Inversion Barrier of Corannulene. A Benchmark for Bowl-to-Bowl Inversions in Fullerene Fragments

Nanohoops in membranes: confined supramolecular spaces within phospholipid bilayer membranes

Nanohoops, an exciting class of fluorophores with supramolecular binding abilities, have the potential to become innovative tools within biological imaging and sensing. Given the biological importance of cell membranes, incorporation of macrocyclic materials with the dual capability of fluorescence emission and supramolecular complexation would be particularly interesting. A series of different-sized nanohoops-ethylene glycol-decorated [n]cyclo-para-pyrenylenes (CPYs) (n = 4-8)-were synthesised via an alternate synthetic route which implements a stannylation-based precursor, producing purer material than the previous borylation approach, enabling the growth of single-crystals of the Pt-macrocycle. Reductive elimination of these single-crystals achieved significantly higher selectivity and yields towards smaller ring-sized nanohoops (n = 4-6). The supramolecular binding capabilities of these CPYs were then explored through host-guest studies with a series of polycyclic (aromatic)hydrocarbons, revealing the importance of molecular size, shape, and CH-π contacts for efficient binding. CPYs were incorporated within the hydrophobic layer of lipid bilayer membranes, as confirmed by microscopic imaging and emission spectroscopy, which also demonstrated the size-preferential incorporation of the five-fold nanohoop. Molecular dynamics simulations revealed the position and orientation within the membrane, as well as the unique non-covalent threading interaction between nanohoop and phospholipid.

Modular Synthesis of a Semibuckminsterfullerene

A convergent synthesis of dibenzochrysenes and diindenochrysenes that proceeds from difluorofluorenes and acetoxyenone 15 has been used to prepare 5,6,11,12-tetrabromosemibuckminsterfullerene (31). The synthesis is highly modular and is distinguished by proceeding through an unsymmetrical intermediate. Our work will enable the straightforward preparation of semibuckminsterfullerenes from diindenochrysenes that lack bilateral symmetry using common reagents and nonpyrolytic conditions.

An Unrevealed Molecular Function of Corannulene Buckybowl Glycoconjugates in Selective Tumor Annihilation by Targeting the Cancer-Specific Warburg Effect

The biomedical application of corannulene π-bowls is historically limited by low solubility and bioavailability despite the potential in their unique electronic properties for new functional materials. Herein, the unexpected role and molecular mechanism of Corranulene π-bowls are uncovered in biomedical applications as an effective anticancer agent for Warburg effect mediated selective tumor targeting. The corannulene triazolyl monosaccharides Cor-sugars exhibit highly potent cytotoxicity against human cancer cells and effectively inhibit xenograft growth of hyperglycolytic tumors. Particularly, the galactose-conjugated Cor-gal exhibits superior in vivo anticancer efficacy in A549 tumor models with outstanding safety profile compared to doxorubicin. Moreover, the combined treatment of Cor-gal with immune checkpoint inhibitor results in an effective synergy in treating H460 human lung carcinoma. An uptake mechanism study reveals that Cor-sugars exploit tumor-specific glucose transporter glucose transporter 1 (GLUT1) for targeted cell delivery and intra-tumoral accumulation through the cancer-specific Warburg effect. Their significant anticancer activity is attributed to multiphasic DNA-binding and cell cycle alteration effects. This study uncovers new molecular properties of corannulene buckybowl and enabling their potential new applications in biomedical engineering.

An unprecedented π-electronic circuit involving an odd number of carbon atoms in a grossly warped non-planar nanographene

The formation of π-aromatic circuits along a grossly warped nanographene, C₈₀H₃₀, containing five- and seven-membered rings inserted into a six-membered mesh, reveals global π-circuits at the edge of the backbone.

