supramolecular Chemistry of Strained Carbon Nanohoops

Since 1996, a growing number of strained macrocycles, comprising only sp² - or sp-hybridized carbon atoms within the ring, have become synthetically accessible, with the [n]cycloparaphenyleneacetylenes (CPPAs) and the [n]cycloparaphenylenes (CPPs) being the most prominent examples. Now that robust and relatively general synthetic routes toward a diverse range of nanohoop structures have become available, the research focus is beginning to shift towards the exploration of their properties and applications. From a supramolecular chemistry perspective, these macrocycles offer unique opportunities as a result of their near-perfect circular shape, the unusually high degree of shape-persistence, and the presence of both convex and concave π-faces. In this Minireview, we give an overview on the use of strained carbon-rich nanohoops in host-guest chemistry, the preparation of mechanically interlocked architectures, and crystal engineering.

Retarded Solid-State Rotations of an Oval-Shaped Guest in a Deformed Cylinder with CH-π Arrays

Upon encapsulating an oval-shaped hydrocarbon guest, a cylindrical host deforms its shape to maximize intermolecular contacts. Structure-assembly relationship studies with a series of hydrocarbon guests disclosed the importance of molecular shapes and CH-π contacts. Multiple contacts and weak CH-π hydrogen bonds resulted in an optimal assembly; however, the shape deformation resulted in severe retardation of rotational motions in the crystal. Thus, unlike a circular guest, the oval-shaped guest did not change its orientation in the host. Unexpectedly, the planar guest did not affect the packing structure to form a double helix in intertwined host arrays.

Unbiased Rotational Motions of an Ellipsoidal Guest in a Tight Yet Pliable Host

In the design of machinery such as steel bearings, a fundamental understanding of material characteristics provides an indispensable basis for the design. Although hydrocarbon cycloarylenes have started to be used for providing unique supramolecular bearings with anomalous dynamic behaviors, their fundamental understanding is immature. A unique property of the cycloarylene host is now reported: the cyclic host is so pliable that it tracks the orientational changes of the ellipsoidal guest, that is, C₇₀ fullerene. Unique structures of the complex were revealed by spectroscopic and crystallographic analyses, and additional theoretical investigations deepened our understanding by revealing the structural changes associated with unbiased rotational motions.

Nuclear spin blockade of laser ignition of intramolecular rotation in the model boron rotor B 13 + 11

The boron rotor B 13 + 11 consists of a tri-atomic inner "wheel" that may rotate in its pseudo-rotating ten-atomic outer "bearing"-this concerted motion is called "contorsion." B 13 + 11 in its ground state has zero contorsional angular momentum. Starting from this initial state, it is a challenge to ignite contorsion by a laser pulse. We discover, however, that this is impossible, i.e., one cannot design any laser pulse that induces a transition from the ground to excited states with non-zero contorsional angular momentum. The reason is that the ground state is characterized by a specific combination of irreducible representations (IRREPs) of its contorsional and nuclear spin wavefunctions. Laser pulses conserve these IRREPs because hypothetical changes of the IRREPs would require nuclear spin flips that cannot be realized during the interaction with the laser pulse. We show that all excited target states of B 13 + 11 with non-zero contorsional angular momentum have different IRREPs that are inaccessible by laser pulses. Conservation of nuclear spins thus prohibits laser-induced transitions from the non-rotating ground to rotating target states. We discover various additional constraints imposed by conservation of nuclear spins, e.g., laser pulses can change clockwise to counter-clockwise contorsions or vice versa, but they cannot stop them. The results are derived in the frame of a simple model.

Concyclic CH-π arrays for single-axis rotations of a bowl in a tube

The hydrogen bond is undoubtedly one of the most important non-covalent interactions. Among the several types of the hydrogen bonds, the CH-π interaction is a relatively new notion that is being recognised in chemistry and biology. Although the CH-π hydrogen bond and conventional hydrogen bonds share common features such as directionality, this weak interaction has played a secondary role in molecular recognition. In this study, we have devised a host-guest complex that is assembled solely by the CH-π hydrogen bonds. Multivalent interactions of a bowl-shaped hydrocarbon with its peripheral hydrogen atoms are made possible via CH-π hydrogen bonds by adopting a tubular hydrocarbon as a host for their enthalpy-driven complexation. Concyclic arrays of weak hydrogen bonds further allow dynamic rotational motions of the guest in the host. Solid-state analysis with crystallographic and spectroscopic methods reveal a single-axis rotation of the bowl in the tube.

Ratchet-free solid-state inertial rotation of a guest ball in a tight tubular host

Dynamics of molecules in the solid state holds promise for connecting molecular behaviors with properties of bulk materials. Solid-state dynamics of [60]fullerene (C₆₀) is controlled by intimate intermolecular contacts and results in restricted motions of a ratchet phase at low temperatures. Manipulation of the solid-state dynamics of fullerene molecules is thus an interesting yet challenging problem. Here we show that a tubular host for C₆₀ liberates the solid-state dynamics of the guest from the motional restrictions. Although the intermolecular contacts between the host and C₆₀ were present to enable a tight association with a large energy gain of –14 kcal mol^–1, the dynamic rotations of C₆₀ were simultaneously enabled by a small energy barrier of +2 kcal mol^–1 for the reorientation. The solid-state rotational motions reached a non-Brownian, inertial regime with an extremely rapid rotational frequency of 213 GHz at 335 K.

