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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Efficient Ytterbium Near-Infrared Luminophore Based on a Nondeuterated Ligand Christian Kruck,† Pariya Nazari,‡ Carolin Dee,† Bryce S. Richards,‡ Andrey Turshatov,*,‡ and Michael Seitz*,† †

Institute of Inorganic Chemistry, University of Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany Institute of Microstructure Technology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

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‡

S Supporting Information *

ABSTRACT: A novel molecular ytterbium complex is reported with a new tetradentate ligand based on the 2,2′-bipyridine-6,6′-dicarboxylic acid scaffold. The photophysical properties are investigated, especially with respect to near-infrared luminescence. The ytterbium complex shows a rather high absolute luminescence quantum yield of Φ = 3.0% and a luminescence lifetime of τobs = 72 μs at room temperature in CD3OD solution.

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INTRODUCTION Near-infrared lanthanoid(III) luminophores are highly interesting for technological applications such as biomedical imaging, optical telecommunication, spectral conversion materials, and many others.1 The main challenge for the development of efficient, molecular, near-IR emitter complexes is the vibrational deactivation of metal-centered states by multiphonon relaxation (MR) where the energy is transferred from the lanthanoid to oscillator overtones in its vicinity.2 The main culprits with respect to MR for trivalent lanthanoids are anharmonic, high-energy oscillators such as O−H (mainly in inner-sphere solvent molecules) and C−H stretching vibrations (e.g., found in the multidentate antenna chelators). The two main strategies to alleviate the problem caused by C−H moieties in the ligands are removing these oscillators by either deuteration2,3 or halogenation.4 Via this approach, considerable progress has been made in recent years, most notably, in the development of extremely efficient, molecular ytterbium(III) near-IR luminophores,5 e.g., by the realization of absolute quantum yields in solution of up to 63% by Zhang and coworkers.5a For a number of reasons, Yb3+ will be very pivotal for future progress in advanced photonic applications of molecular lanthanoid complexes, among others exemplified by the discovery of Yb−Tb upconversion in solution over the past few years.6 While the development of deuterated/halogenated near-IR ytterbium luminophores has made great progress, there is still a need for molecular emitters that are relatively simple to prepare, yet still able to exhibit acceptable luminescence efficiency. Usually, molecular ytterbium(III) © XXXX American Chemical Society

complexes with nondeuterated and/or nonhalogenated ligand scaffolds show rather low quantum yields in solution of below 1%, and only a small number of such species exist which are more efficient emitters reaching ca. 4% in the best cases.7 In an effort to develop new, nondeuterated ligand environments for Yb, we turned to derivatives of tetradentate 2,2′-bipyridine6,6′-dicarboxylate ligands (Figure 1, structure A) which (i) can be employed with different functionalization patterns at the pyridine rings and (ii) form rather stable lanthanoid complexes with a wide range of different lanthanoids.8 In addition, these chelators, while featuring aromatic C−H moieties, do not have high-energy C−H oscillators in close proximity to the metal

Figure 1. Previously reported lanthanoid motifs A based on 2,2′bipyridine-6,6′-dicarboxylic acid (left) and new complex B (Ln = Yb) reported here (right). Received: February 25, 2019

A

DOI: 10.1021/acs.inorgchem.9b00548 Inorg. Chem. XXXX, XXX, XXX−XXX

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Structure Elucidation. 1H NMR spectra of 8-Lu and 8-Yb in CD3OD each show only four resonances for the aromatic protons of the ligand 72− as well as two signals for the three equivalent ethyl groups in triethylammonium (see Figures S2 and S3 in the Supporting Information). The integral ratios between the two sets of signals in each case amount to 2:1 (72−: HNEt3+). This reliably indicates the presence of the anticipated anionic lanthanoid complexes with the stoichiometry 2:1 (ligand/metal) exhibiting D2d symmetry. Semiempirical calculations (MOPAC2016: PM7/SPARKLE)12−14 of the equilibrium geometry of the complex anion in 8-Yb yield this structural element (Figure 2, see the Supporting

