Self-assembled photoactive polyelectrolyte/perylene-diimide composites

Human serum albumin as vehicle for the solubilization of perylene diimides in aqueous solutions

The chemical, physical and photophysical properties of perylene diimides have attracted substantial attention for the potential applications in diverse fields ranging from advanced materials to biomedical applications. Some applications require the diimides to be in aqueous environment where they tend to dissolve poorly. We investigated the use of human serum albumin as a vehicle to increase the aqueous exposure of monomeric perylene diimides. Since studies on the interactions of these compounds with protein is scarce we characterized the binding and the possible effects on the protein. In order to increase the affinity of the dyes to the protein we have used perylene diimides with substituents that replicate the side chains of natural amino acids. The results show that only the dyes containing the side chain of leucine and phenylalanine yield measurable binding. Only the phenylalanine analogue promotes energy transfer with the lone tryptophan residue of albumin indicating different binding modalities for the dyes. In addition, this analogue is the only one which shows photochemical activity that prompts its release from the protein upon laser irradiation.

Probing the limits of the majority-rules principle in a dynamic supramolecular polymer

By systematic variation of the chemical structure of benzene-1,3,5-tricarboxamide (BTA) derivatives, the effect of chemical structure on the amplification of chirality was studied and quantified. In combination with temperature-dependent amplification experiments, the limits of the majority-rules principle were also investigated. For all BTA derivatives a high, constant helix reversal penalty was determined, which is related to the intermolecular hydrogen bonds that are present in all studied derivatives. For asymmetrically substituted BTA derivatives an odd-even effect was found in the degree of chiral amplification when changing the position of the stereogenic center with respect to the amide functionality. It was found that the mismatch penalty could be directly related to the number of stereocenters present in the molecules. Increasing this number from one to three resulted in an increase in this energy penalty while leaving the helix reversal penalty unaffected. For the majority-rules principle this implies that a single stereocenter present in the molecule contains sufficient chiral information at the molecular level to result in a chirally amplified state at the supramolecular level. Further evidence that the mismatch penalty is directly related to the number of stereocenters was obtained from mixed majority-rules experiments where two BTA derivatives with different numbers of stereocenters with opposite stereoconfiguration were studied in a majority-rules experiment. Finally, the ultimate limits of chiral amplification for the majority-rules principle were investigated, revealing that, given a certain helix reversal penalty, there is an optimum to which the mismatch penalty can be reduced while also enhancing the degree of chiral amplification. Temperature-dependent majority-rules experiments could indeed confirm these simulations. These findings show the relevance of both energy penalties when trying to enhance the degree of chiral amplification for the majority-rules principle in a one-dimensional helical supramolecular polymer.

Electrostatic self-assembly of ordered perylene-diimide/polyelectrolyte nanofibers in fluidic devices: from nematic domains to macroscopic alignment

Sequential deposition of nanofibrous composites of charged perylene diimide (PDI) dyes and oppositely charged polyelectrolyte (PE) is demonstrated within fluidic devices. The PDIs employed include an amphiphilic, singly charged PDI (C(7)OPDI(+)) and a doubly charged species (TAPDI(2+)). Anionic poly(acrylate) (PA(-), 5100 and 250K MW) is used as the PE. As previously demonstrated [Weitzel, C. R.; Everett, T. A.; Higgins, D. A. Langmuir, 2009, 25, 1188], dip-coated PDI/PE composites form nanofibrous films that exhibit flow-induced alignment due to gravitational draining of the dipping solution. In this study, the potential for producing patterned, flow-aligned PDI/PE composites by deposition using pressure-driven flow within fluidic channels is explored. The influence of flow profile, PE molecular weight (MW) and PDI structure on deposition efficiency, macroscopic and microscopic morphology, and the potential for nanofiber alignment are also investigated. Optical absorbance microscopy and tapping mode AFM data demonstrate that C(7)OPDI(+)/PA(-) deposition is controlled by PDI aggregation, while TAPDI(2+)/PA(-) composites are more dependent upon PE MW. Optical dichroism images show that C(7)OPDI(+)/PA(-) composites form serpentine, partially aligned nanofibers under all conditions explored, while TAPDI(2+)/PA(-) films incorporate more tightly packed nanofibers that form randomly oriented nematic-like domains when high MW PA(-) is employed. In-plane organization in C(7)OPDI(+)/PA(-) films is concluded to result from flow-induced alignment of solution-formed C(7)OPDI(+) aggregates, while the unaligned domains found in TAPDI(2+)/PA(-) films are concluded to form on the substrate surface by the complexation of small TAPDI(2+) aggregates or monomers with PE.

