Naphthalene Imide Conjugates: Formation of Supramolecular Assemblies,and the Encapsulation and Release of Dyes through DNA‐Mediated Disassembly

We report the synthesis of two new amphiphilic conjugates 1 and 2 based on naphthalene di‐ and monoimide chromophores and the investigation of their photophysical, self‐assembly and DNA‐binding properties. These conjugates showed aqueous good solubility and exhibited strong interactions with DNA and polynucleotides such as poly(dG?dC)–poly(dG?dC) and poly(dA?dT)–poly(dA?dT). The interaction of these conjugates with DNA was evaluated by photo‐ and biophysical techniques. These studies revealed that the conjugates interact with DNA through intercalation with association constants in the order of 5–8×10⁴ M ^?1. Of these two conjugates, bolaamphiphile 1 exhibited a supramolecular assembly that formed vesicles with an approximate diameter of 220 nm in the aqueous medium at a critical aggregation concentration of 0.4 mM , which was confirmed by SEM and TEM. These vesicular structures showed a strong affinity for hydrophobic molecules such as Nile red through encapsulation. Uniquely, when exposed to DNA the vesicles disassembled, and therefore this transformation could be utilised for the encapsulation and release of hydrophobic molecules by employing DNA as a stimulus.     

Bifunctional DNA Architectonics: Three‐Way Junctions with Sticky Perylene Bisimide Caps and a Central Metal Lock

A new type of a bifunctional DNA architecture based on a three way junction is developed that combines the structural motif of sticky perylene bisimide caps with a tris‐bipyridyl metal ion lock in the center part. A clear stabilizing effect was observed in the presence of Fe³⁺, Ni²⁺ and Zn²⁺ by the formation of corresponding bipyridyl complexes in the branching part of the DNA three way junctions. The dimerization of the 5′‐terminally attached perylene diimides (PDI) chromophores by hydrophobic interactions can be followed by significant changes in the UV/Vis absorption and steady‐state fluorescence. The PDI‐mediated DNA assembly occurs at temperatures below the melting temperature and is not influenced by the metal‐ion bipyridyl locks in the central part. The corresponding AFM images revealed the formation of higher‐ordered structures as the result of DNA assemblies mediated by the PDI interactions.     

Supramolecular polymers in aqueous medium: rational design based on directional hydrophobic interactions

Self-assembly in aqueous medium is of primary importance and widely employs hydrophobic interactions. Yet, unlike directional hydrogen bonds, hydrophobic interactions lack directionality, making difficult rational self-assembly design. Directional hydrophobic motif would significantly enhance rational design in aqueous self-assembly, yet general approaches to such interactions are currently lacking. Here, we show that pairwise directional hydrophobic/π-stacking interactions can be designed using well-defined sterics and supramolecular multivalency. Our system utilizes a hexasubstituted benzene scaffold decorated with 3 (compound 1) or 6 (compound 2) amphiphilc perylene diimides. It imposes a pairwise self-assembly mode, leading to well-defined supramolecular polymers in aqueous medium. the assemblies were characterized using cryogenic electron microscopy, small-angle X-ray scattering, optical spectroscopy, and EPR. Supramolecular polymerization studies in the case of 2 revealed association constants in 10(8) M(-1) range, and significant enthalpic contribution to the polymerization free energy. The pairwise PDI motif enables exciton confinement and localized emission in the polymers based on 1 and 2's unique photonic behavior, untypical of the extended π-stacked systems. Directional pairwise hydrophobic interactions introduce a novel strategy for rational design of noncovalent assemblies in aqueous medium, and bring about a unique photofunction.     

Cuboid Vesicles Formed by Frame‐Guided Assembly on DNA Origami Scaffolds

We describe the use of a frame‐guided assembly (FGA) strategy to construct cuboid and dumbbell‐shaped hetero‐vesicles on DNA origami nanostructure scaffolds. These are achieved by varying the design of the DNA origami scaffolds that direct the distribution of the leading hydrophobic groups (LHG). By careful selection of LHGs, different types of amphiphiles (both polymer and small‐molecule surfactants) were guided to form hetero‐vesicles, demonstrating the versatility of the FGA strategy and its potential to construct asymmetric and dynamic hetero‐vesicle assemblies with complex DNA nano‐scaffolds.     

