Effect of side-chain substitutions on the morphology of self-assembly of perylene diimide molecules has been studied with two derivatives modified with distinctly different side-chains, N,N'-di(dodecyl)-perylene-3,4,9,10-tetracarboxylic diimide (DD-PTCDI) and N,N'-di(nonyldecyl)-perylene-3,4,9,10-tetracarboxylic diimide (ND-PTCDI). Due to the different side-chain interference, the self-assembly of the two molecules results in totally different morphologies in aggregate: one-dimensional (1D) nanobelt vs zero-dimensional (0D) nanoparticle. The size, shape, and topography of the self-assemblies were extensively characterized by a variety of microscopies including SEM, TEM, AFM, and fluorescence microscopy. The distinct morphologies of self-assembly have been obtained from both the solution-based processing and surface-supported solvent-vapor annealing. The nanobelts of DD-PTCDI fabricated in solution can feasibly be transferred to both polar (e.g., glass) and nonpolar (e.g., carbon) surfaces, implying the high stability of the molecular assembly (due to the strong pi-pi stacking). The side-chain-dependent molecular interaction was comparatively investigated using various spectrometries including UV-vis absorption, fluorescence, X-ray diffraction, and differential scanning calorimetry. Compared to the emission of ND-PTCDI aggregate, the emission of DD-PTCDI aggregate was significantly red-shifted (ca. 30 nm) and the emission quantum yield decreased about three times, primarily due to the more favorable molecular stacking for DD-PTCID. Moreover, the aggregate of DD-PTCDI shows a pronounced absorption band at the longer wavelength, whereas the absorption of ND-PTCDI aggregate is not significant in the same wavelength region. These optical spectral observations are reminiscent of the previous theoretical investigation on the side-chain-modulated electronic properties of PTCDI assembly.

Nanobelt structures have been fabricated for an n-type semiconductor molecule, N,N'-di(propoxyethyl)perylene-3,4,9,10-tetracarboxylic diimide (PTCDI). The short alkyloxy side chain not only affords effective pi-pi stacking in polar solvents for self-assembling but also provides sufficient solubility in nonpolar solvents for solution processing. As revealed by both AFM and electron microscopies, the nanobelts have an approximately rectangular cross section, with a typical thickness of about 100 nm and a width in the range of 300-500 nm. The length of the nanobelts ranges from 10 to a few tens of micrometers. The highly organized molecular packing (uniaxial crystalline phase) has been deduced from the measurement of electron diffraction and polarized microscopy imaging. The detected optical axis is consistent with the one-dimensional stacking of the molecules.

One-dimensional (1D) self-assembly of planar aromatic (semiconductor) molecules into well-defined nanowires or nanobelts has gained increasing interest 1 since such an approach may extend the optoelectronic properties and applications intrinsic to organic semiconductors from film-based materials into the size ranges and architectures that have never before been achieved. Among the small number of molecules successfully explored in such 1D selfassembly, 2-8 symmetric molecules modified with all identical side chains represent the majority of building block candidates. [2][3][4][5][6] In contrast, there have been much less asymmetric molecules employed in the fabrication of well-defined nanowires or nanobelts, 1b,2,8 although asymmetric molecules may provide more options and adaptability for the surface modification to approach optimized sensing sensitivity and selectivity for the nanomaterials thus fabricated. 8b In this Communication, we report for the first time the fabrication of ultralong nanobelts (>0.3 mm) from an asymmetric perylene tetracarboxylic diimide (PTCDI) molecule as shown in Chart 1. PTCDIs represent a unique class of molecules that demonstrate extremely high thermal and photostability and have been employed in a wide variety of film-based optoelectronic devices. 3,9 While welldefined nanowires have recently been fabricated from various symmetric PTCDIs, 3,5,10 which possess identical linear side chains at the two imide positions, there is no such 1D self-assembly reported on the asymmetric PTCDIs. The new self-assembly approach reported herein on the asymmetric PTCDI will open broader options for both molecular design and engineering to improve the fabrication of 1D nanomaterials. Moreover, the millimeter long nanobelt fabricated in this study will enable more expedient construction of integrated nanoelectronic devices, for which deposition of a wire across multiple parallel electrodes is usually demanded. 11 The polyoxyethylene side-chain attachment makes molecule 1 highly soluble in hydrophilic solvents such as ethanol. Taking advantage of the miscibility between alcohol and water, the solubility (or self-assembly) of the molecule can feasibly be controlled by adjusting the volume ratio of water and alcohol (detailed in the Supporting Information). Upon increasing the water component, the increase in solvent polarity will force solvophobic association between the alkyl side chains, in a similar manner of 1D self-assembly of surfactants and other amphiphilic molecules. [12][13][14] Such hydrophobic interdigitation will bring the molecules in proximity where the π-π interaction dominates the molecular packing configuration. The π-π molecular stacking is likely facilitated by the stretching-out conformation of the polyoxyethylene side chain, which is favored in hydrophilic solvent. 8a Indeed, ultralong nanobelt structure was obtained from the self-assembly of compound 1 in water/ethanol solution at an appropriate volume ratio, ca. 1:1.As demonstrated by the large-area SEM imaging shown in Fig...

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