Highly efficient T-shaped deep-red thermally activated delayed fluorescence emitters: substitution position effect.

Abstract

Modulating the relationship between molecular structures and luminescence properties as well as charge transfer properties of deep-red thermally activated delayed fluorescence (TADF) emitters has always been a great challenge, especially in the solid state. In this work, the light-emitting properties of a T-shaped molecule (pTPA-DPPZ) with donors at the para-position are theoretically investigated in the solid state with the combined quantum mechanics/molecular mechanics (QM/MM) method and the thermal vibration correlation function (TVCF) theory. In comparison with a Y-shaped molecule (oTPA-DPPZ with donors at the ortho-position), pTPA-DPPZ acquires a reduced HOMO-LUMO energy gap and red-shifted emission. In addition, it is found that the transition dipole moment is enhanced and the radiative rate is increased. The stacking pattern of pTPA-DPPZ can effectively suppress the out-of-plane wagging vibration of donors, leading to the reduction of reorganization energy and inhibiting the loss of non-radiative energy in the excited state compared with oTPA-DPPZ. Besides, a larger spin-orbit coupling constant and a smaller energy gap between T2 and S1 (ΔET2-S1 = 0.1 eV) are found in pTPA-DPPZ, and thus a superior TADF emission is obtained. Moreover, the charge transport properties are studied using kinetic Monte Carlo simulations. The calculated mobilities for the electrons and holes of pTPA-DPPZ are all larger than those of oTPA-DPPZ, which is due to close packing modes in the pTPA-DPPZ crystal. Balanced charge transport properties are found, which is helpful for generation of excitons and light emission. The calculation results shed light on the relationship between the molecular structures and light-emitting properties of TADF emitters, which would be helpful for developing efficient non-doped deep-red TADF devices by using T-shaped molecular design.