Self-assembly of coumarin-conjugated acidic proteinoids

Photo- and pH-responsive self assembly was prepared by dispersing coumarin-conjugated acidic proteinoid in an aqueous phase. Proteinoid composed of Asp, Ser and Leu (PASL) was prepared as an acidic one by a melt-condensation method. Epoxypropoxy coumarin (EPC) was conjugated to PASL through its reaction with Ser. Assuming all Ser residues reacted with EPC, the calculated molar ratio of Asp/Ser-EPC/Leu in EPC-PASL conjugate was about 202/18/1 on ¹H NMR spectrum. EPC-PASL conjugates exhibited the dimerization and the de-dimerization of their EPC residues under cyclic irradiation of 365 nm and 254 nm. They were self-assembled into microparticle (about 500 nm), possibly due to the hydrophobic interaction among Leu residues and EPC residues. The EPC-PASL assemblies were linked each other by the photo-dimerization under the UV irradiation of 365 nm. EPCs are the hydrophobic pendants of the PASL conjugates and they could be easy to be hydrophobically assemblied. This may account for why EPC-PASL assemblies were much greater than PASL assemblies. On the contrary, EPC of PASL conjugates was cleavaged by the photo-dedimerization under the UV irradiation of 254 nm. The size of the assemblies increased and decreased in a cyclic manner under the cyclic irradiation, and the size of the photo cross-linked assembly was larger at a higher pH, possibly due to stronger intermolecular repulsion.     

Supramolecular pseudorotaxane polymers from complementary pairs of homoditopic molecules

Self-assembly of supramolecular pseudorotaxane polymers from complementary homoditopic building blocks comprised of bis(dibenzo-24-crown-8) esters derived from the hydroxymethyl crown ether and aliphatic diacid chlorides (CxC, x = number of methylene units in the diacid segment) and 1,10-bis[p-(benzylammoniomethyl)phenoxy]alkane bis(hexafluorophosphate)s (AyA, y = number of methylene units in the linker) has been studied. (1)H NMR spectroscopic studies of bis[(2-dibenzo-24-crown-8)methyl] sebacate (C8C) with dibenzylammonium hexafluorophosphate (6) showed that the two binding sites of the ditopic host are equivalent and independent (no positive or negative cooperativity). Likewise the binding sites in 1,10-bis[p-(benzylammoniomethyl)phenoxy]decane bis(hexafluorophosphate) (A10A) were shown to behave independently with dibenzo-24-crown-8 (1a). Then using (1)H NMR spectroscopy on dilute equimolar solutions (<1 mM) of CxC and AyA association constants were estimated for the formation of the linear (lin-CxC*AyA) and cyclic (cyc-CxC*AyA) dimers, thus enabling effective molarities to be estimated for the various systems. Finally (1)H NMR spectroscopy was used to semiquantitatively or qualitatively demonstrate the formation of linear supramolecular polymers lin-[CxC*AyA](n) in more concentrated solutions (up to 2.0 M) of the complementary pairs of CxC and AyA. The sizes of the assemblies (n values) are not as great as the dilute solution studies predict; this is attributed to the deleterious effect of ionic strength and exo complexation at high concentrations. However, as expected from the dilute solution results, linear extension is indeed favored with the longer building blocks, meaning that "monomer" end-to-end distance is a key factor in reducing the amount of cyclic species that form. Viscosity experiments clearly demonstrate the formation of large noncovalent polymers lin-[CxC*AyA](n) in concentrated solutions. Cohesive film and fiber formation also indicate that supramolecular polymers of sufficient size to enable entanglement self-assemble in these solutions.     

