We discuss the nature of electron-correlation effects in carbon nanorings and nanobelts using an analysis tool known as fractional occupation number weighted electron density (ρ) and the RAS-SF method, revealing for the first time significant differences in static correlation effects depending on how the rings (i.e. chemical units) are fused and/or connected until closing the loop. We choose to study in detail linear and cyclic oligoacene molecules of increasing size, and relate the emerging differences with the difficulties for the synthesis of the latter due to their radicaloid character. We finally explore how minor structural modifications of the cyclic forms can alter these results, showing the potential use of these systems as molecular templates for the growth of well-shaped carbon nanotubes as well as the usefulness of theoretical tools for molecular design.

The scalable production of homogeneous, uniform carbon nanomaterials represents a key synthetic challenge for contemporary organic synthesis as nearly all current fabrication methods provide heterogeneous mixtures of various carbonized products. For carbon nanotubes (CNTs) in particular, the inability to access structures with specific diameters or chiralities severely limits their potential applications. Here, we present a general approach to access solid-state CNT mimic structures via the self-assembly of fluorinated nanohoops, which can be synthesized in a scalable, size-selective fashion. X-ray crystallography reveals that these CNT mimics exhibit uniform channel diameters that are precisely defined by the diameter of their nanohoop constituents, which self-assemble in a tubular fashion via a combination of arene-pefluoroarene and C-H---F interactions. The nanotube-like assembly of these systems results in capabilities such as linear guest alignment and accessible channels, both of which are observed in CNTs but not in the analogous all-hydrocarbon nanohoop systems. Calculations suggest that the organofluorine interactions observed in the crystal structure are indeed critical in the selfassembly and robustness of the CNT mimic systems. This work establishes the self-assembly of carbon nanohoops via weak interactions as an attractive means to generate solid-state materials that mimic carbon nanotubes, importantly with the unparalleled tunability enabled by organic synthesis.

We have recently demonstrated that carbazole-based biradicaloids are promising building blocks in dynamic covalent chemistry. To elucidate their intriguing dynamic covalent chemical properties, it is necessary to understand the physical origin of their biradical nature. To this end, here we focus on two quinoid carbazole systems substituted with dicyanomethylene (DCM) groups via para ( p -Cz-alkyl) or meta positions ( m -Cz-ph), which are able to form cyclophane macrocycles by the formation of long C–C bonds between the bridgehead carbon atoms linked to the DCM groups. We aim at exploring the following questions: (i) How is the biradicaloid character of a quinoid carbazole affected by the substitution position of the DCM groups? (ii) How is the stability of the resulted cyclophane aggregate attained? (iii) How is the dynamic interconversion between the carbazole-based monomers and cyclophane aggregates affected by this subtle change in the substitution pattern position? Density functional theory-based calculations reveal that both p -Cz-alkyl and m -Cz-ph are open-shell biradicals in the ground electronic state, with the DCM substitution in the meta position resulting in a more pronounced biradical character. In contrast, the derivatization via the nitrogen of the carbazole unit is not predicted to affect the biradicaloid character. The spontaneous nature of the cyclophane-based macrocycle formation (i.e., the cyclic tetramer in p -Cz-alkyl and the cyclic trimer and the tetramer in m -Cz-ph) is supported by the negative relative Gibbs free energies calculated at 298 K. Interestingly, cyclic oligomers in which the DCM groups are inserted in the meta position tend to adopt folded conformations with attractive π–π interactions resulting in more stable aggregates; in contrast, note that an extended ring-shaped conformation is acquired for ( p -Cz-alkyl)4. In addition, the larger spin density on the bridgehead carbon atom in the meta-substituted system strengthens the bridging C–C bond in the aggregate forms, hampering its dissociation. In fact, the C–C bond dissociation of ( m -Cz-ph)4 and ( m -Cz-ph)3 was suppressed in solution state, although it was achieved in solid state in response to soft external stimuli (i.e., temperature and grinding). In summary, we report a very comprehensive study aiming at elucidating the challenging chemical properties of carbazole-based biradicaloid systems.

