Lanthanide Complexes with Photochromic Organoboron Ligand

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

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Lanthanide Complexes with Photochromic Organoboron Ligand: Synthesis and Luminescence Study Nan Wang,*,† Junwei Wang,† Dan Zhao,† Soren K. Mellerup,‡ Tai Peng,† Hongbo Wang,§ and Suning Wang†,‡

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†

Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China ‡ Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada § Key Laboratory of Optoelectronic Chemical Materials and Devices of Ministry of Education, School of Chemical and Environmental Engineering, Jianghan University, Wuhan, Hubei 430056, P. R. China S Supporting Information *

ABSTRACT: A novel, photochromic N^C-chelate organoboron functionalized dipicolinic acid (H2L) has been designed and synthesized. Lanthanide(III) complexes based on this ligand (L) with the general formula [NBu4]3[LnL3] (Ln = Eu or Tb) were prepared. The new ligand was found to be effective in both sensitizing and photomodulating the emission of a Eu(III) ion. The photoisomerization conversion of the boryl chromophore attached to the ligand of the lanthanide complex was determined to be quantitative by NMR analysis of the La(III) analogue.

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tives,18,20−23,26,27,29 and azobenzene/stilbene analogues.19,24,25 One example based on a diarylethene ligand reported by Kawai and co-workers is shown in Scheme 1A.27 Despite these new developments, bifunctional ligands, which both sensitize and photomodulate the luminescent properties of Ln(III) ions, remain relatively rare, compared to the plethora of single function antenna ligands explored to date. In 2008, we discovered that N^C-chelate four-coordinate organoboron compounds such as B(ppy)Mes2 (ppy = 2phenylpyridyl, Mes = mesityl), can undergo a highly efficient and thermally reversible photoisomerization upon excitation by light, with a distinct color change (colorless vs dark colored isomers).33 The color of the dark isomers and the isomerization quantum yields are highly tunable through proper modification of the chelation ligands or aryl substituents on boron.34−39 We further demonstrated that the photoisomerization of the organoboron-based system proceeds through a photoactive triplet state, allowing for precise tuning of the isomerization process using transition metals or triplet acceptors.38,40,41 On the basis of these discoveries, we initiated the investigation on photochromic organoboron-functionalized lanthanide complexes, which are currently unknown to the best of our knowledge. The Ln(III) metals used in this work are Tb(III) and Eu(III), which are the most widely studied Ln(III) ions that emit in the visible region. The general structure of the Ln(III)

INTRODUCTION Trivalent lanthanide complexes have remarkable spectroscopic features such as narrow emission bands, large Stokes shifts, long-lived excited states, and a wide variety of emission wavelengths from the visible to near-infrared (NIR) regions, which make them valuable materials for various applications.1−6 However, due to their symmetry-forbidden f−f transitions and shielding of the 4f orbitals, direct excitation of the Ln(III) ions is not efficient. As a consequence, indirect excitation, referred to as the “antenna effect” or sensitization, is often used to overcome this problem. In this process, light is harvested by the ligands bound to the lanthanide ion, and the energy obtained from this photoexcitation is transferred to the metal via the ligand’s triplet state. Considerable research effort has gone into developing effective antenna ligands. For example, triaryl boron functionalization, which is known to effectively enhance fluorescence or phosphorescence of organic molecules and transition metal complexes,7−13 also sensitizes lanthanide ion emissions.14−17 Recently, the rational design and synthesis of Ln(III) complexes containing photochromic units have attracted increasing research attention,18−29 as combining the desirable emission properties of the former with the photoswitchable nature of the latter may lead to promising materials with controllable luminescence. In fact, this strategy has been widely used in designing photochromic transition metal complexes.30−32 Lanthanide complexes containing photochromic ligands have been explored previously, which all employ wellknown photoactive units such as diarylethene deriva© XXXX American Chemical Society

Received: May 3, 2018

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DOI: 10.1021/acs.inorgchem.8b01209 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Scheme 1. Ln(III) Complexes Based on Photochromic Diarylethene Ligand Reported in the Literature (A) and Organoboron Ligand Reported in This Work (B)a

a

The counter cations are omitted for clarity.

Scheme 2. Synthetic Route of Ligand H2L and the Corresponding Ln(III) Complexes

with carboxylates or β-diketonate ligands,15,16 DPA functionalized Ln(III) compounds have little tendency to form oligomeric species and greatly decrease solvent molecule coordination, leading to increased solubility, stability, and emission efficiency of Ln(III) ions. The simple DPA ligands usually require high energy (λ ≤ 300 nm) to sensitize the

complex is depicted in Scheme 1B, in which the photochromic organoboron unit B(ppy)Mes2 is attached to the dipicolinic acid (DPA) unit via a triazole linker. This design is based on the considerations that the DPA ligand can form stable and coordinatively saturated Ln(III) complexes, in addition to sensitizing the Tb(III) or Eu(III) emission.14,42−47 Compared B


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Inorganic Chemistry Ln(III) ions. The organoboron unit functionalized new ligand can also shift the excitation energy to 300−400 nm, which is important for applications in biosensing and materials applications. The results of our investigation revealed that the new photochromic ligand can both effectively sensitize the Eu(III) emission, as well as control the characteristic lanthanide emission intensity of the complex by conversion between its two available states.

