IMIDAZO[1,2-a]PYRIDINE-BASED ORGANIC MATERIALS FOR ELECTRONIC APPLICATIONS

Abstract

Organic π-conjugating small molecules have been utilized in fluorescent OLEDs, and several molecular engineering strategies have been applied to improve the device efficacy. In order to produce efficient OLEDs, the organic materials have to full-fill some criteria such as high photoluminescence quantum efficiency (PLQE), high thermal, electrochemical and morphological stability, and high charge carrier efficiency. Therefore, the selection of core moiety and attached chromophores units are very crucial. Most of the organic emissive materials used in OLEDs consist of nitrogen-containing π-conjugated moieties. Triphenylamine, diphenylamine, carbazole with electron-donating nature and triazine, tri-azole, pyridine, benzimidazole, phenanthroimidazole, and cyano groups with electron-withdrawing ability are a few examples of nitrogen-containing organic materials. Additionally, due to their electron deficient nature these materials can also be used as acceptor to build superior bipolar materials joined with donor groups. Therefore, N-heterocyclic compounds have potential for the development of high-performance host and electron transport materials. In this thesis, we have designed and synthesized pyridine and imidazole fused heterocyclic system, imidazo[1,2-a]pyridine (ImPy) and used as a core structure. It is believed that fused pyridine due to electron-withdrawing nature may impart electron-transporting ability to the resulting materials. It‟s molecular rigidity, high quantum yield, high thermal stability, amorphous nature, easy synthetic procedures and scope for functionality with different linking topologies further proves its advantageous candidacy over other conventional fluorophores. This thesis consists of seven chapters. The first chapter deals with the aim and scope of the work proposed on imidazo[1,2-a]pyridine based organic materials for optoelectronic applications such as organic light-emitting diodes (OLEDs). The second deals with a brief literature survey on the synthesis of pyridine heterocyclic materials and application of six-membered N-heterocyclic such as pyridine, pyrimidine, and triazine and some functional materials constructed by fused N-heterocyclic such as phenanthroimidazole, benzimidazole in the optoelectronic devices. In third chapter, isomeric bipolar materials featuring triphenylamine and carbazole as a donor and 2-phenylimidazo[1,2-a]pyridine electron deficient unit as the core moiety were designed, synthesized, and studied as blue-emitting materials. As expected, their optical and electrochemical properties are prominently dependent on the nature of chromophore loading and their linking topology to the 2-phenylimidazo[1,2-a]pyridine unit. C2 substituted dyes exhibit red-shifted absorption and emission compared to their positional analogs containing donors in C6-position of imidazopyridine. Interestingly, positive solvatochromism is exhibited by triphenylamine containing dyes, which suggests the intramolecular charge transfer from triphenylamine donor to imidazopyridine acceptor in the excited state. All the compounds revealed excellent thermal stability surpassing 355 °C. The dye possessing triphenylamine at C6-position of imidazopyridine showed the comparatively better electroluminescence performance in the series and achieved deep blue color with CIE coordinate (0.17, 0.10) and external quantum efficiency of 2.5 % (1.1 cd/A, 0.7 lm/W).In fourth chapter, newly designed CN-substituted 2-phenylimidazo[1,2-a]pyridine unit was employed as an electron acceptor in D˗A fluorophores. The positions of donors (triphenylamine/carbazole) and cyano substituents are respectively exchanged to get at a series of isomers. All the fluorophores exhibit high fluorescence quantum yield (Фf = < 0.70) and excellent thermal stability (Td10 = < 400 °C). Dye comprising triphenylamine unit at paraposition of C2 phenyl ring exhibited red-shifted emission maxima with larger Stokes shift in the series. Moreover, all the isomers presented excited-state solvatochromism referable to the photo-induced intramolecular charge transfer between donors and cyano acceptor. Addition of the cyano group on phenylimidazopyridine restrained the deposition of aggregates in the solidstate and stabilized the lowest unoccupied molecular orbital. Among the all, doped devices constructed with 3 wt% of 3b revealed the excellent performance in the series with a maximum EQE of 6.9%, 2759 cd/m2 luminance, and CIE coordinates (0.16, 0.08) which is very close to the NTSC standard blue.In fifth chapter, three new bipolar materials based on the phenanthroimidazoleimidazopyridine conjugated system were successfully synthesized and characterized for electronic devices. The position and number of phenanthroimidazole substituents is change to obtain different isomers. All isomers carried superb thermal stability and high photoluminescence quantum yield. Dyes in which phenanthroimidazole chromophore linked at the para-position of C2 phenyl ring showed the red-shifted emission band. The positive solvatochromism was observed with the increase of solvent polarity, associated with the intramolecular charge transfer at excited state. Electrochemical measurement revealed that the fluorophores have low-lying LUMO energy levels. Further, doped devices were fabricated with these emitters to estimate the EL performances. Among all the emitters, the doped device fabricated with a dye containing phenanthroimidazole chromophore at para-position of C2 phenyl showed superior electroluminescence performance with CIE coordinates (0.16, 0.08).In six chapters, 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 change 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 centre core considerably influenced the optical and electroluminescence properties. Among all, triphenylamine substituted on the para-position of the C2 phenyl ring 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 orbital energies attributable to structural isomers is realized. Further, these isomeric dyes used as emitting dopants in multilayer solution-processed organic light-emitting diode. The doped devices fabricated with a dye containing carbazole as donor exhibited deep-blue emission with CIE coordinates (0.16, 0.08) and decent device characteristics.Finally, chapter seven presents a brief summary of the overall work, including analysis and establishment of structure-property relationship among the synthesized emitters. In addition, the future perspectives of imidazopyridine based organic materials for developing efficient OLEDs are also discussed.