K. Yoosaf

Modular Engineering of H-Bonded Supramolecular Polymers for Reversible Functionalization of Carbon Nanotubes

A H-bond-driven, noncovalent, reversible solubilization/functionalization of multiwalled carbon nanotubes (MWCNTs) in apolar organic solvents (CHCl(3), CH(2)Cl(2), and toluene) has been accomplished through a dynamic combination of self-assembly and self-organization processes leading to the formation of supramolecular polymers, which enfold around the outer wall of the MWCNTs. To this end, a library of phenylacetylene molecular scaffolds with complementary recognition sites at their extremities has been synthesized. They exhibit triple parallel H-bonds between the NH-N-NH (DAD) functions of 2,6-di(acetylamino)pyridine and the CO-NH-CO (ADA) imidic groups of uracil derivatives. These residues are placed at 180° relative to each other (linear systems) or at 60°/120° (angular modules), in order to tune their ability of wrapping around MWCNTs. Molecular Dynamics (MD) simulations showed that the formation of the hybrid assembly MWCNT•[X•Y](n) (where X = 1a or 1b -DAD- and Y = 2, 3, or 4 -ADA-) is attributed to π-π and CH-π interactions between the graphitic walls of the carbon materials and the oligophenyleneethynylene polymer backbones along with its alkyl groups, respectively. Addition of polar or protic solvents, such as DMSO or MeOH, causes the disruption of the H-bonds with partial detachment of the polymer from the CNTs, followed by precipitation. Taking advantage of the chromophoric and luminescence properties of the molecular subunits, the solubilization/precipitation processes have been monitored by UV-vis absorption and luminescence spectroscopies. All hybrid MWCNTs-polymer materials have been also structurally characterized via thermogravimetric analysis (TGA), transmission electron microscopy (TEM), atomic force microscopy (AFM), scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS).

A Supramolecular Photosynthetic Model Made of a Multiporphyrinic Array Constructed around a C60 Core and a C60–Imidazole Derivative

The photophysical properties of a supramolecular fullerene-porphyrin ensemble resulting from the self-assembly of a pyrrolidinofullerene-imidazole derivative (F1) with a multimetalloporphyrin array constructed around a hexasubstituted fullerene core (F(ZnP)12) have been investigated. The fullerene hexa-adduct core of the host system does not play any active role in the cascade of photoinduced events of the supramolecular ensemble, indeed no intercomponent photoinduced processes could be observed in host F(ZnP)12. In contrast, upon axial coordination with the monosubstituted fullerene guest F1, a quantitative quenching of the fluorescence signal of the metalloporphyrins was observed for the supramolecular complex [F(ZnP)12(F1)n] both in polar and nonpolar solvents. In toluene, the supramolecular ensemble exhibits a charge transfer emission centered around nm, suggesting the occurrence of intramolecular face-to-face interactions of F1 with neighboring metalloporphyrin moieties within the self-assembled photoactive array. This mechanism is supported by the fact that a one order of magnitude increase in the binding constant was observed for the supramolecular complex [F(ZnP)12(F1)n] when compared with a reference system lacking the pyrrolidinofullerene unit. In benzonitrile, a long-lived charge-separated state (τ=0.3 μs) has been detected for the supramolecular adduct.

Photoinduced electron transfer in a clicked fullerene–porphyrin conjugate

A stable C60 derivative bearing an azide functional group has been prepared and used as a building block for the preparation of a fullerene–porphyrin conjugate (F–P–F) by reaction with a Zn(II)–porphyrin bearing two terminal alkyne groups under the copper mediated Huisgen 1,3-dipolar cycloaddition conditions. The electrochemical and photophysical properties of the resulting multicomponent system have been investigated in detail. In benzonitrile, F–P–F undergoes photoinduced electron transfer and the resulting charge separated state is relatively long lived (τ = 0.48 µs). In contrast, intramolecular energy transfer has been evidenced in toluene, with the generation of the fullerene triplet level upon selective excitation of the porphyrin moiety. In this solvent, a CT emission band is observed in the near-infrared region (λmax = 940 nm) as a consequence of a conformational equilibrium causing, to a minor extent, the formation of intramolecular porphyrin–fullerene tight pairs. This finding is supported by measurements of singlet oxygen sensitization and quenching of the long-lived fullerene centered triplet state in the oxygen free solution.

