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.室温下溶液中地壳丰度金属离子的发光是一项极具挑战性的目标，这源于第一行过渡金属离子（first-row transition metal ions）本身的配体场分裂（ligand field splitting）较弱，进而通过金属中心态发生高效的非辐射失活（nonradiative deactivation）。目前仅有少量3dn金属配合物（3dn metal complexes，n≠10）能在室温下展现出可观的发光现象。近红外光谱区（near-infrared spectral region）的发光更难实现，因为此时会出现更多的非辐射路径（nonradiative pathways）。截至目前，尚无地壳丰度第一行过渡金属配合物能在室温溶液中实现波长大于1000 nm的发射。 本研究报道了一种钒(III)配合物（vanadium(III) complex）mer-[V(ddpd)₂][PF₆]₃，其在77 K的戊腈玻璃态基质中以及室温乙腈溶液中均可产生约1100 nm的磷光，量子产率（quantum yield）为1.8×10⁻⁴%，其中ddpd为N,N′-二甲基-N,N′-二吡啶-2-基吡啶-2,6-二胺（N,N′-dimethyl-N,N′-dipyridine-2-ylpyridine-2,6-diamine）。此外，mer-[V(ddpd)₂][PF₆]₃在室温乙腈溶液中还展现出极强的蓝色荧光（blue fluorescence），量子产率达2%。 本项全面系统的研究表明，具有d²电子构型（d² electron configuration）的钒(III)配合物构成了一类新型的蓝色及NIR-II发光材料（luminophores），可弥补传统昂贵贵金属与稀土元素经典配合物的应用空白。