Spirobifluorene-based pyrazoloquinolines: efficient blue electroluminescent materials

Abstract

We report the synthesis of spirobifluorene-based pyrazoloquinolines, spiro-PAQ-Me and spiro-PAQ-Ph, in which two identical luminophores are connected through an sp³-hybridized carbon atom (a spiro center) and are orthogonally arranged. The incorporation of the rigid spirobifluorene linkage results in significant increases in the glass transition temperatures, which are in the range 246–280 °C. These new materials display the characteristic absorptions of the mono-pyrazoloquinoline (i.e. non-spiro) derivatives, each with a broad, low-energy absorption at ca. 420 nm, and emit photoluminescence efficiently in the blue region. Electrochemical studies reveal that these compounds exhibit reversible reductions and low-lying LUMO energy levels that originate from the electron-deficient nature of the pyrazoloquinoline ring. Multilayer organic electroluminescent devices constructed using spiro-PAQ-Ph as a dopant in the emitting layer produced bright blue emissions with maximum luminescence exceeding 20 000 cd m⁻². For the 2.0%-doped device, a high external quantum efficiency of 3.6% (4.5 cd A⁻¹, 2.02 lm W⁻¹) was achieved at 20 mA cm⁻² and 7.0 V with color coordinates of (0.14, 0.17).