Molecular Packing of Phenoxazine: A Combined Single-Crystal/Crystal Structure Prediction Study

Abstract

Phenoxazine is a heterocyclic molecule, which is used either as a parent molecule or with substituents for applications in various scientific fields: e.g., as a potential antioxidant. The purpose of this work is to present the molecular packing of phenoxazine within the crystalline state, as surprisingly no crystal structure is known so far. The crystal structure solution was performed by single-crystal X-ray investigations. Although the molecule has some potential for intermolecular hydrogen bonding, the observed structure is the classical herringbone packing typical for rodlike conjugated molecules. However, severe substitutional disorder of oxygen and nitrogen atoms is observed over their two opposite positions within the molecule. To get deeper insight into this disorder phenomenon, theoretical studies were performed, including crystal structure prediction using state of the art density functional theory techniques. The theoretical investigations confirm the experimentally observed 50% occupancy of the oxygen and nitrogen atoms.

Engineering of a kinetically driven phase of phenoxazine by surface crystallisation

“…First, there is no consideration for the statistical disorder of the oxygen/nitrogen as present in the bulk single crystal. 41 Second, on inspection of the molecular packing, the low energy packing motifs found in the MD simulation always show a slight bending along the molecule's long axis which does not converge to a specific value, even during longer simulation runs. The angle is in the range of 9.5°, measured between the phenyl rings on each side of the O/N atoms (Fig.…”

“…We compared the peaks with the expected peak positions of the known crystal structure of phenoxazine. 41 This phase is denoted as form 1 or simply as bulk phase. This comparison reveals that the peak at 6.79 nm −1 corresponds to the (100) peak (100-form 1) and the peak at 13.56 nm −1 is the second-order reflection (200-form 1).…”

“…Such a bending is absent in the single-crystal solution. 41 As the parametrization of the molecule in CHARMM-FF includes this bending, i.e. might be an inherent problem of the force field chosen, further optimization of the structure is employed via DFT calculations.…”

“…3b and c), plate-like crystals appeared which are connected to morphologies of form 1. 41 Fig. 3d-f reveal the crystallization at low concentrations; the formation of extended dendrites is observed.…”

“…Quite unexpected is that the first crystal structure solution of phenoxazine was obtained recently, 41 although the molecule is rather small and the tendency of crystallization is high. This might be related to the disorder within the molecular packing: the herringbone pattern forms by partially reversed molecules with an occupation of 50%.…”