Nonradiative Deactivation of Lanthanoid Excited ... - ACS Publications

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Nonradiative Deactivation of Lanthanoid Excited States by InnerSphere Carboxylates Jessica Wahsner‡ and Michael Seitz*,†,‡ †

Institute of Inorganic Chemistry, University of Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany Inorganic Chemistry I, Department of Chemistry and Biochemistry, Ruhr-University Bochum, 44780 Bochum, Germany

‡

S Supporting Information *

ABSTRACT: The vibrational deactivation of metal-centered excited states is one of the fundamental processes that governs the luminescence of inorganic luminophores. In molecular lanthanoid luminescence, the most reliable way to modulate and systematically investigate these processes is deuteration of X−H stretching modes (X = O, N, C). Apart from the effect of these high-energy vibrational motifs, very little is known about the impact of other oscillator fragments present in lanthanoid complexes. We have developed a synthetic protocol to efficiently and selectively label the popular chelator motif “pyridine-2-carboxylic acid” with stable 13C/18O isotope at the carboxylate group. The corresponding isotopologic lanthanoid complexes (Ln = Sm, Eu, Ho) show a decrease of the local-mode carbonyl stretching frequency of up to 5% after isotopic substitution. While this does not seems to have any effect on the luminescence of lanthanoids with medium- to high-energy gaps (Sm and Eu), we have found the first example of a quantifiable luminescence isotope effect for one of the near-IR transitions of holmium (3K8 → 5I5) that only involves the isotopic editing of the vibrational environment at the four carbonyl oscillators.

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INTRODUCTION The vibrational dynamics in molecular metal complexes play a crucial role in determining a variety of important photochemical and photophysical properties, e.g., radiative and nonradiative transitions, intra- and intermolecular energy, and electron transfer, etc.1 Despite their great importance, our understanding of the details and intricacies of the associated phenomena is still rather limited. One area, where this lack of solid knowledge is felt very acutely, is the field of near-IR lanthanoid luminescence which has received a lot of attention over the past years due to the unique photophysical properties of trivalent lanthanoid ions.2 In the latter context, the nature and properties of the oscillator environment have long been known to considerably affect lanthanoid photophysics, e.g., with respect to near-IR luminescence efficiency or the kinetics of nonradiative energy transfer processes. In the field of solid-state phosphors this has been studied in great detail empirically and theoretically,3 whereas for molecular lanthanoid complexes our understanding is much less evolved.4 All luminescent lanthanoids in molecular coordination compounds are prone to quenching via multiphonon relaxation processes mediated by anharmonic, high-energy vibrations such as O−H, N−H, or C− H stretching modes in the vicinity of the metal center. A very useful strategy for the investigation of the associated phenomena is the alteration of the vibrational properties by isotopic substitution. This approach has the advantage that it generally does not substantially change the structural and electronic properties of the molecular entity in question, © XXXX American Chemical Society

making comparisons between different isotopologues very meaningful and informative. The largest effects can be achieved in practice by replacing the lightest isotope 1H with its heavier isotope 2H, which almost doubles the reduced mass μ12 = (m1 × m2)/(m1 + m2), e.g., for a C−H stretching oscillator, and decreases the associated vibrational energy ν̃ by a factor of 1.36 (eq 1). ν12̃ C −1H = ν12̃ C −2 H

μ12C − 2 H μ12C −′1 H

= 1.36 (1)

This difference is large enough that the influence of deuteration on the luminescence properties can usually be detected easily for most lanthanoids. 5,6 With a few exceptions,6e,f deuteration is usually also a reliable strategy in order to enhance near-IR luminescence. In contrast to the situation for deuteration, isotopic substitution and the precise influence of individual oscillators on lanthanoid photophysics for other chemical entities in organic ligands is unknown. In this study, efficient isotopic labeling (13C/18O) of 2,2′bipyridine-6,6′-dicarboxylate moieties7 (Figure 1) is described, the vibrational and photophysical properties of the corresponding metal complexes with representative lanthanoids (Sm, Eu, Ho) are reported, and the impact of the reported isotopic Received: August 20, 2015

A

DOI: 10.1021/acs.inorgchem.5b01920 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Scheme 1. Synthesis of the 13C/18O Isotopologues of 2,2′Bipyridine-6,6′-dicarboxylic Acid (4)

Figure 1. Model system in this study for the investigation of nonradiative deactivation by carbonyl oscillators.

substitution on the luminescence properties of the holmium complexes is analyzed.

