A series of spirofluorene-based host materials for efficient phosphorescent organic light-emitting diodes

A subtle change in molecular structure of OLED material could lead to significant variation in device performance. For a better understanding of the connection between molecular structures and properties, four molecules (1, 2, 3, and 4) based on different spiro-systems, namely spiro[cyclopropane-1,9'-fluorene], spiro [cyclopentane-l,9'-fluorene], 1',3'-dihydrospiro[fluorene-9,2'-indene] and spirobifluorene, were designed and prepared. The influences of different spiro units on photophysical properties and electroluminescence performance were fully studied. The experimental results showed that photophysical properties, electrochemical behaviors, and frontier molecular orbital distributions of these different spirofluorenes are mostly identical because the pi-backbones were less affected by the different cycles at the C-9 position of fluorene. However, green phosphorescent organic light-emitting diodes (PHOLEDs) based on spirobifluorene derivative 4 exhibited the highest electroluminescence performance among these four hosts. Thus, we hope this work could reveal deep understanding for the rational design of host materials based on spirofluorene blocks.