Theoretical studies of a novel carbazole–fluorene macrocycle as a host material for phosphorescent organic light-emitting diodes and the effects of substituents

Phosphorescent organic light-emitting diodes (PhOLEDs) have great potential in display applications due to their high luminous efficiency. Using density functional theory, a series of host molecules labelled as carbazole-fluorene macrocycles (CzFMCs) were designed and characterized to possess high triplet energy levels for application in PhOLEDs, and their accuracy was verified by comparing with experimental values. Our systematic investigation focuses on how the number and position of cyano, ethynyl, and fluoro substitutions affect various electronic properties of these hosts, including triplet energy (E_T), frontier orbital energy levels, absorption spectra, charge-injection barriers, and reorganization energies. These properties were compared with those of reference hosts, emissive layers, and hole and electron transport materials. Our findings highlight that cyano-substituted host molecules exhibit higher E_T, balanced charge transport and low charge injection barriers, making them promising candidates for blue PhOLEDs. Among the host molecules examined in the study, four were identified as the most effective for blue PhOLEDs, showcasing superior electronic properties compared to traditional reference hosts. This research offers theoretical insights into a novel macrocycle design for host materials, warranting further experimental exploration.