New blue-violet emitters based on an indenopyrazine core for OLEDs: Effects of the position of m-terphenyl side group substitution on optical and electroluminescence properties

A promising class of blue–violet emitters based on a new functional core, the indenopyrazine group, has successfully been synthesized for the first time by substitution with bulky m-terphenyl side groups in the ortho, meta, or para positions. There are larger blue shifts in the UV–visible absorption and PL spectra of the synthesized ortho- and para-substituted derivatives than in those of the meta-substituted derivative. Molecular calculations verified that these differences are due to the variation in the π-conjugation length of the derivatives with the position at which the side group is attached to the indenopyrazine core. When the synthesized compounds were used as emitting layers in non-doped OLED devices, a related trend was observed in their optical properties. In particular, the OLED containing the para-substituted derivative was found to exhibit excellent characteristics, with maximum EL emission at 423 nm, a full width at half maximum of 42 nm, pure violet emission with CIE coordinates (0.173, 0.063), and an external quantum efficiency of 1.88%.