Self-assembled molecular network with waterwheel-like architecture: experimental and theoretical evaluation toward electron transport capabilities for optoelectronic devices.

In recent times, self-assembled electron transport materials for optoelectronic devices, both solar cells and organic light-emitting diodes (OLEDs), have been gaining much interest as they help in fabricating high-efficiency devices. However, designing organic small molecular materials with star-shaped self-assembled networks is a challenge. To achieve this sort of target, we chose triazine and benzene-1,3,5-tricarbonyl cores for developing such architecture, and we developed four molecular systems, vizTCpCN, TCmCN, TmCN, and TpCN. Successful isolation of single crystals followed by structural analysis of TmCN revealed interesting molecular arrangements in the solid state resulting in the formation of a waterwheel type architecture with an extended network bearing characteristic voids. Theoretical calculations was carried out to check their electron transportability. The natural transition orbital calculation helped in understanding the locally excited and charge transfer excited states. The low electron reorganization energies of these molecules indicated that these materials may have potential to be used in electron transport layers of optoelectronic devices, particularly in OLEDs. Moreover, the assembled networks have a relatively wide surface area and linked structures, which are advantageous for the conduction of carriers with poor electron recombination inside the ETL, and these may offer a straightforward channel for electron conduction to the emissive layer. Finally, the fabricated electron-only device indicated that the synthesized materials may be used as ETMs in the electron transport layer of optoelectronic devices.

Sterically Crowded Donor-Rich Imidazole Systems as Hole Transport Materials for Solution-Processed OLEDs.

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.