Dynamic multicomponent hemiaminal assembly

A simple approach to generating in situ metal-templated tris-(2-picolyl)amine-like multicomponent assemblies with potential applications in molecular recognition and sensing is reported. The assembly is based on the reversible covalent association between di-(2-picolyl)amine and aldehydes. Zinc ion is best for inducing assembly among the metal salts investigated, whereas 2-picolinaldehyde is the best among the heterocyclic aldehydes studied. Although an equilibrium constant of 6.6×10(3) M(-1) was measured for the assembly formed by 2-picolinaldehdye, di-(2-picolyl)amine, and zinc triflate, the equilibrium constants for other systems are in the 10(2) M(-1) range. X-ray structural analysis revealed that zinc adopts trigonal-bipyramidal geometry within the assembled ligand. The diversity and equilibrium of the assemblies are readily altered by simply changing concentrations, varying components, or adding counteranions.     

Molecular arrangement and assembly guided by hydrophobic cavities inside DNA

DNA is a unique yet useful material to organize nanoscale molecular arrays along the helix axis. In this study, we demonstrate a useful approach for creating molecular arrays inside a double helical DNA. Our approach is based on a host-guest system. Introducing abasic sites into DNA afforded a hydrophobic cavity that serves as a host. A planar aromatic molecule (cationic perylenediimide, PDI) was used as the guest molecule. In an aqueous solution, the PDI molecules tend to aggregate with themselves due to the strong hydrophobicity. In the presence of DNA with the cavity, the binding of the PDI was found to site-specifically occur in the hydrophobic cavity. The unique assembly and arrangement for more than two PDI molecules was achieved by controlling the sizes and positions of the cavities. Our approach would provide a simple and convenient way to construct one-dimensional aromatic arrays in DNA.     

Preparation and characterization of self-assembled nanoparticles of heparin-deoxycholic acid conjugates

Various deoxycholic acid (DOCA) bearing heparin (HD) amphiphilic conjugates with different degrees of substitution (DS) with DOCA were synthesized using heparin as a hydrophilic segment and DOCA as a hydrophobic segment. Structural characteristics of these HD conjugates were investigated using 1H NMR, dynamic light scattering, zeta potential, transmission electron microscopy (TEM), and fluorescence spectroscopy. HD conjugates provided monodispersed self-aggregates in water, with mean diameters decreasing with increasing DOCA DS in the range of 120-200 nm. HD aggregates were covered with negatively charged heparin shells, exhibiting xi potentials near -56 mV. The critical aggregation concentration (cac) of the HD (0.02-0.003 mg/mL) depended upon DOCA DS. TEM images demonstrated that the shape of the self-aggregates was spherical. Partition equilibrium constants, Kv, for pyrene in the self-aggregates in water indicated that increasing DS enhanced the hydrophobicity of the self-aggregate inner core. The mean aggregation number of DOCA per hydrophobic microdomain, estimated by the fluorescence quenching methods using cetylpyridinium chloride, indicated that five to nine of HD chains comprised a hydrophobic domain in the conjugates.     

Ultrasound‐Driven Secondary Self‐Assembly of Amphiphilic β‐Cyclodextrin Dimers

The controlled secondary self‐assembly of amphiphilic molecules in solution is theoretically and practically significant in amphiphilic molecular applications. An amphiphilic β‐cyclodextrin (β‐CD) dimer, namely LA‐(CD)₂, has been synthesized, wherein one lithocholic acid (LA) unit is hydrophobic and two β‐CD units are hydrophilic. In an aqueous solution at room temperature, LA‐(CD)₂ self‐assembles into spherical micelles without ultrasonication. The primary micelles dissociates and then secondarily form self‐assemblies with branched structures under ultrasonication. The branched aggregates revert to primary micelles at high temperature. The ultrasound‐driven secondary self‐assembly is confirmed by transmission electron microscopy, dynamic light scattering, ¹H NMR spectroscopy, and Cu²⁺‐responsive experiments. Furthermore, 2D NOESY NMR and UV/Vis spectroscopy results indicate that the formation of the primary micelles is driven by hydrophilic–hydrophobic interactions, whereas host–guest interactions promote the formation of the secondary assemblies. Additionally, ultrasonication is shown to be able to effectively destroy the primary hydrophilic–hydrophobic balances while enhancing the host–guest interaction between the LA and β‐CD moieties at room temperature.     

