Blue Light-Emitting Electrochemical Cells Based on Angularly Structured Phenanthroimidazole Derivatives

• Published in: Journal of physical chemistry. C 2017-07, Vol.121 (27), p.14811-14818
• Main Authors: Subeesh, Madayanad Suresh, Nguyen, Thao P, Choe, Youngson
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.7b03911

Photophysical properties of four new ionic blue emitting phenanthroimidazole derivatives were investigated. Rarely utilized meta and ortho positions of C2 substituted benzene linker of phenanthroimidazole were functionalized herein to generate blue emitters for light-emitting electrochemical cell (LEC) applications. To this end, pyrene and anthracene units were coupled to phenanthroimidazole core through meta or ortho linkage. This molecular design strategy is anticipated to generate deeper blue emission than the corresponding para isomers due to lack of extended conjugation. Highly contrasting properties were observed between ortho- and meta-coupled fluorophores. Unexpectedly, solubility issues hinder the application of ortho-linked molecules in LEC devices. However, bright blue emissions were achieved in LEC devices when meta derivatives were mixed in the active layer of LEC with poly­(ethylene oxide) and lithium triflate. Notably, the molecule having pyrene and phenanthroimidazole coupled through meta linkage (mPy) presented the best performance. It was identified that a fundamental trade off exists between efficiency and brightness for these tricomponent devices when the ion concentration is increased. Efficient device (device 3, maximum efficiency = 0.43 cdA–1) was the one with low amount of ions, whereas the bright one (device 5, maximum brightness = 585 cdm–2) had high ion concentrations. However, the performances of all of the tricomponent devices were reasonably high in comparison to their single component devices. In addition, modulation of charge transfer band was found feasible for these investigated molecules by switching the linking position, which provides us with an imperative clue to design ideal blue emitters for LEC applications incorporating pyrene and phenanthroimidazole units.