Curved π-conjugated corannulene dimer diradicaloids

So far, most reported open-shell singlet diradicaloids are based on planar π-conjugated molecules. Herein, we report the bridged corannulene dimer diradicaloids, Cor-D1 and Cor-D2, both showing a three-dimensional curved π-conjugated structure. Cor-D1 has a small diradical character (y0 = 5.4%) and behaves more like a closed-shell quinoidal compound at room temperature, while Cor-D2 is a typical open-shell diradicaloid with a larger diradical character (y0 = 16.9%). Both compounds exhibited paramagnetic activity at elevated temperatures, with a singlet-triplet energy gap (ΔES-T) of -8.4 and -3.0 kcal mol-1, respectively. X-ray crystallographic analysis revealed that both molecules have a dumbbell-shaped geometry, with the two terminal corannulene bowls bent to opposite directions. The spin is largely delocalized onto the two bowls in Cor-D2 and there are multiple [CH···π] interactions between the neighboring bowls. Chemical oxidation/reduction to their respective dications/dianions results in global aromaticity with [4n + 2] π-electrons delocalized through the periphery of the whole framework.

A DFT study on the central-ring doped HBC nanographenes

Nanographenes (NGs) are a segment of graphene whose dangling bonds are saturated with hydrogen atoms, introducing different properties and promising applications. Here we investigate the electronic, thermodynamic, optical, and structural properties of four C₃₆X₃Y₃H₁₈ NGs (X=B, and Al; and Y=N, and P) based on the density functional theory calculations. It was mainly found that 1) BN-NG is planar molecule and the others are buckybowl-shaped ones, 2) The bowl-to-bowl inversion Gibbs free energies (ΔG^#) of buckybowl shaped NGs are very huge and the rate constant is very small, hindering the inversion, 3) The relative energetic stability order based on the standard enthalpy of formation (ΔH_f^°) is as BN>AlN>BP>AlP, which the BN, and AlN doped NGs are stable at room temperature but the BP and AlP doped ones are instable, 4) The electrical conductivity order of magnitude is inverse of that of stability, 5) An exciton binding energy is predicted in the range of 0.57-0.75eV for the NGs which corresponds to Frenkel exciton type, 6) the NGs are not soluble in organic solvent in agreement with the experimental results and is partially soluble in water solvent with Gibbs free energy of solvation (ΔG_solv) in the range of -6.1 to -10.1kcal/mol.

The INV24 test set: how well do quantum-chemical methods describe inversion and racemization barriers?

For years, there has been ongoing interest in experimentally and theoretically understanding inversion and racemization processes. However, to the best of our knowledge, there has been no computational study that systematically investigated how well low-level quantum-chemical methods predict inversion barriers. Herein, we provide an answer to this question and we present the INV24 benchmark set of 24 high-level, ab initio inversion barriers. INV24 covers inversion in triatomics, in pyramidal molecules, in one cyclic system, and in various helical and bowl-shaped compounds. Our results indicate that previously applied DFT approximations combined with small basis sets are not reliable enough and that at least a triple-ζ basis is needed for meaningful results. Moreover, we show that intramolecular London dispersion influences the barriers by 2 kcal/mol or more and that dispersion corrections should always be applied to DFT results. With our analysis of 34 DFT approximations, we can reproduce the well-known Jacob’s Ladder scheme with (meta-)generalized-gradient-approximation methods underestimating barriers and global-hybrid DFT functionals performing better. Range-separated hybrids or Minnesota-type hybrids are not particularly superior to more conventional methods, such as B3LYP-D3. The by far best results are achieved with dispersion-corrected double hybrids, which give results below the chemical accuracy target of 1 kcal/mol. They also outperform wave-function second-order perturbation theory approaches and we recommend using them whenever possible. Given that our systematic study of INV24 is the first of its kind, our findings have the potential to change common practice in this field and they will guide future investigations of inversion processes.