Salt effects on poly(N-isopropylacrylamide) phase transition thermodynamics from NMR spectroscopy

NMR spectra were collected for cross-linked poly(N-isopropylacrylamide), poly(NIPAM), hydrogels in the presence of NaCl and CaCl2 aqueous solutions. Intensity variations in the 1H NMR signals of the polymer provide insight into the phase transition process. These data were used to observe a two-stage phase transition process. Thermodynamic quantities were obtained from a van't Hoff analysis of the temperature-dependent equilibrium constants, which were derived from the NMR data. The Delta H degrees and Delta S degrees values for the hydrogel in D2O are 3.4 kJ/mol and 11.2 J/mol.K for stage I, which is attributed to the formation of hydrophobic bonds between neighboring isopropyl groups. The formation of hydrogen bonds during stage II yielded Delta H degrees and Delta S degrees values of 14.8 kJ/mol and 48.4 J/mol.K in D2O. However, the corresponding Delta H degrees values in 150 mM NaCl and 150 mM CaCl2 are reduced to 1.5 and 1.8 kJ/mol for stage I of the dehydration process. This corresponds to the known effect of salts on hydrophobic bond energetics. The value of Delta S degrees also decreased to 4.9 and 5.9 J/mol.K in NaCl and CaCl2 solutions, respectively. However, the thermodynamic values during stage II were only slightly affected by the salts. The lower temperatures required to induce spontaneous precipitation implies that Delta G degrees of precipitation is reduced. With our measurement of equilibrium thermodynamics, we see that 150 mM NaCl and CaCl2 solutions have a greater effect on hydrophobic bond formation associated with the phase transition process. In this manner, these salts aid in solvent reorganization necessary to form the hydrophobic bond, and this suggests that the formation of hydrophobic bonds is a strong determining factor in the stability of poly(NIPAM) hydrogels in water.     

Adsorption of sodium dodecyl sulfate onto clathrate hydrates in the presence of salt

This work presents the effect of NaCl on the adsorption of sodium dodecyl sulfate (SDS) at the cyclopentane (CP) hydrate-water interface. The adsorption isotherms and the SDS solubility in NaCl solutions are obtained using liquid-liquid titrations. The solubility data are determined at typical hydrate forming temperatures (274-287K) to ensure that the adsorption isotherms are obtained within SDS solubility limits in NaCl solutions. The isotherms show L-S (Langmuir-Step) type behaviors with 1mM and 10mM NaCl solutions while L type isotherm is determined for 25mM NaCl solutions due to the low SDS solubility in this salt concentration. Zeta potentials of CP hydrate particles in the aqueous solutions support the shape of the adsorption isotherm with the 1mM NaCl solution. The 1mM NaCl case shows the highest SDS adsorption amount among the cases with 0mM, 10mM, and 25mM NaCl solutions. In this case, the competition for adsorption between Cl(-) and DS(-) is not as strong compared to the 10 and 25mM NaCl cases and the presence of Na(+) ions may reduce the repulsion between DS(-) ions, which results in a higher adsorption of DS(-) ions and enhanced enclathration.     

Hierarchical organization of ferrocene-peptides

Hierarchical self-assembly of disubstituted ferrocene (Fc)-peptide conjugates that possess Gly-Val-Phe and Gly-Val-Phe-Phe peptide substituents leads to the formation of nano- and micro-sized assemblies. Hydrogen-bonding and hydrophobic interactions provide directionality to the assembly patterns. The self-assembling behavior of these compounds was studied in solution by using (1)H?NMR and circular dichroism (CD) spectroscopies. In the solid state, attenuated total reflectance (ATR) FTIR spectroscopy, single-crystal X-ray diffraction (XRD), powder X-ray diffraction (PXRD), and scanning electron microscopy (SEM) methods were used. Spontaneous self-assembly of Fc-peptides through intra- and intermolecular hydrogen-bonding interactions induces supramolecular assemblies, which further associate and give rise to fibers, large fibrous crystals, and twisted ropes. In the case of Fc[CO-Gly-Val-Phe-OMe](2) (1), molecules initially interact to form pleated sheets that undergo association into long fibers that form bundles and rectangular crystalline cuboids. Molecular offsets and defects, such as screw dislocations and solvent effects that occur during crystal growth, induce the formation of helical arrangements, ultimately leading to large twisted ropes. By contrast, the Fc-tetrapeptide conjugate Fc[CO-Gly-Val-Phe-Phe-OMe](2) (2) forms a network of nanofibers at the supramolecular level, presumably due to the additional hydrogen-bonding and hydrophobic interactions that stem from the additional Phe residues.     

