Efficient Blue and Yellow Organic Light-Emitting Diodes Enabled by Aggregation-Induced Emission

A new class of donor–bridge–acceptor
(D−π–A)
π-conjugated light-emitting molecules comprising carbazole as
donor and maleimide (Cbz-MI, Cbz-MI­(d)),
phthalimide (Cbz-Pth) as acceptor units with phenyl ring
as spacer have been synthesized in good yields. These compounds exhibit
high quantum yield with three distinct emission colors yellowish-green
(Cbz-MI), bright yellow Cbz-MI­(d), and sky
blue (Cbz-Pth) in the solid state. Single-crystal X-ray
and quantum chemical calculations reveals that twisting of the phenyl
rings with high torsional angle on maleimide and phthalimide units
reduce the effective inter-chromophore electronic coupling, furnish
dramatic changes in their photophysical properties in solution and
solid states. Intriguingly, Cbz-MI­(d) and Cbz-Pth exhibits a unique aggregation-induced blue-shifted emission (AIBSE)
due to restricted intramolecular rotation (RIR) process, while Cbz-MI shows red-shifted emission in the solid state. The
solvatochromic study reveal that combined RIR and excited state migration
augment AIE (aggregation-induced emission) properties. The electrochemical
properties reveal that Cbz-MI exhibits high oxidation
propensity while Cbz-Pth shows low reduction values.
Subsequently, organic light-emitting diodes (OLEDs) were fabricated
with a simple three-layer device containing Cbz-Pth and Cbz-MI­(d) as emitting layers. Cbz-MI­(d) exhibits
high performance yellow OLED with an external quantum efficiency exceeding
∼4.1% and a brightness exceeding ∼73915 cd/m2, which is among the best performance reported for bright yellow
fluorescence organic light-emitting diodes.