Imidazole, being an interesting dinitrogenic five-membered
heterocyclic
core, has been widely explored during the last several decades for
developing various fascinating materials. Among the different domains
where imidazole-based materials find wide applications, the area of
optoelectronics has seen an overwhelming growth of functional imidazole
derivatives developed through remarkable design and synthesis strategies.
The present work reports a design approach for integrating bulky donor
units at the four terminals of an imidazole core, leading to the development
of sterically populated imidazole-based molecular platforms with interesting
structural features. Rationally chosen starting substrates led to
the incorporation of a bulky donor at the four terminals of the imidazole
core. In addition, homo- and cofunctional molecular systems were synthesized
through a suitable combination of initial ingredients. Our approach
was extended to develop a series of four molecular systems, i.e., Cz3PhI, Cz4I, Cz3PzI, and TPA3CzI, containing carbazole, phenothiazine, and triphenylamine
as known efficient donors at the periphery. Given their interesting
structural features, three sterically crowded molecules (Cz4I, Cz3PzI, and TPA3CzI) were screened by
using DFT and TD-DFT calculations to investigate their potential as
hole transport materials (HTMs) for optoelectronic devices. The theoretical
studies on several aspects including hole reorganization and exciton
binding energies, ionization potential, etc., revealed their potential
as possible candidates for the hole transport layer of OLEDs. Single-crystal
analysis of Cz3PhI and Cz3PzI established
interesting structural features including twisted geometries, which
may help attain high triplet energy. Finally, the importance of theoretical
predictions was established by fabricating two solution-process green
phosphorescent OLED devices using TPA3CzI and Cz3PzI as HTMs. The fabricated devices exhibited good EQE/PE and CE of
∼15%/56 lm/W/58 cd/A and ∼13%/47 lm/W/50 cd/A, respectively,
at 100 cd/m2.