Push–Pull Dyes for Yellow to NIR Emitting Electrochemical Cells

Push–pull (D-π-A) dyes are explored for organic light-emitting electrochemical cells (LECs) taking advantages of their tunability in the intramolecular charge transfer (CT) through a π-bridge between donor and acceptor. These dyes present high thermal stability beyond 290 °C and fluorescence quantum yields (Φ_f), and simple thin film processability. The fluorene-based dyes with diphenylamine donor (DPF) show high tunability in electroluminescence wavelength up to 825 nm that corresponds to the most red-shifted metal-free LECs reported to date. The DPFs show high irradiance under a low working voltage in the presence of poly(9-vinylcarbazole) (PVK) and 2,5-bis(5-tert-butyl-benzoxazol-2-yl)thiophene (BBOT) hosts. As the CT distance of dyes is increased, V_on of the LECs is decreased, since Φ_f and lifetime of dyes are higher. Furthermore, the smaller angle between the π-bridge and acceptor (A_πA) of dye is critical to increase irradiance, for a closer dye–host proximity. Thus a DPF dye with the lowest A_πA (dye I) affords the highest irradiance of 425 µW cm⁻² (>220 Cd m⁻²) among the dyes in this study. Therefore push–pull dyes could provide a rational material design principle toward NIR emitting LECs.