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RESULTS AND DISCUSSION The isotopologic ligands derived from 2,2′-bipyridine-6,6′dicarboxylate (Figure 1) were chosen as our model systems for a number of reasons that make them ideal for the purpose of the study. Most importantly, they feature the very common chelator motif “pyridine-2-carboxylate”, which is one of the mainstays for transition metal and lanthanoid coordination chemistry and is encountered in many successful ligand architectures (Figure 2).8

[18O4]-4 was synthesized in the same way from nonlabeled 2,2′-bipyridine-6,6′-dicarboxylic acid (4). In both cases, deconvolution of the corresponding isotopic patterns in ESI mass spectrometry showed the incorporation of the stable isotopes 13C and 18O to be sufficiently high for our purposes [13C2,18O4]-4, 99% 13C, 85% 18O; and [18O4]-4, 93% 18O (see Figures S1−S2). The corresponding lanthanoid complexes were prepared by standard protocols as depicted in Scheme 2.7 For holmium, the analogous perdeuterated complex [D12]-1-Ho was also synthesized for comparison from the known chelator [D6]-

Figure 2. Isotopologic pyridine-2-carboxylates.

Scheme 2. Complexation Reactions to the Isotopologic Lanthanoid Complexes 1-Ln (Sm,Eu, x = 2; Ho, x = 3)

Second, the corresponding lanthanoid complexes can be prepared reproducibly and cleanly in very small quantities7c,d which is an essential requirement for the rather expensive labeling with 13C and 18O. Lastly, 2,2′-bipyridine-6,6′dicarboxylate has already proven to be a good antenna moiety for the sensitization of luminescence from various lanthanoids, and the previously established ligand-centered triplet energy (≈23 500 cm−1)7c is suitably high as to allow the population of lanthanoid excited states for Sm, Eu, and Ho (as used in this study) by the usual energy transfer mechanisms. The choice of the three specific metal centers was guided by the desire to include lanthanoids with different magnitudes for the energy gaps between the main emitting levels and the corresponding next lower electronic state (Eu, large ΔE ≈ 12 300 cm−1; Sm, intermediate ΔE ≈ 7400 cm−1; Ho, small ΔE < ca. 3000 cm−1).9 The differences in ΔE are generally still considered to be primarily responsible for the extent of multiphonon relaxation according to the “energy gap law” (EGL)3 despite the severe breakdowns that have been discovered recently for near-IR emitting molecular complexes.6e,7d Synthesis of the Model Systems. The preparation of the isotopologic ligand with 13C/18O-labeled carboxylic groups started from readily accessible 6,6′-dibromo-2,2′-bipyridine (2)10 (Scheme 1). Lithium−halogen exchange and methylation with the electrophile [13C]-CH3I at low temperature resulted in the clean formation of [13C2]-6,6′-dimethyl-2,2′-bipyridine ([13C2]-3).10 This compound was then transformed into the dicarboxylic acid [13C2]-4 by chromium(VI)-mediated oxidation of the benzylic methyl groups.11 Acid-catalyzed 16O/18Oexchange to the final chelator [13C2,18O4]-4 was efficiently achieved using a mixture of H218O and a solution of anhydrous HCl (4 M in 1,4-dioxane). In addition, the isotopologue B


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Inorganic Chemistry 4.7c The isotopic enrichment of the complexes as expected from the labeling degree of the enriched ligands was confirmed by ESI-MS (see Figures S3−S8). The bulk composition and stability of the complexes in methanolic solution was established by elemental analysis, supported by NMR (Figure 3) and UV−vis measurements (see Figures S9−S11) of the

Figure 4. IR spectra in the region of the asymmetric and symmetric carboxylate stretching modes in the isotopologues of 1-RE (black, 1RE; red, [18O8]-1-RE; blue, [13C4,18O8]-1-RE). Top: Experimental IR transmission spectra (ATR; RE = Eu). Bottom: Calculated absorption spectra (DFT, B3LYP/LANL2DZ; RE = Y).

ν12̃ C16O = ν12̃ C18O Figure 3. 1H NMR (250 MHz, CD3OD) spectra of Sm-1, Eu-1, and Ho-1.

μ13C = 18O μ12C = 16O

μ12C16O

= 1.024 (3)

This trend is very well-reflected in the experimental IR spectra (Figure 4, top part) if one considers the barycenters of the multipeak bands. The observed effects are naturally much smaller than the one discussed for deuteration (eq 1: 36% for C−H oscillators); the size of the measured decrease in vibrational frequencies for the CO oscillators, however, makes it impossible to simply dismiss any impact of 13C/18O carboxylate labeling on the luminescence properties out of hand. In order to get further insight into the nature and frequencies of the vibrations involved, theoretical calculations were conducted. Gas-phase geometry optimization using DFT methods (B3LYP/LANL2DZ in Gaussian 03) yielded the ground state molecular structure of the corresponding yttrium complex 1-Y (Figure 5).12,14 This species expectedly features an almost ideal D2d-symmetric environment around the metal center, as had already been deduced from the experimental,

