Sustainable phosphorescence based on solution-processable and vacuum-sublimable cationic ruthenium(II) complexes achieved by counter-ion control

Abstract

We designed and synthesized a novel series of red-emitting cationic ruthenium(II) complexes 1–6 with the same coordinated ruthenium(II) cation tris(2,2′-bipyridine)ruthenium(II) while different-sized negative counter-ions, namely chloride (1), tetrafluoroborate (2), hexafluorophosphate (3), perchlorate (4), tetrakis(pentafluorophenyl)borate (5) and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (6), respectively. Their physicochemical characteristics including solubility, photophysical properties and electrochemical behaviors were fully investigated. Experiments indicated that introduction of bulky anions could effectively lower the molecular polarity and lattice energy of these ionic materials, improve their solubility in organic solvents, thus enable constructing solution-processed organic light-emitting diodes (OLEDs) based on complexes 2–6. Furthermore, complex 6 was demonstrated evaporable, owing to the electrostatic interaction strongly reduced by its anion with the largest steric hindrance and well-dispersed charges. We succeeded in the preparation of orange-red-emitting OLEDs fabricated by vacuum evaporation deposition thereof, achieving a maximum brightness of 5345 cd m−2 with color coordinates of (0.51, 0.44). To our knowledge, this is the first report on sublimable cationic ruthenium(II) complexes, suggesting their great potential as a new material system for sustainable phosphorescence.