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

Article abstract of DOI:10.1021/jacs.0c02122

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.

Full text of DOI:10.1021/jacs.0c02122

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Article
A vanadium(III) complex with blue and
NIR-II spin-flip luminescence in solution
Matthias Dorn, Jens Kalmbach, Pit Boden, Ayla Päpcke, Sandra Gomez, Christoph
Förster, Felix Kuczelinis, Luca M. Carrella, Laura Büldt, Nicolas Bings, Eva Rentschler,
Stefan Lochbrunner, Leticia Gonzalez, Markus Gerhards, Michael Seitz, and Katja Heinze
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c02122 • Publication Date (Web): 10 Apr 2020
Downloaded from pubs.acs.org on April 13, 2020
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A vanadium(III) complex with blue and NIR-II spin-flip luminescence in solution
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Matthias Dorn1, Jens Kalmbach2, Pit Boden3, Ayla Päpcke4, Sandra Gómez5, Christoph Förster1, Felix
Kuczelinis1, Luca M. Carrella1, Laura Büldt2, Nicolas Bings1, Eva Rentschler1, Stefan Lochbrunner4, Leticia
González5, Markus Gerhards*3, Michael Seitz*2 and Katja Heinze*1
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Department of Chemistry, Johannes Gutenberg University of Mainz, Duesbergweg 10-14, 55128 Mainz,
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Germany.
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Institute of Inorganic Chemistry, University of Tübingen, Auf der Morgenstelle 18, 72076 Tübingen,
Germany.
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Department of Chemistry and Research Center Optimas, Technical University Kaiserslautern, Erwin-
Schrödinger-Straße, 67663 Kaiserslautern, Germany.
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Institute for Physics and Department of Life, Light and Matter, University of Rostock, 18051 Rostock,
Germany.
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Institute of Theoretical Chemistry, Faculty of Chemistry, University of Vienna, Währinger Straße 17, 1090
Vienna, Austria.
Key words: NIR emitter; Earth-abundant metals, vanadium; nitrogen ligands; quantum chemical calculations;
photophysics; ligand field splitting
Abstract 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 non-radiative deactivation via metal-centered states. Only a handful of 3dn metal complexes (n ≠ 10)
show sizeable luminescence at room temperature. Luminescence in the near-infrared spectral region is even
more difficult to achieve as further non-radiative pathways come into play. No Earth-abundant first-row
transition metal complexes display 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.
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Introduction
Luminescent complexes of Earth-abundant metal ions started to emerge only in the last few years.1-4
Typically, the abundant first row transition metal ions are prone to sub-picosecond non-radiative relaxation
via low-energy metal-centered (MC) states, which precluded any useful emission until recently.4-6 The
majority of the very few reported room-temperature luminescent complexes possess rather long-lived
charge transfer (CT) states thanks to a high energy or absence of the detrimental MC states. A d6-chromium(0)
complex with isocyanide ligands shows 3MLCT (metal-to-ligand CT) emission at 630 nm ( = 2.2 ns,  = 0.001
%).7 d6-Cobalt(III) complexes with tridentate strong field ligands display blue emission from 3LMCT (ligand-to-
metal) states (440/412 nm,  = 5.1/3.2+8.7 ns,  = 0.7/0.4 %).8 Fluorescence from 2LMCT states is induced in
d5-iron(III) complexes using carbene ligands (655 nm,  = 1.96 ns,  = 2.1 %).9,10 Copper(I) complexes can
exhibit quantum yields up tp 100 %, e.g. two-coordinate d10-copper(I) complexes of cyclic
(alkyl)(amino)carbene and amide ligands emit from ligand-to-ligand charge transfer (LLCT) states between
492 – 580 nm ( = 280 – 2500 ns,  = 11 – 100 %).11 Clearly, the visible spectral region started to be covered
by 3d CT emitters, yet room-temperature near-infrared (NIR-I: 780 – 1000 nm; NIR-II: 1000 – 1700 nm)
emission in solution has not yet been reported with CT complexes of Earth-abundant metal ions. On the other
hand, strong NIR luminescence has been achieved exploiting the spin-flip emission of d3-chromium(III)
complexes using tridentate strong field ligands, leading to the so-called molecular rubies [Cr(L)2]3+ (L = e.g.
ddpd, tpe; ddpd = N,N’-dimethyl-N,N’-dipyridine-2-ylpyridine-2,6-diamine; tpe = 1,1,1-tris(pyrid-2-yl)ethane;
782 – 724 nm,  = 670 – 4500 µs,  = 5.2 – 30 %, CH3CN).12-17 However, emission above 900 nm or even above
1000 nm (NIR-II) at room-temperature remains elusive with 3d metal complexes in solution.