center and consequently show reduced MR for many near-IR emitting lanthanoids (e.g., for Tm,8b Dy,8c and Ho8d). Unfortunately, the ytterbium(III) complex A (Figure 1) with the simplest ligand (R = H), which is the obvious candidate for starting this endeavor, was previously reported to only exhibit limited near-IR efficiency, exemplified by the observed luminescence lifetime of τobs = 6.6 μs at room temperature, which indicates a quantum yield well below 1%.8e Nevertheless, we chose this ligand scaffold for initial screening experiments by varying the substitution pattern in the 4- and 4′-positions of the bipyridine units, and we have identified complex B (Figure 1) with appended pyridyl groups as an ytterbium emitter possessing high luminescence efficiency. In this study, we introduce the synthesis of the new ligand, characterize the corresponding ytterbium chelate, and comprehensively investigate the relevant photophysics of this new luminophore.

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RESULTS AND DISCUSSION Synthesis. The synthesis of the new ligand H27 used in structure B (Figure 1) started with the known bipyridine N,N′dioxide 1 (Scheme 1).10 Nitration to 2, followed by

Scheme 1. Synthesis of Ligand H27

Figure 2. Calculated structure of 8-Yb optimized by PM712/ SPARKLE13 in MOPAC2016.14 Representative geometric parameters around the metal center: av distance Yb−N 2.470 Å, av distance Yb− O 2.269 Å, dihedral angle between the two planar ligands (N1−N2− N3−N4) 89° (see the Supporting Information for more details).

Information for more details) with a point group symmetry close to D2d (dihedral angle between the two planar ligand fragments ≈ 89°). The bonding distances Yb−X (X = N, O) are in a range very similar to those observed in related ytterbium complexes obtained by X-ray crystallography on single crystals of complexes A (Figure 1: R = H8e and R = COOMe9a). Importantly, the C−H oscillators, which are mostly responsible for MR in molecular complexes,2 are rather far away as expected, with the closest ones being in the 5- and 5′-positions (rYb−H > 5.53 Å) of the central bipyridine unit. Photophysics. Absorption and emission spectra in the visible region for 8-Lu and 8-Yb at room temperature were measured in CD3OD (Figure 3). The absorption spectra for the two complexes are almost identical and exhibit a pronounced band between ca. 300 and 350 nm, which is typical for bipyridine-based lanthanoid complexes. After excitation at 280 nm, the emission spectrum for 8-Lu features one fluorescence emission band (S1 → S0) centered around ca. 360 nm (Figure 3) with quantum yield of 0.6% and no indication of additional room temperature phosphorescence at higher wavelengths. Interestingly, 8-Yb also shows residual singlet emission, with roughly 20% relative intensity when compared to that of 8-Lu (≡100%). Low-temperature (T = 77 K) emission spectra for 8-Lu do, however, reveal the energetic position of the ligand-based triplet state T1 (Figure 4). Spectral deconvolution of the structured band around 21 000 cm−1 yields a zero-phonon triplet energy of E0→0(T1) ≈ 22 600 cm−1 which is slightly lower than that for the corresponding parent system A (Figure 1, R = H: E0→0(T1) ≈ 23 500 cm−1).8b

nucleophilic aromatic substitution of bromide for the nitro group using HBr in HOAc, yielded 3, which in turn could be reduced to dibromo derivative 4.11 Standard Suzuki coupling of this intermediate with commercial pyridine-4-boronic acid (5) gave tetrapyridine 6 which was fully oxidized at the two benzylic methyl groups with CrO3 in concentrated sulfuric acid to yield the sulfate salt of ligand H27. The targeted ytterbium complex 8-Yb with triethylammonium as countercation was cleanly obtained by reacting 2 equiv of H27 with YbCl3·6H2O in the presence of a slight excess of NEt3 in methanolic solution (Scheme 2). As a diamagnetic and photoinactive control, the corresponding lutetium complex 8-Lu was prepared in the same manner. Scheme 2. Synthesis of the Lanthanoid Complexes 8-Ln

B

DOI: 10.1021/acs.inorgchem.9b00548 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Steady-state emission spectra for the transition 2F5/2 → 2 F7/2 in 8-Yb at room temperature in CD3OD (solid black, 298 K, λexc = 300 nm; dotted black, 298 K, λexc = 940 nm) and at low temperature in a glassy matrix (solid red, 77 K, λexc = 300 nm, CD3OD/C2D5OD, 1:1, v/v).