Aggregation and its influence on macroscopic in-plane organization in thin films of electrostatically self-assembled perylene-diimide/polyelectrolyte nanofibers

The influence of precursor aggregation on materials deposition efficiency, film morphology, and macroscopic in-plane organization is explored for electrostatically self-assembled perylene-diimide/polyelectrolyte (PDI/PE) composites. PDI/PE thin films are prepared from aqueous precursor solutions by sequential dip-coating methods. Three PDI dyes are employed to probe the influence of aggregation on electrostatic self-assembly (ESA) of the composites. These include a singly charged PDI, C(7)OPDI(+), and two doubly charged species, PDISO(3)(2-) and TAPDI(2+). Poly(diallyldimethylammonium) (PDDA(+)) chloride and sodium poly(acrylate) (PA(-)) are used as the PEs. UV-vis absorbance and fluorescence spectroscopies show that all three dyes are heavily aggregated in their respective aqueous solutions. Temperature-dependent fluorescence data and filtration studies show that C(7)OPDI(+) is most strongly associated and also forms the largest aggregates. Absorbance data obtained as a function of the number of deposition cycles employed in film preparation demonstrate that C(7)OPDI(+) is also most efficiently deposited. Atomic force microscopy (AFM) images show that all three PDI/PE films are comprised of similar serpentine nanofibers. Interestingly, bulk absorbance dichroism data and AFM images demonstrate that the C(7)OPDI(+)/PA(-) composites incorporate macroscopically oriented dye and aligned nanofibers. Dye and nanofiber alignment is found to be perpendicular and parallel, respectively, to the dipping direction. No such organization is observed for the other two composites. It is concluded that deposition is strongly influenced by the level of precursor aggregation and that macroscopic in-plane organization in the C(7)OPDI(+)/PA(-) composites results from flow-induced alignment of relatively large preformed C(7)OPDI(+) aggregates during deposition.

Nanopatterned monolayers of an adsorbed chromophore

A simple lift-off process was developed to rapidly fabricate nanopatterned photofunctional surfaces. Dye molecules of a perylene derivative (PDID) were adsorbed irreversibly on clean silicon through the holes of an electron-beam lithographied polymer mask. The subsequent removal of the mask in a proper solvent results in PDID nanosized regions of width as small as 30 nm for stripes and of diameter as small as 120 nm for dots. Numerical analyses of atomic force microscopy and laser-scanning confocal microscopy images show that the dye molecules are confined to the regions defined by the lithographic process, with the integrated fluorescence intensity being essentially proportional to the size of the nanofeatures. This demonstrates that a simple organic lift-off process compatible with clean-room technology, and not involving any chemical step, is able to produce photofunctional nanopatterned surfaces, even though the dye is not chemically bonded to the silicon surface.

Water-soluble perylene diimides: Solution photophysics and layer-by-layer incorporation into polyelectrolyte films

Recently the synthesis of water-soluble and fluorescent perylene diimides has been reported (Müllen, K.; et al. Angew. Chem., Int. Ed. 2004, 43, 1528; Chem.-Eur. J. 2004, 10, 5297). We have characterized the photophysics of two of these compounds (anionic n-PDI, CAS Reg. No. 694438-88-5. and cationic p-PDI, CAS Reg. No. 817207-4-7) in pure water, dimethyl sulfoxide (DMSO), and aqueous NaCl. These studies, supported by molecular dynamics simulations, have led to the conclusion that these compounds form weakly interacting aggregated species in pure water. n-PDI and p-PDI have been incorporated in polyelectrolyte films of poly(styrene sulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDAC) following the layer-by-layer (LBL) methodology. The optical density and fluorescence intensity of the PDI-LBL films grew linearly with the number of layers, and the PDI was not extracted by subsequent polyelectrolyte deposition. The PDI fluorescence quantum yield was substantially diminished in these films, which we interpret as a self-quenching effect, enhanced by inter- and intralayer energy transfer. Energy-transfer studies to the incorporated cationic dye Brilliant Green (BG) has demonstrated that the BG resides in the same PSS-rich region as p-PDI and is largely excluded from the region that contains n-PDI (PDAC-rich).