Highly Congested Nondistorted Diheteroarylnaphthalenes: Model Compounds for the Investigation of Intramolecular π-Stacking Interactions

The rigid, highly congested structure of 1,8-diacridylnaphthalenes has been studied in solution
and in the solid state. The unique geometry of these compounds forces the acridyl rings to undergo
face-to-face interactions while rendering T-shaped orientations and face-to-edge interactions
impossible. Crystallographic analysis shows that splaying between the heteroaryl rings decreases
while twisting between the cofacial rings increases as the acridyl nitrogens of the 1,8-diacridylnaphthalene framework are subsequently oxidized. The peri-acridyl rings are slightly splayed but
remain perfectly planar in all cases. The significant decrease in splaying indicates enhanced π−π-attraction between the electron-rich acridyl N-oxide moieties, which is in agreement with recently
reported symmetry-adapted perturbation theory calculations. The π-stacking and the molecular
geometry between the acridyl rings observed in the solid state have been confirmed through in-solution studies showing characteristic proton NMR upfield shifts and optical properties indicative
of static intramolecular arene−arene interactions. Acridyl protons located directly above the adjacent
aryl moiety as a consequence of twisting between the heteroaryl rings were identified by COSY
NMR measurements and found to intrude into the π-cloud and diamagnetic ring current of the
neighboring acridine. Different shapes and strong red shifts of the fluorescence emission maxima
of the diacridylnaphthalenes in comparison to parental acridyl monomers have been attributed to
static excimer emission.

已在溶液与固态两种状态下，对1,8-二吖啶基萘（1,8-diacridylnaphthalenes）这类刚性且空间高度拥挤的分子结构开展研究。这类化合物的独特几何构型，迫使吖啶基环发生面对面堆积相互作用，同时彻底排除了T型取向与面-边相互作用的可能。晶体学分析结果显示，随着1,8-二吖啶基萘骨架上的吖啶基氮原子逐步被氧化，杂芳环间的环间张开程度逐渐减小，而同面环之间的扭转角则不断增大。peri位吖啶基环始终呈轻微张开状态，但在所有实验条件下均保持完美平面性。环间张开程度的显著减小，表明富电子的吖啶基N-氧化物基团间的π-π吸引作用得到增强，这与近期报道的对称性适配微扰理论（symmetry-adapted perturbation theory）计算结果一致。固态下观察到的吖啶基环间π堆积与分子几何构型，已通过溶液相研究得到验证：溶液中质子核磁共振波谱（Nuclear Magnetic Resonance, NMR）呈现特征性高场位移，其光学性质也符合静态分子内芳环-芳环相互作用的特征。通过二维相关核磁共振波谱（COSY NMR）测试，可识别出因杂芳环扭转而恰好位于相邻芳环正上方的吖啶基质子，这类质子会侵入邻近吖啶环的π电子云与抗磁环电流区域。与母体吖啶单体相比，二吖啶基萘类化合物的荧光发射峰呈现出不同的峰形与显著红移，这一现象可归因于静态激基缔合物发光。