Transfer of pi-conjugated columnar stacks from solution to surfaces

Abstract

Three hydrogen-bonded oligo(p-phenylenevinylene)s, OPV3, OPV4, and OPV5, that differ in conjugation length have been synthesized and fully characterized. All three compounds contain chiral side chains, long aliphatic chains, and a ureido-s-triazine hydrogen bonding unit. (1)H NMR and photophysical measurements show that the OPV oligomers grow hierarchically in an apolar solvent; initially, dimers are formed by hydrogen bonds that subsequently develop into stacks by pi-pi interactions of the phenylenevinylene backbone with induced helicity via the chiral side chains. SANS measurements show that rigid cylindrical objects are formed. Stacks of OPV4 have a persistence length of 150 nm and a diameter of 6 nm. OPV3 shows rigid columnar domains of 60 nm with a diameter of 5 nm. Temperature and concentration variable measurements show that the stability of the stacks increases with the conjugation length as a result of more favorable pi-pi interactions. The transfer of the single cylinders from solution to a solid support as isolated objects is only possible when specific concentrations and specific solid supports are used as investigated by AFM. At higher concentrations, an intertwined network is formed, while, at low concentration, ill-defined globular objects are observed. Only in the case of inert substrates (graphite and silicium oxide) single fibers are visible. In the case of the repulsive surfaces (mica and glass), clustering of the stacks occurs, while, at attractive surfaces (gold), the stacks are destroyed.