Luminescence from the <i>trans</i>-Dioxotechnetium(V) Chromophore

The luminescence of <i>trans</i>-[TcO<sub>2</sub>(L)<sub>4</sub>]<sup>+</sup> (L = pyridine (py) or picoline (pic)) and <i>trans</i>-[TcO<sub>2</sub>(CN)<sub>4</sub>]<sup>3-</sup> at room and low temperature is described and represents the first example of room temperature excited-state luminescence observed for Tc complexes. At room temperature, the complexes exhibited broad luminescence with emission maxima ranging from 745 to 780 nm. Analogous to the Re complexes (emission at 635−655 nm), the low-temperature emission spectra of microcrystalline samples of [TcO<sub>2</sub>(py)<sub>4</sub>]BPh<sub>4</sub> and [TcO<sub>2</sub>(pic)<sub>4</sub>]BPh<sub>4</sub> display the characteristic progressions of the symmetric OTcO and Tc−L stretching modes. DFT/TDDFT calculations were performed on the <i>trans</i>-[MO<sub>2</sub>(L)<sub>4</sub>]<sup>+</sup> (M = Re, Tc) congeners and predicted the dioxotechnetium emission to be 0.41 eV lower in energy than its Re analogue. Low-temperature lifetimes (8 K) ranging from 15 to 1926 μs for the series of Tc complexes are consistent with the Re analogues.