Though dynamics of molecules are generally restricted by intermolecular contacts, C₆₀ fullerene is able to rotate freely despite being tightly bound inside a molecular host. Here, the authors study the solid-state dynamics of this host-guest system to understand the anomalous relationship between tight association and low friction.

Chiral intertwined spirals and magnetic transition dipole moments dictated by cylinder helicity

The presence of anomalous chirality in a roll of graphitic carbon sheets has been recognized since the discovery of carbon nanotubes, which are becoming available in higher quantities through the isolation of chiral single-wall congeners with high purity. Exploration of the properties arising from cylinder chirality is expected to expand the scope of tubular entities in the future. By studying molecular fragments of helical carbon nanotubes, we herein reveal interesting properties that arise from this chirality. The chirality of nanoscale cylinders resulted in chirality of larger dimensions in the form of a double-helix assembly. Cylinder chirality in solution gave rise to a large dissymmetry factor of metal-free entities in circular polarized luminescence. Theoretical investigations revealed the pivotal role of cylindrical shapes in enhancing magnetic dipole transition moments to yield extreme rotatory strength. Unique effects of cylinder chirality in this study may prompt the development of tubular entities, for instance, toward chiroptical applications.

Assembly, Thermodynamics, and Structure of a Two-Wheeled Composite of a Dumbbell-Shaped Molecule and Cylindrical Molecules with Different Edges

A carbonaceous dumbbell was able to spontaneously glue two tubular receptors to form a unique two-wheeled composite through van der Waals interactions, thus forcing the wheel components into contact with each other at the edges. In the present study, two tubular receptors with enantiomeric carbon networks were assembled on the dumbbell joint, and the handedness of the receptors was discriminated, thus leading to the self-sorting of homomeric receptors from a mixture of enantiomeric tubes. The crystal structures of the composites revealed the structural origins of the molecular recognition driven by van der Waals forces as well as the presence of a columnar array of C₁₂₀ molecules in a 1:1 composite.

Oxatub[5,6]arene: synthesis, conformational analysis, and the recognition of C60 and C70

Large oxatub[n]arenes (n = 5, 6) have been synthesized and characterized. Their conformational complexity was analyzed and host-guest chemistry was studied. In particular, the rather flexible oxatub[6]arene is able to accommodate C60 and C70 with moderate binding affinities. Computational results suggest that there are different binding affinities to C60 or C70 for each conformer of oxatub[6]arene. This endows oxatub[6]arene with conformational adaptability according to the needs of its guests.

Chemical Synthesis of Cycloparaphenylenes

Cycloparaphenylenes and analogues thereof are substances having excellent structural and electronic properties due to radial π-conjugation modes and porous structures. Since they are partial structures of carbon nanotubes, they have also attracted attention as a template for carbon nanotube synthesis. In this chapter, we introduce a series of research on the synthesis of cycloparaphenylenes and their analogues.

A universal mechanism of the planar boron rotors B11-, B13+, B15+, and B19-: inner wheels rotating in pseudo-rotating outer bearings

Planar boron clusters B₁₁^-, B₁₃⁺, B₁₅⁺, and B₁₉^- have been introduced recently as molecular Wankel motors or tank treads. Here we present a universal mechanism for these dynamically fluxional clusters; that is, they are molecular rotors with inner wheels that rotate almost freely in pseudo-rotating outer bearings, analogous to rotating molecules trapped in pseudo-rotating cages. This mechanism has significant quantum mechanical consequences: the global-minimum structures of the clusters have C_2v symmetry, whereas the wheels rotating in pseudo-rotating bearings generate rosette-type shapes with D_9h, D_10h, D_11h, and D_13h symmetries. The related rotational/pseudo-rotational energies appear with characteristic band structures, effecting the dynamics.

Self-Sorting of Two Hydrocarbon Receptors with One Carbonaceous Ligand

Non-directional van der Waals forces in biological and synthetic supramolecular systems play important roles in molecular assembly, particularly in determining the distances of the interacting species. The van der Waals forces are normally used in combination with other directional forces and are considered to play a secondary role in achieving specificity and fidelity in molecular recognition. Using an ideal supramolecular system consisting solely of hydrogen and carbon atoms, we found that the van der Waals interactions enable the high-fidelity sorting of two homomeric receptors during ligand-induced assembly. The self-sorting occurred in a narcissistic manner by repulsion of a competing diastereoisomeric receptor from the assembly. The structure-sorting relationship study with enantiomers further revealed the dominant role of the van der Waals forces in shape recognition for high-fidelity self-sorting.