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We systematically investigate the relationships between structural and electronic effects of finite size zigzag or armchair carbon nanotubes of various diameters and lengths, starting from a molecular template of varying shape and diameter, i.e. cyclic oligoacene or oligophenacene molecules, and disclosing how adding layers and/or end-caps (i.e. hemifullerenes) can modify their (poly)radicaloid nature. We mostly used tight-binding and finite-temperature density-based methods, the former providing a simple but intuitive picture about their electronic structure, and the latter dealing effectively with strong correlation effects by relying on a fractional occupation number weighted electron density (ρ FOD ), with additional RAS-SF calculations backing up the latter results. We also explore how minor structural modifications of nanotube end-caps might influence the results, showing that topology, together with the chemical nature of the systems, is pivotal for the understanding of the electronic properties of these and other related systems.

The synthesis of CPPs has been experimentally challenging due to the strain energy needed to close the ring. [7] Vögtle and co-workers [8,9] attempted to synthesize CPPs from macrocycles formed by arene and cyclohexane units to therefore reduce the strain energy of the system. However, the first synthesis of CPPs was afforded only by Jasti et al. [10] in 2008, followed by the first selective synthesis of CPPs by Itami and co-workers [11,12] using a modular approach to build [n]CPPs with n ⩾ 12. Other successfully strategies (Yamago et al. [13] ) use a square-shaped tetra platinum biphenyl complex as an intermediate to selectively synthesize the [8]CPP molecule. In the above mentioned works, it was possible to obtain CPPs in the milligram-scale, with Jasti and co-workers [6,14] producing the first gram-scale synthesis. The smallest CPP synthesized up to date is [5]CPP, [15] which shows the great success achieved so far in overcoming the strain energy.Concerning the supramolecular packing, the solid-state (3D) structure of CPPs has been characterized in several works by means of X-ray diffraction. [16,17] The molecules self-assemble in a herringbone (HB) pattern independently of their size, as it often happens for other π-conjugated systems. An exception is the [6]CPP case, for which three different polymorphs have been found up to date. The first solid-state characterization of [6]CPP revealed a tubular-like structure, [14] but a recent study revealed that the crystallographic structure might depend on the crystallization conditions, also finding a herringbone configuration a few kcal mol −1 more stable than the tubular one. [18] More recently, another polymorph of [6]CPP has been found when the crystallization is carried out by sublimation at 220 °C. In these conditions, no solvent molecules are occluded inside the [6]CPP cavity, and thus, a concave-convex structure appears (i.e., molecules tightly packed in a T-like shape minimizing the internal space inside the cavities). [19] These findings clearly reveal the subtle yet dominant effect of the weak intermolecular interactions driving the supra molecular self-assembly of these systems. Motivated by this, some of us investigated in a previous theoretical work the adsorption energies of [8]CPP on small carbon nanoflakes (circumcirmcumcoronene) and how these through-space weak forces can serve to immobilize the systems in energetically favored configuration. [20] In the present study, we wish to go a step further and employ a tailored force field and molecular dynamics (MD) simulations to predict the supramolecular structure of thin films of CPPs on graphite surfaces. The systematic exploration of the adsorption The nanoscale organization of cycloparaphenylene molecules when physisorbed on a graphite surface is theoretically investigated by means of atomistic molecular dynamics simulations employing a tailored and benchmarked force field. The landing of a single molecule is first considered, to progressively deposit more molecules to finally reach the full coverage...

We theoretically study, by means of dispersion-corrected and cost-effective methods, the strength of non-covalent interactions between cyclic organic nanorings and nano-sized graphene flakes acting as substrates.

We theoretically discuss here the relationships between the structure of recently synthesized nanorings, dubbed as cyclo-2,7-pyrenylene (CPY) and formed upon bending and bonding a finite number of pyrene units until self-cyclation, and a set of chemically relevant properties such as the induced structural and energetical strain, the electronic and optical properties, or the response to charge injection, as well as their transport mechanism through a concerted migration of charge-carriers. We also compare these properties, and their evolution with the number of pyrene-linked units, with those obtained for the closely related cycloparaphenylene (CPP) compounds, trying to disclose the underlying structure-property guidelines. To do it, we always employ dispersion-corrected DFT methods to systematically include the key effects affecting all the properties tackled. A correct match with some available experimental results, for the [4]CPY compound (the only one synthesized so far), anticipates the accuracy of the calculations done for the rest of compounds. Finally, since this kind of systems are envisioned as possible precursors for the fine-tuned and controlled synthesis of carbon nanotubes, we also address the stability of the dimers found in their crystalline structure, and the associated cohesive energy, which may drive the synthesis of the corresponding nanotubes after an adequate dehydrogenation reaction.