Isolated Dimers Versus Solid‐State Dimers of N‐Heteropolycycles: Matrix‐Isolation Spectroscopy in Concert with Quantum Chemistry

Abstract

In this work, matrix isolation spectroscopy and quantum‐chemical calculations are used together to analyse the structure and properties of weakly bound dimers of the two isomers benzo[a]acridine and benzo[c]acridine. Our measured experimental electronic absorbance spectra agree with simulated spectra calculated for the equilibrium structures of the dimers in gas‐phase, but in contrast, disagree with the simulated spectra calculated for the structures obtained by optimising the experimental solid‐state structures. This highlights the sensitivity of the electronic excitations with respect to the dimer structures. The comparison between the solid‐state and gas‐phase dimers shows how far the intermolecular interactions could change the geometric and electronic structure in a disordered bulk material or at device interfaces, imposing consequences for exciton and charge mobility and other material properties.

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

“…For benzo[a]-and benzo[c]acridine, which include an additional benzene ring, the contrast between the isolated and solid-state dimer is already notably more pronounced due to dispersion interactions to surrounding molecules. [24] The Acridine Trimer As noted in the first part, two signals in Figure 3 can be attributed to a trimer. This assignment is additionally supported by infrared (IR) absorption spectra.…”

“…For consistency, the screening procedure as well as the level of theory are exactly the same as in a previously published study. [24] The resulting structure of the most stable ACR dimer exhibits a slip-stacked structure shown in Figure 4. The aromatic planes of the two molecules are co-planar at a distance of 3.39 Å.…”

“…[43,44] But, through the investigation of benzoacridine dimers, we have shown that these dimers do not show Davydov splitting. [24] To better understand the influence of dimerisation in ACR, the energies of the energetically lowest eight electronic singlet excitations of the dimer are calculated at varying distances between the two molecular planes between the equilibrium distance of 3.39 Å and an additional 5.0 Å. This is visualised in Figure 6.…”

“…[12] Furthermore, we were able to observe trimers of the polycyclic compounds benzo[a]-and benzo[c]acridine next to dimers and monomers based on their red-shifted electronic absorption in Ne matrices as well. [24] However, the signals were weak and the assignments were made tentatively.…”

The Electronic Structures of Azaphenanthrenes and Their Dimers

“…IR/UV ion dip experiments using tunable mid-IR radiation provided by a free electron laser (FEL) confirmed the computationally predicted lower energy of the π-stacked dimers, so dispersion dominates over H bonds. In the Ne matrix, π-stacks have also been found for benzoacridine dimers, 35 whereas work on the pyridine dimer found a preference for H bonding. 36 However, there are two structures of almost equal energy, labeled pi and pi2.…”