Inverse size exclusion chromatography (ISEC)

In this work, an improved protocol for inverse size-exclusion chromatography (ISEC) was established to assess important pore structural data of porous silicas as stationary phases in packed chromatographic columns. After the validity of the values generated by ISEC was checked by comparison with data obtained from traditional methods like nitrogen sorption at 77 K (Study A), the method could be successfully employed as valuable tool at the development of bonded poly(methacrylate)-coated silicas, while traditional methods generate partially incorrect pore structural information (Study B). Study A: Different mesoporous silicas were converted by a pseudomorphical transition into ordered MCM-41-type silica while maintaining the particle-size and -shape. The essential parameters like specific surface area, average pore diameter and specific pore volume, the pore connectivity from ISEC remained nearly the same which was reflected by the same course of the theoretical plate height vs. linear velocity curves. Study B: In the development of bonded poly(methacrylate)-coated silicas for the reversed phase separation of biopolymers, ISEC was the only method to generate valid pore structural information of the polymer-coated materials. Synthesis procedures were developed to obtain reproducibly covalently bonded poly(methacrylate) coatings with good thermal stability on different base materials, employing as well particulate and monolithic materials.

Complex materials via colloidal crystallization

The work presented in this thesis deals with complex materials, which were obtained by self-assembly of monodisperse colloidal particles, also called colloidal crystallization. Two main fields of interest were investigated, the first dealing with the fabrication of colloidal monolayers and nanostructures, which derive there from. The second turned the focus on the phononic properties of colloidal particles, crystals, and glasses. For the fabrication of colloidal monolayers a method is introduced, which is based on the sparse distribution of dry colloidal particles on a parent substrate. In the ensuing floating step the colloidal monolayer assembles readily at the three-phase-contact line, giving a 2D hexagonally ordered film under the right conditions. The unique feature of this fabrication process is an anisotropic shrinkage, which occurs alongside with the floating step. This phenomenon is exploited for the tailored structuring of colloidal monolayers, leading to designed hetero-monolayers by inkjet printing. Furthermore, the mechanical stability of the floating monolayers allows the deposition on hydrophobic substrates, which enables the fabrication of ultraflat nanostructured surfaces. Densely packed arrays of crescent shaped nanoparticles have also been synthesized. It is possible to stack those arrays in a 3D manner allowing to mutually orientate the individual layers. In a step towards 3D mesoporous materials a methodology to synthesize hierarchically structured inverse opals is introduced. The deposition of colloidal particles in the free voids of a host inverse opal allows for the fabrication of composite inverse opals on two length scales. The phononic properties of colloidal crystals and films are characterized by Brillouin light scattering (BLS). At first the resonant modes of colloidal particles consisting of polystyrene, a copolymer of methylmethacrylate and butylacrylate, or of a silica core-PMMA shell topography are investigated, giving insight into their individual mechanical properties. The infiltration of colloidal films with an index matching liquid allows measuring the phonon dispersion relation. This leads to the assignment of band gaps to the material under investigation. Here, two band gaps could be found, one originating from the fcc order in the colloidal crystal (Bragg gap), the other stemming from the vibrational eigenmodes of the colloidal particles (hybridization gap).

From large polycyclic aromatic hydrocarbons to extended aromate-rich networks

The common ground of this study is the development of novel synthetic strategies to extended one-, two- and three-dimensional aromate-rich systems for which a number of applications are envisaged. rnThe point of departure is the synthesis and characterization of highly symmetric macrocyclic PAHs (polycyclic aromatic hydrocarbons) for which various aspects of supramolecular chemistry will be investigated. The versatility of the Yamamoto macrocyclization will be demonstrated on the basis of a set of cyclic trimers that exhibit a rich supramolecular chemistry. 1,10-phenanthroline, triphenylene and ortho-terphenyl building blocks have been successfully assembled to the corresponding macrocycles following the newly developed synthetic route. Scanning-tunneling microscopy (STM) and two-dimensional wide-angle X-ray scattering (2D-WAXS) were used to study the two- and three-dimensional self-assembly, respectively.rnSecondly, the development of chemical approaches to highly shape-anisotropic graphene nanoribbons (GNRs) and related nanographene molecules shall be discussed. Aryl-aryl coupling was used for the bottom-up fabrication of dendronized monomers, polymers and model compounds. Subsequently, these structures were converted into the final graphene material using oxidative (Scholl-type) cyclodehydrogenation. The GNRs thus obtained are characterized by an unprecedented length and lateral extension. The relevance of structural tailoring in the field of well-defined graphene materials is discussed in detail as only the chemical approach provides full geometry control. rnLastly, novel pathways towards the synthesis of extended three-dimensional networks that are dominated by nitrogen-rich motifs will be presented. If porous, these materials hold a great potential in the fields of gas and energy storage as well as for applications in catalysis. Hence, poly(aminal) networks based on melamine as crosslinking unit were synthesized and characterized with respect to the applications mentioned above. As set of conjugated poly(azomethine) networks was investigated regarding their use as a novel class of organic semiconductors for photocatalytic water splitting. The network structures described in this chapter can also be subjected to a controlled pyrolysis yielding mesoporous, nitrogen-rich carbon materials that were evaluated as active component for supercapacitors.rn

