Unraveling the Mechanism of Circularly Polarized Thermally Activated Delayed Fluorescence (CP-TADF) in Chiral Two-Coordinated Cu(I) Emitters: A Comprehensive Theoretical Exploration

Abstract

The circularly polarized thermally activated delayed fluorescence (CP-TADF) processes of four pairs of Cu(I)-based chiral enantiomers were calculated by employing the path integral approach to dynamics. The calculated results reveal that the introduction of different substituents at various positions on the planar chiral CzP units significantly impacts on the dissymmetry factor g_CPL of CP luminescence (CPL). Substituting the sixth position hydrogen on the chiral CzP unit with −CN or −Cl groups increases the g_CPL factor to 2.1 × 10^–3 and 2.2 × 10^–3 in Sp-MAC-Cu-CNCzP-1 and Sp-MAC-Cu-ClCzP, respectively. This is due to a small electric transition dipole moment |μ| and a relatively large magnetic transition dipole moment |m|. The small |μ| results from the CT excitation with spatially separated highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) transitions in the S₁ state, leading to a small ΔE(S₁ – T₁) and more efficient TADF. Interestingly, the seventh-substituted MAC-Cu-CNCzP-2 complex exhibits a substantial increase in the ΔE(S₁ – T₁) of 0.346 eV, which consequently leads to a significant decrease in the k_RISC rate, down to 3.32 × 10³ s^–1, indicating that further efficient reverse intersystem crossing (RISC) processes are failed. Thus, to design efficient CP-TADF molecules, the key lies in regulating and balancing the factors affecting both TADF and CPL properties.