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.