Figure 3. UV−vis absorption (solid lines) and singlet steady-state emission (dashed lines, λexc = 280 nm) spectra for 8-Lu (black) and 8Yb (red) in CD3OD (c = 10 μM) at room temperature (the relative scales of the emission spectra reflect the actual intensity differences).

and triplet states and the metal-centered 2F5/2 level, it is not easy to imagine that near-IR luminescence is directly sensitized via singlet/triplet energy transfer from the ligand to the metal. An alternative explanation is photosensitization of Yb3+ via internal redox processes involving electron transfer as proposed in the past in other systems.1c,15 The involvement of LMCT states in 8-Yb, however, seems unlikely due to the observation of identical residual singlet emission of exactly the same magnitude in the corresponding erbium complex16 and the rather electron-deficient nature of the ligands in our case. Therefore, a conclusive answer regarding the nature of the sensitization pathway remains unclear. Gratifyingly, the measurement of the absolute quantum yield at room temperature in CD3OD revealed a rather high value of ΦLLn = 3.0% (Table 1), which compares well with other relatively efficient systems reported so far in the literature for nondeuterated, molecular ytterbium luminophores in solution. In order to get more insight into the sensitization process, we set out to determine a number of important parameters according to eq 11 τobs ΦLLn = ηsensΦLn Ln = ηsens τrad (1)

Figure 4. Low-temperature steady-state emission spectra (λexc = 305 nm, T = 77 K) for 8-Lu in a glassy matrix (CD3OD/C2D5OD, 1:1, v/ v).

After excitation with UV light, the near-IR emission spectra of 8-Yb originating from the transition 2F5/2 → 2F7/2 in CD3OD at room temperature can easily be detected (Figure 5). Interestingly, direct f−f excitation at λexc = 940 nm of the same sample results in an emission spectrum that shows a broadly similar emission band but where emission from a higher ligand-field state (≈ 975 nm) is more prominent compared to the spectra after sensitized excitation via the ligand (Figure 5). The emission after ligand excitation exhibits a monoexponential luminescence lifetime τobs = 72 μs (Table 1), which is comparatively long for molecular species of this kind. The analogous spectra at T = 77 K in a glassy matrix of CD3OD/C2D5OD are very similar to the room temperature signatures (Figure 5) with a slightly longer, monoexponential lifetime τobs = 96 μs. Surprisingly, near-IR luminescence of 8Yb is insensitive to molecular oxygen present in the solvent. Measurements performed in deoxygenated solutions revealed no difference in luminescence intensity as compared to the ones where dioxygen has not been removed. Thus, we assume that the triplet state of the ligand is not of primary importance for the photosensitization of Yb3+ or is very short-lived. Considering the large mismatch in the energy of ligand singlet

with ηsens as the sensitization efficiency, ΦLn Ln as the intrinsic quantum yield, and τrad being the radiative (intrinsic) luminescence lifetime. Among the three unknown parameters so far (ΦLn Ln, ηsens, τrad), the radiative lifetime τrad can be determined if the emissive transition in question terminates in the ground state. This is the case for ytterbium, and τrad can be extracted from the molar extinction spectrum corresponding to the f−f transition 2F5/2 → 2F7/2 (eq 2):17 8πcn2νm̃ 2 ( 2Jl + 1) 1 = 2303 τrad NA ( 2Ju + 1) with νm̃ =

∫ ε(ν)̃ dν ̃

∫ νε̃ (ν)̃ dν ̃ ∫ ε(ν)̃ dν ̃

(2)

This equation includes the following parameters and constants: c speed of light (in cm s−1), NA Avogadro’s constant, n C

DOI: 10.1021/acs.inorgchem.9b00548 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Luminescence Data for 8-Yb in CD3OD ΦLn Ln = entry

compd

a τ298K [μs] obs

a τ77K obs [μs]

τradb [ms]

