Rational Modulation of Small Molecules Based on Methyl Substituted Dibenzothiophene-S,S-Dioxide Unit for Deep-Blue Light-Emitting Diodes

Abstract

Small-molecule organic light-emitting diodes (OLEDs) with deep-blue emission play an important role in full-color flat panel displays and solid-state lighting. Here, a series of luminogens (BM1−BM4) are developed and constituted of methyl-modified dibenzothiophene-S,S-dioxide (SO) with different linking patterns and fluorene unit with grafted carbazole-functionalized side chains. The linking position of SO unit largely affects the conjugation degree, band gap, molecular geometry, and electron cloud distribution of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which ultimately regulate the electroluminescence performance of OLED devices. Besides, the functionalized methyl substitutions endow all luminogens with relatively large torsion angles. Hence, they present a peak wavelength (λPL) of 423−439 nm in the film states, which renders them as appealing deep blue emitters. Apart from BM1 because of excimers, single-layer OLED devices based on BM2−4 displayed typical deep-blue emissions in the wavelength range of 426−438 nm, furthermore, the optimized multilayer non-doped devices adopting PFN as an electron injection layer yield a decent maximum luminous efficiency (LEmax) of 4.33 cd A−1, and Commission Internationale de l’Eclairage (CIE) coordinates of (0.16, 0.11) due to the more balanced hole and electron fluxes.

Keywords: OLED, deep-blue emission, dibenzothiophene-S, S-dioxide, methyl substitute