Association in Unsalted and Brine Solutions of a Water-Soluble Terpolymer with <Emphasis Type="Italic">p</Emphasis>-Vinylbenzyl-Terminated Octylphenoxy Poly(ethylene oxide)

An associating terpolymer (PAOE) of acrylamide (AM), sodium 2-acrylamido-2-methylpropane sulphonate (NaAMPS), and a novel macromonomer: p-vinylbenzyl-terminated octylphenoxy poly(ethylene oxide) (VOE, degree of polymerization: 4) was synthesized by aqueous free-radical copolymerization. The PAOE polymer exhibited excellent thickening properties in unsalted and brine solutions. The electrostatic shielding of repulsive interactions of the polymer was much weaker than that of the linear associating polymers with small hydrophobic monomers. This brine solution exhibited unexpected salt-thickening behaviors twice, and good resistance to salt and ageing. The intermolecular hydrophobic association in unsalted and brine PAOE solutions, as functions of polymer and NaCl concentration, were characterized by fluorescence spectroscopy. With the addition of NaCl, the polymer chains were comparatively extended and continuous network structures were formed via the intermolecular hydrophobic association in brine solutions as well as in unsalted solutions at 0.15–0.25 g⋅dL⁻¹ PAOE, as observed by a scanning electron microscope (SEM).     

Well-defined critical association concentration and rapid adsorption at the air/water interface of a short amphiphilic polymer, amphipol A8-35: a study by Förster resonance energy transfer and dynamic surface tension measurements

Amphipols (APols) are short amphiphilic polymers designed to handle membrane proteins (MPs) in aqueous solutions as an alternative to small surfactants (detergents). APols adsorb onto the transmembrane, hydrophobic surface of MPs, forming small, water-soluble complexes, in which the protein is biochemically stabilized. At variance with MP/detergent complexes, MP/APol ones remain stable even at extreme dilutions. Pure APol solutions self-associate into well-defined micelle-like globules comprising a few APol molecules, a rather unusual behavior for amphiphilic polymers, which typically form ill-defined assemblies. The best characterized APol to date, A8-35, is a random copolymer of acrylic acid, isopropylacrylamide, and octylacrylamide. In the present work, the concentration threshold for self-association of A8-35 in salty buffer (NaCl 100 mM, Tris/HCl 20 mM, pH 8.0) has been studied by F?rster resonance energy transfer (FRET) measurements and tensiometry. In a 1:1 mol/mol mixture of APols grafted with either rhodamine or 7-nitro-1,2,3-benzoxadiazole, the FRET signal as a function of A8-35 concentration is essentially zero below a threshold concentration of 0.002 g·L(-1) and increases linearly with concentration above this threshold. This indicates that assembly takes place in a narrow concentration interval around 0.002 g·L(-1). Surface tension measurements decreases regularly with concentration until a threshold of ca. 0.004 g·L(-1), beyond which it reaches a plateau at ca. 30 mN·m(-1). Within experimental uncertainties, the two techniques thus yield a comparable estimate of the critical self-assembly concentration. The kinetics of variation of the surface tension was analyzed by dynamic surface tension measurements in the time window 10 ms-100 s. The rate of surface tension decrease was similar in solutions of A8-35 and of the anionic surfactant sodium dodecylsulfate when both compounds were at a similar molar concentration of n-alkyl moieties. Overall, the solution properties of APol "micelles" (in salty buffer) appear surprisingly similar to those of the micelles formed by small, nonpolymeric surfactants, a feature that was not anticipated owing to the polymeric and polydisperse nature of A8-35. The key to the remarkable stability to dilution of A8-35 globules, likely to include also that of MP/APol complexes, lies accordingly in the low value of the critical self-association concentration as compared to that of small amphiphilic analogues.     

