Dorin V. Preda

Modeling of Supramolecular Properties of Molecular Tweezers, Clips, and Bowls

Abstract The electrostatic potential surface (EPS) is calculated for molecular tweezers, clips, and bowls at different levels of theory (semiempirical AM1, ab initio HF/6-31G*, and density functional theory pBP/DN**). According to these calculations, the molecular electrostatic potential (MEP) on the concave side of the molecular tweezers and clips is suprisingly negative for hydrocarbons. This finding seems to be a general phenomenon in nonconjugated π-electron systems with concave-convex topology and it explains the receptor properties of the molecular tweezers and clips. Analogous calculations performed for the conjugated aromatic molecular bowls show different results. The DFT calculations predict that in these systems the more negative MEP lies on the concave side similar to the findings for the nonconjugated molecular tweezer- and clip-systems, whereas the AM1 calculation leads to the opposite result that the MEP is more negative on convex side of the bowl-systems.

Single and Double Ionization of Corannulene and Coronene

Electron-transfer processes that involve single and doubly charged cations of corannulene (C20H10) and coronene (C24H12) are examined by three different mass-spectrometric techniques. Photoionization studies give first-ionization energies of IE(C20H10)=7.83±0.02 eV and IE(C24H12)=7.21 ±0.02 eV. Photoionizations of the neutrals to the doubly charged cations occur at thresholds of 20.1±0.2 eV and 18.5±0.2 eV for corannulene and coronene, respectively. Energy-resolved charge-stripping mass spectrometry yields kinetic energy deficits of Qmin(C20H=13.8±0.3 eV and Qmin(C24H=12.8±0.3 eV for the transitions from the mono- to the corresponding dications in keV collisions. Reactivity studies of the C20H and C24H dications in a selected-ion flow-tube mass spectrometer are used to determine the onsets for the occurrence of single-electron transfer from several neutral reagents to the dications, affording two different monocationic products. With decreasing IEs of the neutral reagents, electron transfer to doubly charged corannulene is first observed with hexafluorobenzene (IE=9.91 eV), while neutrals with lower IEs are required in the case of the coronene dication, e.g., NO2 (IE=9.75 eV). Density-functional theory is used to support the interpretation of the experimental data. The best estimates of the ionization energies evaluated are IE(C20H10)=7.83±0.02 eV and IE(C24H12)=7.21 ±0.02 eV for the neutral molecules, and IE(C20H)=12.3±0.2 eV and IE(C24H)=11.3±0.2 eV for the monocations.

Addition of dihalocarbenes to corannulene. A fullerene-type reaction☆

Dihalocarbenes (:CCl2, :CBr2, and :CI2) add preferentially to one of the radial double bonds of corannulene, rather than to the rim. These cyclopropanations strongly resemble the additions of dihalocarbenes to fullerenes, which likewise occur at 6:6-double bonds, destroy the cyclic conjugation in two adjacent benzene rings, and give ‘closed’ adducts. An explanation is offered for the abnormally high reactivity of the interior carbon atoms of corannulene.

Corannulene–Helicene Hybrids: Chiral π-Systems Comprising Both Bowl and Helical Motifs

Two distinct structural elements that render π-systems nonplanar, i.e., geodesic curvature and helical motifs, have been combined into new polyarenes that contain both features. The resultant corannulene-[n]helicenes (n = 5, 6) show unique molecular dynamics in their enantiomerization processes, including inversion motions of both the bowl and the helix. Optical resolution of a corannulene-based skeletally chiral molecule was also achieved for the first time, and the influence of the bowl-motif annulation on the chiroptical properties was investigated.

Indeno-Polycyclic Aromatic Hydrocarbons: A Versatile New Synthetic Route

Abstract Acylation of a polycyclic aromatic hydrocarbon (PAH) adjacent to a ring junction with 2-bromobenzoyl chloride followed by Flash Vacuum Pyrolysis (FVP) of the resulting bromoketone affords the corresponding indeno-annulated PAH. The new method is illustrated by syntheses of indeno[1,2,3-cd]pyrene (1) from pyrene and indeno[1,2,3-cd]fluoranthene (2) from fluoranthene. The formation of indeno[1,2,3,4-defg]chrysene (11) from FVP of 8-(2-bromobenzoyl)-fluoranthene (10), and as a secondary product from FVP of 3-(2-bromobenzoyl)fluoranthene (9), reveals the ability of phenyl groups to migrate around the periphery of a didehydro-PAH. Mechanisms involving reversible hydrogen atom transfers are proposed. Diacylation of fluoranthene with 2-bromobenzoyl chloride followed by FVP gives the previously unknown, nonplanar, nonalternant, C28H14 PAH diindeno[1,2,3,4-defg: 1,2,3-rs]chrysene (21) in just two steps.