Jon R. Parquette

Self-Assembly of 1-D n-Type Nanostructures Based on Naphthalene Diimide-Appended Dipeptides

n-Type 1D nanostructures are formed from the beta-sheet assembly of dipeptides bearing a 1,4,5,8-naphthalenetetracarboxylic acid diimide (NDI) side chain into either helical nanofibers or twisted nanoribbons. Amyloid-like 1-D helical nanofibers and twisted nanoribbons assemble in an aqueous solution depending on the placement of the NDI group. beta-Sheet-type hydrogen bonding and pi-pi association play important roles in directing the assembly process. A delicate balance between electrostatic repulsion and hydrophobic interactions is critical for self-assembly. Fluorescence lifetime and anisotropy experiments indicate that the nature of the intermolecular organization and packing within the nanostructures critically impacts intermolecular energy migration pi-electron delocalization.

Amphiphilic Self‐Assembly of an n‐Type Nanotube

The electronic properties of p-conjugated materials depend on the nature of the interactions among the constituent chromophores. The p–p stacking interactions present in aggregated arrays of semiconductors provide pathways for charge transport and energy migration. Thus, the selfassembly of p-conjugated building blocks into discrete, onedimensional (1D) nanostructures is a powerful strategy to tune the properties of organic electronic materials. The majority of these approaches have produced twisted nanofibers of p-type chromophores. The exceptional electronic characteristics of carbon nanotubes have also inspired interest in versatile supramolecular approaches toward p-conjugated nanotubes. The availability of self-assembled organic nanotubes would provide greater modularity in their design and functionalization. However, examples of p-conjugated systems that assemble into well-defined nanotubes are relatively uncommon. Herein, we describe a 1D n-type nanotube formed by the bolaamphiphilic self-assembly of 1,4,5,8-naphthalenetetracarboxylic acid diimide (NDI) with l-lysine headgroups (Figure 1). We recently reported a simple method for fabricating n-type 1D nanostructures by the b-sheet assembly of dipeptide–NDI conjugates into either helical nanofibers or twisted nanoribbons. Time-resolved fluorescence anisotropy experiments showed enhanced energy migration within these nanostructures. Herein, we explore how the intermolecular electrostatic interactions derived from the lysine headgroups in bolaamphiphile A (Figure 1 a), in conjunction with p–p association among the NDI chromophores, drive the self-assembly process in water toward soluble, well-ordered 1D nanotubes. Bolaamphiphile A was constructed by imidation of 1,4,5,8-naphthalenetetracarboxylic acid dianhydride with two equivalents of Boc-l-lysine, followed by TFA deprotection (Supporting Information, Scheme S1). Bolaamphiphile A formed a transparent gel in water at concentrations as low as 1% (w/w) (1.9 mm ; Figure 1b, red inset), and was stable in the gel state for several months. Transmission electron microscopy (TEM) of a negatively stained sample of A revealed the formation of micrometer-long nanotubes with uniform diameters of (12 1) nm (Figure 1b). The nanotubes appeared as two white, parallel lines separated by a dark center, which is consistent with the cross-sectional view of a hollow tubular structure filled with the negative stain, uranyl acetate (Figure 1 b). The thickness of the wall was approximately (2.5 0.5) nm. A few nanorings, albeit rare, could also be observed in the TEM images (red arrows in Figure 1 b), with external diameters of 12 nm and wall thicknesses of 2.5 nm, which are identical with the nanotube dimensions. Tapping-mode AFM imaging of dilute bolaamphiphile A gel samples (250 mm) on mica also revealed high-aspect ratio assemblies with cross-sectional heights of about 9 nm, which were slightly smaller than those observed by TEM; this effect Figure 1. a) Structures of lysine-based bolaamphiphiles A (R =O ) and B (R = OMe) and the assembly of A into rings, which stack to give tubes. The blue sections of A undergo hydrophobic p–p stacking interactions, and the red sections electrostatic interactions. b) TEM image of bolaamphiphile A in water (250 mm ; carbon-coated copper grid); 2% (w/w) uranyl acetate as negative stain. Blue insets: Two nanotubes and one nanoring. c) Tapping-mode AFM image of bolaamphiphile A in water (250 mm) on freshly cleaved mica. Red inset: Section analysis showing uniform height of the assemblies. Height indicated by red arrows: ca. 9 nm.

A π-conjugated hydrogel based on an Fmoc-dipeptide naphthalene diimide semiconductor

The beta-sheet self-assembly of an n-type NDI-dipeptide into a transparent, self-supporting hydrogel at low concentrations is described. The nanostructure of the gel is stabilized by the intermolecular pi-pi association of the NDI units and pi-pi interdigitation of the fluorene groups.

