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Spin-Selection using Chirality: A Supramolecular Approach

Periodic Reporting for period 1 - Supra spin-selection (Spin-Selection using Chirality: A Supramolecular Approach)

Reporting period: 2016-03-01 to 2018-02-28

In recent times there is an impetus towards more sustainable and efficient ways of living, with an aim to lowering our dependence on non-renewable resources of energy. We are increasingly using electronic gadgets with advanced features, better contrast and smart lighting technologies. Prime examples are the use of light-emitting diodes (LEDs) over incandescent bulbs and LED displays over conventional liquid crystal displays. Many of the LED devices now-a-days make use of organic based materials because of the lower cost and flexibility to adopt on surfaces of different topologies. However, still the performance of these materials is not comparable to those of well-established inorganic materials. Thus, there is a need for new innovative concepts to realize efficient devices with organic materials.
One of the new concepts (CISS effect) developed by Naaman and Waldeck harnesses spin of chiral organic or soft materials to achieve efficient OLEDs. However, only proof-of-concept studies have been carried out so far and this idea has not yet been applied to organic systems which can transport charges to realize its full potential in OLEDs. In the current project, such an effort was undertaken and a summary of the results are presented below.
The CISS effect was successfully tested on a state-of-the-art organic polymer with high chiral organization in an OLED architecture. However, the efficiency of the effect was low (~5%). In order to further increase the efficiency, a new, generally applicable and practical approach was developed to improve the chiral organization of polymeric systems. In addition, control over the chiral organization of such systems was demonstrated using light as a stimuli to control the device performances.
The project started with the synthesis, purification and self-assembly study of a chiral π-conjugated polymer to test the CISS-effect. The polymer was chosen such that, the CISS effect could be tested in an OLED device architecture with the chiral polymer being the active material. The chiral polymer was synthesized according to the know procedure previously established in our laboratory. It is well documented that the metallic impurities in polymers adversely affect the photoluminescence and charge-transport properties of the polymer in the device. Thus the polymer was further purified extensively to ensure that the polymer was free of any trace metallic impurities (Palladium catalyst traces).
The self-assembly of the polymer was studied in thin-film. It was observed that the studied polymer gives high degree of absorption of one of the handedness of circularly polarized light over the other, thus indicating high level of chiral organization in the thin-film. To further understand the system quantitatively, a new method based on the use of linearly polarized light was developed and this method confirmed the cholesteric organization of the polymer chains with a pitch of ~600 nm. The new method developed has been published in The Journal of Physical Chemistry B (10.1021/acs.jpcb.7b10236)
Having established the self-assembly in thin-films, we further collaborated with the group of Prof. Richard H. Friend at University of Cambridge to test the materials in OLED devices. The tested devices showed exceptional performance with ~40% of emitted light having one of the handedness. Such performance are crucial for high-contrast displays. This part of the work has been recently published in the high-impact journal ACS Nano (10.1021/acsnano.7b07390).
With the comprehensive understanding of the system both in the thin-film and OLED device, we further collaborated with the group of Prof. Ron Naaman at the Weizmann Institute of Science to test if the CISS effect could be operative in such π-conjugated polymers. In order to test the CISS effect, again the OLED architecture was employed, but with magnetizable top electrodes to externally switch the magnetization and thus the spin-injection. It was observed that the direction of the magnetization of the OLED reproducibly affected the circular polarized emission from the devices. This is a direct evidence of the CISS-effect. However, the effect of external magnetic field was only 4-5% on the emission of circularly polarized light. Nonetheless, we believe that further optimization of the device will improve the magnitude of the CISS effect in such devices. This work is currently at the stage of submission for publication.
In addition to the above mentioned π-conjugated covalent polymers, control over the chiral organization of π-conjugated supramolecular polymers was also pursued. It was observed that for a coronene bisimide based supramolecular polymer, the handedness of the system was completely opposite at two different temperature. This work has been published in The Journal of the American Chemical Society (10.1021/jacs.7b07639). Such a supramolecular polymer with temperature sensitive helicity could be used to probe the CISS effect in π-conjugated systems. Presently, efforts in this direction are being carried out in collaboration with the group of Prof. Ron Naaman at Weizmann Institute of Science.
There are two main scientific progress beyond the state-of-the art in the field and are outlined below. Firstly, during the course of this project, we observed that the high degree of chiral organization which is necessary to harness CISS effect or applications in display technology is only limited to a small number of polymers. Thus, we have developed a new, cheap, generally applicable and practicable method to induce high degree of chiral organization in polymeric systems. This method could potentially lead to highly efficient OLED displays covering the visible spectrum. Secondly, a new functional conjugated polymer based photo-switchable system was constructed in which both the handedness of the organization and the direction of alignment of the polymer chains could be controlled with either circularly or linearly polarized light. Such systems incorporating photo-switchability and electronic functionality are ideal for modulating the device performance with a non-invasive stimuli such as light. We hope to develop such devices in the near future.
The demonstration of efficient OLEDs and methods to control the chiral organization of polymeric materials might impact the society in the long-run through construction of cheap and efficient lighting and display technologies.
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