Electronic structure of V3+ in NaMgAl(ox)3·9H2O probed by Fourier transform spectroscopy

Abstract

a b s t r a c tHigh-resolution Fourier transform absorption and luminescence spectroscopy reveal axial and rhombic zero-field splittings of the spin-forbidden electronic origins of V 3+ in NaMgAl(ox) 3 Á 9H 2 O (ox ¼ oxalate) single crystals below 25 K. The temperature dependence of the integrated absorption of the split features display behavior consistent with a Boltzmann distribution within the zero-field split 3 Â 2 ground state of V 3+ . Weak luminescence is observed in the near-IR from the lowest energy spinforbidden transition with a luminescence lifetime of less than 0.5 ms at 11 K and an estimated quantum efficiency of the order of 10 À5 .

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

“…The 3 T 1g → 3 T 2g ligand field transition (in octahedral symmetry) is split due to the lower symmetry (calcd 441 and 413 nm) and superimposed onto the stronger 3 LMCT bands in this spectral region (Figure c). As compared to vanadium­(III) complexes with oxygen donor ligands, for example, V­(acac) 3 , − , the ligand field splitting of [V­(ddpd) 2 ] 3+ (Δ o ≈ 23 440 cm –1 ) is larger by more than 2500 cm –1 , confirming our conceptual approach of using ddpd as a strong ligand to prevent back-ISC …”

“…Compared to vanadium(III) complexes with oxygen donor ligands, e.g. V(acac) 3 , [37][38][39][40][41]48 the ligand field splitting of [V(ddpd) 2 ] 3+ ( o  23440 cm -1 ) is larger by more than 2500 cm -1 confirming our conceptual approach of using ddpd as strong ligand to prevent back-ISC. 46 Expectedly, the 3 T 1g ground state splits as well, yet the TD-DFT-UKS calculated energy gaps are unreasonably large (Supporting Information, Table S3).…”

“…The long lifetime is in a similar range compared to the excited singlet state lifetime reported for V 3+ doped in NaMgAl(ox) 3 at 11 K ( < 500 ns). 41 All emission bands show vibrational progression, while the ground state splitting might also account for the observed pattern. Based on the quantum chemical calculations (Figure 1d), the emission energies and the lifetimes (Supporting Information, Figures S12 -S13, Table S7), we assign the high-energy emission bands (374, 447 and 660 nm) and the NIR-I/NIR-II emission bands (982 and 1088/1109 nm) to fluorescence and phosphorescence, respectively.…”

“…However, the known vanadium(III) compounds display after excitation and intersystem crossing (ISC) only very weak phosphorescence at low temperature, if phosphorescence is observed at all. [37][38][39][40][41] For example, the phosphorescence intensity of V 3+ :Al 2 O 3 is 3-4 orders of magnitude smaller than that of the R lines of ruby (Cr 3+ :Al 2 O 3 ) and the vanadium(III) spin-flip emission has been only detected in the solid state at low temperatures so far. 37,38 Instead, (delayed) fluorescence from vanadium centered 3 T 2g states occurs in compounds with ligand field strengths close to 17.3 B.…”

“…We hypothesized that this situation can be realized by octahedral vanadium­(III) complexes in a strong ligand field. The energy of potentially emissive singlet states of vanadium­(III) has been determined by absorption spectroscopy on classical vanadium­(III) compounds with O-donor ligands, such as V 3+ :Al 2 O 3 , [V­(H 2 O) 6 ] 3+ , [V­(urea) 6 ] 3+ , and [V­(ox) 3 ] 3– , − and by 2p3d resonant inelastic X-ray scattering on V­(acac) 3 as ∼10 000 cm –1 (∼1000 nm) (urea = OC­(NH 2 ) 2 , ox = [C 2 O 4 ] 2– , acac = [CH 3 (CO)­CH­(CO)­CH 3 ] − ) . This energy excellently matches the targeted window for luminescence applications in the NIR-II region.…”