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

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 12+ and 22+ was primarily based on the theoretical calculations, and the
open-shell singlet biradical or triplet character was anticipated
for tetraazacoronene 32+ and
triazacoronene 42+ with the aid
of theoretical calculations. These polycyclic aromatic hydrocarbons
(PAHs) represent the first series of nitrogen-containing PAHs that
can be multiply oxidized.

本研究以相应六芳基苯的氧化环脱氢反应为关键步骤，合成了一类全新的吡咯稠合氮杂蔻烯（pyrrole-fused azacoronene）家族化合物，并解析了四氮杂蔻烯3b与三氮杂蔻烯4a的晶体结构。研究人员在室温与77 K条件下，采用稳态紫外-可见吸收/发射光谱以及时间分辨光谱技术（含发射光谱与寿命测试），对中性化合物1至4的光物理性质开展了系统探究。通过同时观测到的荧光与磷光信号，我们估算出单重态-三重态能级差（ΔE_S-T）分别为：1a对应0.35 eV，2a对应0.26 eV，4a对应0.36 eV。与此前报道的六吡咯并六氮杂蔻烯1（hexapyrrolohexaazacoronene 1，HPHAC）类似，电化学氧化实验显示所有新型化合物均存在至多四组可逆氧化过程。针对化学法制备的自由基阳离子与双阳离子，研究人员借助紫外-可见-近红外吸收光谱（ultraviolet-visible-near infrared absorption spectroscopy，UV-vis-NIR）、电子顺磁共振光谱（electron spin resonance spectroscopy，ESR）与核磁共振光谱（nuclear magnetic resonance spectroscopy，NMR），对该系列氮杂蔻烯π体系的电荷与自旋离域特性进行了表征。结合理论计算结果，实验数据清晰证实：以二烷氧基苯取代吡咯环的设计，对电子通讯过程起到关键调控作用；共振结构可显著提升阳离子电荷/自旋的热力学稳定性，并决定自旋多重度。对于HPHAC 1与五氮杂蔻烯2，其闭壳层双阳离子1²+与2²+的整体芳香性预测主要依托理论计算；而四氮杂蔻烯3²+与三氮杂蔻烯4²+的开壳层单线态双自由基或三线态特性，则通过理论计算得以验证。此类多环芳烃（polycyclic aromatic hydrocarbons，PAHs）为首例可被多次氧化的含氮多环芳烃系列。