Self-assembled organic nanostructures: effect of substituents on the morphology

Three perylene-3,4;9,10-tetracarboxydiimide (PTCDI) compounds with two dodecyloxy or thiododecyl chains attached at the bay positions of the perylene ring, PTCDIs 1-3, were fabricated into nanoassemblies by a solution injection method. The morphologies of these self-assembled nanostructures were determined by transmission electronic microscopy (TEM), scanning electronic microscopy (SEM), and atomic force microscopy (AFM). PTCDI compound 1, with two dodecyloxy groups, forms long, flexible nanowires with an aspect ratio of over 200, while analogue 3, with two thiododecyl groups, self-assembles into spherical particles. In line with these results, PTCDI 2, with one dodecyloxy group and one thiododecyl group, forms nanorods with an aspect ratio of around 20. Electronic absorption and fluorescence spectroscopy results reveal the formation of H-aggregates in the nanostructures of these PTCDI compounds owing to the pi-pi interaction between the substituted perylene molecules and also suggest a decreasing pi-pi interaction in the order 1>2>3, which corresponds well with the morphology of the corresponding nanoassemblies. On the basis of DFT calculations, the effect of different substituents at the bay positions of the perylene ring on the pi-pi interaction between substituted perylene molecules and the morphology of self-assembled nanostructures is rationalized by the differing degree of twisting of the conjugated perylene system caused by the different substituents and the different bending of the alkoxy and thioalkyl groups with respect to the plane of the naphthalene.