Pyrrole-Fused Azacoronene Family: The Influence of Replacement with Dialkoxybenzenes on the Optical and Electronic Properties in Neutral and Oxidized States

Abstract

A novel pyrrole-fused azacoronene family was synthesized via oxidative cyclodehydrogenation of the corresponding hexaarylbenzenes as the key step, and the crystal structures of tetraazacoronene 3b and triazacoronene 4a were elucidated. The photophysical properties for neutral compounds 1-4 were investigated using steady-state UV-vis absorption/emission spectroscopy and time-resolved spectroscopy (emission spectra and lifetime measurements) at both room temperature and 77 K. The observation of both fluorescence and phosphorescence allowed us to estimate the small S1-T1 energy gap (ΔES-T) to be 0.35 eV (1a), 0.26 eV (2a), and 0.36 eV (4a). Similar to the case of previously reported hexapyrrolohexaazacoronene 1 (HPHAC), electrochemical oxidation revealed up to four reversible oxidation processes for all of the new compounds. The charge and spin delocalization properties of the series of azacoronene π-systems were examined using UV-vis-NIR absorption, ESR, and NMR spectroscopies for the chemically generated radical cations and dications. Combined with the theoretical calculations, the experimental results clearly demonstrated that the replacement of pyrrole rings with dialkoxybenzene plays a critical role in the electronic communication, where resonance structures significantly contribute to the thermodynamic stability of the cationic charges/spins and determine the spin multiplicities. For HPHAC 1 and pentaazacoronene 2, the overall aromaticity predicted for closed-shell dications 1(2+) and 2(2+) was primarily based on the theoretical calculations, and the open-shell singlet biradical or triplet character was anticipated for tetraazacoronene 3(2+) and triazacoronene 4(2+) with the aid of theoretical calculations. These polycyclic aromatic hydrocarbons (PAHs) represent the first series of nitrogen-containing PAHs that can be multiply oxidized.

New advances in nanographene chemistry

“…By contrast, bottom-up organic synthesis can provide structurally defined heteroatom-doped graphene fragments with perfect control not only of the size, periphery and substituent, but also of the doping concentration and position. 142,143 These PAHs incorporating electron-rich pyrrole-type nitrogens exhibited up to four reversible oxidation processes in the electrochemical measurements. 41,60,[139][140][141] As typical examples of nitrogen (N)-doped graphene molecules, we developed a series of pyrrole-fused azacoronenes 52-55 ( Fig.…”

Synthetic Applications of Oxidative Aromatic Coupling—From Biphenols to Nanographenes

“…More recently, the authors have extended this synthetic method to include analogues of 177 with one, two, or three pyrrole moieties replaced by benzene units (e.g. 178 ) …”

Methoxy functionalisation: exerting synthetic control of the supramolecular and electronic structure of nitrogen-doped nanographenes

“…[4][5][6] As many aspects in the formation of large graphitic sheets are difficult to control, synthetic chemists have pioneered routes to nanographenes with impressive and varying dimensions, and an array of peripheral substituents. [7][8][9][10] Dependent on the latter are a range of material characteristics e.g. the nature of the p-p interactions, HOMO-LUMO gaps and film-forming capabilities.…”

“…[11][12][13][14] The ability to manufacture graphene systems to such specificity is notably absent in exfoliation 15 /epitaxial growth 16 processes, and this makes the further development of bottom-up synthetic methodologies for the formation of next-generation and heteroatomcontaining graphenes all the more important. 10,17 Oxidative cyclodehydrogenation is a key step in the chemical formation of planarised ring systems. All-carbon systems cyclise rapidly, while heteroatom-containing polyphenylenes appear to adopt a more complex mechanism for ring closure, e.g.…”