Unveiling the Impact of Aggregation on Optical Anisotropy of Triazaacephenanthrylene Single Crystals. A Combined Quantum Crystallography and Conceptual Density Functional Theory Approach

Triazaacephenanthrylene (TAAP) triclinic single crystals show substantial optical anisotropy of absorption and fluorescence. The maximum effect can be correlated with the direction perpendicular to the plane of chromophores connected in a head-to-tail manner via weak dispersive interactions. This phenomenon is uncommon as usually the existence of postulated π···π interactions between the molecules forming dimers or stacks cause quenching of fluorescence. Herein we present a comprehensive study of inter- and intramolecular interactions in the crystal of TAAP enriched with the investigation of aromaticity. Our results show that intramolecular interactions stabilize the overall conformation of the molecule whereas dispersive forces determine the aggregation between TAAP molecules. In fact, there is no conventional π···π interaction between the molecules in the dimer. Instead, we observed a close contact between the lone pair of the bridgehead N10B atom and π-deficient pyrazine ring from an adjacent molecule. Optical anisotropy in TAAP crystals was directly correlated with the alignment of the molecular transition dipole moments caused by specific molecular self-assembly.