Flexibility Coexists with Shape-Persistence in Cyanostar Macrocycles

Shape-persistent macrocycles are attractive functional targets for synthesis, molecular recognition, and hierarchical self-assembly. Such macrocycles are noncollapsible and geometrically well-defined, and they are traditionally characterized by having repeat units and low conformational flexibility. Here, we find it necessary to refine these ideas in the face of highly flexible yet shape-persistent macrocycles. A molecule is shape-persistent if it has a small change in shape when perturbed by external stimuli (e.g., heat, light, and redox chemistry). In support of this idea, we provide the first examination of the relationships between a macrocycle's shape persistence, its conformational space, and the resulting functions. We do this with a star-shaped macrocycle called cyanostar that is flexible as well as being shape-persistent. We employed molecular dynamics (MD), density functional theory (DFT), and NMR experiments. Considering a thermal bath as a stimulus, we found a single macrocycle has 332 accessible conformers with olefins undergoing rapid interconversion by up-down and in-out motions on short time scales (0.2 ns). These many interconverting conformations classify single cyanostars as flexible. To determine and confirm that cyanostars are shape-persistent, we show that they have a high 87% shape similarity across these conformations. To further test the idea, we use the binding of diglyme to the single macrocycle as guest-induced stimulation. This guest has almost no effect on the conformational space. However, formation of a 2:1 sandwich complex involving two macrocycles enhances rigidity and dramatically shifts the conformer distribution toward perfect bowls. Overall, the present study expands the scope of shape-persistent macrocycles to include flexible macrocycles if, and only if, their conformers have similar shapes.

Induced-fit catalysis of corannulene bowl-to-bowl inversion

Stereoelectronic complementarity between the active site of an enzyme and the transition state of a reaction is one of the tenets of enzyme catalysis. This report illustrates the principles of enzyme catalysis (first proposed by Pauling and Jencks) through a well-defined model system that has been fully characterized crystallographically, computationally and kinetically. Catalysis of the bowl-to-bowl inversion processes that pertain to corannulene is achieved by combining ground-state destabilization and transition-state stabilization within the cavity of an extended tetracationic cyclophane. This synthetic receptor fulfils a role reminiscent of a catalytic antibody by stabilizing the planar transition state for the bowl-to-bowl inversion of (ethyl)corannulene (which accelerates this process by a factor of ten at room temperature) by an induced-fit mechanism first formulated by Koshland.

Chemical mimicry of viral capsid self-assembly

Stable structures of icosahedral symmetry can serve numerous functional roles, including chemical microencapsulation and delivery of drugs and biomolecules, epitope presentation to allow for an efficient immunization process, synthesis of nanoparticles of uniform size, observation of encapsulated reactive intermediates, formation of structural elements for supramolecular constructs, and molecular computing. By examining physical models of spherical virus assembly we have arrived at a general synthetic strategy for producing chemical capsids at size scales between fullerenes and spherical viruses. Such capsids can be formed by self-assembly from a class of molecules developed from a symmetric pentagonal core. By designing chemical complementarity into the five interface edges of the molecule, we can produce self-assembling stable structures of icosahedral symmetry. We considered three different binding mechanisms: hydrogen bonding, metal binding, and formation of disulfide bonds. These structures can be designed to assemble and disassemble under controlled environmental conditions. We have conducted molecular dynamics simulation on a class of corannulene-based molecules to demonstrate the characteristics of self-assembly and to aid in the design of the molecular subunits. The edge complementarities can be of diverse structure, and they need not reflect the fivefold symmetry of the molecular core. Thus, self-assembling capsids formed from coded subunits can serve as addressable nanocontainers or custom-made structural elements.

Density Functional Theory Study on the Effect of Substitution and Ring Annelation to the Rim of Corannulene

B3LYP/6-311G calculations indicate that annelation of a five-membered ring to the rim of corannulene and substitution to all the rim carbons lowers the barrier for bowl-to-bowl inversion. Singlet-triplet energy differences, frontier orbital analysis, and nucleus-independent chemical shift (NICS) values indicate significant enhancement of the reactivity when the substitutions involve exocyclic double bonds. Bowl-to-bowl inversion barrier, curvature, and reactivity for unsaturated and saturated five-membered ring-annelated corannulenes are analogous to decamethyl- and decamethylene-substituted corannulenes.