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In principle, near-infrared (NIR) luminescence enables exciting applications in telecommunications, laser and
LED/OLED or LEC technologies (OLED = organic light emitting diode, LEC = light-emitting electrochemical cell)
as well as in bio-analysis and bio-imaging.18-22 NIR-II luminescence can be particularly beneficial for in vivo
imaging with respect to tissue penetration as well as reduced auto-fluorescence of the background and
scattering losses.18,19 Current materials are based on rare-earth elements, precious metals and organic dyes
often in conjunction with precious metals.23-28 The rare-earth ions typically display comparably sharp
emission bands, yet split by electron-electron repulsion, spin-orbit coupling and the crystal field.29 Organic
emitters often show broad emission bands which typically merely tail into the NIR spectral region, hence,
some emission still occurs in the visible spectral region.
A fundamental problem of low-energy emission derives from the energy-gap law especially for distorted
excited states. The rate constant for non-radiative decay that reduces the luminescence quantum yield 
increases with decreasing energy difference between the emitting and the ground state.30,31 Nested, i.e.
weakly distorted states (such as f-f and d-d spin-flip transitions) are hence better suited for NIR emission.
Furthermore, high-energy oscillators, e.g. OH from water or CH from coordinating ligands, can increase non-
radiative decay via energy transfer to suitable OH/CH vibrational overtones.32,33 Lanthanide compounds, e.g.
Nd, Sm, Dy or Yb, very often embedded in nanoparticles which lack detrimental CH oscillators, achieve NIR-
II luminescence due to their favorable excited state levels originating from f-f transitions and the absence of
high-energy oscillators.18,19,29 The quantum yields for molecular NIR emissive lanthanide complexes are rather
small, although deuterated cage and fluorinated porphyrin complexes of Yb3+ showing emission around 1000
nm recently achieved exceptionally high quantum yields.34-36 However, rare-earth elements and precious
metals are difficult and expensive to mine and purify, so that a huge interest arose in replacing these
elements with Earth-abundant metals in luminescence applications.1-4
Analogous to the d3 electron configuration of the abundant metal chromium(III),2,12-17 an excited state
ordering with low-energy spin-flip states (1Eg/1T2g) emerges from a d2 electron configuration in an octahedral
ligand field (3T1g ground state), provided that a strong ligand field is imposed to increase the energy gap
between the interconfigurational triplet state (3T2g) and the intraconfigurational singlet states (1Eg/1T2g < 3T2g
at o > 17.3 B in the standard Tanabe-Sugano diagram; see Supporting Information, Figure S1; o and B are
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the ligand field parameters). 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
V3+:Al2O3, [V(H2O)6]3+, [V(urea)6]3+ and [V(ox)3]3-,37-41 and by 2p3d resonant inelastic X‑ray scattering on
V(acac)3 as ~10000 cm–1 (~ 1000 nm) (urea = OC(NH2)2, ox = [C2O4]2–, acac = [CH3(CO)CH(CO)CH3]–).42 This
energy excellently matches the targeted window for luminescence applications in the NIR-II region. 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.37-41 For example, the
phosphorescence intensity of V3+:Al2O3 is 3-4 orders of magnitude smaller than that of the R lines of ruby
(Cr3+:Al2O3) and the vanadium(III) spin-flip emission has been only detected in the solid state at low
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temperatures so far.37,38 Instead, (delayed) fluorescence from vanadium centered T2g states occurs in
compounds with ligand field strengths close to 17.3 B.43,44 Consequently, back-ISC from 1Eg/1T2g to 3T2g states
must be suppressed. This can in principle be achieved by further increasing the ligand field strength thus
shifting the 3T2g states to higher energy relative to the singlet states. On the other hand, to promote ISC from
the triplets to the singlet manifold besides large spin-orbit coupling, a high density of singlet states in the
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energy region of the T2g states is beneficial.45 According to ligand field theory, an additional singlet term
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(1A1g) crosses the T2g term at very high ligand field strengths (o > 36.6 B in the standard Tanabe-Sugano
diagram; Supporting Information, Figure S1). Consequently, an even larger ligand field splitting than o >>
17.3 B, namely o  36.6 B, could be advantageous to promote ISC to the singlet states via the high-energy
1A1g state in vanadium(III) complexes. In this study, such a large ligand field splitting will be targeted.
