ABSTRACT Tetracoordinated heteroleptic Cu (I) complexes have gained prominence as prospective thermally activated delayed fluorescence (TADF) emitters, where both the structural framework and substituent effects of the diimine ligands critically influence their photophysical behavior. Herein, we report the design, synthesis, and characterization of two Cu (NN) (PP) PF 6 complexes, T1 and P1, featuring an imidazo4, 5‐f1, 10phenanthroline (Imphen) diimine ligand functionalized with 3, 6‐di‐ tert ‐butylcarbazole (t Cz) donor units or H in combination with conventional chelating diphosphine ligands. The optimization of the diimine structure for T1 in which the 3, 6‐di‐ tert ‐butylcarbazole (t Cz) was introduced to the Imphen fragment to form a donor–acceptor (D–A) type diimine ligand facilitates pronounced spatial separation of the frontier molecular orbitals, leading to a remarkably small singlet–triplet energy gap (Δ E ST) of 516 cm −1, as confirmed by photophysical and theoretical analyses. The complex T1 exhibit microsecond‐range delayed fluorescence (τ = 12. 05 μs) with a radiative decay rate (k r) of 1. 33 × 10 4 s −1. The theoretical calculations further attribute the moderate oscillator strength and spin–orbit coupling (SOC) to the separation of frontier orbitals. Notably, T1 displays a distinct “turn‐on” luminescent response upon deprotonation of the Imphen ligand in different basic media (e. g. , NaOH, triethylamine, and DBU), underscoring its potential in pH‐responsive sensing. These results demonstrate that heteroleptic Cu (Imphen) (PP) + complexes incorporating D–A type diimine ligands represent a versatile platform for the development of efficient TADF materials and stimuli‐responsive optical devices.
Qiao et al. (Thu,) studied this question.