Unidirectional Molecular Stacking of Tribenzotriquinacenes in the Solid State: A Combined X-ray and Theoretical Study

exo-6b2-Methyl-Substituted Pentabenzocorannulene: Synthesis, Structural Analysis, and Properties

exo-6b2-Methyl-substituted pentabenzocorannulene (exo-PBC-Me) was synthesized by the palladium-catalyzed cyclization of 1,2,3-triaryl-1H-cyclopenta[l]phenanthrene. Its bowl-shaped geometry with an sp3 carbon atom in the backbone and a methyl group located at the convex (exo) face was verified by X-ray crystallography. According to DFT calculations, the observed conformer is energetically more favorable than the endo one by 39.9 kcal/mol. Compared to the nitrogen-doped analogs with intact π-conjugated backbones (see the main text), exo-PBC-Me displayed a deeper bowl depth (avg. 1.93 Å), redshifted and broader absorption (250-620 nm) and emission (from 585 to more than 850 nm) bands and a smaller optical HOMO-LUMO gap (2.01 eV). exo-PBC-Me formed polar crystals where all bowl-in-bowl stacking with close π ⋅ ⋅ ⋅ π contacts is arranged unidirectionally, providing the potential for applications as organic semiconductors and pyroelectric materials. This unusual structural feature, molecular packing, and properties are most likely associated with the assistance of the methyl group and the sp3 carbon atom in the backbone.

Centrohexaindane, a Unique Polyaromatic Hydrocarbon Bearing the Rare Cq (Cq )4 Core: Inelastic Neutron Scattering, Infrared and Raman Spectroscopy

The structure and vibrational spectroscopy of centrohexaindane, 1, was investigated. This unusual molecule has a quaternary carbon atom that is coordinated to four further such quaternary carbon atoms as its core, each pair of which is bonded to an ortho-phenylene unit. Previous NMR studies have shown that the molecule has tetrahedral (Td ) symmetry in solution. The infrared and Raman spectra of chloroform and deuterochloroform solutions of 1 are completely in agreement with this conclusion, as the only modes that are visible are those allowed for Td symmetry. This is not the case in the solid state: X-ray powder diffraction indicates that the unit cell is triclinic or monoclinic with a volume in excess of 4000 Å3 . The vibrational spectroscopy is consistent with C1 site symmetry and the presence of at least two molecules in the primitive cell. It is likely that the space group is centrosymmetric.

Controlled Formation of Porous 2D Lattices from C3 -symmetric Ph6 -Me-Tribenzotriquinacene-OAc3

The on-surface self-assembly of molecules to form holey nanographenes is a promising approach to control the properties of the resulting 2D lattice. Usually, planar molecules are utilized to prepare flat, structurally confined molecular layers, with only a few recent examples of warped precursors. However, control of the superstructures is limited thus far. Herein, we report the temperature-controlled self-assembly of a bowl-shaped, acetylated C₃ -symmetric hexaphenyltribenzotriquinacene derivative on Cu(111). Combining scanning tunneling microscopy (STM) and density functional theory (DFT) confirms the formation of highly differing arrangements starting with π-stacked bowl-to-bowl dimers at low coverage at room temperature via chiral honeycomb structures, an intermediate trigonal superstructure, followed by a fully carbon-based, flattened hexagonal superstructure formed by on-surface deacetylation, which is proposed as a precursor for holey graphene networks with unique defect structures.

Donor-Acceptor Tribenzotriquinacene-Based Molecular Wizard Hats Bearing Three ortho-Benzoquinone Units

Two π-extended bay-bridged tribenzotriquinacenes ("TBTQ wizard hats") 12 and 16 bearing three mutually conjugated, alternating veratrole-type and ortho-benzoquinone units were synthesized. The electronic properties of these complementarily arranged, nonplanar push-pull systems are affected by the fusion with the rigid, C₃ -symmetric TBTQ core to a different extent, as revealed by X-ray structural analysis, UV-vis spectroscopy and cyclovoltammetry. The combination of three quinone units within the original TBTQ core and three veratrole-type bay bridging units in 12 gives rise to a more efficiently π-conjugated chromophore, as reflected by the shallower shape of wizard hat and its absorption in the visible up to 750 nm in comparison to 16. Congener 12 contains an aromatic 18-π electron system in contrast to the cross-conjugated analog 16. X-ray structure analysis of the precursor dodecaether 15 revealed the formation of a cage-like supramolecular dimer, in which the peripheral dioxane-type ether groups interlace by twelve noncovalent C-H⋅⋅⋅⋅⋅O bonds.

