Sky-Blue Thermally Activated Delayed Fluorescence from a Zirconium(IV) Complex

Earth-abundant early transition-metal complexes with luminescent ligand-to-metal charge-transfer (LMCT) excited states remain an attractive yet underexplored alternative to widely used precious-metal emitters. A fundamental understanding of the molecular design principles governing the photophysical properties of LMCT emitters is critical to further developing cost-effective and sustainable photochemical applications using this compound class. Here, we report the synthesis of a novel bis(4,5-ditolylamido-9,9-dimethylxanthene)zirconium complex, Zr(TolXDA)2, that shows efficient photoluminescence due to thermally activated delayed fluorescence (TADF) at room temperature. Steady-state and time-resolved spectroscopic studies at room temperature reveal highly efficient and long-lived emission in the sky-blue and green wavelength range for solid samples (λPL = 480 nm, ΦPL = 0.60, τPL = 4-14 μs) and THF solutions (λPL = 548 nm, ΦPL = 0.30, τPL = 8 μs) of Zr(TolXDA)2, respectively. Despite the relatively large singlet-triplet energy gap (ΔEST ≥ 0.26 eV), TADF emission in Zr(TolXDA)2 is unambiguously confirmed by temperature-dependent emission studies. This is likely facilitated by the high LMCT character of the S1 and T1 states, a consequence of strategic ligand design, which allows for rapid forward and reverse intersystem crossing. This hypothesis is supported by femtosecond transient absorption spectroscopy, which confirms facile forward and reverse intersystem crossing in Zr(TolXDA)2 (τISC = 15 ± 5 ps, τrISC = 0.48 ± 0.39 μs), and TD-DFT calculations, which establish significant negative charge transfer to zirconium in the lowest energy excited states.