Theoretical study for constructing red multiple-resonance thermally activated delayed fluorescence molecules with narrowband emission by donor engineering strategy

The exploration of reducing the full-width at half-maximum (FWHM) without enlarging energy gaps (∆E_ST) in red multi-resonance thermally activated delayed fluorescence (MR TADF) molecule is highly demanded. Herein, based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations coupled with the thermal vibration correlation function (TVCF) method, we study the excited-state properties and luminescence mechanisms of two reported TADF molecules (PXZ-R-BN and BCz-R-BN). Drawing inspiration from these, we theoretically design four novel red MR TADF molecules by donor engineering strategy with increased electron donation abilities and extended π-conjugations. To accurately predict the excited state energies of these MR TADF molecules, we employ the range-separated double hybrid density functional (B2PLYP), effectively refining the ∆E_ST values. Results show that different donor groups influence the frontier molecular orbitals, thereby reducing the energy gaps. These adjustments effectively optimize the energy levels and transition properties of the excited states, leading to the substantial acceleration of the reverse intersystem crossing (RISC) processes. Furthermore, the designed molecules with extended π-conjugations possess small reorganization energies and minimal nuclear displacements, coupled with short-range charge transfer characteristics. These features contribute significantly to the narrowband emissions. Therefore, it is verified that these four novel red MR TADF molecules strike a favorable balance between the ∆E_ST and the FWHM values. Meanwhile, the intrinsic relationship between molecular structures and photophysical properties is elucidated, paving the way for developing novel and efficient red MR TADF emitters.