Self-assembled hybrid oligo(p-phenylene vinylene) gold nanoparticles : synthesis, characterisation and manipulation

Abstract

Hybrid materials based on inorganic nanoparticles and p-conjugated segments are of growing interest to scientists in the field of nanotechnology who work with opto-electronic devices. The combination of inorganic and organic units gives rise to new functionalities and unexpected opto-electronic properties. The device properties depend on the chemical structure and morphology of the active layer. However, so far only limited control is afforded by self-assembly strategies over the internal organisation of electronic devices based on these materials and their potential has not been fully explored. In Chapter 1 of this thesis, first a brief introduction is given on p-conjugated organic materials, inorganic nanoparticles and self-assembly. Subsequently, an overview follows on the field of inorganic nanoparticles that are decorated with p-conjugated molecules. Herein, it is shown that hardly any self-assembly strategies have been developed for these hybrid particles. Finally, the aim – being the development of self-assembly strategies for inorganic nanoparticles functionalised with p-conjugated ligands – and the outline of this thesis are addressed. In the second Chapter, the synthesis of a set of oligo(p-phenylene vinylene) (OPV) chromophores bearing sulfur functionalities is described. Three routes to the decoration of gold nanoparticles with these p-conjugated segments via the sulfur linkages are developed and the step-wise ligand exchange is investigated in detail with optical spectroscopy techniques. Binding of OPV molecules to nanometer-sized gold particles reduces the quantum yield for fluorescence by at least an order of magnitude. Photo-induced absorption spectroscopy reveals a very short lifetime (ps), much shorter than the lifetime of the unbound OPV (1.3 ns). The reduction of the S1 lifetime is ascribed to resonance energy transfer from the OPV to the nanoparticle via the Forster mechanism. The self-assembly in n-butanol of gold nanoparticles functionalised with OPVs having long, aliphatic tails is developed in Chapter 3. By means of temperature and concentration dependent optical absorption measurements, the aggregate formation is studied and the self-assembly process is found to be reversible. Dynamic light scattering measurements indicate that spherical aggregates exist in solution. Transmission electron microscopy indicates that these assemblies are constructed from intercalated OPV-gold nanoparticles; the assemblies collapse upon transfer from solution to the solid state. Atomic force microscopy shows spheres that are built up from smaller spheres, pointing to fractal-like self-assembly. The self-assembly of gold nanoparticles decorated with OPV chromophores bearing long aliphatic tails and a hydroxy functionality was investigated in Chapter 4. Here, spherical, micrometer-sized aggregates were obtained that reversibly self-assemble in n-heptane. Optical absorption spectroscopy and transmission electron microscopy studies show that the shape persistency of these assemblies can be tuned by modification of the ligands, adjustment of the core size and variation of the concentration. The aggregates are built up from intercalated OPV-gold nanoparticles whose mutual interactions are sufficiently strong enough that the shapes of the aggregates remain intact upon transfer from solution to the apolar substrates. Due to these properties it is possible to manipulate such an assembly with the tip of an atomic force microscope: an aggregate can be pushed over the surface at least 20 times with nanometer precision and without substantial loss of material. By making use of a scaffolding OPV gelator that forms tapes in toluene, OPV-gold nanoparticles can be organised in solution into aggregates with a high aspect ratio (Chapter 5). These hybrid tapes gel in toluene and detailed microscopy studies show that they feature linear arrays of OPV-gold nanoparticles along the edges of the scaffolding tape. The ligands interact with the tapes via non-covalent interactions that are similar to those that hold the tape together. Spectroscopic measurements show that energy transfer takes place from the OPV scaffold to the docked OPV-gold nanoparticles. The hybrid gels can be aligned by means of a magnetic field. Furthermore, as a result of drying effects the OPV-gold nanoparticles themselves self-assemble into fibrous monolayers upon drop-casting from toluene. In Chapter 6, amphiphilic gold nanoparticles are synthesised via functionalisation with both hydrophobic and hydrophilic ligands. In this simple method, the ratio of two different types of ligands on these so-called Janus particles can be varied easily so that these particles self-assemble in water. Microscopy and scattering studies reveal the formation of bilayered disk-shaped micelles with an average diameter of approximately 0.5 µm and a height of about 8 nm. Below the so-called critical aggregation concentration (CAC) the functionalised particles exist as single species in water, whereas above the CAC they self-assemble into aggregates. Initial experiments show that this approach can be extended to create Janus gold nanoparticles that are decorated with OPV ligands. In this thesis, it is shown that hybrid OPV-gold nanoparticles can be self-assembled. This occurs through intercalation of the OPV ligands. The aggregates can be tailored by making use of the same supramolecular interactions that have been used to self-assemble organic OPV systems (i.e. p-p stacking and Van der Waals interactions). By altering various parameters, hybrid self-assembled architectures are obtained of which the properties, like size, shape and shape-persistency, can be manipulated. These first steps taken in this field of hybrid materials may find future applications in (nano-)electronics.