Synthesis and Optical Properties of Molecular Rods Comprising a Central Core-Substituted Naphthalenediimide Chromophore for Carbon Nanotube Junctions

Abstract The synthesis of a series of molecular rods 1?5, designed to bridge the gap of a carbon nanotube junction in order to emit light as a characteristic signal of integrated molecules, is reported. The molecular rods consist of a central naphthalenediimide (NDI) core, which itself is substituted with benzylamino and benzylsulfanyl groups, providing distinct absorption and emission properties. The NDI core is embedded in an oligo(phenylene ethynylene) (OPE) system providing the rod-like structure required to bridge gaps between nanoelectrodes. The number of repeating units of the OPE is varied to adjust the length of the target compounds between 2.3 and 6.6 nm. The OPE parts are terminally functionalized with polyaromatic hydrocarbon groups (naphthalene, phenanthrene, anthracene or pyrene), which possess affinity with the surface of the carbon nanotubes due to van der Waals interactions. Synthetic protocols based on Sonogashira?Hagihara couplings were developed to build up the OPE backbone. Bifunctional iodophenyl acetylene derivative 33 served as a key building block in a coupling?deprotecting?coupling sequence. The NDI building block was synthesized by an aromatic nucleophilic substitution reaction of 2,6-dichloro-1,4,5,8-tetracarboxylic acid naphthalenediimide derivative 9 and the corresponding amine and sulfide (i.e., 11, 12), respectively. The convergent synthesis allows modular assembly of the NDI and OPE parts in a final Sonogashira?Hagihara coupling reaction. The target structures were fully characterized by NMR spectroscopy and mass spectrometry. Further, the optical properties of compounds 3?5 in solution, and on a graphene surface were qualitatively investigated. A Dexter-type energy transfer from the OPE unit to the NDI unit was observed. The studies of target structures 3?5 revealed that diamino-functionalized compound 3 is ideally suited for the envisaged single molecule electroluminescence experiments.