Hybrid materials consisting of nanocrystals functionalized with organic polymers are currently attracting increasing attention for use in light-weight and flexible optoelectronic devices and biological labelling because of the possibility of combining the desirable characteristics of each component. II-VI nanocrystals can be synthesized with excellent control over their size and shape via colloidal methods, but for the most part, this requires the use of long chain aliphatic ligands with coordinating head groups which play a critical role during synthesis by simultaneously providing solubility, diffusion barriers for growth, and changing the reactivity of monomers. After synthesis has been completed, ligands provide further roles in passivation, processability, and drastically affect the processed morphology. These insulating ligands tend to prevent efficient charge separation at the donor/acceptor interface, as well as inhibit charge transport to the electrodes, even with the strong tendency of nanocrystals to aggregate in the solid state. The most common method for controlling the surface chemistry of nanoparticles is via ligand exchange reactions, which allow for the replacement of ligands critical for good synthetic conditions with those optimized for specific applications, including inorganic ions, short chain oligomers, or a variety of end functionalized polymers, among others. The effectiveness of ligand exchange reactions is largely dependent on the affinity of the incoming ligand for the nanocrystal core, the sterics of all ligands at play, and strength of interaction between each different ligand with the nanocrystal core. Ligand exchange processes can often result in the use of a large excess of the desired capping ligand, and require long reactions at elevated temperatures for often incomplete exchange, and in the case of end-functionalized polymers, these realities can prove prohibitive for implementation. While some research has been done on the attachment of π-conjugated molecules to II-VI nanocrystals, they have relied heavily on ligand exchange processes or by the growth or attachment of conjugated polymers on preexisting nanoparticles, often requiring two or three step processes.
We recently developed a one-pot method for attaching π-conjugated small molecules to CdSe during nanoparticle synthesis, which we would like to apply to systems with extended π-conjugation and large steric hindrance. Using our unique methodology we aim to facilitate charge transfer between donor and acceptor materials, and reduce the complexity of processing for the functionalization of semiconducting quantum dots while simultaneously decreasing the amount of material required for functionalization and thereby cutting the overall production cost. Our primary goal is to improve upon the design and miscibility of polymer/nanocrystal hybrid materials by directly attaching semiconducting polymers to the surface of II-VI nanocrystals, without relying on ligand exchange reactions, coupling processes, or post-processing steps.
In this presentation, a new method for the attachment of semiconducting polymers that takes advantage of a chemical reaction with the polymer acting as a reagent in the synthesis of II-VI nanomaterials will be described. We first expand on a method for the end-functionalization of P3HT via thiol quenching, followed by a sulphur-phosphorous coupling reaction, resulting in a thio-phosphonate terminated polymer. This end-functionalization results in a polymer end-group that resembles many chalcogenide reagents used in the synthesis of II-VI nanomaterials. We confirm that these materials can be used for the direct attachment of semiconducting polymers to nanocrystal cores, as shown by NMR and TEM. Furthermore, we use steady state and time-resolved optical spectroscopies performed on dilute solutions to verify the attachment of these polymers to CdSe nanocrystals and provide insight into the excited state processes that occur between these chromophores.
