Molecular self-assembly into one-dimensional nanostructures

Abstract

Self-assembly of small molecules into one-dimensional nanostructures offers many potential applications in electronically and biologically active materials. The recent advances discussed in this Account demonstrate how researchers can use the fundamental principles of supramolecular chemistry to craft the size, shape, and internal structure of nanoscale objects. In each system described here, we used atomic force microscopy (AFM) and transmission electron microscopy (TEM) to study the assembly morphology. Circular dichroism, nuclear magnetic resonance, infrared, and optical spectroscopy provided additional information about the self-assembly behavior in solution at the molecular level. Dendron rod-coil molecules self-assemble into flat or helical ribbons. They can incorporate electronically conductive groups and can be mineralized with inorganic semiconductors. To understand the relative importance of each segment in forming the supramolecular structure, we synthetically modified the dendron, rod, and coil portions. The self-assembly depended on the generation number of the dendron, the number of hydrogen-bonding functions, and the length of the rod and coil segments. We formed chiral helices using a dendron-rod-coil molecule prepared from an enantiomerically enriched coil. Because helical nanostructures are important targets for use in biomaterials, nonlinear optics, and stereoselective catalysis, researchers would like to precisely control their shape and size. Tripeptide-containing peptide lipid molecules assemble into straight or twisted nanofibers in organic solvents. As seen by AFM, the sterics of bulky end groups can tune the helical pitch of these peptide lipid nanofibers in organic solvents. Furthermore, we demonstrated the potential for pitch control using trans-to-cis photoisomerization of a terminal azobenzene group. Other molecules called peptide amphiphiles (PAs) are known to assemble in water into cylindrical nanostructures that appear as nanofiber bundles. Surprisingly, TEM of a PA substituted by a nitrobenzyl group revealed assembly into quadruple helical fibers with a braided morphology. Upon photocleavage of this the nitrobenzyl group, the helices transform into single cylindrical nanofibers. Finally, inspired by the tobacco mosaic virus, we used a dumbbell-shaped, oligo(phenylene ethynylene) template to control the length of a PA nanofiber self-assembly (<10 nm). AFM showed complete disappearance of long nanofibers in the presence of this rigid-rod template. Results from quick-freeze/deep-etch TEM and dynamic light scattering demonstrated the templating behavior in aqueous solution. This strategy could provide a general method to control size the length of nonspherical supramolecular nanostructures.

FIGURE 1

1D assemblies of DRC molecules: (a) TEM microscopy of the nanofiber structure; (b) molecular graphics representation of the nanofiber ribbon. Adapted with permssion from ref . Copyright 2001 American Chemical Society.

FIGURE 2

Chemical structure of DRC 1 and structural modifications, such as the number of hydrogen-bonding groups on the dendron (blue) and changes to the length of the rod (red).

FIGURE 3

In the crystalline state, the dendron–rod 2 crystallizes as a stack of cyclic tetramers. Hydrogen bonds between adjacent tetramers are indicated by dashed lines. Reproduced with permission from ref . Copyright 2001 American Chemical Society.

FIGURE 4

Chemical structure of oligothiophene-containing DRC 3.

FIGURE 5

(a) Molecular structure of chiral DRC 4, (b) AFM images of (R)-4 (left) and (S)-4 (right) drop cast on mica from acetonitrile solutions–the horizontal and vertical scale bars are identical for both images, and the white lines indicate handedness, and (c) the CD signal at 300 nm is plotted as a function of mole percent (S)-4. Adapted with permission from ref . Copyright 2005 American Chemical Society.

FIGURE 6

AFM images of the aggregates formed by compounds (a) 5 and (b) 6 in chlorocyclohexane. The height profile shows that these helices have a pitch of 22 ± 2 nm. Scale bars = 100 nm. Adapted with permission from ref . Copyright 2007 Wiley-VCH.

FIGURE 7

Steric interactions between the bulky end groups on the periphery of the nanostructure (left, green spheres) cause a torque, which is relieved (right) as the nanofiber forms a superhelix. Adapted with permission from ref . Copyright 2007 Wiley-VCH.

FIGURE 8

Reversible control of superhelical pitch by trans-cis photoisomerization of peptide lipid 16. Adapted with permission from ref . Copyright 2007 Wiley-VCH.

FIGURE 9

TEM micrographs of PA 17 (pH 11, 7.4 × 10−⁴ M, annealed at 80 °C and slowly cooled to 25 °C) (a) before and (b) after photoirradiation (350 nm, 250 W, 5 min) to give 18 and (c) TEM showing that the unirradiated helices (yellow arrow) were composed of braids of four individual fibrils (red arrows). Adapted with permission from ref . Copyright 2008 American Chemical Society.

FIGURE 10

Strategies for controlling length of self-assemblies: (a) vernier approach requires two complementary but different recognition groups (indicated as a ball and socket); (b) a small amount of a rigid-rod template (blue) can limit the length of a 1D nanostructure (green).

FIGURE 11

Chemical structures of (a) peptide amphiphile monomer 19 and (b) PEG-terminated oligo(phenylene ethynylene) template 20 (R = 2-octyldodecyl).

FIGURE 12

AFM height images of (a) peptide amphiphile 1 alone (heights = 5.3 ± 0.6 nm) or (b) 200:1 molar ratio mixture of 19 and 20 showed no fibers and (c) QFDE TEM of 200:1 molar ratio mixture of 19 to 20 revealing the aspect ratio of the small aggregates. Reproduced with permission from ref . Copyright 2008 American Chemical Society.