nonlabeled species 1-Ln. The 1H NMR data of the paramagnetic chelates are fully consistent with the formation of anionic complexes with 2:1 stoichiometry (ligand/metal), the exclusive formation of D2d-symmetric species, and the presence of one triethylammonium countercation. Concentrationdependent UV−vis absorption measurements in methanolic solution and in DMSO also showed that the complexes retain their structural integrity upon dilution from the concentrations used for NMR measurements (c ≈ 0.4 mM) down to the ones for typical luminescence measurements (c ≈ 10 μM) performed in this study (see Figure S12).12 Vibrational Properties. The vibrational characteristics of the isotopologic complexes were measured experimentally for the europium species using solid-state ATR-FTIR spectroscopy (Figure 4, top part). These spectra show multiple peaks between ca. 1700−1500 cm−1, which corresponds to the typical range for asymmetric carboxylate stretching modes, for 1-Eu (black) and [ 18O 8]-1-Eu (red), whereas the one for [13C4,18O8]-1-Eu (blue) shows only one major and relatively broad band. This behavior has been seen in similar instances upon isotopic substitution and is likely to be due to the modulation of accidental Fermi resonances.13 The region of the corresponding symmetric carboxylate stretching vibration (ca. 1460−1400 cm−1) only shows rather weak absorption or minor isotopic shifts and was not analyzed further. Assuming a local-mode, harmonic model for the diatomic carbonyl oscillators of 1-RE, the associated frequencies should decrease upon isotopic substitution by 2.4% for [18O8]-1-RE and by 4.9% for [13C4,18O8]-1-RE according to the differences in reduced masses μ (eqs 2 and 3). ν12̃ C = 16O = ν13̃ C = 18O

μ12C18O

Figure 5. Optimized molecular structure (DFT, B3LYP/LANL2DZ) of model complex 1-Y. Color scheme: H, light gray; C, gray; N, blue; O, red; Y, green.

= 1.049 (2) C


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Inorganic Chemistry solution-phase 1H NMR spectra (Figure 3). The calculation of the vibrational spectra (Figure 4, bottom) for the different isotopologues was performed on the same level of theory using the obtained ground state geometry and the associated Hessian matrix for 1-Y by variation of specific atomic masses according to the respective isotope positions. The calculated isotope effects almost perfectly matched the trends and barycenter frequencies seen in the experimental IR spectra (Figure 4). The vibrational data for the carbonyl region of the experimental and calculated IR spectra are summarized in Table 1. Table 1. Experimental and Calculated Vibrational Frequencies of Isotopologic 1-Ln in the Carbonyl Region (ca. 1500−1700 cm−1) expta [cm−1]

compd nonlabeled

1595 1624 1643 1570 1589 1619 1549

[18O8]

[13C4,18O8]

ν̃12C16O/ν̃xCyO (expt)b

ν̃12C16O/ν̃xCyO (calcd)

calcdc [cm−1]

1

1617

1

1.022

1594

1.014

1.048

1551

−1

Figure 7. Steady-state emission for 1-Sm (black) and [13C4,18O8]-1Sm (red) in CD3OD (c = 10 μM). Left: λexc = 311 nm, 2.0 nm bandwidth. Right: λexc = 316 nm, 6.0 nm bandwidth. Emission: long pass filter RG780.

1.043

ATR. ν̃ C O ≡ 1624 cm used for the calculation. DFT (B3LYP/ LANL2DZ); no scaling factors for the obtained wavenumbers were applied. a

b

12

16

c

Analysis of the calculated vibrations responsible for the observed carbonyl bands in the isotopologues in 1-Y revealed the main component to be an asymmetrically coupled motion of the two carbonyl oscillators (CO with the oxygen not bound to the metal center) located on the same bipyridine ligand (see Figure 6). There are also minor contributions from Figure 8. Steady-state emission for 1-Eu (black) and [13C4,18O8]-1-Eu (red) in CD3OD (c = 10 μM, λexc = 320 nm, 0.8 nm bandwidth).

(Eu, 5D0; Sm, 4G5/2) in these isotopologues were identical within the margin of error for each lanthanoid (Table 2, entries 1−4). Taken together, these results strongly suggest that isotopologic substitution in η1-bound carboxyl oscillators is generally very unlikely to show measurable effects on the luminescence efficiencies in lanthanoids with medium (Sm) or large (Eu) energy gaps ΔE.

Figure 6. Illustration of the molecular motion of the calculated (DFT, B3LYP/LANL2DZ) vibration at 1551 cm−1 in [13C4,18O8]-1-Y. Predominant carbonyl stretching character.

