Novel 1,8-naphthalimide derivatives for standard-red organic light-emitting device applications

Abstract

Three red-emissive D–π–A-structured fluorophores with an aromatic amine as the donor, ethene-1,2-diyl as the π-bridge, and 1,8-naphthalimide as the acceptor subunit, namely, (E)-6-(4-(dimethylamino)styryl)-2-hexyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (Nap1), (E)-2-(2,6-di(isopropyl)phenyl)-6-(4-(dimethylamino)styryl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (Nap2) and (E)-2-(2,6-di(isopropyl)phenyl)-6-(2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)vinyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (Nap3), were designed and synthesized. In-depth investigations on the correlations between their molecular structures and photophysical characteristics revealed that the presence of an electron-rich 4-dimethylaminophenyl donor moiety in compound Nap1 could endow it with a red emission (e.g., λ_PL max = 641 nm in the host–guest blend film with a 14 wt% guest composition); moreover, the replacement of the n-hexyl group of Nap1 bonding to the imide nitrogen atom for a more bulky 2,6-di(isopropyl)phenyl one would result in compound Nap2 with more alleviated concentration quenching. Alteration of the 4-(dimethylamino)phenyl donor subunit of Nap2 into a more electron-donating 1,1,7,7-tetramethyljulolidin-9-yl substituent would render compound Nap3 with more improved chromaticity (e.g., λ_PL max = 663 nm in a 14 wt% guest-doped film). Consequently, Nap3 could not only emit standard-red fluorescence with satisfactory chromaticity, but it also showed suppressed intermolecular interactions. Using Nap3 as the dopant, a heavily doped standard-red organic light-emitting diode (OLED) with the device configuration of ITO/MoO₃ (1 nm)/TcTa (40 nm)/CzPhONI:Nap3 (14 wt%) (20 nm)/TPBI (45 nm)/LiF (1 nm)/Al (80 nm) was fabricated, and the Commission Internationale de L’Eclairage coordinates, maximum external quantum efficiency and maximum current efficiency of this OLED were (0.67,0.32), 1.8% and 0.7 cd A⁻¹, respectively. All these preliminary results indicated that 1,8-naphthalimide derivatives could act as quite promising standard-red light-emitting materials for OLED applications.