By adding different electron donor moieties to the ancillary ligand in ruthenium(ii) phenanthroimidazole complexes, we successfully designed near infrared light emitting complexes suitable for light emitting electrochemical cells (LECs).
Turn-on
time is a key factor for lighting devices to be of practical
application. To decrease the turn-on time value of a deep-red light-emitting
electrochemical cells (DR-LECs), two novel approaches based on molecularly
engineered ruthenium phenanthroimidazole complexes were introduced.
First, we found that with the incorporation of ionic methylpyridinium
group to phenanthroimidazole ligand, the turn-on time of the DR-LECs
device was dramatically reduced, from 79 to 27 s. By complexation
of ruthenium emitter with Ag+, the turn-on time was improved
by 85%, and the EQE of DR-device was increased from 0.62 to 0.71%.
These results open a new avenue in decreasing the turn-on time without
adding ionic electrolytes, leading to an efficient LEC.
Light-Emitting Electrochemical Cells (LECs) with a simple device structure ITO/Ru complex/Ga:In were prepared by using heteroleptic ruthenium(ii) complexes containing 2-(2-hydroxyphenyl)-1-(4-bromophenyl)-1h-imidazo[4,5-f][1,10]phenanthroline (hpbpip) as the π-extended ligand. After ancillary ligand modification, the [Ru(hpbpip)(dmbpy)2](ClO4)2 complex shows a deep red electroluminescence emission (2250 cd m(-2) at 6 V) centered at 685 nm, 65 nm red-shifted compared to the [Ru(bpy)3](ClO4)2 benchmark red-emitter at a very low turn voltage (2.6 V), demonstrating its potential for low-cost deep-red light sources. Moreover, the PL quantum yield of the [Ru(hpbpip)(bpy)2](ClO4)2 complex was revealed to be higher (0.121) than the benchmark standard [Ru(bpy)3](2+) (0.095).
Near-infrared light-emitting electrochemical cell (NIR-LEEC) has emerged as a new and promising lighting sourcewhich could serve as low-cost alternatives in NIR light-emitting sources which are typically expensive. LECs were also shown advantages such as light weight, simplicity and low operation voltages. However, only a few examples of NIR-LEEC are reported in which external quantum efficiency(EQE) of devices limited to 0.1%. Here, we report, efficient NIR-LEEC based of two novel binuclear ruthenium phenanthroimidzole complex which differ by employing the type of ancillary ligand including 2, 2′bipyridine (bpy) (B1) and 4, 4′ dimethyl bpy (B2) that realize maximum EQE of 0.14 and 0.68% and extremely long excited state lifetimes of 220 and 374 ns for thin film were estimated, respectively, indicating that influences of substitution on ancillary ligand. Moreover, this substitution dramatically influences other electroluminescence metrics including decreasing turn on voltage from 4.5 to 3.1 V, increasing maximum luminance (Lmax) from 193to 742 cd.m−2 and increasing lifetime from 539 to 1104 second, which are the best value among the binuclear ruthenium polypyridyl complexes to date.
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