The incorporation of a boron-nitrogen unit in polycyclic aromatic hydrocarbons (PAHs) can have a strong impact not only on the single molecules’ optical properties but may also influence the degree of intermolecular interactions like the formation of dimers in the excited state. Pyrene is the textbook 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. Herein, we present the syntheses of a thienyl-flanked pyrene with internal BN-substitution pattern (BNP) and its all-carbon analog (CCP) and provide an in-depth comparison of their properties in solution and in solid state by means of concentration-dependent absorption and photoluminescence (PL) spectroscopy, time-correlated single photon counting (TCSPC), NMR spectroscopy and crystallography.
The differences in the spectrooptical behavior of both compounds in solution are immense. Compared to CCP, significantly bathochromic shifts of the absorption and PL bands were observed for BNP. PL and TCSPS measurements at different concentrations clearly indicate that CCP forms excimers at increased concentrations (at c = 10.0 mg/mL: λ_{em monomer} = 400 nm, λ_{em excimer} = 494 nm with two lifetime components), while BNP is present only as monomers (λ_{em monomer} = 531 nm).
Despite their structural similarity as single molecules, the crystal packings of BNP and CCP differ considerably, too. While CCP exhibits classical π-π-interactions and a plane-to-plane overlap area of 58%, the BN-unit of BNP induces a higher electrostatic term to the attraction forces. This leads to a more displaced packing with a significantly reduced overlap area (35%). As the solid state PL characteristics are comparable to the results in solution for both derivatives, this distinction can be seen as the crucial determinant for excimer suppression in BNP. This knowledge could be fundamental for developing optically active BN-materials with precisely attuned PL maxima due to the lack of supramolecular aggregation.

Nadja Bigall

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