Table 2. Luminescence Data for the Isotopologues of Complexes 1-Ln in CD3OD (Sm, Eu) or DMSO-[D6] (Ho)

pyridine ring deformations and hydrogen wagging, but the overwhelming character of the oscillator is stemming from carbonyl stretching modes. Due to the η1-binding mode of the carboxylates in 1-Y, the carbonyl oscillators are located ca. 4.5 Å from the lanthanoid center (Ln-O). Photophysical Properties. Samarium/Europium. The photophysical properties of the isotopologues were investigated in CD3OD for the rather brightly emissive Sm/Eu-complexes. For the UV−vis absorption (see Figures S9−S10) and steadystate emission spectra (Figures 7 and 8) of the nonlabeled species 1-Ln (Ln = Sm, Eu) and the corresponding fully labeled [13C 4,18O8]-1-Ln, we obtained virtually superimposable features without any measurable differences in absorption/ emission intensities. In particular, even the hypersensitive transition 5D0 → 7F2 of europium is not affected at all. In addition to the steady-state spectra, the luminescence lifetimes

entry 1 2 3 4 5 6 7 8

compd 1-Eu [13C4,18O8]-1-Eu 1-Sm [13C4,18O8]-1-Sm 1-Ho [18O8]-1-Ho [13C4,18O8]-1-Ho [D12]-1-Ho

τobsa [ms]

I/I0e

b

1.83 1.84b 0.28c 0.28c d d d d

1e 1.07e 1.45e 4.07e

Estimated uncertainties: τ ± 5%; I/I0 ± 5%. bλexc = 320 nm, λem = 610 nm: 5D0 → 7F2. cλexc = 306 nm, λem = 644 nm: 4G5/2 → 6H9/2. d Intensity too weak to measure lifetimes. eλexc = 306 nm, I/I0: integrated intensities for the bands between 900 and 1100 nm relative to the intensity I0 of 1-Ho. a

D


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Inorganic Chemistry Holmium. The luminescence properties of the isotopic complexes of 1-Ho were measured in DMSO-[D6] (instead of CD3OD used for Sm and Eu) because holmium emission in solution is usually quite weak and its intensity greatly suffers from multiphonon quenching induced by the O−D oscillators of deuterated methanol in the second sphere around the complex. The steady-state near-IR emission spectra for the complexes (Figure 9) show multiple emission features. The

In order to get a better idea of the magnitude of the observed effects, we also measured steady-state spectra of the corresponding deuterated complex [D12]-1-Ho and compared its emission intensity quantitatively with the other complex isotopologues (Figure 10). Expectedly, removal of the 12 high-

Figure 9. Quantitative steady-state emission spectra for 1-Ho (black), [18O8]-1-Ho (red), and [13C4,18O8]-1-Ho (blue) in DMSO-[D6] (c = 10 μM, λexc = 306 nm, 24 nm bandwidth). *See text for assignment.

Figure 10. Quantitative steady-state emission spectra for [13C4,18O8]1-Ho (blue) and [D12]-1-Ho (green) in DMSO-[D6] (c = 10 μM, λexc = 306 nm, 24 nm bandwidth). Inset: Deconvolution of the bands at ca. 1000 nm for 1-Ho (one Gaussian) and [D12]-1-Ho (green shows cumulative fit function composed of two individual Gaussians in gray).

bands around λem = 1180 nm and λem = 1475 nm could unambiguously be assigned to the commonly observed transitions 5I6 → 5I8 and 5F5 → 5I6.2,9a The large band at λem = 974 nm would ordinarily be assigned as originating from the often observed transition 5F5 → 5I7, but in our case, there are other possibilities as well that have to be considered. We know from previous work7c that the ligand-centered triplet state of 1Y, which is assumed to transfer the excitation energy to the lanthanoid (“antenna effect”), is located at an energy of ca. 23 500 cm−1 which in principle is high enough to populate the holmium magnifold consisting of 3K8 (21 370 cm−1), 5F2 (21 100 cm−1), and 5F3 (20 600 cm−1).9a It is unclear, however, if very fast equilibration of these very close levels is really operative, which would render mainly the lowest one (5F3) as the emitting state. There are two arguments against this scenario. First, there is the spin-forbidden nature of a potential 3 K8−5Fx equilibrium (x = 2, 3). Second, with the assumption of the validity of the usual energies for the holmium states,9a any transition originating from the 5F3 state is relatively far off the observed emission wavelength (Figure 9, λem = 974 nm), whereas the transition 3K8 → 5I5 would ideally fit (λ = 976 nm) this criterion. For these reasons, we tentatively assign the observed band to the latter transition. The overall shapes of the bands are very similar for all isotopologues, but they do indeed exhibit a gradual increase in emission intensity of the 3K8 → 5I5 (Figure 9, λem = 974 nm) with falling vibrational energy of the carboxylate groups. Going from 1-Ho to [13C4,18O8]-1-Ho, the emission intensity increases by 7% for the 18O-labeled complex and by a remarkable 45% for the 13C/18O-carboxylates (Table 2). The other two bands are not affected to the same extent by isotopic substitution and within the margin of error retain their emission intensities throughout the isotopologic series. Luminescence lifetimes measurements to corroborate the findings were not possible with our experimental setup due to the low emission intensities.

