Add time:07/13/2019         Source:sciencedirect.com

A class of four-coordinate boron complexes is reported that contain both electron-rich and electron-poor functional groups. Judicious selection of donor and acceptor moieties with the use of a boron atom as a separating node yields charge transfer complexes capable of thermally activated delayed fluorescence (TADF). Complexes were prepared by a modular method providing access to a wide range of emission colors. The singlet (S1) and triplet (T1) energies are independently tuned to achieve a small S1-T1 gap. Raising and lowering of S1 and T1 states can be predicted using cyclic voltammetry, NTO analysis, and spin density distribution as determined using Density Functional Theory; separation of the hole and electron wavefunction for S1 excitation and delocalization of spin density distribution in the T1 state can help in achieving negligible S1-T1 gap. Although photoluminescent quantum yields of the boron complexes in a host matrix are less than 65%, OLED device external quantum efficiencies of up to 8.1% have been achieved at a luminance of 1000 cd/m2. Selection of a boron emitter with a gap of less than 0.01 eV between the singlet and triplet excited state enables the fabrication of a device with low efficiency roll-off and long lifetime.

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