Influence of alkyl substituents on the solution- and surface-organization of hexa-peri-hexabenzocoronenes

Three hexa-peri-hexabenzocoronenes (HBCs) with branched, bulky alkyl substituents of different lengths in the periphery of the aromatic core have been synthesized to tune the self-association properties in solution. H-1 NMR and photophysical measurements were used to probe the solution organization in comparison to the known hexa-dodecyl-substituted HBC in different solvent systems. Thermodynamic parameters for the self-association in solution, obtained by curve fitting of the concentration- and temperature-dependent NMR data using van't Hoff analysis, indicated that the self-association is an enthalpically driven process that is entropically disfavored. Photoluminescence and NMR results were both employed to determine the critical concentration where no self-association for different compounds occurred. The interactions between the molecules could be controlled by varying the nonsolvent content in the solvent mixtures, supporting the model of solvophobic effects. The spatial demand of the solubilizing side chains modulated the self-association in solution. This behavior was translated into the solution casting process, where the kinetic in addition to the thermodynamic parameters played an essential role for structure formation. The study illuminates the relationship between the solution association of HBCs and the morphology, when processed on a surface. These results are essential for the application of these materials in devices.