Two-state resonance deep-red narrowband OLED emitters

Wide color gamut Organic Light Emitting Diode (OLED) materials require a small full-width at half-maximum (FWHM) to achieve superior display performance, and a high molar extinction coefficient (ε) is beneficial to improving energy transfer efficiency and further reducing power consumption. Merocyanine derivatives can regulate the two-state resonance between neutral and zwitterionic states through donor-acceptor modulation, and the resonance intensity can be quantified by the combination coefficient c ² parameter. A c ² = 0.5 corresponds to nearly degenerate two-state energy levels, where strong resonance suppresses ground-state energy dissipation, promotes strong emission, and simultaneously realizes high ε and small FWHM. Herein, relying on the two-state resonance theory, we select xanthene as the core chromophore and phenylamine as the donor, and further construct three novel narrowband-emission molecules (2EtN-O-2CN, 2EtN-O-S, and 2EtN-O-OCN) with different electron-withdrawing acceptors, which are different from boron-nitrogen materials. With the gradual increase of acceptor strength, the calculated c ² values of the three molecules are 0.40, 0.45, and 0.46, respectively. Photophysical results demonstrate that 2EtN-O-S and 2EtN-O-OCN with strong resonance exhibit a high ε of approximately 10⁵ L mol^-1 cm^-1 at the absorption peak and a small FWHM of 0.12 eV, much better than 2EtN-O-2CN (FWHM = 0.19 eV, ε = 5 × 10⁴ L mol^-1 cm^-1). Impressively, this type of non-multiple-resonance material can achieve efficient deep-red emission at relatively low molecular weights below 465. The 2EtN-O-S-based OLED achieves deep-red emission at 712 nm with a small FWHM of 0.16 eV. This work reveals the great potential of two-state resonance emitters for high-efficiency narrowband deep-red OLEDs.