1

8-Yb

72

96

1.3

c

298K τobs τrad

[%] 5.5

ηsens = ΦLLnd [%] 3.0

e

ΦLLn ΦLn Ln

[%]

54%

Luminescence lifetimes: λexc = 300 nm, λem(298 K) = 1012 nm, λem(77 K) = 1010 nm, estimated uncertainty ±10% for 8-Yb. bRadiative lifetime: calculated using eq 2, estimated uncertainty ±20%. cIntrinsic quantum yield. dAbsolute quantum yield, estimated uncertainty ±15%. eSensitization efficiency. a

refractive index, ν̃m barycenter of the transition to the ground state (as defined above), 2Jl + 1 degeneracy of the lower (ground) state, 2Ju + 1 degeneracy of the upper (excited) state, and ε(ν̃) molar extinction coefficient (in units of [M−1 cm−1]) of the transition to the ground state vs the wavenumber. The analysis was performed on the basis of the measured extinction spectrum (Figure 6) yielding the value τrad = 1.3 ms for 8-Yb

purification steps. Isotopically labeled solvents (CDCl3, CD3OD, [d6]DMSO, CD2Cl2, CD3CD2OD) for NMR and optical spectroscopies had at least 99.8% isotope content. Unless indicated, reactions were carried out under an atmosphere of argon (dry, O2-free) using standard Schlenk methodology. Preparative column chromatography was carried out using SiO2 60 (Merck KGaA, 0.040−0.063 mm), whereas TLC was performed with silica gel 60 plates (Merck, coated on aluminum sheets, with fluorescence indicator F254). Mass spectrometry was performed using a Bruker Daltonics Esquire 3000plus instrument (ESI mode). NMR spectra were acquired on a Bruker instrument Avance II+400 (1H 400 MHz, 13C 101 MHz). All chemical shifts (δ) for the NMR spectra are given in parts per million (ppm) and relative to the usual standard (tetramethylsilane, TMS). The chemical shift range for each spectrum was calibrated using the residual solvent signals as internal reference. Synthesis. 6,6′-Dimethyl-4,4′-dinitro-2,2′-bipyridine-N,N′-dioxide (2).11 6,6′-Dimethyl-2,2′-bipyridine-N,N′-dioxide (1)10 (5.60 g, 25.7 mmol, 1.0 equiv) was dissolved in concentrated sulfuric acid (50 mL). Concentrated nitric acid (17 mL) was added dropwise with a dropping funnel in the course of approximately 20 min, followed by continued stirring at 100 °C for 4 h. The resulting solution was allowed to attain room temperature and was poured onto crushed ice (200 g). After the ice melted, the pH was adjusted to ca. 9 by the dropwise addition of aqueous NaOH (10 wt % NaOH), resulting in precipitation of the product. It was collected, washed with ice cold water, and dried in vacuo. The product was obtained as a yellow solid (6.6 g, 84%). 1H NMR ([d6]-DMSO, 400 MHz): δ = 8.60 (s, 2 H), 8.53 (s, 2 H), 2.48 (s, 6 H) ppm. 4,4′-Dibromo-6,6′-dimethyl-2,2′-bipyridine-N,N′-dioxide (3)11. A flask was charged with 6,6′-dimethyl-4,4′-dinitro-2,2′-bipyridineN,N′-dioxide (2) (6.60 g, 21.6 mmol, 1.0 equiv), and a solution of hydrogen bromide (33 wt % HBr in acetic acid, 150 mL) was added. The resulting mixture was stirred at 90 °C for 44 h and allowed to come to ambient temperature, and the pH was cautiously adjusted to approximatey 9 using a solution of sodium hydroxide (10 wt % NaOH in water). After extraction of the aqueous phase with CH2Cl2 (3 × 150 mL), the combined organic phases were dried (MgSO4) and concentrated under reduced pressure. The crude title compound was purified by chromatography (SiO2, CH2Cl2−MeOH, gradient 50:1 → 9:1), yielding the product as a light yellow solid (1.40 g, 19%). 1H NMR ([d6]-DMSO, 400 MHz): δ = 7.92 (s, 2 H), 7.84 (s, 2 H), 2.36 (s, 6 H) ppm. TLC: Rf = 0.23 (SiO2, CH2Cl2/MeOH 9:1, detection UV). MS (ESI, pos mode): m/z (%) = 374.9 (100, [M + H]+, Br2 pattern). 4,4′-Dibromo-6,6′-dimethyl-2,2′-bipyridine (4). 4,4′-Dibromo6,6′-dimethyl-2,2′-bipyridine-N,N’-dioxide (3) (2.18 g, 5.83 mmol, 1.0 equiv) was suspended in CH3CN (200 mL, HPLC grade). PBr3 (11.0 mL, 31.6 g, 117 mmol, 20 equiv) was added dropwise, and the red mixture was heated under reflux for 18 h. The green/brown suspension was poured onto crushed ice (ca. 150 g), and the pH was adjusted to ≈8 with saturated aqueous Na2CO3. The aqueous phase was extracted with CHCl3 (1 × 200 mL, 2 × 100 mL). The combined organic phases were dried (MgSO4) and concentrated under reduced pressure. The product was obtained as a light yellow solid (1.24 g, 62%) and was used without further purification for the next step. 1H NMR (CDCl3, 400 MHz): δ = 8.40 (s, 2 H), 7.36 (s, 2 H), 2.60 (s, 6 H) ppm.

