Synthesis and optoelectronic properties of spirofluorenexanthene-based carbazole host materials

A series of carbazole substituted spiro[fluorene-9,9'-xanthene] (SFX) derivatives, namely SFX-2-Cz, SFX-2'-Cz, SFX-3'-Cz, SFX-2,7-DCz, SFX-2',7'-DCz and SFX-3',6'-DCz, were designed and prepared as hosts for phosphorescent organic light emitting diodes (PhOLEDs). All the target compounds exhibit excellent thermal stability, especially for the substitution onto the 3',6'-position of SFX (SFX-3',6'-DCz, upto 500 °C). The results of low temperature phosphorescence show that the triplet energy level (T1) will be reduced when carbazole group is attached to the fluorene unit of SFX, however, the T1 is nearly intact when carbazoles were introduced into the xanthene moiety of SFX. In the following investigation of blue, green, and red PhOLEDs based on these SFX-carbazole hosts, all the host materials can realize effective energy transfer to dopant phosphors under electric field excitation. For the blue PhOLEDs, the SFX-3',6'-DCz-based device reached the maximum current efficiency (CE) of 9.7 cd A-1, power efficiency (PE) of 6.4 lm W-1 and external quantum efficiency (EQE) of 5.1% due to high T1. However, the SFX-2'-Cz-based green PhOLED and the SFX-2,7-DCz-based red PhOLED show better performance of 28.2 cd A-1, 24.7 lm W-1, 8.5% and 17.4 cd A-1, 15.6 lm W-1, 10.1%, respectively, attributable to their well-matched frontier molecular orbitals energy levels with corresponding phosphorescent dopants. This work provides a systematic scheme for the rational design of efficient SFX-based hosts by carbazolyl functionalization.