Pyrrole-Fused Azacoronene Family: The Influence of
Replacement with Dialkoxybenzenes on the Optical and Electronic Properties
in Neutral and Oxidized States
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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 <b>3b</b> and triazacoronene <b>4a</b> were elucidated. The photophysical
properties for neutral compounds <b>1</b>–<b>4</b> 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 S<sub>1</sub>–T<sub>1</sub> energy gap (Δ<i>E</i><sub>S–T</sub>) to be 0.35 eV (<b>1a</b>),
0.26 eV (<b>2a</b>), and 0.36 eV (<b>4a</b>). Similar
to the case of previously reported hexapyrrolohexaazacoronene <b>1</b> (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 <b>1</b> and pentaazacoronene <b>2</b>, the
overall aromaticity predicted for closed-shell dications <b>1</b><sup><b>2+</b></sup> and <b>2</b><sup><b>2+</b></sup> was primarily based on the theoretical calculations, and the
open-shell singlet biradical or triplet character was anticipated
for tetraazacoronene <b>3</b><sup><b>2+</b></sup> and
triazacoronene <b>4</b><sup><b>2+</b></sup> with the aid
of theoretical calculations. These polycyclic aromatic hydrocarbons
(PAHs) represent the first series of nitrogen-containing PAHs that
can be multiply oxidized