The strong-field ligand ddpd already enabled spectacular room temperature quantum yields above 10 % and
µs-lifetimes in solution in the prototypical molecular ruby [Cr(ddpd)2]3+.12 Ligand and solvent deuteration
even boosted the quantum yield to 30 % and the lifetime to 2300 µs.13 However, an analogous coordination
chemistry of vanadium(III) and oligopyridine ligands is underdeveloped.46,47 cis-fac-[V(ddpd)2][PF6]3x3 CH3CN
is the first reported stable homoleptic polypyridine vanadium(III) complex.46 Yet, the bond lengths in cis-fac-
[M(ddpd)2]n+ isomers are typically longer than in mer-isomers due to the ligand deformation, which reduces
the actual ligand field strength.46
Here we demonstrate, that the strong field ligand ddpd promotes ISC in a novel close-to-octahedral
vanadium(III) complex mer-[V(ddpd)2][PF6]3. Consequently, the complex shows NIR-II phosphorescence at
low as well as at room temperature in solution. In addition, a strong blue fluorescence is observed. To deeper
elucidate the key challenges for luminescent vanadium(III) complexes, we recorded fs-transient absorption
spectra and ns-step-scan FTIR spectra for mer-[V(ddpd)2][PF6]3. In order to judge the influence of
multiphonon relaxation via CH overtones, we prepared and studied the deuterated analogue [V([D17]-
ddpd)2][PF6]3. Static quantum chemical and non-adiabatic molecular dynamics simulations provide a
comprehensive insight into the electronic excited state properties and ultrafast dynamics of mer-
[V(ddpd)2]3+.
Results
Synthesis, X-ray structure and Characterization of [VIII(ddpd)2]3+
The dark green complex mer-[VIII(ddpd)2][PF6]3 was efficiently synthesized from [V(CH3CN)6][BPh4]2 and
ddpd46 followed by oxidation with ferrocenium hexafluorophosphate (Figure 1a). Full characterization is
collected in the Supporting Information, Figures S2 – S5. The starting vanadium material VCl3(CH3CN)3 was
checked for chromium trace impurities by ICP-MS yielding (4.010.31) µg g–1 Cr, assuring that emission from
conceivable luminescent chromium(III) species will not interfere. The deuterated complex [V([D17]-
ddpd)2][PF6]3 was prepared from [D17]-ddpd in an analogous manner with a total degree of deuteration of
93.6 %D in the ligand and in the complex according to ESI+ mass spectrometry (Supporting Information,
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Figures S6 – S9). Importantly, the -protons, closest to the vanadium center, are almost quantitatively
substituted by deuterium (99 % by 1H NMR spectroscopy).
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The vanadium(III) oxidation state in mer-[V(ddpd)2][PF6]3 was confirmed by elemental analysis, mass
spectrometry, magnetometry, cyclic voltammetry and single crystal X-ray diffraction analysis (Figure 1b and
Supporting Information, Figures S4 – S5, Table S1). The magnetic moment of [V(ddpd)2][PF6]3 between 300
and 50 K amounts to T  1.0 cm3 K mol–1 as expected for an S = 1 ground state. Below 50 K, the magnetic
moment decreases due to zero-field splitting. The low temperature part was modeled with g = 1.946 and a
zero-field splitting of Dexp = +2.87(11) cm−1 (Supporting Information, Figure S4).47,48 The vanadium(III) complex
is reversibly reduced at –0.275 V vs. ferrocene (Supporting Information, Figure S5). In the solid state, the
central ion in [V(ddpd)2]3+ is coordinated by six pyridine donors. The [VN6] coordination geometry is nearly
octahedral as shown by the octahedral shape parameter being quite close to zero (S(OC-6) = 0.63; Supporting
Information, Tables S1 – S2). The [VN6] symmetry is, however, less perfect than in [CrIII(ddpd)2]3+ complexes
(S(OC-6) = 0.29 – 0.4346), as a consequence of the Jahn-Teller active partially filled t2g shell. Compared to the
metrics of the cis-fac-[V(ddpd)2]3+ isomer, the V-N bond lengths to the four terminal pyridines in mer-
[V(ddpd)2]3+ decreased by 0.025 – 0.053(9) Å, while V-N distances to the central pyridines increased only
slightly by 0.008/0.022(8) Å.46 The overall smaller [VN6] coordination cage of mer-[V(ddpd)2]3+ and its close-
to-octahedral symmetry imposes the desired large ligand field splitting o.
