Cooperative Self-Assembly of Helical Exciton-Coupled Biosurfactant-Functionalized Porphyrin Chromophores.

Biobased, self-organizing molecules are of considerable interest as functional materials due to their structural versatility, sophisticated nanoarchitectures, and sustainable biosynthesis. Here, we study the self-assembly of a systematic series of bioconjugate sophorolipid-functionalized zinc porphyrin complexes with potential applications in optoelectronic devices. Our results provide insight into the molecular features that control the polymerization pathway, in particular the influence of carbohydrate chirality and noncovalent hydrogen-bonding on biosurfactant-driven self-organization of sophisticated light-absorbing supramolecular polymers. The self-assembly process is driven by a combination of hydrogen-bond, steric, and π-π interactions. Compounds under investigation were synthesized to examine the influence of peripheral chiral carbohydrate hydrogen bonding on chromophore aggregation and physicochemical properties through selective acetylation of the sophorose moiety. In dilute methanol/water solution, we found that excitonically coupled helical structures form by strong carbohydrate hydrogen-bonding interactions, in contrast to weakly coupled J-type aggregate formation with acetyl-group substitution of sugar hydroxyl moieties. Temperature-dependent UV/vis absorption and circular dichroism revealed that supramolecular polymerization proceeds through a cooperative mechanism of self-assembly for compounds bearing free OH groups capable of forming hydrogen-bond interactions. In contrast, per-acetylation of the sophorolipid's sugar group leads to micellar aggregation that is governed by a sterically driven isodesmic (noncooperative) assembly mechanism.

Biosurfactant-functionalized porphyrin chromophore that forms J-aggregates.

Structurally complex biosynthesized building blocks whose structures can be systematically varied are of great interest for the synthesis of manipulable self-organizing supramolecular systems. Sophorolipids (SLs) are an important class of glycolipid biosurfactants that consists of a sophorose (glucose disaccharide) polar head group that allows structural diversification by full or selective acetylation at the 6'- and 6''-positions. Porphyrins are a group of naturally-occurring heterocyclic macromolecular organic compounds that have efficient charge transfer properties. Herein we describe the synthesis of SL-porphyrin conjugates where the number of sophorolipid arms, availability of hydrogen bonding sophorose hydroxyl groups and rigidity of the lipid chain were systematically varied. SLs differing in 'sophorose acetylation' and 'lipid unsaturation' were conjugated to zinc-porphyrin dyes by copper(i)-catalyzed azide-alkyne cycloaddition (CuAAC) 'click' chemistry. Mono-, di-, and tetra-conjugation of SL-arms to the zinc-porphyrin core provided variation in SL-arm steric effects. UV-vis spectra in methanol/water reveal features indicative of supramolecular J-type aggregates. The synthesized compounds were designed to provide a library of unique bio-based molecules with built-in variation in non-covalent interactions, hydrogen-bonding, π-π stacking, metal-ligand coordination, dipole-dipole, van der Waals, and hydrophobic interactions for future interrogation of supramolecular self-assembly into functional materials for electro-optical applications.

Dielectric Properties of Bio-Based Diphenolate Ester Epoxies.

Thermoset bio-based diglycidyl ether of diphenolate esters (DGEDP) exhibit comparable mechanical properties as petroleum-derived diglycidyl ether of bisphenol A (DGEBA), whereas DGEDP is derived from levulinic acid, a safe and readily renewable feedstock. To determine the potential replacement of DGEBA as dielectric materials, a series of DGEDP-esters (i.e., methyl, ethyl, propyl, and butyl esters) were synthesized and studied. Broadband dielectric spectroscopy revealed that DGEDP-propyl has the highest dielectric constant in the series, comparable to DGEBA. Differences in the dielectric properties of DGEDP-esters is attributed to the interplay of segmental, small local, and side-chain motions on one hand and free volume and steric hindrance on the other.

Chromatic control in coextruded layered polymer microlenses.