energy, aromatic C−H oscillators leads to a substantial increase of the major band with an improvement factor of I/I0 = 4.07 (compared to 1-Ho). The two lower-energy bands are unaffected again as seen for the other isotopologues. The apparent shift of the major band from λem = 974 nm for 1-Ho to λem = 988 nm for [D12]-1-Ho can be taken as a clear indication of the presence of additional emission components in the latter compound. We therefore attempted spectral deconvolution of this band in [D12]-1-Ho by a fitting procedure using two Gaussian functions (Figure 10, inset). Without any constraints imposed, the fit naturally converged to a situation where one minor component (30% by integrated area) was located at exactly the wavelength measured for the transition 3K8 → 5I5 in the nondeuterated isotopologues (λem = 974 nm, see Figure 9). In addition, a new and comparatively larger contribution (70%) was determined to originate from a band at λem = 997 nm which was not observed in the C−Hcontaining complexes. For the assignment of this band, the most likely candidate is the transition (5F4/5S2) → 5I6 (Figure 11) with a calculated emission wavelength of λem = 1008 nm.9a This transition should indeed be greatly affected by the presence of C−H oscillators due to its relevant energy gap of ΔE = 3000 cm−1 (5F4/5S2−5F5)9a which should be in good resonance with the stretching modes of typical aromatic C−H modes with energies ν̃C−H ≈ 3050 cm−1. This phenomenon also sets this transition apart from the 3K8 → 5I5 band (with ΔE = 2870 cm−1 for the gap 3K8−5F4/5S2) and could consequently be responsible for the fact that the latter transition is indeed observed even for the nondeuterated complexes. A very crude estimation of the relative magnitudes of the effect of carboxylate 13C/18O-labeling versus C−H deuteration is possible by taking into account the inverse sixth-power dependence of the quenching ability of multiphonon relaxation and neglecting other factors (e.g., the differences in spectral overlap, etc.). Using the lanthanoid−oscillator distances obtained from the DFT calculation for 1-Y as a good model E


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Inorganic Chemistry

The main benefit of our model system, however, is not the possibility for luminescence efficiency gains but the potential to further study and modulate nonradiative deactivation and energy transfer processes in the future in photophysically complex and interesting lanthanoids such as holmium. Investigations in this direction are currently underway.

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Figure 11. Energy level diagram for Ho(III) with energies as reported by Carnall et al.9a with selected transitions (green) and energy gaps (red).

for the holmium complexes (Ln−H3 = 5.61 Å, Ln−H4 = 6.37 Å, Ln−H5 = 5.46 Å, Ln−Ocarbonyl = 4.49 Å), eq 4 gives the normalized (i.e., distance-independent) quenching power for three aromatic C−H oscillators relative to one carboxyl group as