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Figure 1. Synthesis, molecular structure, absorption spectrum of [V(ddpd)2][PF6]3 and Jablonski diagram of
[V(ddpd)2]3+. (a) Preparation of [V(ddpd)2][PF6]3. (b) Molecular structure of the cation of [V(ddpd)2][PF6]3x3
CH3CN. Thermal ellipsoids are shown at 50 % probability. Hydrogen atoms, counter ions and CH3CN molecules
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are omitted for clarity. (c) Absorption spectrum and photograph of [V(ddpd)2][PF6]3 in CH3CN. The vertical
green bars correspond to spin-allowed transitions calculated by TD-DFT-UKS calculations (unshifted). (d)
Jablonski diagram constructed from TD-DFT-UKS (3LMCT states in green) and CASSCF-NEVPT2 calculations
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(3MC states in blue; MC states in red); notation of MC states according to Oh symmetry; ISC = intersystem
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crossing; IC = internal conversion; VC = vibrational cooling; NIR = NIR emission; VIS = emission in the visible
spectral region; CH overtones = multiphonon relaxation (experimental overtone energies from ref. 13). (e)
Representative microstates of the most relevant metal-centered triplet and singlet states.
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The electronic absorption spectrum of mer-[V(ddpd)2][PF6]3 in CH3CN reveals a broad absorption band around
597 nm (Gaussian deconvolution: 616, 570 nm), a shoulder at ~475 nm, an asymmetric band at 422 nm
(Gaussian deconvolution: 429, 412 nm) and very intense bands at 301 nm (Figure 1c; Supporting Information,
Figure S10). All bands are quite intense ( >> 1000 M–1 cm–1) compared to typical metal-centered transitions,
suggesting significant CT character. These intense CT bands conceal the weaker Laporte-forbidden d-d
transitions, preventing their observation and hence experimental determination of o. To assign the
absorption bands and to obtain the excited state level ordering including the MC states, quantum chemical
calculations were performed (Supporting Information, Tables S1 – S4).
Static Quantum Chemical Calculations.
The overall geometry of [V(ddpd)2]3+ is well reproduced by unrestricted Kohn-Sham Density Functional
Theory (DFT-UKS) calculations (CPCM(acetonitrile)-RIJCOSX-UB3LYP-D3BJ-ZORA/def2-TZVPP) with
a
maximum deviation in V-N bond lengths of 0.028 Å and N-V-N bond angles of 1.3° (Supporting Information,
Table S1; octahedral shape parameter S(OC-6) = 0.68). Mulliken spin population analysis yields 2.07 at the
vanadium center, similar to that calculated for [V(bpy)3]3+ (2.11),47 accounting for the two unpaired electrons.