Figure 6. Quantitative absorption spectrum of the f−f transition 2F7/2 → 2F5/2 in 8-Yb (c = 0.6 mM, path length d = 5 cm) in CD3OD.

(see the Supporting Information for more details). This value is on the higher side of the usual range (τrad ≈ 0.6−1.3 ms)18 in molecular complexes and very similar to the one found (τrad = 1.31 ms) in Na3[Yb(dpa)3]19 (with dpa = pyridine-2,6dicarboxylate) which is structurally very similar to 8-Yb. The high value of τrad is a reflection of the relatively high symmetry (D2d) of the inner coordination sphere around the metal center. With the key parameter τrad in hand, the full evaluation according to eq 1 could be performed, giving an intrinsic quantum yield ΦLn Ln = 5.5% and a sensitization efficiency ηsens = 54% for 8-Yb (Table 1). The value for ΦLn Ln is quite high for complexes of this type, while ηsens is rather low, reflected also by the previous observation of residual singlet emission (vide supra) in 8-Yb.

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CONCLUSION In conclusion, we have identified a new chelator based on 2,2′bipyridine-6,6′-dicarboxylate for the efficient sensitization of ytterbium near-IR luminescence. This antenna ligand allows the realization of rather high absolute luminescence quantum yields of 3% in solution, which is quite rare for a nondeuterated and/or nonhalogenated sensitizer of this kind.

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EXPERIMENTAL SECTION

General. All commercially available chemicals for the synthetic and analytical work were purchased and used without any further D