(dLn − Ocarbonyl)−6 ∑i (dLn − Hi)−6

EXPERIMENTAL SECTION

General. The isotopically labeled starting materials [13C]-CH3I (Sigma-Aldrich, stabilized with copper, ≥99% 13C) and H218O (SigmaAldrich, >97% 18O) were used as received. Anhydrous hydrogen chloride (4 M in 1,4-dioxane) was purchased from Sigma-Aldrich. Solvents were dried by standard procedures (THF: Na/benzophenone; MeOH, Mg/I2, NEt3, and CH2Cl2: CaH2). Air-sensitive reactions were carried out under a dry, dioxygen-free atmosphere of N2 using Schlenk techniques. Column chromatography was performed with silica gel 60 (Merck KGaA, 0.063−0.200 mm). Analytical thin layer chromatography (TLC) was done on silica gel 60 F254 plates (Merck, coated on aluminum sheets). ESI mass spectrometry was measured using Bruker Daltonics Esquire6000. NMR spectra were measured on a Bruker DPX-250 (1H 250 MHz; 13C 62.9 MHz) and DPX-200 (1H 200 MHz). UV−vis absorption spectra were recorded on a Jasco-670 spectrophotometer using 1.0 cm quartz cuvettes. Luminescence Spectroscopy. Steady-state emission spectra at room temperature were acquired on a PTI Quantamaster QM4 spectrofluorimeter using 1.0 cm quartz cuvettes at RT (room temperature). The excitation light source was a 75 W continuous xenon short arc lamp. Emission was monitored at 90° using a PTI P1.7R detector module (Hamamatsu PMT R5509-72 with a Hamamatsu C9525 power supply operated at −1500 V and a Hamamatsu liquid N2 cooling unit C9940 set to −80 °C). Spectral selection was achieved by single grating monochromators (excitation, 1200 grooves/mm, blazed at 300 nm; visible emission, 1200 grooves/ mm, blazed at 400 nm). Luminescence lifetimes were determined with the same instrumental setup. The light source for these measurements was a xenon flash lamp (Hamamatsu L4633, 10 Hz repetition rate, pulse width ca. 1.5 μs fwhm). Lifetime data analysis (deconvolution, statistical parameters, etc.) was performed using the software package FeliX32 from PTI. Lifetimes were either determined by fitting the middle and tail portions of the decays or by deconvolution of the decay profiles with the instrument response function which was determined using a dilute aqueous dispersion of colloidal silica (Ludox AM-30). The estimated uncertainties in τobs are ±10%. All measured values are averages of three independent experiments. DFT Calculations. All gas-phase calculations were performed using the Gaussian 03W package15 on the corresponding yttrium complexes without any symmetry constraints. The molecular geometry optimization of the model system 1-Y in the ground state was achieved by DFT methods (B3LYP16 in conjunction with the LANL2DZ basis set17). The vibrational frequencies of the different isotopologues of 1-Y were calculated at the same level of theory using in each case the Hessian obtained from the geometry optimization of 1-Y and only varying the contents of the stable isotopes 13C and 18O. No scale factors were applied for the resulting frequencies. Synthesis. [18O4]-2,2′-Bipyridine-6,6′-dicarboxylic acid ([18O4]-4). In a thick-walled glass pressure tube (3 mL with Teflon screw), 47a (32.0 mg, 131 μmol, 1.0 equiv) was suspended in H218O (97% 18O, 221 mg, 11.2 mmol, 91 equiv), and 4 M HCl in dioxane (400 μL, 1.6 mmol, 13 equiv) was added. The vessel was sealed, and the suspension was heated with stirring under autogenous pressure at 100 °C (bath temperature) for 47 h. The suspension was cooled in an ice bath for 1 h, and the solid was collected on a Büchner funnel and dried in vacuo at 100 °C (bath temperature) for 1 h. The title compound was obtained as an off-white solid (24.3 mg, 78%, 93% 18O). 1 H NMR (200 MHz, DMSO-d6): δ = 8.90−8.64 (m, 2 H), 8.33− 8.04 (m, 4 H) ppm. MS (ESI+): m/z (%) = 244.77 (100, [M + Na]+, see Figure S1 in the Supporting Information for isotopic pattern). [13C2]-6,6′-Dimethyl-2,2′-bipyridine ([13C2]-3).10 Under N2, [13C]iodomethane (1.00 g, 7.0 mmol, 10.0 equiv) was rapidly18 added to a

= 1.44 (4)

With the inclusion of this geometric factor, the luminescence efficiency improvement for the deuteration of a hypothetical complex with the C(H/D) oscillators at the same distance as the 4 carboxylates in the isotopologues of 1-Y would increase from the measured factor of 4.07 (Table 2, entry 8) for [D12]1-Ho (relative to 1-Ho) to 5.86. Further taking into account the different numbers of oscillators (12 CH vs 4 CO), we arrive at the result that the deuteration of an aromatic CH oscillator has as a rule of thumb that there is the same effect on holmium luminescence improvement (for the transition around 980 nm) as the 13C/18O-isotopic substitution of two η1-bound carboxylates.

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CONCLUSION In summary, we have realized the first efficient and selective synthetic route to 13C/18O-labeled pyridine 2-carboxylate chelators and have shown that this modification is suitable for the rational decrease of the vibrational frequencies of the relevant carbonyl oscillators (of up to ca. 5%). This change in the vibrational properties does not seem to have any noticeable effect on the luminescence properties in lanthanoids such as Sm and Eu with relatively large energy gaps. In contrast to this, with the observation of a significant luminescence efficiency increase of the transition 3K8 → 5I5 in the corresponding nearIR luminescent holmium complexes, we have found the first example where isotopic substitution other than deuteration induces a detectable change in luminescence properties in a molecular lanthanoid compound. As a byproduct of this finding, we have obtained indications that aromatic C−H oscillators seem to be very efficient at shutting down any emission from the holmium 5F4/5S2 magnifold, and consequently, these transitions can only be observed after complete deuteration in our complexes. As a rule of thumb, it appears that isotopic 13C/18O substitution in our system is roughly half as effective for the purpose of luminescence enhancement as the deuteration of one aromatic local-mode C−H oscillator. F