From molecular to macroscopic engineering: shaping hydrogen-bonded organic nanomaterials.

The self-assembly and self-organization behavior of chromophoric acetylenic scaffolds bearing 2,6-bis(acetylamino)pyridine (1, 2) or uracyl-type (3-9) terminal groups has been investigated by photophysical and microscopic methods. Systematic absorption and luminescence studies show that 1 and 2, thanks to a combination of solvophilic/solvophobic forces and π-π stacking interactions, undergo self-organization in apolar solvents (i.e., cyclohexane) and form spherical nanoparticles, as evidenced by wide-field optical microscopy, TEM, and AFM analysis. For the longer molecular module, 2, a more uniform size distribution is found (80-200 nm) compared to 1 (20-1000 nm). Temperature scans in the range 283-353 K show that the self-organized nanoparticles are reversibly formed and destroyed, being stable at lower temperatures. Molecular modules 1 and 2 were then thoroughly mixed with the complementary triply hydrogen-bonding units 3-9. Depending on the specific geometrical structure of 3-9, different nanostructures are evidenced by microscopic investigations. Combination of modules 1 or 2 with 3, which bears only one terminal uracyl unit, leads to the formation of vesicular structures; instead, when 1 is combined with bis-uracyl derivative 4 or 5, a structural evolution from nanoparticles to nanowires is observed. The length of the wires obtained by mixing 1 and 4 or 1 and 5 can be controlled by addition of 3, which prompts transformation of the wires into shorter rods. The replacement of linear system 5 with the related angular modules 6 and 7 enables formation of helical nanostructures, unambiguously evidenced by AFM. Finally, thermally induced self-assembly was studied in parallel with modules 8 and 9, in which the uracyl recognition sites are protected with tert-butyloxycarbonyl (BOC) groups. This strategy allows further control of the self-assembly/self-organization process by temperature, since the BOC group is completely removed on heating. Microscopy studies show that the BOC-protected ditopic modules 8 self-assemble and self-organize with 1 into ordered linear nanostructures, whereas BOC-protected tritopic system 9 gives rise to extended domains of circular nano-objects in combination with 1.

Thermosolutal self-organization of supramolecular polymers into nanocraters.

The ability of two complementary molecular modules bearing H-bonding uracilic and 2,6-(diacetylamino)pyridyl moieties to self-assemble and self-organize into submicrometer morphologies has been investigated by means of spectroscopic, thermogravimetric, and microscopic methods. Using uracilic (3)N-BOC-protected modules, it has been possible to thermally trigger the self-assembly/self-organization process of the two molecular modules, inducing the formation of objects on a mica surface that exhibit crater-like morphology and a very homogeneous size distribution. Confirmation of the presence of the hydrogen-bonding-driven self-assembly/self-organization process in solution was obtained by variable-temperature (VT) steady-state UV-vis absorption and emission measurements. The variation of the geometric and spatial features of the morphologies was monitored at different T by means of atomic force microscopy (AFM) and was interpreted by a nonequilibrium diffusion model for two chemical species in solution. The formation of nanostructures turned out to be affected by the solid substrate (molecular interactions at a solid-liquid interface), by the matter-momentum transport in solution (solute diffusivity D(0) and solvent kinematic viscosity ν), and the thermally dependent cleavage reaction of the BOC functions (T-dependent differential weight loss, θ = θ(Τ)) in a T interval extrapolated to ∼60 K. A scaling function, f = f (νD(0), ν/D(0), θ), relying on the onset condition of a concentration-driven thermosolutal instability has been established to simulate the T-dependent behavior of the structural dimension (i.e., height and radius) of the self-organized nanostructures as ⟨h⟩ ≈ f (T) and ⟨r⟩ ≈ 1/f (T).

Photo-induced Energy Transfer in a Th-Symmetrical Hexakis-adduct of C60 Substituted with π-Conjugated Oligomers

A stilbene derivative bearing a terminal alkyne unit has been prepared and grafted onto a Th-symmetrical C60 hexakis-adduct building block under alkyne/azide copper mediated Huisgen 1,3-dipolar cycloaddition conditions. The photophysical properties of the resulting fullerene derivative surrounded by 12 conjugated oligomers have been investigated. Upon excitation of the peripheral chromophores, an efficient intramolecular energy transfer to the C60 core has been evidenced.