Study of Potential Energy Surfaces towards Global Reaction Route Mapping

The potential energy surface (PES) is just a theoretical construct based on the Born-Oppenheimer approximation, but it underlies various phenomena, including molecular vibrations, collisional ionizations, and chemical reactions. This account describes how a new idea for global reaction route mapping (GRRM), which had seemed to be impossible for chemical systems with more than three atoms, was born and has been developed during the course of the study of the PES. GRRM has pioneered new fields of chemistry. Furthermore, techniques for GRRM are still developing, and GRRM is further extending its application to various areas of chemistry and chemical physics.

Reply to the 'Comment on "Theoretical studies on a carbonaceous molecular bearing: association thermodynamics and dual-mode rolling dynamics"' by E. M. Cabaleiro-Lago, J. Rodriguez-Otero and A. Gil, Chem. Sci., 2016, , DOI: 10.1039/C5SC04676A

We reply to the comments raised by Cabaleiro-Lago et al. on our article (H. Isobe et al., Chem. Sci., 2015, 6, 2746-2753). Here we appreciate the common views we share and welcome the clarity this gives, and we discuss and question some of their criticisms of the original piece and clarify our opinion in this area. The most important answer can be found in the equation, ΔG = ΔH - TΔS, derived from one of the best established fields in physics.

Comment on "Theoretical studies on a carbonaceous molecular bearing: association thermodynamics and dual-mode rolling dynamics" by H. Isobe, K. Nakamura, S. Hitosugi, S. Sato, H. Tokoyama, H. Yamakado, K. Ohno and H. Kono, Chem. Sci., 2015, , 2746

The LC-BLYP functional accompanied with proper calculations leads to unreliable results for systems governed by π···π interactions. It seems quite clear that a good representation of dispersion interactions is required, so DFT must be supplemented (through the DFT-D formalism or the many-body dispersion method) in order to afford good results.

Cycloparaphenylene as a molecular porous carbon solid with uniform pores exhibiting adsorption-induced softness

The molecular carbon nanoring, cycloparaphenylene (CPP), is fascinating as a new class of carbonaceous porous solids with the uniform structure of an all-benzene surface.

The molecular carbon nanoring, cycloparaphenylene (CPP), is fascinating as a new class of carbonaceous porous solids with the uniform structure of an all-benzene surface. We explored the feasibility of [12]CPP as a carbon-based porous material and uncovered its unique adsorption properties due to its shape and highly nonpolar surface. Unlike other porous carbon solids, [12]CPP shows stepwise adsorption behaviors sensitive to the functionalities of the guest molecules. In situ powder X-ray diffraction and infrared spectra provided insights into how [12]CPP accommodates the guest molecules with structural deformation retaining its structural periodicity during the whole adsorption process, which exemplifies that this molecular nanoring represents an unprecedented carbon-based soft porous solid.

Organic nanotubes and belt shaped molecules based on norbornadiene tethers

One dimensional materials based on norbornadiene tethers showed outstanding electronic properties and can host large fullerenes with high affinity.

Highlights from the 51st EUCHEM conference on stereochemistry, Bürgenstock, Switzerland, May 2016

On Sunday 1st May 2016, 104 chemists made their way to the picturesque town of Brunnen, located on the banks of the Vierwalstättersee (Lake Lucerne) to participate in the 51st EUCHEM conference on stereochemistry.

Modulation of Energy Conversion Processes in Carbonaceous Molecular Bearings

The energetics and photodynamics of carbonaceous molecular bearings with discrete molecular structures were investigated. A series of supramolecular bearings comprising belt-persistent tubular cycloarylene and fullerene molecules accepted photonic stimuli to afford charge-separated species via a photoinduced electron transfer process. The energy conversion processes associated with the photoexcitation, however, differed depending on the molecular structure. A π-lengthened tubular molecule allowed for the emergence of an intermediary triplet excited state at the bearing, which should lead to an energy conversion to thermal energy. On the other hand, low-lying charge-separated species induced by an endohedral lithium ion in fullerene enabled back electron transfer processes to occur without involving triplet excited species. The structure-photodynamics relationship was analyzed in terms of the Marcus theory to reveal a large electronic coupling in this dynamic supramolecular system.

Cycloparaphenylenes and related nanohoops

The first synthesis of a cyclic oligophenylene possessing a radial π system was reported in 2008. In the short period that has elapsed since, there has been an ever-increasing level of interest in molecules of this type, as evidenced by the volume of publications in this area. This interest has been driven by the highly unusual properties of these molecules in comparison to their linear oligoarene analogues, as well as the diverse array of potential applications for them. Notably, CPPs and related structures were proposed as viable templates for the bottom-up synthesis of single-walled carbon nanotubes (SWCNTs), a proposition which has recently been realised. This review gives a comprehensive and strictly chronological (by date of first online publication) treatment of literature reports from the inception of the field, with emphasis on both synthesis and properties of CPPs and related nanohoops. (The scope of this review is restricted to molecules possessing a radial π system consisting entirely of subunits which are aromatic in isolation, e.g. CPPs, but not cycloparaphenyleneacetylenes or cyclopolyacetylenes).