Realization of deep-blue TADF in sterically controlled naphthyridines for vacuum- and solution-processed OLEDs

Abstract

Narrow-band deep-blue (emission peak < 460 nm) TADF emitters are in demand for commercial OLED display applications, yet the development of efficient emitters with low efficiency roll-off is very challenging. To address this issue, herein, we studied carbazole–naphthyridine (donor–acceptor)-based blue-emitting TADF compounds, which were designed by using both the H-bonding and sterically controlled charge-transfer (CT) interactions between D and A units. Methyl substitution employed at the first position of t-butyl-carbazole donors was found to affect CT strength and consequently the TADF properties of the studied compounds, enabling a significant reduction of delayed fluorescence lifetime (down to 3.1 μs) and enhancement of reverse intersystem crossing rate (up to 10⁶ s⁻¹). The naphthyridines were demonstrated to hold great potential as deep-blue TADF emitters suitable for both vacuum- and solution-processed TADF OLEDs. The optimized devices with 7 wt% naphthyridine emitter in a weakly polar mCP host delivered external quantum efficiencies (EQEs) of up to ∼17.6% and ∼13.5% for vacuum- and solution-processed OLEDs, respectively. Unsubstituted naphthyridine exhibited deep-blue (λ_max < 460 nm) and narrow-band (FWHM = 66 nm) electroluminescence, whereas the more twisted methyl-substituted compound expressed broader band (FWHM > 80 nm) sky-blue (λ_max ≈ 480 nm) emission. The demonstrated emitters are among the best-performing conventional D–A-type blue/deep-blue TADF emitters in terms of EQE and efficiency roll-off properties of their devices.