Highly Emissive Luminogens Based on Imidazo[1,2-a]pyridine for Electroluminescent Applications

• Published in: Chemistry, an Asian journal an Asian journal, 2014-01, Vol.9 (1), p.294-304
• Main Authors: Nagarajan, Natarajan, Velmurugan, Gunasekaran, Prakash, Asit, Shakti, Nanda, Katiyar, Monica, Venuvanalingam, Ponnambalam, Renganathan, Rajalingam
• Format: Article
• Language: English
• Subjects: Chemistry; density functional calculations; donor-acceptor systems; dyes/pigments; Electrochemical Techniques; fluorescence; Fluorescent Dyes - chemical synthesis; Fluorescent Dyes - chemistry; Luminescent Measurements; Molecular Structure; nitrogen heterocycles; Pyridines - chemical synthesis; Pyridines - chemistry
• ISSN: 1861-4728; 1861-471X
• DOI: 10.1002/asia.201301061

A search for novel organic luminogens led us to design and synthesize some N‐fused imidazole derivatives based on imidazo[1,2‐a]pyridine as the core and arylamine and imidazole as the peripheral groups. The fluorophores were synthesized through a multicomponent cascade reaction (A3 coupling) of a heterocyclic azine with an aldehyde and alkyne, followed by Suzuki coupling and a multicomponent cyclization reaction. All of the compounds exhibited interesting photophysical responses, especially arylamine‐containing derivatives, which displayed strong positive solvatochromism in the emission spectra that indicated a more polar excited state owing to an efficient charge migration from the donor arylamine to the imidazo[1,2‐a]pyridine acceptor. The quantum yields ranged from 0.2 to 0.7 and depended on the substitution pattern, most notably that based on the donor group at the C2 position. Moreover, the influence of general and specific solvent effects on the photophysical properties of the fluorophores was discussed with four‐parameter Catalán and Kamlet–Taft solvent scales. The excellent thermal, electrochemical, and morphological stability of the compounds was explored by cyclic voltammetry, thermogravimetric analysis, and AFM methods. Furthermore, to understand the structure, bonding, and band gap of the molecules, DFT calculations were performed. The performance of the electroluminescence behavior of the imidazo[1,2‐a]pyridine derivative was investigated by fabricating a multilayer organic light‐emitting diode with a configuration of ITO/NPB (60 nm)/EML (40 nm)/BCP (15 nm)/Alq3 (20 nm)/LiF (0.5 nm)/Al(100 nm) (ITO=indium tin oxide, EML=emissive layer, BCP=2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline, Alq3=tris(8‐hydroxyquinolinato)aluminum), which exhibited white emission with a turn‐on voltage of 8 V and a brightness of 22 cd m−2. Core developments: The electroluminescence characteristics of new donor–acceptor–acceptor conjugated materials that consist of imidazo[1,2‐a]pyridine, imidazole, and arylamine are reported (see picture). The compounds exhibit interesting photophysical properties that are highly dependent on the conjugation length and nature of the terminal functional groups.