The Electronic Structures of Azaphenanthrenes and Their Dimers

Abstract

Insertion of a nitrogen atom modifies the electronic structures and photochemistry of polycyclic aromatic hydrocarbons by introducing nπ* states into the molecules. To better understand the electronic structures of isolated polycyclic aromatic nitrogencontaining hydrocarbons (PANHs) and their dimers as well as the influence of the position of the nitrogen atom in the molecule, we investigate three different azaphenanthrenes, benzo[f ]quinoline, benzo[h]quinoline, and phenanthridine, in a joint experimental and computational study. Experimentally, resonance-enhanced multiphoton ionization (REMPI) spectroscopy is applied to characterize the excited electronic states. The REMPI spectra of the azaphenanthrene monomers have a rather similar appearance, with origins between 3.645 and 3.670 eV for the 1ππ* ← S 0 transition. In contrast to the phenanthrene parent, 2ππ* ← S 0 is broad and unstructured even at the band origin. The experiments are accompanied by density functional theory computation, and vibrationally resolved spectra are simulated using a time-independent approach. The differences between phenanthrene and the azaphenanthrenes are assigned to perturbations due to the low-lying 1 (nπ*) state, which accelerates nonradiative deactivation. For the dimers, it is found that two π-stacked isomers with two electronic transitions each contribute to the electronic spectrum, leading to overlapping bands that are difficult to assign.

Aggregation of N‐Heteropolycyclic Aromatic Molecules: The Acridine Dimer and Trimer

“…Such studies have been conducted for the systems listed above, [8][9][10] but also for nitrogen-containing aromatic molecules like imidazole, [11] pyrrole, [12] pyridine [13] and phenanthrenes. [14] Mainly dimers were observed, but in a few studies, such as those of imidazole [11] and naphthalene, [15] even trimers were clearly identified.…”