Stacking Is Favored over Hydrogen Bonding in Azaphenanthrene Dimers

Abstract

N-Doped polycyclic aromatic hydrocarbons have recently emerged as potential organic electronic materials. The function of such materials is determined not only by the intrinsic electronic properties of individual molecules but also by their supramolecular interactions in the solid state. Therefore, a proper characterization of the interactions between the individual units is of interest to materials science since they ultimately govern properties such as excitons and charge transfer. Here, we report a joint experimental and computational study of two azaphenanthrene dimers to determine the structure and the nature of supramolecular interactions in the aggregates. IR/UV double-resonance experiments were carried out using far- and mid-infrared free-electron laser radiation. The experimental spectra are compared with quantum chemical calculations for the lowest-energy π-stacked and hydrogen-bonded structures. The data reveal a preference of the π-stacked structure for the benzo­[f]­quinoline and the phenanthridine dimer.

The Electronic Structures of Azaphenanthrenes and Their Dimers

“…Note that the [1 + 1′]-REMPI spectra of the dimers of 1 and 3 around the S 1 ← S 0 origin transition were also recorded using ns lasers. They are already presented in the SI of ref , and the assignments have been confirmed.…”

“…The spectrum of the phenanthridine dimer is given in Figure . A short [1 + 1′]-REMPI spectra of the dimer of 3 was recorded using ns lasers and is given in the SI of ref , confirming the position of the major band. The transition densities indicate collective excitations for pi2 , so the oscillator strength of the S 1 ← S 0 (blue) of pi2 is small, and its contribution to the spectrum can be ignored.…”

“…For the dimers of all three molecules, two almost degenerate π-stacked structures were computed to represent the lowest-energy structures. For benzo[ f ]quinoline and phenanthridine, this has been confirmed by IR/UV experiments . Both π-stacked structures are expected to be present in the molecular jet.…”

“…Recently, we investigated the structure of the dimers of 2 and 3 by IR/UV ion dip spectroscopy and identified a π-stacked structure rather than a H-bonded one for both dimers. 26 The UV wavelengths employed in that work were chosen based on the REMPI spectra of the dimers. Here, we present the electronic spectra of the three azaphenanthrenes shown in Scheme 1 and compare them with quantum-chemical simulations.…”

Structures of (Pyrazine)2 and (Pyrazine)(Benzene) Dimers Investigated with Infrared–Vacuum Ultraviolet Spectroscopy and Quantum-Chemical Calculations: Competition among π–π, CH···π, and CH···N Interactions

“…In our previous study of the pyridine dimer, (Pyd) 2 , we also showed that the π–π stacked structure has the lowest energy. It should also be noted that recent work on the azaphenanthrene dimer also showed that the π–π stacked structure is more stable than the H-bonded planar structure . In Table , we listed the energies of three lowest-energy isomers of (Pyra)­(Bz) calculated with the B3PW91 + D3/aug-cc-pVDZ, ωB97XD/aug-cc-pVDZ, and CCSD­(T)/aug-cc-pVTZ//ωB97XD/aug-cc-pVDZ methods.…”