Pure Hydrocarbons:An Efficient Molecular Design Strategy for the Next Generation of Host Materials for Phosphorescent Organic Light-Emitting Diodes

CONSPECTUS:Fantastic progresses have been made since the discovery of phosphorescent organic light-emitting diodes(PhOLEDs)in 1998(Nature 1998,395,151).In the emissive layer of a PhOLED,a heavy-metal complex(emitter)is dispersed as a guest in an organic semiconductor(host)in order to harvest both singlet and triplet excitons.This host/guest technique is essential to reach highperformance PhOLEDs.Undoubtedly,designing high-efficiency host materials for red,green,and blue phosphors has been the main driving force of the field.Nowadays,the most efficient PhOLEDs report very high external quantum efficiency(EQE)above 27%for the three colors.The molecular design of these high-efficiency host materials is based on small functional units,which have a crucial role in the device performance.These functional units are constructed on the assembly of electron-rich and/or electron-poor molecular fragments(such as carbazole,phenylamine,phosphine oxide,oxadiazole,pyridine,sulfone,etc.),which allows to finely tune the molecular orbitals energy levels(HOMO and LUMO)and the mobility of the charge carriers(hole and electrons).However,these molecular fragments incorporate heteroatoms,which in turn increase the molecular design complexity and production cost of the target materials and also create potential instability in the device.Indeed,the fragile C−N,C−P,and C−S bonds of such heteroatom-based hosts are involved,in part,in the OLED instability.As the instability of OLEDs is one of the most important problem to address at the current stage of development,developing new generations of host materials,without heteroatoms,has appeared as an important challenge in the field.In recent years,this new generation of host materials,called Pure HydroCarbons(PHC),only incorporating carbon and hydrogen atoms,has drawn the attention of the scientific community.PHC host materials only contain benzene rings,one of the most robust and inert building unit,which are assemble with suitable linkages to adjust their energy levels.However,reaching high-performance PhOLEDs with PHC-based hosts has been a real challenge.In the present account,thanks to a structure−properties relationship study,we recount the history of PHC-based host materials for PhOLEDs from the first example published in 2005(Org.Lett.2005,7,5131),to the recent advances,which have shown that PHC hosts can overpass the performance of heteroatom-based hosts(Chem.Sci.2020,11,4887).Our group has contributed over the years to unravel the design principles of PHC hosts in order to improve the devices performance.In the light of recent results,PHC hosts now enter a new era and this work shows that the PHC design strategy is promising for the future development of OLED industry as a high-performance and low-cost option.