Contributions of hydroxyethyl groups to the DNA binding affinities of anthracene probes

Contributions of hydroxyethyl functions to the DNA binding affinities of substituted anthracenes are evaluated by calorimetry and spectroscopy. Isothermal titration calorimetry indicated that binding of the ligands to calf thymus DNA (5 mM Tris buffer, 50 mM NaCl, pH 7.2, 25 degrees C) is exothermic. The binding constants increased from 1.5 x 10(4) to 1.7 x 10(6) M(-1) as a function of increase in the number of hydroxyethyl functions (0-4). DNA binding was accompanied by red-shifted absorption (approximately 630 cm(-1)), strong hypochromism (>65%), positive induced-circular dichroism bands, and negative linear dichroism signals. DNA binding, in general, increased the helix stabilities to a significant extent (DeltaT(m) approximately 7 degrees C, DeltaDeltaH approximately 3 kcal/mol, DeltaDeltaS approximately 6-20 cal/K.mol). The binding constants showed a strong correlation with the number of hydroxyethyl groups present on the anthracene ring system. Analysis of the binding data using the hydrophobicity parameter (Log P) showed a poor correlation between the binding affinity and hydrophobicity. This observation was also supported by a comparison of the affinities of probes carrying N-ethyl (Kb = 0.8 x 10(5) M(-1)) versus N-hydroxyethyl side chains (Kb = 5.5 x 10(5) M(-1)). These are the very first examples of a strong quantitative correlation between the DNA binding affinity of a probe and the number of hydroxyethyl groups present on the probe. These quantitative findings are useful in the rational design of new ligands for high-affinity binding to DNA.     

Effects of electrolyte and pH on the behavior of cross-linked films of ferrocene-modified poly(ethylenimine)

Ferrocene redox polymers based on the coupling of ferrocenecarboxaldehyde to both linear and branched poly(ethylenimine) (PEI) have been prepared to investigate the effects of pH, electrolyte, and cross-linking on electron charge transport and film swelling. The redox behavior of both ferrocene-modified linear PEI and ferrocene-modified branched PEI was investigated by cyclic voltammetry, while electron diffusion coefficients reported for PEI-based redox polymers were determined by electrochemical impedance spectroscopy. In phosphate solutions at pH>7, cross-linked films of both redox polymers exhibited multiple redox wave behavior and were unstable. In contrast, in non-phosphate solutions, cross-linked films exhibited stable electrochemical behavior and fast electron transfer in solutions with pH<11. Gel swelling experiments suggested that the multiple wave behavior and instability exhibited in either phosphate solutions or at high pH in non-phosphate solutions were related to a combination of film collapse and electrolyte binding within the hydrogel. The electron diffusion coefficients for these polymers are on the order of 10-8 (mol cm(-2) s(-1/2)), which are approximately 40 times greater than other ferrocene-modified polymers. Incorporation of the enzyme, glucose oxidase, into these films demonstrated that these redox polymers were able to electrically communicate with the enzyme's flavin adenine dinucleotide (FAD) redox centers. Glucose sensors based on these films exhibited enzyme saturation current densities that ranged from 240 to 480 microA/cm2 in response to glucose, which were dependent upon the supporting electrolyte and pH. The sensitivity of these sensors at 5 mM glucose ranged from 10 to 48 microA.cm(-2).mM(-1).     

