Self-Consistent Quantum Mechanics/Embedded Charge Study on Aggregation-Enhanced Delayed Fluorescence of Cu(I) Complexes: Luminescence Mechanism and Molecular Design Strategy

Abstract

To elucidate the luminescence mechanism of highly efficient blue Cu(N^N)(POP)⁺-type thermally activated delayed fluorescence (TADF) materials, we have selected Cu(pytfmpz)(POP)⁺ (<b>1</b>) and Cu(pympz)(POP)⁺ (<b>2</b>) as targets to investigate the photophysical properties in both solution and solid phases. The self-consistent electrostatic potential (ESP) embedded charge within the quantum mechanics/molecular mechanics (QM/MM) method demonstrates a greater advantage over the charge equilibrium (QEQ) in accurately calculating atomic charges and reasonably describing the polarization effect, ultimately resulting in a favorable consistency between simulation and experimental measurements. After systematic and quantitative simulation, it has been found that complex <b>2</b>, with an electron-donating group of -CH₃, exhibits a much more blue-shifted spectrum and a significantly enhanced efficiency in comparison to complex <b>1</b> with -CF₃. This is due to the widened HOMO-LUMO gap as well as the narrowed energy gap between the lowest singlet and triplet excited states (Δ<i>E</i>_ST), respectively. Then, the designed complex <b>3</b> is introduced with a stronger electron donor and larger <i>tert</i>-butyl group, which plays a key role in simultaneously suppressing the structural distortion and reducing the Δ<i>E</i>_ST. This leads to a faster reverse intersystem crossing process than that of the two experimental complexes in solution, turning out to be a new deep-blue-emitting material with excellent TADF performance.