Chromenopyrazole-based TADF emitters for bluish-green and sky-blue OLEDs

Abstract

Thermally activated delayed fluorescence (TADF) materials have emerged as efficient, sustainable alternatives to phosphorescent emitters in organic light-emitting diodes (OLEDs), as they do not require heavy metals which are expensive and damaging to the environment. Nevertheless, the development of bright and high-efficiency bluish-green TADF emitters remains a significant challenge. Herein, we present a series of novel molecules based on a chromenopyrazole (CP) acceptor with various donor moieties, phenoxazine, phenothiazine, and phenoselenazine, demonstrating TADF emission. TADF has been confirmed by luminescence spectroscopy methods and quantum chemical TD-DFT calculations. Among the investigated dyes, phenoxazine-containing dye (POZCP) has demonstrated the most favorable photophysical characteristics for OLED applications, including a delayed fluorescence lifetime of 1.8 µs in the doped PMMA film. An OLED device employing POZCP exhibited bluish-green emission with a maximum luminance of 13 864 cd m⁻², a current efficiency of 11.6 cd A⁻¹, a peak EQE of 14.6%, a T₉₀ of 20 h and a low turn-on voltage of 3.0 V. Structural modification of POZCP by incorporating an additional phenyl ring as a π-spacer resulted in a broader electroluminescence spectrum with sky-blue emission; however, this significantly suppressed the TADF process. These findings establish CP-based molecules as promising candidates for efficient bluish-green and sky-blue TADF emitters and highlight their potential in next-generation OLED technologies.