Regioconvergent Synthesis of a π-Extended Tribenzotriquinacene-Based Wizard Hat-Shaped Nanographene

The successful enlargement of the curved π-electron periphery of the wizard hat-shaped polycyclic aromatic compound 1 is described. The target structure 2 features an m,m,p,m,m,p,m,m,p-nonaphenylene belt fused to a central tribenzotriquinacene unit. The synthesis involves a multiple regioconvergent Scholl-type dehydrocyclization as the key step. Spectroscopic, structural, and electronic properties of the title compound 2 are reported.

Spectroscopic characterisation of centropolyindanes

A highly promising class of three-dimensional polyaromatic hydrocarbons comprises the centropolyindanes. The characteristic feature of these compounds is the mutual fusion of several molecules of indane along the saturated C-C bonds of their cyclopentane rings. Among the polycyclic aromatic hydrocarbons, the centropolyindanes are special because of the saturated core of sp3-hybridised carbon atoms embedded in a three-dimensional environment of aromatic building blocks. While the centropolyindanes and their numerous derivatives have been studied in detail by NMR spectroscopy, mass spectrometry and X-ray diffraction, investigation of their vibrational features, and especially those of the neopentane core present in most cases, have not been performed so far. In the present paper, we report the first systematic study of a set of centropolyindanes by vibrational spectroscopy, using inelastic neutron scattering (INS), infrared and Raman spectroscopies.

Simplified DFT methods for consistent structures and energies of large systems

Kohn-Sham density functional theory (DFT) is routinely used for the fast electronic structure computation of large systems and will most likely continue to be the method of choice for the generation of reliable geometries in the foreseeable future. Here, we present a hierarchy of simplified DFT methods designed for consistent structures and non-covalent interactions of large systems with particular focus on molecular crystals. The covered methods are a minimal basis set Hartree-Fock (HF-3c), a small basis set screened exchange hybrid functional (HSE-3c), and a generalized gradient approximated functional evaluated in a medium-sized basis set (B97-3c), all augmented with semi-classical correction potentials. We give an overview on the methods design, a comprehensive evaluation on established benchmark sets for geometries and lattice energies of molecular crystals, and highlight some realistic applications on large organic crystals with several hundreds of atoms in the primitive unit cell.

Stereoselective synthesis of enantiomerically pure bowl-shaped hydroxytribenzotriquinacenes

A bowl-shaped (M)-2-hydroxytribenzotriquinacene was obtained in enantiomerically pure form by stereoselective enzyme-catalysed ester hydrolysis of the racemic TBTQ-based aryl acetate.

Single functionalization of fenestrindane and centrohexaindane at the molecular periphery

The single functionalization of the parent centropolyindane hydrocarbons fenestrindane (2) and centrohexaindane (3) at the molecular arene periphery has been studied. The monoformylation of 2 and 3 using the Rieche method resulted in the corresponding aldehydes, 7 and 10, in 23% and 35% yield, respectively. The Friedel-Crafts acetylation of 2 furnished 2-acetylfenestrindane (8) in 53% yield, and the Baeyer-Villiger oxidation of 7 followed by hydrolysis resulted in the 2-hydroxyfenestrindane (9) in 72% yield. The results show that electrophilic attack at one of the eight or, respectively, even 12 equivalent positions of 2 and 3 is a viable method for the monofunctionalization of these polycyclic aromatic hydrocarbons.

Centrohexaindane: six benzene rings mutually fixed in three dimensions - solid-state structure and six-fold nitration

The solid-state molecular structure of centrohexaindane (), a unique hydrocarbon comprising six benzene rings clamped to each other in three dimensions around a neopentane core, and the molecular packing in crystals of ·CHCl3 are reported. The molecular Td-symmetry and the Cartesian orientation of the six indane wings of in the solid state have been confirmed. The course and limitation of electrophilic aromatic substitution of are demonstrated for the case of nitration. Based on nitration experiments of a lower congener of , tribenzotriquinacene , the six-fold nitrofunctionalisation of has been achieved in excellent yield, giving four constitutional isomers, two nonsymmetrical ( and ) and two C3-symmetrical ones ( and ), all of which contain one single nitro group in each of the six benzene rings. The relative yields of the four isomers (∼3 : 1 : 1 : 3) point to a random electrophilic attack of the electrophiles at the twelve formally equivalent outer positions of the aromatic periphery of , suggesting electronic independence of its six aromatic π-electron systems. In turn, the pronounced conformational rigidity of the centrohexacyclic framework of enables the unequivocal structural identification of the isomeric hexanitrocentrohexaindanes by (1)H NMR spectroscopy.

