Strongly fluorescent spiro-type tetracoordinate complexes of dibenzo[b,e][1,4]thiaborinine dioxide with functionalized 2-(benzo[d]heterazol-2-yl)phenolate ligands displaying TADF

Abstract

A series of eight luminescent tetracoordinate organoboron chelate complexes 7a–7d and 8a–8d, featuring spiro geometry of the boron centre, were synthesized by combining strongly electron-acceptor dibenzo[b,e][1,4]thiaborinine 5,5-dioxide (SO2B) with 2-(benzo[d]heterazol-2-yl)phenolate ligands comprising an attached 3,6-bis(tert-butyl)carbazole as a strong electron donor. Two analogous reference systems 9 and 10 bearing BF₂ and BAr₂ (Ar = 2,6-F₂-C₆H₃) cores, respectively, were also prepared and characterized. The obtained complexes exhibit bright blue, green or yellow photoluminescence in solution and Zeonex thin films with quantum yields reaching unity in some systems. Our detailed photophysical analysis of these complexes indicates the presence of thermally activated delayed fluorescence (TADF). Time-resolved photoluminescence experiments performed at RT and 77 K show that the energy difference between the lowest excited singlet and triplet states (ΔE_ST) is in the range of 0.22–0.36 eV. The most effective TADF emitters 7a and 7b were selected as luminescent dopants in OLEDs, showing external quantum efficiency (EQE) of up to 6.7% for the 7a-based device and maximum luminance of up to 9400 cd m⁻² for the 7b-based device.