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.