Synthesis and optical properties of the isomeric phenanthroimidazole Imidazo[1,2-a]pyridine conjugates: Effects of donor and their linking topology

Abstract

In the present work, four imidazopyridine based isomeric emitters were synthesized as deep-blue emitters for application in organic light-emitting diodes. The position of triphenylamine/carbazole and phenanthroimidazole is changed to obtain different isomers. Their structure-property relationships were investigated through photophysical, electrochemical, thermal and electroluminescence properties. All the isomers exhibited excellent thermal stability and high fluorescence quantum efficiency. The position of the triphenylamine, carbazole and phenanthroimidazole units on the 2-phenylimidazopyridine core considerably influenced the optical and electroluminescence properties. Among all, triphenylamine substituted on the para-position of the phenyl ring attached to the C2 of the imidazopyridine exhibited the longest wavelength emission maximum. The positive solvatochromism and larger Stokes shifts of this compound revealed the participation of ICT from triphenylamine to imidazopyridine unit. Electrochemical measurement presented the high-lying HOMO for the triphenylamine containing dyes and low lying LUMO for the carbazole containing dyes. Thus, variation in the orbital energies attributable to the structural isomers is realized. Further, these isomeric dyes were used as emitting dopants in multilayer solution-processed organic light-emitting diodes. The doped devices fabricated with a dye containing carbazole as a donor exhibited deep-blue emission with CIE coordinates (0.16, 0.08) and decent device characteristics.