DOI: 10.1021/acs.inorgchem.9b00548 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Dimethyl Tetrapyridine 6. A two-neck Schlenk-flask equipped with a reflux condenser was charged with 4,4′-dibromo-6,6′-dimethyl-2,2′bipyridine (4) (1.0 g, 2.9 mmol, 1.0 equiv), 4-pyridylboronic acid (5) (1.1 g, 8.8 mmol, 3.0 equiv), Pd2dba3 (107 mg, 117 μmol, 4.0 mol %), and K3PO4 (3.10 g, 14.6 mmol, 5.0 equiv). The apparatus was set under inert gas (argon), and 1,4-dioxane (degassed, 140 mL, 3 × freeze−pump−thaw) was added, followed by PCy3 (66.0 mg, 285 μmol, 8.0 mol %) and deoxygenated H2O (50 mL). The reaction mixture was heated under reflux for 48 h and allowed to attain ambient temperature. Water (20 mL) was added under atmospheric conditions, and the pH of the mixture was adjusted to ≈9 using saturated aqueous Na2CO3. The mixture was extracted with CH2Cl2 (3 × 70 mL). The combined organic phases were dried (MgSO4), and the solvent was evaporated. The residue was purified by column chromatography (SiO2, CH2Cl2/MeOH, gradient 50:1 → 25:1), yielding the title compound as a colorless solid (513 mg, 52%). 1H NMR (CD2Cl2, 400 MHz): δ = 8.64 (dd, J = 4.5 Hz, 1.7 Hz, 4 H), 8.48 (s, 2 H), 7.76 (dd, J = 4.5 Hz, 1.7 Hz, 4 H), 7.54 (s, 2 H), 2.69 (s, 6 H) ppm. 13C NMR (CD2Cl2, 100.6 MHz): δ = 159.6, 156.8, 151.1, 147.3, 146.5, 122.1, 121.6, 116.5, 25.0 ppm. MS (ESI, pos mode): m/z (%) = 339.1 (100, [M + H]+), 361.1 (26, [M + Na]+), 376.9 (21, [M + K]+). Bipyridinium Dicarboxylic Acid Sulfate H27. Dimethyl tetrapyridine 6 (641 mg, 1.90 mmol, 1.0 equiv) was slowly added as a solid to concentrated sulfuric acid (10 mL) while the mixture was stirred vigorously. The viscous solution was brought to an internal temperature of 65 °C before solid chromium(VI) oxide (0.83 g, 8.3 mmol, 4.4 equiv) was added at such a pace as to not let the reaction temperature (measured internally) rise beyond 70 °C. The green solution was stirred at 70 °C (internal temperature) for 3 h, allowed to cool to room temperature, and poured onto crushed ice (100 g). The resulting solution was stored in a refrigerator (4 °C) for 24 h. The precipitate was filtered over a Büchner funnel, washed with ice cold H2O, and dried under reduced pressure. The product was obtained as a colorless solid (612 mg, 81%). 1H NMR ([d6]-DMSO, 400 MHz): δ = 9.18 (d, J = 1.6 Hz, 2 H) 8.97 (d, J = 6.2 Hz, 4 H), 8.59 (d, J = 1.6 Hz, 2 H), 8.30 (d, J = 6.1 Hz, 4 H) ppm. 13C NMR ([d6]-DMSO, 100.6 MHz): δ = 165.4, 155.2, 149.6 145.9, 145.3, 145.0, 124.6, 123.7, 122.6, 119.6 ppm. MS (ESI+): m/z (%) = 399.0 (100, [M + H]+). Anal. Calcd for C22H14N4O4·H2SO4·4H2O (Mr = 568.51): C, 46.48; H, 4.26; N, 9.86; S, 5.64. Found: C, 46.38; H, 4.49; N, 9.69; S, 5.49. Lanthanoid Complexes 8-Ln. General Procedure. H27 (2.0 equiv) and LnCl3·6H2O (1.0 equiv) were treated with anhydrous MeOH (80 μL per μmol of H27), followed by dry NEt3 (10.0 equiv). The mixture was stirred at ambient temperature for 2 h, and the solid formed during this time was filtered over a Büchner funnel. It was subsequently washed with ice cold, anhydrous methanol and dried under reduced pressure. Lutetium Complex 8-Lu. The general procedure using H27 (50.0 mg, 87.9 μmol), LuCl3·6H2O (18.0 mg, 44.0 μmol), and NEt3 (61.3 μL, 439.7 μmol) yielded a light yellow solid (42.0 mg, 87%). 1H NMR (CD3OD, 400 MHz): δ = 9.40 (d, J = 1.5 Hz, 4 H), 8.78 (dd, J = 4.6 Hz, 1.6 Hz, 8 H), 8.52 (d, J = 1.2 Hz, 4 H), 8.10 (dd, J = 4.6 Hz, 1.6 Hz, 8 H), 3.14 (s br, 6 H), 1.28 (t, J = 6.9 Hz, 9 H) ppm. MS (ESI, neg mode): m/z (%) = 967.4 (100, [M]−). Anal. Calcd for [C44H24N8O8Lu] (HNEt3)·CH3OH·6H2O (Mr = 1210.01): C, 50.62; H, 4.66; N, 10.42. Found: C, 50.48; H, 4.94; N, 10.40. Ytterbium Complex 8-Yb. The general procedure using H27 (100.0 mg, 175.9 μmol), YbCl3·6H2O (34.3 mg, 88.0 μmol), and NEt3 (122.6 μL, 879.5 μmol) yielded a colorless solid (79.0 mg, 82%). 1H NMR (CD3OD, 400 MHz): δ = 18.90 (s br, 4 H), 9.02 (s br, 16 H), 7.58 (s br, 4 H), 3.13 (s br, 6 H), 1.27 (s br, 9 H) ppm. MS (ESI, neg mode): m/z (%) = 966.3 (100, [M]−, Yb pattern). Photophysics. The NIR f−f absorption spectrum for 8-Yb was measured using a spectrophotometer (Jasco V770) with long-path (5.0 cm) quartz cuvettes (see the Supporting Information). Steady-state emission spectra at ambient temperature (298 K) were collected on a Cary Eclipse G9800A fluorimeter. The luminescence lifetime measurements were performed using a multichannel scaling

