The color-tuning mechanism in multi-resonance thermally activated delayed fluorescence emitters: site effects in peripheral modification and skeleton fusion

Ultra-narrowband, highly modifiable multi-resonance thermally activated delayed fluorescence (MR-TADF) materials are critical for high-performance, wide-color-gamut display applications, yet balancing broad color tunability while maintaining high color purity remains challenging. We present a proof-of-concept study into 7,10-di-<i>tert</i>-butyl-5-mesityl-5<i>H</i>-benzo[5,6][1,4]azaborinin-o[3,2,1 <i>jk</i>]carbazole (BIC) derivatives with a B/N MR skeleton to elucidate the site effects of peripheral modification and skeleton fusion on MR-TADF properties. Incorporating electron donating groups (EDGs) in a <i>meta</i>-(N-π-N) configuration forms D-void-A motifs, which retain or even enhance the MR characteristics, achieving a blue-shifted emission. In contrast, EDG modifications in a <i>para</i>-(N-π-N) configuration result in conjugative D-π-A motifs, leading to a red-shift in emission. Rigid skeleton fusion in an extended D-π-A motif compensates for the potential increase in vibronic coupling, which typically arises from a decline in MR properties. Furthermore, the centrosymmetric fused skeleton suppresses the excited-state structural relaxation in dielectric environments, achieving narrowband green emission (528 nm, 30 nm or 929 cm^-1 in emission FWHM). This work elucidates the intrinsic mechanism of the widely applicable site effects in regulating MR characteristics and color-tuning <i>via</i> peripheral modification and skeleton fusion, providing a theoretical foundation for the design of ultrapure long-wavelength emission MR-TADF materials.