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Inorganic Chemistry

7.8 Hz, 4 H), 7.01 (d, J = 7.6 Hz, 4 H), 3.07 (q, J = 7.3 Hz, 6 H), 1.21 (t, J = 7.3 Hz, 9 H) ppm. MS (ESI−): m/z (%) = 636.5 (100, [M]−). Anal. Calcd for C30H28EuN5O8·2H2O (Mr = 774.57): C, 46.52; H, 4.16; N, 9.04. Found: C, 46.61; H, 4.15; N, 8.96. [18O8]-1-Eu. 9.7 mg (64%) from 9.7 mg (38.5 μmol, 93% 18O) of [18O4]-4. MS (ESI−): m/z (%) = 650.7 (100, [M]−, see Figure S4 in the Supporting Information for isotopic pattern). [13C4,18O8]-1-Eu. 7.7 mg (68%) from 7.5 mg (29.5 μmol, > 85% 18 O, 99% 13C) of [13C2,18O4]-4. MS (ESI−): m/z (%) = 654.7 (100, [M]−, see Figure S5 in the Supporting Information for isotopic pattern). Holmium. 1-Ho. 266 mg (81%) from 199 mg (815 μmol) of 4. 1H NMR (250 MHz, CD3OD): δ = 13.6 (br, 4 H), 3.87 (br, 6 H), 2.03 (br, 9 H), −9.28 (br, 4 H), −39.1 (br, 4 H) ppm. MS (ESI−): m/z (%) = 648.7 (100, [M]−). Anal. Calcd for C30H28HoN5O10·3H2O (Mr = 805.55): C, 44.73; H, 4.25; N, 8.69. Found: C, 44.47; H, 4.19; N, 8.67. [18O8]-1-Ho. 3.7 mg (40%) from 5.7 mg (22.6 μmol, 93% 18O) of [18O4]-4. MS (ESI−): m/z (%) = 664.6 (100, [M]−, see Figure S6 in the Supporting Information for isotopic pattern). [13C4,18O8]-1-Ho. 3.6 mg (71%) from 3.1 mg (29.5 μmol, >85% 18O, 99% 13C) of [13C2,18O4]-4. MS (ESI−): m/z (%) = 668.6 (100, [M]−, see Figure S7 in the Supporting Information for isotopic pattern). [D12]-1-Ho. 3.6 mg (71%) from 3.1 mg (29.5 μmol, >99% D) of [D6]-4.7c MS (ESI−): m/z (%) = 660.7 (100, [M]−, see Figure S8 in the Supporting Information for isotopic pattern).

