Ethyl 4-(9H-carbazol-9-yl)benzoate: fivefold superstructure with ten crystallographically independent molecules...

The photophysical properties of organic fluorophores are sensitive to the local sterics of the surrounding environment. Restriction of torsional motion in aggregates and crystals can give rise to enhanced emissive behavior. N-Aryl-substituted carbazoles serve an essential role as ubiquitous host matrices for organic light-emitting diodes, due to their large band gaps and high triplet energies, and so studies connecting photophysical behaviors with detailed crystallographic structural information are important. To elucidate the structural changes involved in the excited-state charge-transfer processes of N-aryl-substituted carbazoles with ester withdrawing groups, ethyl 4-(9H-carbazol-9-yl) benzoate, C₂₁H₁₇NO₂, was synthesized. The compound crystallizes with ten independent molecules in the asymmetric unit that pack together through moderate C-H...π interactions between carbazole units (2.5-2.9 Å) and π-stacks of benzoate groups (3.8-3.9 Å) between neighboring molecules. Four of the ten independent molecules show disorder by rotation of the ethyl carboxylate groups, with major occupancy rates between 0.931 (3) and 0.840 (3). The attached benzoate groups are also disordered, with identical occupancies, to compensate for the altered steric profile of the misaligned ethyl ester groups. For two molecules, the disorder extends to the entire carbazole units as well. Torsion angles between the nonplanar carbazole and benzoate groups range from θ = 44.8 to 57.2°, while those between the benzoate planes and the carboxylate COO atoms vary from α = 6.4 to 15.7°. The crystal is twinned by pseudomerohedry. The superstructure can be reduced to a hypothetical averaged parent structure in the space group Pbcn with Z' = 1, displaying fourfold disorder. Variable-temperature data collection shows that there is no phase transition between the disordered supercell and the hypothetical parent structure; supercell reflections persist up to 350 K. We propose that the disorder and variation in torsion angles result from frustrated close-packing and necessitate a unit cell with a high Z' number.