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.