New blue emitting indenopyrazine derivatives for OLEDs: Improving the EL properties through substitution effects by using terphenyl side group

Two new blue emitting materials of o,p-TP-EPY (6,6,12,12-tetraethyl-2,8-bis-[1,1′;3′,1″]terphenyl-4′-yl-6,12-dihydro-diindeno[1,2-b;1′,2′-e]pyrazine) and m,m-TP-EPY (6,6,12,12-tetraethyl-2,8-bis-[1,1′;3′,1″]terphenyl-5′-yl-6,12-dihydro-diindeno[1,2-b;1′,2′-e]pyrazine) were synthesized using two bulky m-terphenyl side groups with different substitution positions in the new core system of indenopyrazine (IPY). The o,p-TP-EPY more effectively prevented intermolecular interactions compared to m,m-TP-EPY, resulting in more blue-shifted emission in the thin film state. When the synthesized compounds were used as emitting layers in non-doped OLED devices, an identical trend in electroluminescent spectra was observed. EL spectrum of o,p-TP-EPY (λmax,EL = 440 nm) was more blue shifted than that of m,m-TP-EPY (λmax,EL = 452 nm), and external quantum efficiency (E.Q.E.) of o,p-TP-EPY (2.68%) was also showed large improvement of about 60% compared to efficiency of m,m-TP-EPY (1.61%).