BN-Substitution in Dithienylpyrenes Prevents Excimer Formation in Solution and in the Solid State

Boron-nitrogen substitutions in polycyclic aromatic hydrocarbons (PAHs) have a strong impact on the molecules’ optical properties due to a significantly more heterogeneous electron distribution. However, besides these single molecule properties, the observed optical properties of PAHs critically depend on the degree of intermolecular interactions like π-π-stacking, dipo-lar interactions or the formation of dimers in the excited state. Pyrene is the most prominent example showing the latter, as it exhibits a broadened and strongly bathochromically shifted emission band at high concentrations in solution, compared to the respective monomers. In the solid state, the impact of intermolecular interactions is even higher, as it determines the crystal packing crucially. In this work, a thiophene-flanked BN-pyrene (BNP) was synthesized and compared with its all-carbon analog (CCP) in solution and in solid state by means of crystallography, NMR spectroscopy, UV-vis as well as pho-toluminescence (PL) spectroscopy. In solution, PL spectroscopy revealed the solvent-dependent presence of excimers of CCP at high concentrations. In contrast, no excimers were found in BNP. Clear differences were also observed in the single crystal packing motifs. While CCP revealed overlapped pyrene-planes with centroid distances in the range of classical π-stacking interactions, the BNP scaffolds were displaced and significantly more spatially separated.