Heterobuckybowls: a theoretical study on the structure, bowl-to-bowl inversion barrier, bond length alternation, structure-inversion barrier relationship, stability, and synthetic feasibility

Hybrid density functional theory (DFT) calculations at the B3LYP/cc-pVDZ level have been performed on a series of heterobuckybowls, 3X, C(18)X(3)H(6) (X = O, NH, CH(2), BH, S, PH, PH(3), Si, SiH(2), and AlH). The minimum energy conformations and the transition states for bowl-to-bowl inversion, where the geometry is bowl shaped, are computed and characterized by frequency calculations. The geometries of heterotrindenes, 2X, C(12)X(3)H(6) (X = O, NH, CH(2), BH, S, PH, PH(3), Si, SiH(2), and AlH), were obtained, and the bond length alternation (Delta) in the central benzenoid ring shows remarkable sensitivity as a function of substituent with a wide range of fluctuations (-0.014 to +0.092 A). The Delta computed in 2BH was found to be comparable with the highest bond alternation reported to date in benzenoid frameworks. The inversion dynamics of these heterobowls and their bowl depths were fit to a mixed quartic/quadratic function. The size of the heteroatom seems to exclusively control the bowl depth and rigidity as well as the synthetic feasibility. In contrast, the bond length alternation seems to be controlled by electronic factors and not by the size of the substituted atom either in trindenes or in heterosumanenes. The thermodynamic stability of this class of compounds is very much comparable with trithiasumanene (3S), which has been synthesized recently. The chemical hardness (eta) was measured to assess the stability of the heterosumanenes. The strain energy buildup in a sequential ring closure strategy along two synthetic routes, namely a triphenylene route and a trindene route, were explored, and the trindene route was found to be highly favorable for making such compounds compared to the triphenylene route. However, in both routes the ease of the synthetic feasibility increases as the size of the heteroatom increases.

Corannulene as a Lewis base: computational modeling of protonation and lithium cation binding

A computational modeling of the protonation of corannulene at B3LYP/6-311G(d,p)//B3LYP/6-311G(d,p) and of the binding of lithium cations to corannulene at B3LYP/6-311G(d,p)//B3LYP/6-31G(d,p) has been performed. A proton attaches preferentially to one carbon atom, forming a sigma-complex. The isomer protonated at the innermost (hub) carbon has the best total energy. Protonation at the outermost (rim) carbon and at the intermediate (bridgehead rim) carbon is less favorable by ca. 2 and 14 kcal mol(-)(1), respectively. Hydrogen-bridged isomers are transition states between the sigma-complexes; the corresponding activation energies vary from 10 to 26 kcal mol(-)(1). With an empirical correction obtained from calculations on benzene, naphthalene, and azulene, the best estimate for the proton affinity of corannulene is 203 kcal mol(-)(1). The lithium cation positions itself preferentially over a ring. There is a small energetic preference for the 6-ring over the 5-ring binding (up to 2 kcal mol(-)(1)) and of the convex face over the concave face (3-5 kcal mol(-)(1)). The Li-bridged complexes are transition states between the pi-face complexes. Movement of the Li(+) cation over either face is facile, and the activation energy does not exceed 6 kcal mol(-)(1) on the convex face and 2.2 kcal mol(-)(1) on the concave face. In contrast, the transition of Li(+) around the corannulene edge involves a high activation barrier (24 kcal mol(-)(1) with respect to the lowest energy pi-face complex). An easier concave/convex transformation and vice versa is the bowl-to-bowl inversion with an activation energy of 7-12 kcal mol(-)(1). The computed binding energy of Li(+) to corannulene is 44 kcal mol(-)(1). Calculations of the (7)Li NMR chemical shifts and nuclear independent chemical shifts (NICS) have been performed to analyze the aromaticity of the corannulene rings and its changes upon protonation.