Ring-Size Effect on Molecular Assembly Structures and Optical Properties of C₃-Symmetrical Dehydrobenzoannulene Derivatives

We reported that C8[12]DBA, a C3-symmetric hexadecahydrotribenzo[12]annulene ([12]DBA) with a pore surrounded by three -C≡C- bonds, forms a stable molecular glass during cooling from the liquid state when an octylbenzoate group was introduced at the molecular terminal. In this study, to understand the influence of the molecular structure of C3-symmetric DBA derivatives on molecular assembly structures and optical properties, we newly synthesized C8[18]DBA, which has a pore surrounded by more elongated -C≡C-C≡C- bonds. Furthermore, a comparison was made with an octylbenzoate-substituted triphenylene derivative (C8Trip) that has C3-symmetry and no pore at the molecular center. The strength of intermolecular interactions decreased in the order C8[18]DBA > C8Trip > C8[12]DBA. C8[18]DBA formed π-dimers and gave crystalline molecular assemblies, whereas in C8[12]DBA, the suppression of π-π interactions stabilized the glass state. C8[18]DBA did not exhibit dielectric anomalies, whereas C8Trip and C8[12]DBA showed dielectric relaxation associated with the thermal motion of the octylester chains. Furthermore, while C8[12]DBA exhibited similar fluorescence spectra in the solution and solid states, both C8[18]DBA and C8Trip exhibited aggregation-induced quenching with a significant decrease in fluorescence intensity in the solid state.