Consistent structures and interactions by density functional theory with small atomic orbital basis sets

A density functional theory (DFT) based composite electronic structure approach is proposed to efficiently compute structures and interaction energies in large chemical systems. It is based on the well-known and numerically robust Perdew-Burke-Ernzerhoff (PBE) generalized-gradient-approximation in a modified global hybrid functional with a relatively large amount of non-local Fock-exchange. The orbitals are expanded in Ahlrichs-type valence-double zeta atomic orbital (AO) Gaussian basis sets, which are available for many elements. In order to correct for the basis set superposition error (BSSE) and to account for the important long-range London dispersion effects, our well-established atom-pairwise potentials are used. In the design of the new method, particular attention has been paid to an accurate description of structural parameters in various covalent and non-covalent bonding situations as well as in periodic systems. Together with the recently proposed three-fold corrected (3c) Hartree-Fock method, the new composite scheme (termed PBEh-3c) represents the next member in a hierarchy of "low-cost" electronic structure approaches. They are mainly free of BSSE and account for most interactions in a physically sound and asymptotically correct manner. PBEh-3c yields good results for thermochemical properties in the huge GMTKN30 energy database. Furthermore, the method shows excellent performance for non-covalent interaction energies in small and large complexes. For evaluating its performance on equilibrium structures, a new compilation of standard test sets is suggested. These consist of small (light) molecules, partially flexible, medium-sized organic molecules, molecules comprising heavy main group elements, larger systems with long bonds, 3d-transition metal systems, non-covalently bound complexes (S22 and S66×8 sets), and peptide conformations. For these sets, overall deviations from accurate reference data are smaller than for various other tested DFT methods and reach that of triple-zeta AO basis set second-order perturbation theory (MP2/TZ) level at a tiny fraction of computational effort. Periodic calculations conducted for molecular crystals to test structures (including cell volumes) and sublimation enthalpies indicate very good accuracy competitive to computationally more involved plane-wave based calculations. PBEh-3c can be applied routinely to several hundreds of atoms on a single processor and it is suggested as a robust "high-speed" computational tool in theoretical chemistry and physics.

From Fragmentation to Construction--from Void to Massive: Fascination with Organic Mass Spectrometry and the Synthesis of Novel Three-Dimensional Polycyclic Aromatic Hydrocarbons

Detailed insights gained from our research into the gas-phase chemistry of ionized and protonated diphenylalkanes and their congeners, obtained by extended synthesis of isotopically labeled model compounds and mass spectrometry, are presented and merged with those acquired during our development of a new family of polycyclic hydrocarbons, the centropolyindanes. Aside from a Personal Account that describes "two scientific lives in one", it is demonstrated, on the one hand, how our understanding of organic chemistry can help to shed light on the details of mass spectrometric fragmentation and to unravel, in a more fundamental way, the unimolecular reactivity of gaseous ions. On the other hand, it is shown how unexpected reactivity of related ions in solution, being subject to the very same fundamentals of organic chemistry, can lead to the construction of novel and, in part, unique three-dimensional polycyclic structures that may contribute to future research in material science. Two such apparently independent fields of organic chemistry may be seen as joint contributions of the art of science.

Monometalated tribenzotriquinacene: exo and endo coordination of sodium and potassium with a rigid bowl-shaped hydrocarbon anion

Monometalated tribenzotriquinacene forms an exo coordinated arrangement with sodium, while potassium is found in an endo position inside the bowl.

Dispersion corrected hartree-fock and density functional theory for organic crystal structure prediction

We present and evaluate dispersion corrected Hartree-Fock (HF) and Density Functional Theory (DFT) based quantum chemical methods for organic crystal structure prediction. The necessity of correcting for missing long-range electron correlation, also known as van der Waals (vdW) interaction, is pointed out and some methodological issues such as inclusion of three-body dispersion terms are discussed. One of the most efficient and widely used methods is the semi-classical dispersion correction D3. Its applicability for the calculation of sublimation energies is investigated for the benchmark set X23 consisting of 23 small organic crystals. For PBE-D3 the mean absolute deviation (MAD) is below the estimated experimental uncertainty of 1.3 kcal/mol. For two larger π-systems, the equilibrium crystal geometry is investigated and very good agreement with experimental data is found. Since these calculations are carried out with huge plane-wave basis sets they are rather time consuming and routinely applicable only to systems with less than about 200 atoms in the unit cell. Aiming at crystal structure prediction, which involves screening of many structures, a pre-sorting with faster methods is mandatory. Small, atom-centered basis sets can speed up the computation significantly but they suffer greatly from basis set errors. We present the recently developed geometrical counterpoise correction gCP. It is a fast semi-empirical method which corrects for most of the inter- and intramolecular basis set superposition error. For HF calculations with nearly minimal basis sets, we additionally correct for short-range basis incompleteness. We combine all three terms in the HF-3c denoted scheme which performs very well for the X23 sublimation energies with an MAD of only 1.5 kcal/mol, which is close to the huge basis set DFT-D3 result.