Hierarchical Organization of Ferrocene–Peptides

Hierarchical self‐assembly of disubstituted ferrocene (Fc)–peptide conjugates that possess Gly‐Val‐Phe and Gly‐Val‐Phe‐Phe peptide substituents leads to the formation of nano‐ and micro‐sized assemblies. Hydrogen‐bonding and hydrophobic interactions provide directionality to the assembly patterns. The self‐assembling behavior of these compounds was studied in solution by using ¹H NMR and circular dichroism (CD) spectroscopies. In the solid state, attenuated total reflectance (ATR) FTIR spectroscopy, single‐crystal X‐ray diffraction (XRD), powder X‐ray diffraction (PXRD), and scanning electron microscopy (SEM) methods were used. Spontaneous self‐assembly of Fc–peptides through intra‐ and intermolecular hydrogen‐bonding interactions induces supramolecular assemblies, which further associate and give rise to fibers, large fibrous crystals, and twisted ropes. In the case of Fc[CO‐Gly‐Val‐Phe‐OMe]₂ ( 1 ), molecules initially interact to form pleated sheets that undergo association into long fibers that form bundles and rectangular crystalline cuboids. Molecular offsets and defects, such as screw dislocations and solvent effects that occur during crystal growth, induce the formation of helical arrangements, ultimately leading to large twisted ropes. By contrast, the Fc–tetrapeptide conjugate Fc[CO‐Gly‐Val‐Phe‐Phe‐OMe]₂ ( 2 ) forms a network of nanofibers at the supramolecular level, presumably due to the additional hydrogen‐bonding and hydrophobic interactions that stem from the additional Phe residues.     

Peptide Conjugates for Directing the Morphology and Assembly of 1D Nanoparticle Superstructures

Designed peptide conjugates molecules are used to direct the synthesis and assembly of gold nanoparticles into complex 1D nanoparticle superstructures with various morphologies. Four peptide conjugates, each based on the gold‐binding peptide (AYSSGAPPMPPF; PEP_Au), are prepared: C₁₂H₂₃O‐AYSSGAPPMPP ( 1 ), C₁₂H₂₃O‐AYSSGAPPMPPF ( 2 ), C₁₂H₂₃O‐AYSSGAPPMPPFF ( 3 ), and C₁₂H₂₃O‐AYSSGAPPMPPFFF ( 4 ). The affect that C‐terminal hydrophobic F residues have on both the soft‐assembly of the peptide conjugates and the resulting assembly of gold nanoparticle superstructures is examined. It is shown that the addition of two C‐terminal F residues ( 3 ) leads to thick, branched 1D gold nanoparticle superstructures, whereas the addition of three C‐terminal F residues ( 4 ) leads to bundling of thin 1D nanoparticle superstructures.     

Excited-state symmetry breaking in cofacial and linear dimers of a green perylenediimide chlorophyll analogue leading to ultrafast charge separation

Photoexcitation of chromophoric dimers constrained to a symmetric pi-stacked geometry by their molecular structure usually produces excimers independent of solvent polarity, while dimers with edge-to-edge perpendicular pi systems undergo excited-state symmetry breaking in highly polar solvents leading to intradimer charge separation. We present direct evidence for symmetry breaking in the lowest excited singlet state of a symmetric cofacial dimer of 1,7-bis(pyrrolidin-1'-yl)-perylene-3,4:9,10-bis(dicarboximide) (5PDI) in the low polarity solvent toluene to produce a radical ion pair quantitatively. This dimer, cof-5PDI2, was synthesized by attaching two 5PDI chromophores via imide groups to a xanthene spacer. For comparison, a linear symmetric dimer, lin-5PDI2, was prepared in which the 5PDI chromophores are linked end-to-end via a N-N single bond between their imides. The edge-to-edge pi systems of the 5PDI chromophores within lin-5PDI2 are perpendicular to one another. Ground-state absorption spectra of both 5PDI dimers show exciton coupling, which is consistent with the orientation of the 5PDI chromophores relative to one another. Ultrafast transient absorption spectroscopy following excitation of the dimers with 700 nm, 100 fs laser pulses shows that quantitative intradimer electron transfer occurs in cof-5PDI2 in toluene with tau = 0.17 ps followed by charge recombination to the ground state with tau = 222 ps. Similar measurements on lin-5PDI2 reveal that photoinduced electron transfer does not occur in toluene, but occurs in more polar solvents such as 2-methyltetrahydrofuran, wherein tau = 55 ps for charge separation and tau = 99 ps for charge recombination. Excited-state symmetry breaking in 5PDI dimers provides new routes to biomimetic charge separation and storage assemblies that can be more easily prepared and modified than those based on multiple tetrapyrrole macrocycles.