Please outline your proposed research project at the University of Cambridge. If you have already begun the research project, please also briefly describe the progress in the last 12 months, including, if any, brief particulars of conference(s) attended, paper(s) submitted and publications(s) (with names of contributors). Please attach additional sheets if necessary. Design, Synthesis and Fabrication of Self-Assembling Luminescent MetallopolymersSupervisor: Prof. Jonathan Nitschke Introduction: Subcomponent self-assembly is a synthetic strategy that utilizes the interplay of numerous weak non-covalent interactions to prepare complex and well-defined architectures from simple building blocks. Self-assembling polymers have been of growing interest in recent years as a fascinating new functional material owing to their dynamic behavior and inherent error-checking mechanism.
Metallopolymers formed by such interactions make use of the metal templation under which monomers are brought together by metal coordination to form polymers. These polymers have exhibited high luminescence efficiencies, stimuli-responsive behavior and self-healing properties. Many of these materials have been used to construct energy-efficient optoelectronic devices such as organic light-emitting diodes (OLEDs), light-emitting electrochemical cells (LECs), sensors and dye-sensitized solar cells (DSSCs). LECs, in particular, have emerged as a modified OLED technology, in which they share a similar underlying principle but differ in architecture. Unlike an OLED device that construct to have many different layers, the active part of LEC is composed only of a single layer of electroluminescent materials sandwiched between two electrodes. Together with their insensitivity towards the electrode work functions, LECs are the simplest class of electroluminescent device with minimal complexity in the fabrication process. While the development of metallopolymers has gone through several evolutionary steps in the past years, our understanding of their structure-property relationships remains shallow.
Recent studies have demonstrated the generation ofopolymers wrapping around an array of copper (I) metal ions, wherein monomers with amine and aldehyde functionalities condense around the metals to form dynamic covalent imine (C=N) and coordinative (NàMetal bonds) linkages. The incorporation of transition metals is beneficial for the performance of LEDs or LECs because the metal centers can augment spin-orbit coupling to allow light harvest from both triplet and singlet excitons, offering internal quantum efficiencies of up to 100%. That being said, metallopolymers that are specially designed to emit light can potentially drive solution in energy-efficient lightings to make energy-saving displays that we all use every day. Further advancement of such metallopolymers into unique helical structure has shown promising electroluminescent properties and is anticipated to possess exciting conducting properties. These helical motifs are of biological relevance and are of interest in the field of foldamers, polarized light emission and spintronics.
Chiral helical metallopolymers can be adopted for discriminative sensing of enantiomers as well as to address challenging questions related to biological homochirality and the origin of life. Furthermore, a helical metallopolymer can be used as a source of circularly polarized light (CPL) attributed to its handedness (left-handed or right-handed) that can be manipulated by the introduction of chiral groups. To date, many strategies have been used to produce CPL, such as cholesteric liquid crystals and lanthanide complexes, but chirality never comes from a helical metallopolymer. It is noteworthy that CPL emission is an intrinsic property of the polymers, meaning there is no need to organize them, unlike in the cholesteric films. Although prior studies have revealed circular dichroism behavior of these helical polymers in absorption, the polarization of light has not been comprehensively studied. Moreover, their photoluminescence is not strong enough to make excellent electronic devices. The range of monomers used so far is also very limited and no alternative polymers, such as co-polymers, has been reported.
In this sense, it would be of great interest to design new metallopolymers with enhanced luminescence properties that can be thoroughly studied and translated into functional LEC devices. This project aimed to develop processes that can utilize energy in a more efficient manner which can lead to their potential application in flat-panel displays, solid-state lighting and sensing. Looking beyond the current technology and deepen our understanding in fundamental synthetic inorganic chemistry is the key to realize the ultimate goal in building a sustainable energy future where lights of all types would power our world. Project plan: The project aims to target molecular design of self-assembling metallopolymers through manipulating the delicate balance between the nature of metal centers, ligands (monomers), coordination modes and polymer length which in term would result in altering the luminescence properties. In the initial stage, the project will focus on controlling the emission behavior of the metallopolymers such as wavelength (colour), emission intensity and lifetime via the judicious design and modification of the monomers. Thiophene-based monomers bearing amine and aldehyde functionalities, for instance, are an interesting alternative for a copper (I) metal coordination, as favoured by the hard and soft acids and bases (HSAB) sense of Pearson. With the recent breakthrough in controlling over the length and the regiochemistry, helical polymers of consistent properties can be prepared by stoichiometric control of end-capping groups. While the double helical structure is currently limited to 4-coordinated metal systems, 6-coordinated systems can be explored with the use of other transition metals.
Meanwhile, efforts will be placed on developing conjugating metallopolymers with good dispersity in the solvent to enable the dynamic covalent polymerization process to take place under a milder condition and render better polymer processability. Aside from modifying the side chains, one alternate method to do this would be the fabrication of nanoparticles through polymerization, emulsion or nanoprecipitation. The growth of metallopolymers can be followed by nuclear magnetic resonance (NMR) spectroscopy and the purification can be achieved by standard membrane dialysis.
While the polymer size distribution can be determined by dynamic light scattering (DLS), which has proven to be useful in previous literatures, larger aggregates can be observed by multi-angle light scattering (SLS). Their absorption and luminescence profiles can be analysed using a UV-vis spectrometer and fluorometer. Meanwhile, the measurement of light polarization can be done with quarter wave plates (QWP), polarizers and lenses along the path of the beam, as reported in the literature on CPL emission. In order to gain insights into energy levels of frontier molecular orbitals as well as the stability of polymer against decomposition, the electrochemical properties of these complexes can be studied with cyclic voltammetry (CV) and if time permits, molecular modeling can be employed to visualize the three-dimensional structure of polymer strands. In the late stage of the project, active excitonic compounds with tunable properties will be integrated into LEC through thin-layer fabrication in a glovebox and this will be done at the physics department in Prof. Richard Friend’s group. The management at the end of the project will be highly dependent on the Friend group availabilities but synthesis and characterization can be done in parallel. As soon as metallopolymers with good luminescent properties are obtained, we will integrate them into electronic devices.
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