Synthesis and Characterization of Peralkylated Pyrrole-Fused Azacoronene

Abstract

A hexapyrrolohexaazacoronene (HPHAC) with 12 less-bulky peripheral ethyl groups than its aryl-containing HPHAC counterpart was synthesized to investigate the innate character of HPHAC. X-ray diffraction analysis revealed that HPHAC had a planar structure and close packing because of CH−π interactions between the alkyl groups and the HPHAC core. Compared to the previously reported HPHAC decorated with 12 peripheral aryl groups, this electron-rich π-system exhibited reversible multistep oxidations at low potentials and easily formed mono- and dicationic salts and charge-transfer (CT) complexes with 7,7,8,8-tetracyano-p-quinodimethane. These oxidized species exhibited clear changes in the bond-length alternation of the pyrrole units in the crystal state, indicating charge and spin delocalization. The distinct upfield shift of the central carbon signal of the dication in the 13C NMR spectrum affirms the global aromaticity from the viewpoint of a magnetic criterion. In the UV–vis/NIR spectra, broad absorption in the NIR region was observed only for HPHAC2+ and not the structurally similar cyclo[6]­pyrrole. Magnetic circular dichroism measurements and time-dependent density functional theory calculations revealed that the broad absorption was assigned to the CT transition from the central benzene ring to the outer pyrrole rings.

Recent Advances in Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds

“…Variants of this method provide access to a range of structurally diverse molecules. The use of ethyl substitution introduced by Uno and Takase et al provides access to more electron-rich and potentially more reactive HPHAC derivatives, such as 13.1 , 16 which is easily oxidizable to the typical globally aromatic dication 13.1 2+ containing a 22 π-electron conjugation pathway. According to an MCD analysis and DFT calculations, the NIR absorption band observed for the dication but absent in the spectrum of the parent cyclo[6]pyrrole macrocycle is attributed to a CT transition from the central benzene to the peripheral pyrrole moieties.…”

“… Reagents and conditions: (a) 16 NOSbF 6 (2.0 equiv) and DCM, rt, 10 min; (b) 18 3,4-dihexylpyrrole, NaH, DMF, rt; (c) FeCl 3 , CH 3 NO 2 , rt; (d) I 2 (excess), reflux, under N 2 flow; (e) 17 AgNO 2 (9.7 equiv), DCM, rt; (f) 21 DMF or N , N- dimethylbenzamide, POCl 3 , KPF 6 , DCE; (g) NOSbF 6 or BAHA, DCM. …”

Non-Planar Polycyclic Aromatic Molecules Including Heterole Units

“…61 Very recently, dodecaethyl-substituted HPHAC 137d was synthesized by Takase and Uno. 62 After that report, several groups developed the chemistry of HPHAC through structural modification such as the peripheral expansion of the macrocyclic circuit, core-expansion, and peripheral substitution/πextension (Scheme 17b).…”

Novel Synthetic Approach to Heteroatom Doped Polycyclic Aromatic Hydrocarbons: Optimizing the Bottom-Up Approach to Atomically Precise Doped Nanographenes

“…Although hexapyrrolohexaazacoronenes (HPHACs) functionalized with aryl substituents are well known and characterized, only recently was a peralkylated analogue, 64 ( Scheme 16 ), synthetized [ 63 ]. The synthetic approach was similar to previous studies although variation of the reaction condition allowed the isolation of a partially fused intermediate, which could be converted in the fully fused derivative using the standard Rathore conditions [ 64 ].…”