Narrow-Band Orange–Red Emission Organic Luminophore with Dominant Low-Frequency Vibronic Coupling

High-resolution organic light-emitting diode (OLED) display industries are in significant demand for organic light-emitting materials of narrow-band emission. However, current organic light-emitting materials, especially the orange–red luminophores, usually exhibit broad emission as a result of the large structural relaxation as well as vigorous vibronic couplings in the excited state. In this study, a novel molecule, 2,2′-(7-phenylquinolino[3,2,1-de]acridine-5,9-diylidene)dimalononitrile (QADMA-Ph), with methylenemalononitrile as the acceptor and acridine as the donor, is designed and synthesized. According to the analysis of photophysical properties and theoretical computation, we found that the strong intramolecular charge transfer character endows QADMA-Ph with a favorable bandgap to the orange–red emission region. Benefiting from the confined π conjugation of the acceptor and donor within the locked-in and nonplanar structure, the excited state of QADMA-Ph presents dominant low-frequency vibronic coupling arising from the twisting vibration of the nonplanar core and the rotating vibration of the peripheral substituent. Consequently, QADMA-Ph exhibits a narrow-band emission at 568 nm with a full width at half maximum (fwhm) of 41 nm/0.13 eV in the diluted toluene solution. The corresponding OLED emitted a narrow electroluminescence at 592 nm with a fwhm of 54 nm/0.19 eV.