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

Polycyclic aromatic hydrocarbons and their nitrogen-substituted analogues are of great interest for various applications in organic electronics. The performance of such devices is determined not only by the properties of the single molecules, but also by the structure of their aggregates, which often form via self-aggregation. Gaining insight into such aggregation processes is a challenging task, but crucial for a fine-tuning of the materials properties. In this work, an efficient approach for the generation and characterisation of aggregates is described, based on matrix-isolation experiments and quantum-chemical calculations. This approach is exemplified for aggregation of acridine. The acridine dimer and trimer are thoroughly analysed on the basis of matrix-isolation spectroscopy over two spectral ranges and quantum chemical calculations, which agree well with each other. Thereby a novel structure of the acridine dimer is found, which disagrees with a previously reported one. In addition, a structure of the trimer is determined and new insight into the photophysics of small acridine aggregates is gained. Finally, an outlook is given on how even higher aggregates can be made accessible through experiment.