Application of a hybrid blocking layer in dye-sensitized solar cells

In dye-sensitized solar cells a blocking layer between the transparent electrode and the mesoporous titanium dioxide film is used to prevent short-circuits between the hole-conductor and the front electrode. The conventional approach is to use a compact layer of titanium dioxide prepared by spin coating or spray pyrolysis. The thickness of the blocking layer is critical. On one hand, the layer has to be thick enough to cover the rough substrate completely. On the other hand, the serial resistance increases with increasing film thickness, because the layer acts as an ohmic resistance itself. In this thesis an amphiphilic diblock copolymer is used as a functional template to produce an alternative, hybrid blocking layer. The hybrid blocking layer is thinner than the conventional, compact titanium dioxide film and thereby possesses a higher conductivity. Still, this type of blocking layer covers the rough electrode material completely and avoids current loss through charge recombination. The novel blocking layer is prepared using a tailored, amphiphilic block copolymer in combination with sol-gel chemistry. While the hydrophilic poly(ethylene oxide) part of the polymer coordinates a titanium dioxide precursor to form a percolating network of titania particles, the hydrophobic poly(dimethylsiloxane) part turns into an insulating ceramic layer. With this technique, crack-free films with a thickness down to 24 nm are obtained. The presence of a conductive titanium dioxide network for current flow, which is embedded in an insulating ceramic material, is validated by conductive scanning force microscopy. This is the first time that such a hybrid blocking layer is implemented in a solar cell. With this approach the efficiency could be increased up to 27 % compared to the conventional blocking layer. Thus, it is demonstrated that the hybrid blocking layer represents a competitive alternative to the classical approach.

Charge generation and recombination in solid-state dye-sensitized solar cells

Diese Doktorarbeit befasst sich mit Ladungsgeneration und – rekombination in Feststoff-Farbstoffsolarzellen, die spiro-OMeTAD als Lochleiter verwenden. Die vorliegende Arbeit ist in drei Fallstudien unterteilt: i.) Kern-erweiterte Rylen-Farbstoffe, ii.) ein Perylenmonoimid-Farbstoff und iii.) Donor-π verbrückte (Cyclopentadithiophen)-Akzeptor-Farbstoffe. Trotz ihres hohen molaren Extinktionskoeffizienten und der hohen Absorbanz der sensibilisierten Filme, zeigen einige dieser Farbstoffmoleküle nur geringe photovoltaischen Effizienzen. Um den Ursprung des geringen Wirkungsgrades herauszufinden, wurde breitbandige, ultraschnelle transiente Absorptionsspektroskopie an Solarzellen durchgeführt.rnInsbesondere die Auswirkungen verschiedender Ankergruppen, Dipolmomente, Photolumineszenzlebenszeiten, Lithium-Kationensensitivität und Ladungsträgerdynamik, die alle einen großen Einfluss auf den Wirkungsgrad der Solarzelle besitzen, wurden untersucht. In der ersten Fallstudie zeigte ein kurzer Rylen-Farbstoff aufgrund deutlich verlängerter Lebenszeiten die beste Effizienz im Vergleich zu größeren Kern-erweiterten Rylen-Farbstoffen. Die Lebenszeit wurde weiter reduziert, wenn Maleinsäure als Ankergruppe unter einer Ringöffnungsreaktion an die mesoporöse Oberfläche des Metalloxid-Halbleiters adsorbierte. Dies konnte mit Hilfe von Berechnungen mittels der Dichtefunktionaltheorie (DFT, B3LYP) auf die Differenz des Dipolmoments zwischen Grundzustand und angeregtem Zustand zurückgeführt werden. Die Berechnungen bekräftigen die unvorteilhafte Injektion von Ladungen durch die Änderung der Richtung des Dipolmoments, wenn eine Ringöffnung der Anhydridgruppe stattfindet. In der zweiten Studie zeigte das Perylenmonoimid-Derivat ID889 einen Wirkungsgrad von 4.5% in Feststoff-Farbstoffsolarzellen, wobei ID889 sogar ohne Zuhilfenahme eines Additivs in der Lage ist langlebige Farbstoffkationen zu bilden. Die Verwendung von Lithium-Kationen stabilisiert jedoch sowohl den Prozess der Ladungsgeneration als auch den der Ladungsregeneration. Des Weiteren wurde in ID889-sensitivierten Bauteilen kein reduktives Löschen beobachtet. Dabei wurde die Dynamik der Exzitonen mittels einer soft-modelling Methode Kurvenanalyse aus den Daten der transienten Absorptionsspektroskopie gewonnen. Zuletzt wurden Strukturen mit Cyclopentadithiophen(CPDT)-Baustein untersucht, die eine typische D-π-A Molekülstruktur bilden. FPH224 und 233 zeigten dabei eine bessere Effizienz als FPH231 und 303 aufgrund einer großen Injektionseffizienz (IE) und längerer Lebenszeit der angeregten Zustände. Dies kann auf reduktives Löschen in FPH231 und 303 zurückgeführt werden, wohingegen FPH224 und 233 einen moderaten Zerfall des Spirokationensignals zeigten.