Two-channel emission controlled by a conjugation valve for the color switching of thermally activated delayed fluorescence emission

Abstract

The development of two-channel emission featuring deep-blue thermally activated delayed fluorescent (TADF) emitters is highly challenging due to their unavoidably extended conjugated structures. Herein, we propose a simple and effective strategy involving engineering of the interconnection position and formation of a “conjugation valve” to control their emission colors, including enabling emission of deep-blue light with an uncompromised external quantum efficiency (EQE). Two novel TADF emitters, namely 2,3′,4,5′-tetra(9H-carbazol-9-yl)-[1,1′-biphenyl]-3,4′-dicarbonitrile (34CzBN) and 3,3′,5,5′-tetra(9H-carbazol-9-yl)-[1,1′-biphenyl]-4,4′-dicarbonitrile (44CzBN), were developed to produce a two-channel-emission path by directly connecting two identical TADF units in two different ways to produce different degrees of conjugation. The non-conjugative mode of connection in 34CzBN resulted in severe steric hindrance at the central core, which interrupted the extensive conjugation and resulted in significant blue-shifts of the absorption and emission compared to those of its isomer 44CzBN. The two-channel-emission process provided a four-times higher EQE than the single-channel-emission process. The 34CzBN-based TADF device demonstrated deep-blue emission with color coordinates of (0.14, 0.14) due to a closed conjugation channel, and displayed an EQE of 15.2%. In contrast, the 44CzBN device showed green emission with color coordinates of (0.20, 0.44), due to the open conjugation channel, and an EQE of 13.8%. Additionally, the emission peak in the spectrum of the 34CzBN-based device became narrow (full width at half-maximum of ∼63 nm) due to restricted molecular motion resulting from the interlocked carbazole donor at the central core.