Shape controlled synthesis of multi-branched gold nanocrystals through a facile one-pot bifunctional biomolecular approach

Anisotropic nanocrystals of gold and silver are promising candidates for sensing and therapeutic applications because of their high extinction coefficient, increased NIR response and localization of hot spots at their tips. Herein, we report a viable room temperature synthetic strategy to prepare multi-branched gold nanocrystals of varying morphologies without the aid of additional nanoseeds or shape directing agents. By systematically modulating the bifunctional ligand to the Au3+ ion molar ratio ([L-DOPA]/[HAuCl4] = 0.15–1), the plasmon absorption was tuned from visible (530 nm) to NIR (930 nm). The corresponding microscopic studies showed a gradual transformation of the nanomaterials morphology from multiply twinned spheres to branched stars and flowers. The detailed spectroscopic and microscopic studies have revealed that evolution of these branched nanocrystals proceeds through aggregation and subsequent overgrowth of initially produced spherical particles.

Engineering supramolecular photoactive nanomaterials by hydrogen-bonding interactions

The photophysical properties of molecules containing anthracene, pyrene, or phenyleneethynylene chromophores bearing complementary triple H-bonding terminal units, namely, 2,6-di(acetylamino)pyridine (donor–acceptor–donor, DAD) and uracyl (acceptor–donor–acceptor, ADA) have been investigated as a function of solvent polarity. For asymmetric systems, presenting only one H-bonding unit, a solvatochromic effect is found, suggesting a charge-transfer character of the lowest electronic excited state. Systematic absorption and emission studies carried out as a function of temperature show that phenylene-ethynylenes having linear geometry and H-bonding functionalities at both ends undergo reversible self-aggregation in cyclohexane (CHX), leading to the formation of spherical nanoparticles, as evidenced by wide-field fluorescence microscopy (WFM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). A combination of an anthracene derivative bearing only one ADA terminal functionality and a linear phenylene-ethynylene derivative possessing two DAD terminal groups in CHX (2:1 molecular ratio) leads to the formation of vesicular nanostructures. The interaction of linear phenylene-ethynylenes possessing two terminal 2,6-di(acetylamino)pyridine functionalities with that bearing bis uracylic units gives origin to nanofibers, while the assembly of the former with bisuracylic units exhibiting bent geometry leads to the formation of helical nanofibers. The length of these fibers can be controlled by addition of the anthracene derivative having only one uracyl group which effectively blocks the extent of H-bonding, prompting the formation of shorter nanorods.

Electrostatically driven self-assembly of CdTe nanoparticles with organic chromophores probed via Ham effect

We demonstrate that the polarity influence on the pyrene fluorescence band ratios (I3/I1) termed as the Ham effect can be effectively utilized to study the electrostatic interaction of CdTe nanoparticles (ζ = −46 mV) with the oppositely charged organic molecule, pyrenemethylamine hydrochloride (PMAH). As compared to water, the less polar nature of the QDs surface resulted in an increase in I3/I1 peak ratio of PMAH (from 0.34 to 0.54) when brought into close proximity. The lowering of the asymmetric stretching frequency of the carboxylate group of thioglycolic acid (1570 cm−1 to ∼1550 cm−1) and the drop in zeta potential of the nanoparticle from −46 mV to −8 mV indicate the interaction of PMAH with the nanoparticles. Control experiments with capping ligands, QDs with different surface charges (−46, −39, −35, −31, −20 and +21 mV) and negatively charged pyrene molecules invariably support the role of nanoparticles and their surface charge. The combined effects of charge neutralization and hydrophobicity increase lead to the organization of QDs into 2D sheet like superstructures, preserving their initial individuality.

Interfacial properties of hybrid nanomaterials

A brief summary of our ongoing efforts to understand the surface properties of nanoparticles using fluorophores, namely pyrene alkanethiols, is presented. Excited state interactions were investigated by varying the length of the spacer group and the concentration of fluorophore. The flexible long alkyl chain tethering pyrene inAu-P2/Au-P3 allows free interaction between fluorophores resulting in excimer formation whereas the intermolecular interactions are limited in theAu-P 1 system due to the restriction imposed by the curvature of spherical gold nanoparticle. A gradual increase in the peak intensity ratio of III/I band of the normal fluorescence of pyrene was observed indicating that the surface of nanoparticle is more polar than the bulk solvent (toluene)