We describe the formation, characterization and theoretical understanding of microlenses comprised of alternating polystyrene and polymethylmethacrylate layers produced by multilayer coextrusion. These lenses are fabricated by photolithography, using a grayscale mask followed by plasma etching, so that the refractive index alternation of the bilayer stack appears across the radius of the microlens. The alternating quarter-wave thick layers form a one-dimensional photonic crystal whose dispersion augments the material dispersion, allowing one to sculpt the chromatic dispersion of the lens by adjusting the layered structure. Using Huygen's principle, we model our experimental measurements of the focal length of these lenses across the reflection band of the multilayer polymer film from which the microlens is fashioned. For a 56 µm diameter multilayered lens of focal length 300 µm, we measured a ∼ 25% variation in the focal length across a shallow, 50 nm-wide reflection band.

Spectral aspects of cavity tuned absorption in organic photovoltaic films.

Concentration of light and infrared capture are two favored approaches for increasing the power conversion efficiency (PCE) of photovoltaic devices. Using optical transfer matrix formalism, we model the absorption of organic photovoltaic films as a function of active layer thickness and incident wavelength. In our simulations we consider the absorption in the optical cavity formed by the polymer bulk heterojunction active layer (AL) between the aluminum cathode and indium tin oxide (ITO) anode. We find that optical absorption can be finely tuned by adjusting the ITO thickness within a relatively narrow range, thus eliminating the need for a separate optical spacer. We also observe distinct spectral effects due to frequency pulling which results in enhanced long-wavelength absorption. Spectral sculpting can be carried out by cavity design without affecting the open circuit voltage as the spectral shifts are purely optical effects. We have experimentally verified aspects of our modeling and suggest methods to improve device design.

Folding flexible co-extruded all-polymer multilayer distributed feedback films to control lasing.

We report on improved gain and spectral control in co-extruded all-polymer multilayer distributed feedback (DFB) lasers achieved by folding and deliberate modification of the center "defect" layer. Because DFB laser gain is greater at spectral defects inside the reflection band than at the band edges, manipulation of structural defects can be used to alter spectral defects and thereby tune the output wavelength and improve laser efficiency. By experimentally terracing the layer that becomes the center of the fold, we tuned the lasing wavelength across the reflection stop-band (∼25 nm) in controllable, discrete steps. The increased density of states associated with the defect resulted in a lower lasing threshold and, typically, a 3- to 6-fold increase in lasing efficiency over non-folded samples.

Spectral aspects of cavity tuned absorption in organic photovoltaic films.

Concentration of light and infrared capture are two favored approaches for increasing the power conversion efficiency (PCE) of photovoltaic devices. Using optical transfer matrix formalism, we model the absorption of organic photovoltaic films as a function of active layer thickness and incident wavelength. In our simulations we consider the absorption in the optical cavity formed by the polymer bulk heterojunction active layer (AL) between the aluminum cathode and indium tin oxide (ITO) anode. We find that optical absorption can be finely tuned by adjusting the ITO thickness within a relatively narrow range, thus eliminating the need for a separate optical spacer. We also observe distinct spectral effects due to frequency pulling which results in enhanced long-wavelength absorption. Spectral sculpting can be carried out by cavity design without affecting the open circuit voltage as the spectral shifts are purely optical effects. We have experimentally verified aspects of our modeling and suggest methods to improve device design.

Tailored one- and two-dimensional self-assembly of a perylene diimide derivative in organic solvents.

We report on studies of the tailored self-assembly of the perylene diimide derivative, N,N'-ditridecylperylene-3,4,9,10-tetracarboxylic diimide, into structures with fibrous gel-type, one-dimensional, and two-dimensional morphologies. This approach for producing highly ordered nanostructures of well-defined morphologies utilizes a property of π-conjugated molecules to assemble in poor organic solvents due to π-π interaction between the aromatic cores and takes advantage of the temperature dependence of solubility. The morphology control is based on a fine-tuning of anisotropic, intermolecular solute-solute interactions that are attenuated by the solute-solvent interaction in organic solvents of different chemical structure. We discuss the role of light illumination in the self-assembly process as well as application of ultrasonic treatment as a way of mechanical tailoring of morphology. This approach paves the way toward the molecular-scale tailoring of structural properties of organic semiconducting materials for electronic and optoelectronic applications.