solution of 6,6′-dilithio-2,2′-bipyridine (0.7 mmol, 1.0 equiv., prepared from 0.22 g of 6,6′-dibromo-2,2′-bipyridine 2 and 0.9 mL of 1.6 M nBuLi in hexanes)10 in dry THF (25 mL) at −90 °C. The mixture was allowed to come to −30 °C over the course of 5 h. HCl (2 M, 15 mL) was cautiously added, THF was removed under reduced pressure, and the remaining aqueous phase was neutralized (pH 7) with sat. aqueous Na2CO3. The mixture was extracted with CHCl3 (3 × 20 mL), and the combined organic phases were dried (MgSO4) and concentrated. The crude material was subjected to column chromatography (SiO2, CH2Cl2/MeOH 100:1, detection UV) to yield the product as a beige solid (95 mg, 73%, >99% 13C in the benzylic position as determined by ESI+ mass spectrometry). 1 H NMR (200 MHz, CDCl3): δ = 8.19 (d, J = 7.7 Hz, 2 H), 7.68 (t, J = 7.7 Hz, 2 H), 7.15 (dd, J = 7.7, 1.9 Hz, 2 H), 2.63 (d, 1JC−H = 127 Hz, 6 H) ppm. TLC: Rf = 0.11 (CH2Cl2/MeOH 25:1, detection UV) [13C2]-2,2′-Bipyridine-6,6′-dicarboxylic acid ([13C2]-4).11 [13C2]6,6′-Dimethyl-2,2′-bipyridine ([13C2]-3) (90 mg, 0.48 mmol, 1.0 equiv) was added in portions to conc H2SO4 (2.2 mL), and the mixture was heated to 65 °C (bath temperature). CrO3 (0.22 g, 2.2 mmol, 4.6 equiv) was added at a rate such that the internal temperature did not rise above 70 °C. After complete addition, the dark solution was heated at 70 °C (bath temperature) for an additional 60 min. The mixture was poured onto crushed ice (ca. 4 g) and diluted to a volume of ca. 15 mL with water, and the green solution was stored at 4 °C overnight. The precipitate was collected on a filter, washed with ice-cold water, and dried under reduced pressure at 100 °C for several hours. The product [13C2]-4 was obtained as a light-yellow solid (44 mg, 37%, 99% 13C). 1 H NMR (200 MHz, DMSO-d6): δ = 8.88−8.64 (m, 2 H), 8.32− 8.05 (m, 4 H) ppm. 13C NMR (62.9 MHz, [D6]-DMSO): δ = 165.8 (99% 13C), 154.4 (d, J = 6.8 Hz), 147.8 (d, J = 19.6 Hz), 138.9 (d, J = 3.4 Hz), 125.1 (d, J = 6.7 Hz), 124.1 ppm. MS (ESI+): m/z (%) = 268.82 (100, [M + Na]+) [13C2,18O4]-2,2′-Bipyridine-6,6′-dicarboxylic acid ([13C2,18O4]-4). In a thick-walled glass pressure tube (3 mL with Teflon screw), [13C2]-4 (26.0 mg, 106 μmol, 1.0 equiv) was suspended in H218O (97% 18O, 375 mg, 18.7 mmol, 177 equiv), and 4 M HCl in dioxane (350 μL, 1.4 mmol, 13 equiv) was added. The vessel was sealed, and the suspension was heated with stirring under autogenous pressure at 100 °C (bath temperature) for 72 h. The suspension was cooled in an ice bath for 1 h, and the solid was collected on a Büchner funnel and dried in vacuo at 100 °C (bath temperature) for 1 h. The title compound was obtained as an off-white solid (18.5 mg, 69%, 99% 13C, >85% 18O, see Figure S2 in the Supporting Information for isotopic pattern). 1 H NMR (200 MHz, DMSO-d6): δ = 8.89−8.66 (m, 2 H), 8.31− 8.07 (m, 4 H) ppm. 13C NMR (62.9 MHz, [D6]-DMSO, only enriched positions): δ = 165.8 (99% 13C). MS (ESI+): m/z (%) = 276.95 (100, [M + Na]+) Synthesis of Ln Complexes. General Procedure. Compound 4, [18O4]-4, [D6]-4, or [13C2,18O4]-4 (2.0 equiv) was suspended in dry MeOH (10 mL per mmol of isotopologic 4), and a solution of LnCl3· 6H2O (>99.9% Ln, 1.0 equiv) in dry MeOH (10 mL per mmol of isotopologic 4) was added. The cloudy solution was stirred at ambient temperature for 1 min, and dry NEt3 (10 equiv) was added. Stirring was continued at room temperature for 20 min, and the precipitate was collected on a membrane filter (nylon, 0.45 μm) and washed with a minimum of dry, ice-cold MeOH. The lanthanoid complexes were obtained as colorless (Sm, Eu) or faintly pink (Ho) solids. Samarium. 1-Sm. 164 mg (69%) from 150 mg (614 μmol) of 4. 1 H NMR (250 MHz, CD3OD): δ = 8.30 (d, J = 7.6 Hz, 4 H), 8.10 (t, J = 7.8 Hz, 4 H), 7.87 (d, J = 7.9 Hz, 4 H), 3.16 (q, J = 7.3 Hz, 6 H), 1.28 (t, J = 7.6 Hz, 9 H) ppm. MS (ESI−): m/z (%) = 635.7 (100, [M]−). Anal. Calcd for C30H28SmN5O8·2H2O (Mr = 772.96): C, 46.62; H, 4.17; N, 9.06. Found: C, 47.00; H, 4.18; N, 9.04. [13C4,18O8]-1-Sm. 3.5 mg (64%) from 3.5 mg (13.8 μmol, >85% 18 O, 99% 13C) of [13C2,18O4]-4. MS (ESI−): m/z (%) = 655.6 (100, [M]−, see Figure S3 in the Supporting Information for isotopic pattern). Europium. 1-Eu. 234 mg (74%) from 199 mg (815 μmol) of 4. 1H NMR (250 MHz, CD3OD): δ = 9.72 (d, J = 8.0 Hz, 4 H), 8.83 (t, J =

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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.5b01920. Isotopic patterns for the ESI mass spectra of isotopically labeled ligands 4 and complexes 1-Ln, and UV−vis absorption spectra for isotopologues of complexes 1-Ln (PDF) Molecular geometry of 1-Y obtained by DFT calculations (PDB)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support is gratefully acknowledged from DFG (Emmy Noether and Heisenberg Fellowships for M.S., Research Grant SE 1448/6-1), International Isotope SocietyCentral European Division (predoctoral fellowship for J.W.).

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REFERENCES

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Inorganic Chemistry

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