Self-assembled aggregates of amphiphilic perylene diimide-based semiconductor molecules: effect of morphology on conductivity

Two amphiphilic perylenetetracarboxylic diimide derivatives modified with different side chains at imide nitrogen, N-n-hexyl-N'-(2-hydroxyethyl)-1,7-di(4'-t-butyl)phenoxy-perylene-3,4:9,10-tetracarboxylic diimide (PDI 1) and N,N'-di(2-hydroxyethyl)-1,7-di(4'-t-butyl)phenoxy-perylene-3,4:9,10-tetracarboxylic diimide (PDI 2), were fabricated into organic nanostructures via solution-phase self-assembly. Their self-assembling properties in methanol and n-hexane have been comparatively studied by electronic absorption, fluorescence, and Fourier transform infrared spectroscopy (FT-IR). The morphologies and structures of the self-assemblies were examined by scanning electronic microscopy (SEM), atomic force microscopy (AFM), as well as X-ray diffraction (XRD) techniques. The conducting properties were evaluated by current-voltage (I-V) measurements. Due to the presence of different number of hydroxyethyl groups in the molecule of PDI 1 and PDI 2, the self-assembly of the two molecules in methanol and n-hexane results in nanostructures with distinctly different morphology as follows: nanobelts and nanoleaves for PDI 1 and nanobelt dendrites and nanosheets for PDI 2, respectively. Analysis of the spectral change for the aggregates relative to that of monomeric PDI in solution revealed that in polar and apolar solvents, both nanobelts and nanoleaves precipitated from PDI 1 adopt the H aggregation mode, whereas nanobelt dendrites and nanosheets from PDI 2 adopt H and J aggregation mode, respectively, implying the effect of both side-chain substituent and solvent on tuning the intermolecular stacking. Furthermore, the conductivity of the aggregates of either PDI 1 or PDI 2 from methanol is more than ca. 1 order of magnitude higher than those from n-hexane. In particular, the well-defined, one-dimensional (1D) nanobelts of PDI 1 show excellent semiconducting property with the electrical conductivity as high as 3.3×10(-3) S cm(-1), which might serve as promising candidates for applications in nano-electronics.