Self-Organized Perylene Diimide Nanofibers Ping Yan, Arindam Chowdhury, Michael W. Holman, and David M. Adams* Department of Chemistry, Columbia UniVersity, 3000 Broadway, New York, New York 10027 ReceiVed: August 26, 2004; In Final Form: October 22, 2004 A propeller-shaped perylene diimide trimer was synthesized and a simple evaporation method was used for the self-organization of trimer molecules into fluorescent nanofibers. The sizes of these fiberssfrom 4 to 150 nm in diameterswere measured by atomic force microscopy and can be controlled by adjusting the concentration of the initial solution. The aspect ratios (length/height) are around 500. The plane of the trimer was determined by polarized scanning confocal microscopy to be perpendicular to the axis of the fibers, in agreement with molecular mechanics calculations. UV/vis and NMR spectroscopies were used to monitor concentration-dependent π-π stacking in solution. Single-fiber fluorescence imaging and spectroscopy were performed using a total internal reflection fluorescence microscope equipped with a digital color camera and imaging CCD spectrometer. Strongly red-shifted fluorescence from these fibers indicates a high degree of electronic delocalization, and breaking up this delocalization by photobleaching blue-shifts the emission toward that of an isolated noninteracting molecule. The delocalization along these nanofibers and the ability to study the electronic structure using fluorescence make them potentially useful in nanoscale devices, such as field effect transistors and photoconductors. Introduction During the past decade, self-organization has received increased attention as a tool for the design and synthesis of well- defined organic nanostructures. 1-9 Organic nanotubes and nanofibers are structures of particular interest, since, comple- mentary to carbon nanotubes 10,11 and inorganic semiconductor nanowires, 12 they can potentially be formed by low-cost processes such as spin-coating, printing, and evaporating and are compatible with plastic substrates. 13 To date, several organic systems have been found which self-organize into nanofibers and nanotubes, including cyclic peptides, 14,15 amphiphilic pep- tides, 16,17 crowded aromatics, 18-21 hexa-peri-hexabenzocoro- nenes, 22,23 porphyrins, 24-26 polyanilines, 27-29 phthalocyanines, 30 cyanines, 31,32 and merocyanines. 33,34 It has been shown that single crystals of organic materials can have high charge carrier mobilities, 35-37 but single crystals are generally difficult to obtain, so self-organization provides an alternative route to creating molecular electronic materials at low cost. 38-40 Although the relationship between molecular packing and charge carrier mobility is complex, 41,42 strong π-π interactions are generally considered desirable for charge transport. 43-47 Perylene diimides are excellent candidates for creating a self-organized molecular electronic material, since they are one of few n-type organic semiconductor systems that have demonstrated high charge carrier mobilities in thin film devices and structures 13,47,48 and since perylene diimides have strong, well-studied π-π interactions which can be used to direct self-organization. 49-64 In this paper we report the self-organization of discrete nanofibers from a perylene diimide trimer (1), shown in Figure 1A. The three perylene diimide subunits strengthen the π-π interactions and are arranged around a central core to give the molecule a propeller-like shape which directs one-dimensional self-organization, 65,66 as seen in molecular mechanics calcula- tions on a group of six trimers (Figure 1B,C). The phenylene linkers between the central benzene and three perylene diimide subunits allow some free rotation for the perylene diimide subunits to achieve ideal π-π stacking. The secondary alkyl chains improve solubility in organic solvents, which makes the synthesis and self-organization from solution phase feasible, and likely provide additional van der Waals attractions between adjacent molecules within the stacks and between adjacent stacks in larger nanostructures. Self-organization depends on the delicate balance between molecule-molecule and molecule- solvent interactions, 67 and hence solvents always play crucial roles in these systems. Trimer 1 is highly soluble in CH 2 Cl 2 and CHCl 3 but not in MeOH, so solvent mixtures were used to tune the solubility for solution studies and for self-organization. Results and Discussion Synthesis. As shown in Scheme 1, the trimer (1) was obtained in satisfactory yield (64%) by triple condensation between anhydride 4 and triamine 7. It should be noted that direct condensation between a similar triamine, 1,3,5-triaminobenzene, and 4 only gave monochromophoric derivatives. 68 Anhydride 4 in turn was synthesized by a two-step procedure: 69 condensa- tion between 1-hexylheptylamine and perylene-3,4:9,10-tetra- carboxyldianhydride gave the symmetric diimide 3, followed by partial saponification. Triamine 7 was also synthesized by a two-step procedure: 70 acid-catalyzed triple condensation of 4-nitroacetophenone gave 1,3,5-tris(4-nitrophenyl)benzene, fol- lowed by reduction with hydrazine. UV/Vis Spectroscopy. UV/vis absorption spectra are sensi- tive to interchromophore distance and orientation 71,72 and have been widely used to study π-π stacking of perylene dyes. 53,54,57,61,73 Figure 2 shows absorption spectra of 1 at different concentrations in 1:1 MeOH/CHCl 3 . At low concentra- tion (0.067 μM) the spectra are similar to that of monomeric perylene diimide 3, 74 while at higher concentration (0.67 μM) * To whom correspondence should be addressed. E-mail: dadams@ chem.columbia.edu. 724 J. Phys. Chem. B 2005, 109, 724-730 10.1021/jp046133e CCC: $30.25 © 2005 American Chemical Society Published on Web 12/16/2004