card (Timeharp 260, PicoQuant) with modulation of the pulsed diode (M300L4: λem = 300 nm, Thorlabs) being achieved by a builtin function generator of the corresponding driver unit. Spectral selection was achieved using a double monochromator (DTMS300, Bentham), and the luminescence band at 1012 nm was detected via a photomultiplier tube (R928P, Hamamatsu, operated in a thermoelectrically cooled CoolOne PMT housing unit from Horiba). For the absolute quantum yield measurements, the beam from the diode (M300L4: λem = 300 nm, Thorlabs) was focused by a lens (focal length: 75 cm) into an integrating sphere (Labsphere, ⌀ 15 cm). The emitted light was transferred from the integrating sphere into the spectrometer (AvaSpec-ULS2048LTEC spectrometer, Avantes) with the help of an optical fiber (FP1000URT, Thorlabs, ⌀ 1 mm). A calibration lamp (HL-3plus-INT-CAL, Ocean Optics) was used to calibrate the entire system’s optical response. The relative quantum yield of 8-Lu was measured against the standard 9,10-diphenylanthracene20 (Φ = 0.90 in cyclohexane).21 Steady-state luminescence at 77 K (liquid N2 cuvette) of 8-Lu (CD3OD/C2D5OD, 1:1, v/v) was collected with the help of a Horiba Fluorolog-3 DF fluorimeter (excitation, 450 W continuous xenon lamp; detection, Hamamatsu R2658P PMT) equipped with double-grating DFM/DFX monochromators (excitation, 1200 grooves/mm, blazed at 330 nm; emission, 1200 grooves/mm, blazed at 500 nm). The lifetime of 8Yb at the same temperature was determined with excitation from a pulsed xenon flash lamp (70 W, pulse width ca. 1.5 μs fwhm) by tailfitting the decays using the DAS software package from Horiba.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00548. 1 H NMR spectra for H27, 8-Lu, and 8-Yb; details for the geometry optimization of 8-Yb; luminescence decay profiles for 8-Yb; and details for the determination of τrad in 8-Yb (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Bryce S. Richards: 0000-0001-5469-048X Andrey Turshatov: 0000-0002-8004-098X Michael Seitz: 0000-0002-9313-2779 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support is gratefully acknowledged from DFG (Research Grants TU487/2-1 and SE1448/6-1) within the Priority Program SPP 1928: COORNET. P.N., B.S.R, and A.T. acknowledge the Helmholtz Energy Materials Foundry (HEMF).

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REFERENCES

(1) (a) Bünzli, J.-C. G.; Eliseeva, S. V. Photophysics of Lanthanoid Coordination Compounds. In Comprehensive Inorganic Chemistry II; Reedijk, J., Poeppelmeier, K., Eds.; Elsevier: Amsterdam, 2013; Vol 8, pp 339−398. (b) Bünzli, J.-C. G.; Eliseeva, S. V. J. Lanthanide NIR luminescence for telecommunications, bioanalyses and solar energy conversion Rare Earths. J. Rare Earths 2010, 28, 824−842. (c) Comby, S.; Bü n zli, J.-C. G. Lanthanide Near-Infrared Luminescence in Molecular Probes and Devices. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Jr., E

DOI: 10.1021/acs.inorgchem.9b00548 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.9b00548 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b00548 Inorg. Chem. XXXX, XXX, XXX−XXX