To assign the intense CT bands, time-dependent DFT-UKS calculations at the same level of theory were
employed (Supporting Information, Table S3 – S4; Figure 1c). The absorption band at 597 nm corresponds to
3LMCT transitions from the bridging nitrogen atoms to vanadium (calcd. 631/616 nm; Figures 1c, 1d). The
shoulder and bands between 500 and 390 nm originate from 3LMCT transitions as well (calcd. 503, 453, 444
nm; Figures 1c, 1d). The 3T1g  3T2g ligand field transition (in octahedral symmetry) is split due to the lower
symmetry (calcd. 441 and 413 nm) and superimposed onto the stronger 3LMCT bands in this spectral region
(Figure 1c). Compared to vanadium(III) complexes with oxygen donor ligands, e.g. V(acac)3,37-41,48 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 3T1g ground state splits as well, yet the TD-DFT-UKS calculated energy gaps are unreasonably
large (Supporting Information, Table S3). To better describe the MC triplet and singlet states (Figure 1e) and
the ground state splitting, complete-active-space self-consistent field (CASSCF) calculations including spin-
orbit coupling (SOC) in conjunction with fully internally contracted N-electron valence perturbation theory to
second order (FIC-NEVPT2) SOC-CASSCF(6,12)-FIC-NEVPT2 on optimized [V(ddpd)2]3+ were performed
(Supporting Information, Tables S5 – S6, Figure S11).39,48 Figure 1d depicts the calculated order of these MC
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triplet and singlet states derived from CASSCF-NEVPT2 and the LMCT states derived from TD-DFT-UKS
calculations. The 3T1g ground state splits with E = 1873 and 2687 cm–1, the lowest excited triplet state 3T2g
splits into 20844, 22706, 22803 cm–1. The zero-field splitting was calculated as Dcalcd = +6.8 cm–1, larger than
the experimental value Dexp = +2.87(11) cm–1, yet similar to the experimental and calculated data of V(acac)3
(Dexp = +6.9 cm–1; Dcalcd = +9.6 cm–1).48
A series of five singlet states originating from the 1T2g and 1Eg terms is located between the triplet ground and
excited states. This excited state ordering confirms the large ligand field splitting with o > 17.3 B. The two
lowest singlet states centered at 9612 and 9736 cm–1 (Supporting Information, Figure S11) are only weakly
split by E = 124 cm–1. A sixth (isolated) singlet state (1A1g) is predicted at 23283 cm–1, slightly above the
highest level of the 3T2g states by 480 cm–1. This calculation suggests that o is slightly smaller than 36.6 B (in
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a standard Tanabe-Sugano diagram with C/B = 4.42, Supporting Information, Figure S1). The combined TD-
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DFT-UKS and CASSCF-NEVPT2 calculations indicate that several states (3LMCT, T2g and A1g) lie in the 3.1 –
2.6 eV (400 – 480 nm) region.
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Steady-state and Time-resolved Spectroscopy.
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Steady-state emission spectra with excitation at 306 nm (4.05 eV) at 77 K in a valeronitrile glass matrix show
rich spectral features between 370 and 1150 nm (Figure 2a). Obviously, several states are emissive upon
high-energy excitation. The NIR-II emission is also observed in butyronitrile glass at 10 K with excitation
wavelengths between 306 and 450 nm. The highest intensity occurs at an excitation wavelength of 360 nm,
where the luminescence is twice as strong as at 306 nm excitation, while the luminescence almost vanishes
when exciting with 450 nm (Figure 2c). Tuning the excitation wavelength populates different electronically
excited triplet states which might exhibit different ISC efficiencies as sketched in Figure 1d and consequently
lead to different phosphorescence intensities. In the solid state (KBr pellet; Supporting Information, Figures
S14a, S14b) the NIR-II emission is not observed, probably resulting from self-quenching. The emission in
solution is sensitive to the presence of oxygen. A biexponential lifetime of several hundred nanoseconds is
associated with the sharp NIR emission bands at 1100 nm (790 ns (93 %), 8800 ns (7 %), Supporting
Information, Table S7, Figure S12). The long lifetime is in a similar range compared to the excited singlet state
lifetime reported for V3+ 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.
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Figure 2. (a) Normalized luminescence spectra of [V(ddpd)2][PF6]3 (black) and [V([D17]-ddpd)2][PF6]3 (red) (exc
= 306 nm, nBuCN, 77 K). Intensities in the different spectral regions are not to scale due to different detectors
and gratings used. GG455 and RG850 long pass filters were employed for the low-energy spectra,
respectively. (b) Normalized luminescence spectra of [V(ddpd)2][PF6]3 (exc = 306 nm, CD3CN, 298 K).
Intensities in the different spectral regions are not to scale due to different detectors and gratings used. The
integral ratio of the two bands amounts to I(VIS) : I(NIR) = 2.1 : 1.8×10–4. Purple vertical bars indicate relevant
CH and CD overtones. Their length approximately illustrates the 3CH : 4CD intensity ratio.13 The photograph
illustrates the blue fluorescence of [V(ddpd)2][PF6]3 (exc = 350 nm, CH3CN, 298 K). (c) Luminescence spectra
of [V(ddpd)2][PF6]3 (exc = 306 – 450 nm, butyronitrile glass, 20 K). (d) Ground-state FTIR (black) and step-scan
FTIR spectrum (red) of [V(ddpd)2][PF6]3 at 20 K (exc = 355 nm, KBr pellet, 0 – 300 ns).
