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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 experimental and calculated UV and IR absorption spectra, which agree well with each other. Thereby a novel structure of the acridine dimer is found, which disagrees with a previously reported one. The calculations also show the changes from excitonic coupling towards orbital interactions between two molecules with decreasing distance to each other. In addition, a structure of the trimer is determined. Finally, an outlook is given on how even higher aggregates can be made accessible through experiment. [ABSTRACT FROM AUTHOR]

N‐Heteropolycycles are attractive as materials in organic electronic devices. However, a detailed understanding of the low‐energy electronic excitation characteristics of these species is still lacking. In this work, the matrix isolation technique is applied to obtain high‐resolution absorbance spectra for a series of tetracene and core‐substituted N‐analogues. The experimental electronic excitation spectra obtained for matrix‐isolated molecules are then analysed with the help of quantum‐chemical calculations. Additional lower energy excitation bands in the spectrum of the core‐substituted N‐derivatives of tetracene could be explained in terms of intensity borrowing from dipole‐forbidden transitions due to Herzberg–Teller vibronic coupling. In the case of tetracene, evidence for the additional formation of London dimers (J aggregates) is found at higher tetracene concentrations in the matrix. [ABSTRACT FROM AUTHOR]