Photophysical Studies of a Zr(IV) Complex with Two Pyrrolide-Based Tetradentate Schiff Base Ligands.

The reaction of two equivalents of ,-bis(2-pyrrolylmethylidene)-1,2-phenylenediamine (Hbppda) with tetrabenzylzirconium provided the air- and moisture-stable eight-coordinate complex Zr(bppda). Temperature-dependent steady-state and time-resolved emission spectroscopy established weak photoluminescence (Φ = 0.4% at 293 K) by a combination of prompt fluorescence and thermally activated delayed fluorescence (TADF) upon visible light excitation at and around room temperature. TADF emission is strongly quenched by O and shows highly temperature-sensitive emission lifetimes of hundreds of microseconds. The lifetime of the lowest energy singlet excited state, S, was established by transient absorption spectroscopy and shows rapid deactivation (τ = 142 ps) by prompt fluorescence and intersystem crossing to the triplet state, T. Time-dependent density functional theory (TD-DFT) calculations predict moderate ligand-to-metal charge transfer (LMCT) contributions of 25-30% for the S and T states. A comparison of Zr(bppda) to related zirconium pyridine dipyrrolide complexes, Zr(PDP), revealed important electronic structure changes due to the eight-coordinate ligand environment in Zr(bppda), which were correlated to differences in the photophysical properties between the two compound classes.