Enantioselective desymmetrization of meso-aziridines with TMSN3 or TMSCN catalyzed by discrete yttrium complexes.

using a variety ofnucleophiles have been the subject of extensive research. Theless developed ring-opening reactions, those of meso-aziri-dines by carbon and nitrogen nucleophiles, give direct accessto enantiopure b-amino acids and 1,2-diamines—two classesof compounds which have broad chemical and pharmaceut-ical relevance. Li, Fernndez, and Jacobsen first reportedenantioselective ring-opening of meso-aziridines withTMSN

Controllable Peptide–Dendron Self‐Assembly: Interconversion of Nanotubes and Fibrillar Nanostructures

Roll up: A peptide-dendron hybrid (PDH) is capable of self-assembling into either a soluble nanotube or an amyloid-like fibrillar network. The structures interconvert on adjusting the salt concentration or pH value. Their hydrophobic interfaces efficiently encapsulate hydrophobic molecules in water which can then be released by lowering the pH value.

Regiodivergent Ring Opening of Chiral Aziridines

A bimetallic catalyst selects different ring-opening sites, depending on the chiral configuration of the substrate. Kinetic resolution of strained three-membered rings has proven broadly useful for the generation of enantiopure organic intermediates. Herein we demonstrate a complementary approach whereby a single catalyst transforms a racemic mixture of aziridines to a pair of regioisomeric products, each in good yield with exceptionally high enantioselectivity. Specifically, the dimeric yttrium salen catalyst accelerates the ring opening of aliphatic aziridines by trimethylsilylazide, inducing nucleophilic attack at the primary position of one enantiomer and the secondary position of the other. Both rate and selectivity are highly sensitive to the catalyst structure.

Self-assembly of a donor-acceptor nanotube. A strategy to create bicontinuous arrays.

The self-assembly of bolaamphiphile 1 into nanotubes containing a nanostructured electron donor/acceptor heterojunction is reported. In 10% MeOH/H(2)O, the tetraphenylporphyrin (TPP) and 1,4,5,8-naphthalenetetracarboxylic acid diimide chromophores engage in strong J-type π-π interactions within monolayer rings that further stack into the nanotube assemblies. In 10% MeOH/H(2)O at pH 1 or 11 or in pure MeOH, assembly is driven exclusively by the TPP ring, leading to the formation of nonspecific, unstructured aggregates. Steady-state, time-resolved fluorescence and femtosecond transient absorption spectroscopy revealed a strong dependence of the fluorescence decay and electron-transfer/charge-recombination time constants on the nature of the assemblies. These studies highlight the importance of local nanostructure in determining the photophysical properties of optoelectronic materials.

Conformationally Driven Asymmetric Induction of a Catalytic Dendrimer

Natures catalysts promote the reactions necessary for life with extremely high specificity by folding into specific shapes capable of communicating remote structural information to an active site. Achieving this objective in synthetic systems has been hampered by the lack of information concerning how dynamic conformational chirality can influence the stereoselectivity of a catalytic process. Herein, we report the first illustration of a catalytic dendrimer that achieves high enantioselectivity by amplifying/propagating local chirality via a dynamically folded structure. Experimental evidence supports a chiral relay mechanism that propagates local terminal chirality of the dendron to the axial chirality of the biphenyl core through the helical secondary structure of the dendron.

A model for the controlled assembly of semiconductor peptides

The self-assembly of small molecules provides a potentially powerful method to create functional nanomaterials for many applications ranging from optoelectronics to oncology. However, the design of well-defined nanostructures via molecular assembly is a highly empirical process, which severely hampers efforts to create functional nanostructures using this method. In this review, we describe a simple strategy to control the assembly of functionalized peptides by balancing attractive hydrophobic effects that drive assembly with opposing electrostatic repulsions. Extended π-π contacts are created in the nanostructures when assembly is driven by π-stacking interactions among chromophores that are appended to the peptide. The formation of insoluble β-sheet aggregates are mitigated by incorporating charged side-chains capable of attenuating the assembly process. Although the application of this approach to the assembly of organic semiconductors is described, we expect this strategy to be effective for many other functional organic materials.

The Self‐Assembly of Anticancer Camptothecin–Dipeptide Nanotubes: A Minimalistic and High Drug Loading Approach to Increased Efficacy

20-(S)-Camptothecin (CPT)-conjugated dipeptides are reported that preassemble into nanotubes with diameters ranging from 80-120 nm. These nanoassemblies maintain a high (∼47 %) drug loading and exhibit greater drug stability (i.e., resistance to lactone hydrolysis), and consequently greater efficacy against several human cancer cells (HT-29, A549, H460, and H23) in vitro compared with the clinically used prodrug irinotecan. A key and defining feature of this system is the use of the CPT-conjugated dipeptide as both the drug and precursor to the nanostructured carrier, which simplifies the overall fabrication process.