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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. The NIR-I and NIR-II phosphorescence originates from the two almost degenerate lowest excited
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singlet states (with Eg and T2g parentage) decaying radiatively to the split ground state levels (Figure 1d).
From these NIR-I/NIR-II phosphorescence bands, the experimental ground state splitting is estimated as 1080
cm–1 (averaged 1088/1109 nm), which is somewhat smaller than that obtained by the CASSCF-NEVPT2
calculations (1873 cm–1; Figure 1d). As a reference, a splitting of 880 cm–1 was calculated for V(acac)3 by SOC-
CASSCF(12,10)-NEVPT2,48 while a value of 1400 cm–1 was experimentally determined for [V(urea)6][ClO4]3.40
Excitation of 3LMCT states at lower energy (600 nm) does not lead to the NIR-II luminescence, demonstrating
that these 3LMCT states do not efficiently evolve to the emissive singlet states (Figure 1d).
In diluted deaerated CD3CN solution, dual emission at 396 and 1109/1123 nm occurs even at 298 K upon
excitation at 306 nm (Figure 2b). The high-energy blue emission is even observable by the naked eye at room
temperature in solution and in a KBr pellet (Figure 2b; Supporting Information, Figure S14c). The quantum
yield of the high energy fluorescence is very high with  = 2.1 % and surpasses the quantum yield of the blue
emission of the recently reported blue emissive cobalt(III) complexes ( = 0.7/0.4 %).8 The emission decay at
396 nm is biexponential with 396 of 3.2 (56 %) and 8.2 (44 %) ns and 1.7 (51 %) and 6.3 (49 %) ns at 298 K and
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77 K. respectively. We assign this fluorescence to T1g(P)3T1g or to LMCT transitions (Figure 1d). Indeed,
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two LMCT states are calculated around 3 eV by TD-DFT calculations and T2g(P) states around 3.46 eV by
CASSCF-NEVPT2 calculations, which could account for the biexponential decay as sketched in Figure 1d with
blue vertical arrows (Supporting Information, Tables S3 and S6).
Beyond the intense blue emission of [V(ddpd)2][PF6]3, the observed room temperature NIR-II
phosphorescence of [V(ddpd)2][PF6]3 in solution is unique. The NIR quantum yield of  = 1.810–4 % is not
very high but the rather sharp emission band (FWHM ca. 325 cm–1) is clearly observable. To the best of our
knowledge, no other complexes with Earth-abundant metal ions display NIR-II luminescence in frozen and in
fluid solution (Figures 2a – 2c). The ground state splitting of the 3T1g levels (Figure 1d) rather than vibrational
progression likely accounts for the asymmetric band shape. This differs from NIR emission bands of
chromium(III) complexes, which possess an orbitally non-degenerate ground state (4A2) and consequently
deliver sharp phosphorescence bands.12-17
In order to obtain information on the long-lived excited singlet states, the complex (in a KBr pellet) was
excited with 355 nm at 20 K and probed by step-scan FT-IR spectroscopy. The measured FTIR ground state
spectrum is in excellent agreement with the DFT-UKS calculated spectrum of [V(ddpd)2]3+ (Supporting
Information, Figure S15). The recorded step-scan difference spectrum collected between 0 and 300 ns after
the pulse shows the ground state bleach (negative bands) and excited state absorption (positive bands)
(Figure 2d). The latter vibrational bands correspond to the lowest energy long-lived singlet state. A
monoexponential lifetime of about 290 ns was determined by performing a global exponential fit for the
most prominent positive and negative peaks in the step-scan difference spectrum (Supporting Information,
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Figure S16). The observation of step-scan FTIR spectra further confirm the presence of long-lived excited
states.
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The lifetime of this emissive excited state depends on the efficiency of non-radiative decay processes.
