Supramolecular Ordering in Oligothiophene-Fullerene Monolayers

Scanning tunneling microscopy (STM) of monolayers comprising oligothiophene and fullerene molecular semiconductors reveals details of their molecular-scale phase separation and ordering with potential implications for the design of organic electronic devices, in particular future bulk heterojunction solar cells. Prochiral terthienobenzenetricarboxylic acid (TTBTA) self-assembles at the solution/graphite interface into either a porous chicken wire network linked by dimeric hydrogen bonding associations of COOH groups (R (8)) or a close-packed network linked in a novel hexameric hydrogen bonding motif (R-6(6)(24)). Analysis of high-resolution STM images shows that the chicken wire phase is racemically mixed, whereas the close-packed phase is enantiomerically pure. The cavities of the chicken wire structure can efficiently host C-60 molecules, which form ordered domains with either one, two, or three fullerenes; per cavity. The observed monodisperse filling and long-range co-alignment of fullerenes is described in terms of a combination of an electrostatic effect and the commensurability between the graphite and molecular network, which leads to differentiation of otherwise identical adsorption sites in the pores.