Blue emitters with various electron-donors attached to the 9-phenyl-9-phosphafluorene oxide (PhFIOP) moiety and their thermally activated delayed fluorescence (TADF) behavior

Abstract

A series of luminescent molecules have been developed through attaching various electron-donors to the 4- and/or 6-positions of the 9-phenyl-9-phosphafluorene oxide (PhFIOP) moiety. Furthermore, their photophysical, thermal, electrochemical and electroluminescent properties have been characterized in detail. Critically, it has been found that introducing two electron-donors to the 4- and 6-positions of the PhFIOP moiety can furnish a smaller singlet–triplet energy gap (ΔE_ST) than the analogues with a single electron-donor at the 4-position of the PhFIOP moiety. Hence, PhFIOP-based emitters with two phenoxazine and two acridine moieties as electron-donors can show thermally activated delayed fluorescence (TADF) behavior with a high reverse inter-system crossing constant (k_RISC) in the order of 10⁶ s⁻¹. In contrast, TADF features cannot be observed in PhFIOP-based emitters with only one electron-donor. In addition, these luminescent molecules can also exhibit high photoluminescence quantum yields (PLQYs) ranging from 70% to 90% in doped films, which results in excellent electroluminescence (EL) performance. To validate this idea, monochromatic OLED devices have been fabricated and the results show a maximum current efficiency (η_L) of 55.6 cd A⁻¹, a power efficiency (η_P) of 49.2 lm W⁻¹, and an external quantum efficiency (η_ext) of 27.3%. Moreover, blue EL emission has also been achieved with CIE (0.17, 0.14) as well as high EL efficiencies of 18.7 cd A⁻¹, 15.6 lm W⁻¹, and 21.9%, respectively. Thus, these PhFIOP-based emitters will enrich the structural diversity of APO-type TADF molecules with high EL efficiency.