Especially, NIR luminescence is prone to multiphonon relaxation via CH overtones which could be diminished
by deuteration.13,32,33 Indeed, the second aromatic CH overtone (3 = 8972 cm–1 as estimated from CH
CH
overtone IR spectroscopy of 6,6’-dimethyl-2,2’-bipyridine)13 has a significant spectral overlap with the NIR-II
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emission bands of [V(ddpd)2][PF6]3 (Figure 2b). For the deuterated derivative, the third CD overtone at 4
=
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CD
8755 cm–1 is the most relevant one (Figure 2b).13 Due to the lower energy and lower extinction coefficient of
the third CD overtone 4CD, the spectral overlap with the vanadium NIR-II emission band should be smaller.
Consequently, the non-radiative rate constant for the deuterated complex should be smaller than for the
protio derivative. As long as this multiphonon pathway dominates the decay of the singlet states, the NIR
quantum yield and lifetime of [V([D17]-ddpd)2]3+ should be larger than those of [V(ddpd)2]3+. However, within
experimental uncertainty, we cannot observe significant differences between the NIR emission intensities
and lifetimes of [V([D17]-ddpd)2]3+ and [V(ddpd)2]3+ at 298 K and 77 K (Supporting Information, Figures S12 –
S13 and S17 – S18, Table S7). This suggests that other non-radiative pathways dominate the decay of the
lowest energy singlet states in [V(ddpd)2]3+. Presumably, the split ground state levels favor ISC to the ground
state (Figure 1d).
The dual emission of [V(ddpd)2][PF6]3 in the blue and NIR-II spectral region demonstrates the huge energy
difference of ca. 2 eV between these two luminescent states. The observed intense fluorescence of
[V(ddpd)2]3+ suggests that the ISC rates from the triplets to the singlets are not particularly large. This slow
ISC then opens other pathways (fluorescence, non-radiative decay from the triplets) before the singlet states
are populated to higher extents.
The initial dynamics was then probed by fs-transient absorption (TA) spectroscopy. TA spectra of
[V(ddpd)2][PF6]3 in CH3CN recorded after excitation with femtosecond laser pulses at 620 nm (~2 eV) show
excited state absorptions (ESA) at around 500 and 670 nm (Supporting Information, Figure S19). With a time
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constant of 6.3 ps the ground state is essentially recovered. Hence, the LMCT states do not significantly
evolve to the emissive singlets as already demonstrated by the steady-state luminescence data. This is
consistent with the assumption that efficient ISC requires a high density of final states in the energy region
of interest.45 Around 2 eV, such singlet states are unavailable for [VIII(ddpd)2]3+ (Figure 1d). However, a singlet
state is present at higher energy (1A1g, 2.89 eV from the CASSCF-NEVPT2 calculation; Figure 1d).
TA spectra recorded after excitation at 400 nm (3.1 eV; 3LMCT and 3T2g states) exhibit a dominant ultrafast
decay to the ground state with a time constant of 4.9 ps and a second weaker time constant of 74 ps (Figure
3, Supporting Information, Figure S20). On the nanosecond time scale, a small component persists showing
an excited state absorption around 460 nm (Figures 3c and 3d). We assign this component to the long-lived
singlet state(s), similar to the analogous amplitude spectrum of [Cr(ddpd)2]3+, representing its emissive
doublet states, at longer times scales (Figure 3d).49 These results agree with the observation, that a small
fraction of the triplet population evolves to the singlet states via ISC. Obviously, the low-energy 3LMCT states
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are less competent to feed the spin-flip singlet states than the T2g and higher energy LMCT states.
Conceptually, the energy tuning of 3LMCT and 3T2g states as well as identification of suitable antenna systems
should enable an even more efficient population of the emissive singlet states of vanadium(III) complexes.
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Figure 3. fs-Transient absorption spectroscopy of [V(ddpd)2][PF6]3. (a) Transient absorption spectra of
[V(ddpd)2][PF6]3 in CH3CN excited with 400 nm laser pulses. (b) Bleach (blue) and decay associated amplitude
spectra labelled with the corresponding time constants: 1 = 4.9 ps (~70 %, black), 2 = 74 ps (~20 %, grey)
and long-living component with a time constant 3 > 2 ns (~10 %, green). (c) Transient signals as a function of
the pump–probe delay time observed at 670 nm (black), 550 nm (blue) and 500 nm (red). The fits to the
experimental data correspond to triexponential decays with 1,2 = 4.9, 74 ps and a time constant 3 > 2 ns. (d)
Scaled amplitude spectra of the long-lived components of [V(ddpd)2][PF6]3 (green) and [Cr(ddpd)2][BF4]3
(orange) (chromium(III) data from ref. 49).
