Efficient and color-purity blue electroluminescence by manipulating the coupling forms of D‒A hybrids with phenothiazine as the strong donor

Controlling the conjugate degree of the molecules and regulating the properties of excited states are critical for determining the photoluminescence (PL) and electroluminescence (EL) properties of twisted donor–acceptor (D–A) type blue emitters. Here, two novel phenothiazine-based blue emitters derivatives, 10-ethyl-3-(4-(1-phenyl-1H-phenanthro [9,10-d]imidazol-2-yl)phenyl)-10H-phenothiazine (PHPPI) and 10-ethyl-3-(3-(1-phenyl-1H-phenanthro [9,10-d]imidazol-2-yl)phenyl)-10H-phenothiazine (MPHPPI), are successfully synthesized and their thermal, photophysics, theoretical calculations and electronic properties are fully investigated. By merging two simple donor and acceptor units together through different linking positions of the bridging benzene ring (para- and meta-position), the π-conjugation length is effectively controlled to ensure the deep-blue emission. The non-doped devices using the PHPPI and the MPHPPI emitters exhibit EQEmax (external quantum efficiency) of 4.14% and 2.23% with maximum luminous efficiency (LEmax) of 11.06 cd A−1 and 3.52 cd A−1, and EL emission peak of 496 nm and 464 nm, respectively. Whereas, the doped MPHPPI-based device exhibits color-pure deep blue EL performance with an emission peak at 460 nm, corresponding to the Commission Internationale de L'Eclairage coordinates of (0.15, 0.17), and a maximum EQE of 3.96%. To the best of our knowledge, this is the most efficient and color-pure deep blue OLED based on phenothiazine as a donor material reported thus far. The results of this study not only provide a better understanding of the relationship between the structure of the D-A system and the EL properties, but also provide a novel and versatile method for the design of deep blue emitters based on phenothiazine as a donor material.