A Vanadium(III) Complex with Blue and NIR-II Spin-Flip Luminescence in Solution

Luminescence
from Earth-abundant metal ions in solution at room
temperature is a very challenging objective due to the intrinsically
weak ligand field splitting of first-row transition metal ions, which
leads to efficient nonradiative deactivation via metal-centered states.
Only a handful of 3dn metal complexes
(n ≠ 10) show sizable luminescence at room
temperature. Luminescence in the near-infrared spectral region is
even more difficult to achieve as further nonradiative pathways come
into play. No Earth-abundant first-row transition metal complexes
have displayed emission >1000 nm at room temperature in solution
up
to now. Here, we report the vanadium­(III) complex mer-[V­(ddpd)2]­[PF6]3 yielding phosphorescence
around 1100 nm in valeronitrile glass at 77 K as well as at room temperature
in acetonitrile with 1.8 × 10–4% quantum yield
(ddpd = N,N′-dimethyl-N,N′-dipyridine-2-ylpyridine-2,6-diamine).
In addition, mer-[V­(ddpd)2]­[PF6]3 shows very strong blue fluorescence with 2% quantum
yield in acetonitrile at room temperature. Our comprehensive study
demonstrates that vanadium­(III) complexes with d2 electron
configuration constitute a new class of blue and NIR-II luminophores,
which complement the classical established complexes of expensive
precious metals and rare-earth elements.