Curved Polycyclic Aromatic Molecules That Are π-Isoelectronic to Hexabenzocoronene

Abstract

Reported here are two types of curved π-molecules that are π-isoelectronic to planar hexabenzocoronene (HBC) but are forced out of planarity either by an embedded seven-membered ring or by atom crowding at the fjord region. Embedding a heptagon in HBC leads to a novel saddle-shaped molecule 1, whose π-backbone is slightly less curved than the previously reported [7]circulene in terms of the average Gauss curvature, but surprisingly much more rigid than [7]circulene. Overcrowded fjord regions in novel derivatives of hexabenzoperylene (HBP) 2a,b lead to both chiral twisted and antifolded conformers. The successful synthesis of 1 and 2a,b is related to introducing alkoxyl groups to unprecedented positions of hexaphenylbenzenes. It is found that the red twisted isomer of 2b isomerizes at elevated temperature to the yellow anti-folded conformer. This finding along with the study on the thermodynamics and kinetics of the thermal isomerization has improved the early understandings on the conformation of HBP. In the crystals, 1 lacks π-π interactions between neighboring molecules, while twisted-2a exhibits both face-to-face and edge-to-face π-π interactions. Twisted-2b is found to function as a p-type semiconductor in thin film transistors, but the thin films of 1 appear insulating presumably due to lacking π-π interactions. By exploring three different types of curvatures in 1 and the two isomers of 2b, this study has revealed that the curvature of π-face plays a role in determining the frontier molecular orbital energy levels and π-π interactions and thus needs to be considered when one designs new organic semiconductors.

Aromatic Saddles Containing Two Heptagons

“…1,7 However, these interesting carbon nanostructures with negative curvature remain imaginary, and heptagonembedded polycyclic arenes that can serve as segments, model compounds, and in principle, synthetic precursors for them are still rare. Recent approaches to negatively curved carbon nanostructures with saddle-shaped polycyclic arenes 8 include a heptagon-embedded peri-hexabenzocoronene (HBC) 9 and C 80 H 30 , a grossly warped nanographene containing five heptagons. 10 Here we report derivatives of C 70 H 26 (1a,b) and C 70 H 30 (2a,b), two new saddle-shaped polycyclic arenes containing two heptagons as shown in Figure 1.…”

“…The positions of the four alkoxy groups were important to activate the cyclization at the desired position as noted earlier with the synthesis of the heptagonembedded HBC. 9 The propyl groups of 3c were then replaced with other alkyl chains yielding 3a,b. The propyl group was chosen as the solubilizing substituent in this synthesis because it not only provided reasonable solubility to 3c for convenient purification but also, upon dealkylation with BBr 3 , resulted in 1-bromoproane, which was easily removed under reduced pressure before the subsequence alkylation.…”

Size-dependent melting of polycyclic aromatic hydrocarbon nano-clusters: A molecular dynamics study

“…Goddard et al [20] synthesised nearly pure hexabenzocoronene and benzodicoronene crystal samples from pyrene. Unlike usual planar hexabenzocoronene, a novel saddle-shaped molecule was synthesised by embedding a seven-membered ring [30]. The difficulty in the preparation of the bulk system highlights the advantage of computer simulations.…”

Heptagons in Aromatics: From Monocyclic to Polycyclic

“…Heptagon-embedded hexaperi-benzocoronene (HBC) 7 was synthesized from 10 using FeCl3 as both Lewis acid and oxidant for the Scholl-type cyclodehydrogenation reaction. [13] It was found that the position of alkoxyl groups plays a key role in this reaction. It is necessary to equip the heptagon-containing hexaphenylbenzene with alkoxyl groups ortho or para to the reaction sites since the Scholl-type reaction is reported to form C-C bonds preferably ortho or para to activating substituents.…”