Study On Synthesis, Optical And Electroluminescent Properties Of Red Emitters Based On Dithienylbenzothiadiazole

Organic light-emitting diodes (OLEDs) have drawn considerable interest among researchers and industries because of their great prospective in the field of flat panel display and efficient solid-state lighting. For full-color applications, efficient emitting materials of all three primary colors (red, green, and blue) are required. It is essential to improve the performance of red-emitting materials. In this paper, we described the recent research progress of red-emitting materials. And four red emitters 1-4 based on a dithienylbenzothiadiazole emissive core and naphthalene end groups, were synthesized by Suzuki and Stille coupling reaction. Fluorescence spectra of the four compounds in the solution and the solid state showed red emission, with maximum photoluminescence wavelengths varied between 605 nm and 666 nm.The control of emission wavelength has been achieved by modulating the position of end group and altering conjugation length. Compounds 2 and 4 with end group 2-naphthalene have a longer conjugation length, when compared to compounds 1 and 3 with end group 1-naphthalene, displayed obvious red-shift. We lowered the LUMO levels of the compounds by using strongly electron-withdrawing group (trifluoromethylphenyl), the LUMO levels of the compounds with trifluoromethylphenyl group is lower 0.03 ev than the compounds with electron-donating group (4-sec-butoxyphenyl). In addition, when the 4-sec-butoxyphenyl was substituted by trifluoromethylphenyl, the emission wavelength displayed obvious blue shift. Especially for compound 4, the maximum emission showed blue shift about 30 nm.Compounds 1 and 2 have good thermal stability. During the DSC test, the glass-transition temperature (Tg) was detected in the second heating process, no other exothermic peak was observed, which indicated a stable amorphous was formed after the heat treatment. Therefore, multi-layer OLEDs were fabricated by thermal deposition in a vacuum system. The devices ITO/MoO3(6 nm)/NPB(100 nm)/EML(30 nm)/Alq3(30 nm)/LiF(1.5 nm)/Al(200 nm) showed the maximum luminous efficiency were 2.72 cd A-1 and 0.89 cd A-1 for 1 and 2, respectively. Due to the strong electron-withdrawing ability of trifluoromethylphenyl, 4 grew as nano-wire structure in the solvent, which is promising for optical waveguide purpose.