T₂ state energy modulation through molecular engineering: controlling the tug of war between thermally activated...

Single-component, metal-free organic emitters capable of both thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) hold great potential for next-generation optoelectronic devices, yet precise control over these emissions remains challenging. The manipulation of dark triplet states may offer a promising strategy in this regard. Here, we strategically designed four donor-acceptor molecules, QnD1Br, QnD2, QnPh1Br, and QnPh2, and their photophysical properties were investigated through both experimental and theoretical spectroscopy. Brominated derivatives with a single donor unit (QnD1Br and QnPh1Br) show dual emission: TADF results from reverse intersystem crossing (RISC) via the higher-lying T₂ state to S₁, while RTP from the T₁ state. Conversely, molecules with multiple donor units (QnD2 and QnPh2) exhibit only TADF, as increased π-conjugation stabilises T₂ and subsequently reduces the T₁-T₂ gap, thereby enhancing vibronic coupling (VC), which facilitates RISC via the T₁-T₂-S₁ pathway. Interestingly, these molecules do not show any RTP because at elevated temperature, the enhanced vibronic coupling facilitates the non-radiative decay from T₁-S₀. These findings highlight that tailoring higher triplet-state energies through molecular design enables control over emission pathways. This consolidated research described herein may open up new avenues for designing TADF/RTP-based potential organic OLED materials.