Mode delocalization in 1D photonic crystal lasers.

We have investigated the formation of in-bandgap delocalized modes due to random lattice disorder as determined from the longitudinal mode spacing in a distributed Bragg laser. We were able to measure the penetration depth, and from transfer matrix simulations, determine how the localization length is altered for disordered lattices. Transfer matrix simulations and studies of the ensemble average were able to connect the gap delocalized modes to localized modes outside of the gap as expected from consideration of Anderson localization, as well as identify the controlling parameters.

Melt-processed all-polymer distributed Bragg reflector laser.

We have assembled and studied melt-processed all-polymer lasers comprising distributed Bragg reflectors that were fabricated in large sheets using a co-extrusion process and define the cavities for dye-doped compression-molded polymer gain core sheets. Distributed Bragg reflector (DBR) resonators consisting of 128 alternating poly(styrene) (PS) and poly(methyl methacrylate) (PMMA) layers were produced by multilayer co-extrusion. Gain media were fabricated by compression-molding thermoplastic host poly notmers doped with organic laser dyes. Both processing methods can be used in high-throughput roll-to-roll manufacturing. Optically pumped DBR lasers assembled from these components display single and multimode lasing in the reflection band of the resonators, with a slope efficiency of nearly 19% and lasing thresholds as low as 90microJ/cm(2). The lasing wavelength can be controlled via the layer thickness of the DBR resonator films, and variation of the laser dye. Studies of threshold and efficiency are in agreement with models for end-pumped lasers.

Third harmonic generation in self-focused filaments in liquids.

Intense filaments of third harmonic generation have been observed in a variety of liquids when pumped by 1300 nm wavelength femtosecond light. The onset of third harmonic generation coincides with the threshold for self-focusing as deduced from n(2) measurements performed on the liquids using the z-scan technique. We have found self-phase modulation above the threshold leading to continuum generation at about 3 times threshold. In the third-harmonic regime, the third harmonic output power is independent of the input power. Our observations are consistent with those for similar processes in gases as described by a phase-locking model. The measured conversion efficiency was approximately 10(6).

Controlled self-assembly of triphenylene-based molecular nanostructures.

Molecular nanostructures of the disc-shaped molecule hexapentyloxytriphenylene have been fabricated on length scales ranging from 30 nm to 1.5 mum following self-assembly arising from pi-pi interactions in organic solvents. The size and density of the molecular nanostructures deposited onto glass and indium tin oxide-coated glass substrates were characterized by atomic force microscopy. Dynamic light scattering and spectroscopic evidence of predeposition aggregation in solution are presented, suggesting that the nanostructures are organized in solution and then deposited onto the substrate. Correlations between the relative solvent polarity and the size of molecular nanostructures as well as between the solute concentration in dilute solutions and their density on the substrate are discussed.

High charge carrier mobility in conjugated organometallic polymer networks.

The improvement of charge transport in conjugated polymers is a focal point of current research. It is shown here that the carrier mobility can be substantially increased through the introduction of conjugated cross-links between the conjugated chains. Novel organometallic polymer networks based on a poly(p-phenylene ethynylene) (PPE) derivative and Pt0 were synthesized by ligand-exchange reactions between the linear PPE and a low-molecular Pt complex. Time-of-flight measurements revealed ambipolar charge carrier mobilities of up to 1.6 x 10-2 cm2 V-1 s-1 for these materials, which are an order of magnitude higher than those of the neat polymer and represent the highest mobilities yet observed in disordered conjugated polymers.