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 exhibited excellent thermal stability, especially for the substitution onto the 3′,6′-position of SFX (SFX-3′,6′-DCz, up to 500 °C). The results of low temperature phosphorescence show that the triplet energy level (T1) will be reduced when the 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 could 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 orbital energy levels with the corresponding phosphorescent dopants. This work provides a systematic scheme for the rational design of efficient SFX-based hosts by carbazolyl functionalization.