Molecular dynamics calculations.
In order to gain deeper insight into the complex spin-flip dynamics of [V(ddpd)2]3+ after excitation, dynamics
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simulations starting from the LMCT manifold in acetonitrile were performed. Specifically, non-adiabatic
surface hopping including arbitrary couplings (SHARC) dynamics simulations with a linear vibronic coupling
(LVC) model were employed.50 Initially, 100 fs were propagated from the four lowest 3LMCT states located at
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3.5 – 3.7 eV (354 – 333 nm (Supporting Information, Table S3, Figures S21 – S22), finding one LMCT state
with a significant amount of trapped population. Subsequently and starting from this state (23LMCT) located
at 3.59 eV (345 nm), we propagated during 50000 fs and observed IC to the 3MC states (3T2g and 3T1g ground
state) and ISC to a singlet state 21MC (Figure 1d, Figure 4).
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Figure 4. Time-evolution of the population of the excited states during 50000 fs (in logarithmic scale) after
starting the dynamics on the 23LMCT state (the longest lived triplet state after 100 fs, Supporting Information,
Figure S22). The label “other 3LMCT” stands for the population in any other state but the 23LMCT. The label
“3MC” corresponds to the sum of populations in every triplet MC state, indicating the total amount of internal
conversion. Population in any other singlet state other than 21MC is negligible and not plotted.
1
The singlet LMCT manifold acts as a mere spectator during the dynamics. The most populated metal-
centered singlet state 21MC was then optimized within the LVC method. The optimized geometry (Supporting
Information, Table S8) lies 1.10 eV (1127 nm) above the 3T1g ground state, in very good agreement with the
experimental emission energy, thus validating the dynamical approach. The dynamics calculations confirm
that the 23LMCT state at high energy (3.59 eV) undergoes ISC to a singlet state of MC character while no
lower energy 1LMCT states around 2 eV are involved in the excited state evolution towards the emissive 1MC
state, as already suggested by the steady-state and time-resolved experimental data.
Conclusion
Our study extends the very series of luminescent metal complexes with Earth-abundant 3d metal ions (Cr,
Fe, Co, Cu) to the early 3d transition metal vanadium. The vanadium(III) complex mer-[V(ddpd)2]3+ with a
tuned large ligand field splitting exhibits a deep-blue emission at room temperature in solution and a long-
lived phosphorescence above 1000 nm. This expands the spectral region previously accessible with 3d metal
complexes (Vis and NIR-I) to the NIR-II region for the first time. Conceptual design principles for NIR-II
luminescent vanadium(III) complexes evolved, highlighting the distinct roles of ligand-to-metal charge
transfer states, intersystem crossing and multiphonon relaxation. This first success in designing blue and NIR-
II emissive complexes of the Earth-abundant metal vanadium raises the prospect of employing luminescent
vanadium complexes in the future. Especially, dual emissive complexes could find applications as optical
probes with a built-in internal reference for chemical or physical perturbations. Work in this direction is
currently in progress.
ASSOCIATED CONTENT
Supporting Information
Data of quantum chemical calculations, synthetic procedures, analytical and spectroscopic data of
[V(ddpd)2][PF6]3. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
* katja.heinze@uni-mainz.de (ORCID 0000-0003-1483-4156), gerhards@chemie.uni-kl.de (ORCID 0000-
0002-8748-2940), michael.seitz@uni-tuebingen.de (ORCID 0000-0002-9313-2779).
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
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Financial support from the Deutsche Forschungsgemeinschaft [DFG, Priority Program SPP 2102 “Light-
controlled reactivity of metal complexes” (GE 961/10-1, HE 2778/15-1, LO 714/11-1, SE 1448/8-1, GO 1059/8-
1] is gratefully acknowledged. Parts of this research were conducted using the supercomputer Mogon and
advisory services offered by Johannes Gutenberg University Mainz (https://hpc.uni-mainz.de), which is a
member of the AHRP and the Gauss Alliance e.V. as well as at the Vienna Scientific Cluster (VSC3).
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