Table 1. Photophysical Properties of Compound H2L and Lanthanide Complexes compound H2L Eu1

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Tb1

RESULTS AND DISCUSSION Syntheses. The free ligand H2L was prepared using the procedures shown in Scheme 2. The 4-azido DPA derivative A and the boron unit B were synthesized in high yields following literature procedures.14,43 A “click” reaction was used to connect the photochromic boron unit B with the anchor DPA, producing the ester compound C in high yield (87%). Due to the competitive chelation effect of the DPA unit, utilizing simple CuSO4 and ascorbic acid did not give satisfactory yield in this reaction. Instead, [Cu(CH3CN)4]PF6 along with stabilizing ligand tris[(1-benzyl-1H-1,2,3triazol-4-yl)methyl]amine (TBTA) was used as the catalyst and diisopropylethylamine (DIPEA) was used as the base. The subsequent saponification of compound C provided the desired ligand H2L in high yield. The structure of H2L was determined by X-ray diffraction analysis, and the data are provided in the Supporting Information. As shown in Figure 1, the triazolyl ring is nearly coplanar with the DPA unit (dihedral angle ≈ 7.3(7)°).

λabsa (nm) (ε, M−1 cm−1) 288 (28100), 353 (sh, 13400) 289 (78200), 345 (sh, 43800) 288 (83600), 346 (sh, 47500)

λema (nm)

τb (ms)

c QLLn/QLn Ln

1.17

0.13/0.35

523 525, 594, 615, 652, 696 515

In methanol at 3.0 × 10−6 M. bDetermined for the Ln(III) emission peak under N2 in methanol solvent. cQLLn and QLn Ln are the ligandsensitized and intrinsic luminescence quantum yield of Ln(III) measured in acetonitrile with λex = 365 nm by integration sphere at room temperature. a

Figure 1. Crystal structure of ligand H2L with 35% thermal ellipsoids. Figure 2. Absorption (top) and emission (bottom) spectra of the free ligand H2L and Ln(III) complexes in degassed methanol solution.

The Ln(III) complexes were prepared according to a literature method.14 The free ligand H2L was first reacted with [NBu4][OH] in water, after which Eu(NO3)3·6H2O or Tb(NO3)3·6H2O salts were added in a 3:1 ratio and the Ln(III) complexes gradually precipitated out of solution as they formed. The final products were isolated in 65−70% yield. Efforts to grow single crystals suitable for X-ray diffraction analysis were unsuccessful. Absorption and Luminescent Spectra. The absorption spectra of the free ligand and lanthanide complexes were studied in degassed methanol at room temperature, and the data are summarized in Table 1. As shown in Figure 2, the absorption spectra of the complexes resemble the free ligand with λmax at 288 nm as well as a low energy shoulder band at 345 nm. Compared to the nonfunctionalized [Ln(DPA)3]3− complexes48 and triarylboron functionalized [Eu(DPA)3]3−

analogues,14 the absorption bands of the new lanthanide complexes [LnL3]3− are red-shifted by 30 and 45 nm, respectively, which is caused mainly by the extended πconjugation and the chelate boron unit. Additionally, the chelate boron containing ligand has a large molar extinction coefficient, similar to the triarylboron functionalized DPA ligand,14 which in turn allows the lanthanide complexes to absorb strongly in the UV region (288−360 nm). To further understand the electronic properties of the free ligand and complexes, TD-DFT calculations were performed based on the optimized ground-state geometries of the free ligand and its sodium complex Na2L. As shown in Figure 3, the HOMO of H2L mainly resides on the mesityl of the C


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Inorganic Chemistry

the energy gap between the energy transferring state (usually T1 state of the ligand) and the accepting state of the Ln center (Tb(III) (5D4, 20 500 cm−1) and Eu(III) (5D0, 17 250 cm−1)) needs to be ∼3000 cm−1.49,50 Thus, ligand H2L is expected to only sensitize Eu(III) emission. The room temperature emission spectra were recorded for both Eu1 and Tb1 in degassed methanol solutions (Figure 2), which match well with the TD-DFT predicted outcome. Ligand L2− was found to effectively sensitize Eu(III) emission as evidenced by the characteristic Eu(III) emission bands at 594, 615, 651, and 695 nm corresponding to 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3, and 5 D0 → 7F4 transitions, respectively, with a minimal ligand contribution. The ligand-sensitized luminescence quantum yield of Eu1 is 0.13. In contrast, the emission spectrum of Tb1 is almost entirely from the ligand, which is further confirmed by the nanosecond decay lifetime, excluding the possibility of Tb(III) ion emission. It is well-known that the charge transfer transition energy can be tuned by temperature.51 Furthermore, intraligand CT states have been demonstrated to be a possible mechanism to sensitize lanthanides.50 Consequently, it is highly possible that, with decreasing temperature, ligand L2− may be able to activate the Tb(III) ion emission. To verify this hypothesis, temperature-dependent emission measurements were performed. As shown in Figure 4, the characteristic Tb(III) ion emission profile becomes gradually visible as the temperature drops

Figure 3. Key contributions to S0 → S1, S0 → S2, and S0 → S3 transitions of the ligand and sodium complex obtained from TD-DFT data (MOs are plotted with an isocontour value of 0.03).

B(ppy)Mes2 chromophore, while the LUMO has a large contribution from the triazolyl-DPA π* orbitals. The vertical excitation to S1 displays appreciable oscillator strength of 0.0434 (HOMO →LUMO, 90%) and is believed to be responsible for the absorption band at 345 nm. The vertical excitations to S2 and S3 are similar, which are believed to be responsible for the absorption at 288 nm. Unlike the free ligand, the LUMO orbital of Na2L mainly resides on the conjugated triazolyl phenylpyridyl backbone. Contribution of the DPA unit to the LUMO energy level is significantly reduced, and the HOMO orbital is dominated by both mesityl π-character and the phenylpyridine C−B σ-bond. The S0 → S1 excitation is almost entirely HOMO → LUMO in nature, with significant oscillator strength, which could be described as mesityl → conjugated π-backbone charge transfer transitions. The profiles of the calculated absorption spectra match well with the experimental results (see Figures S2 and S3). The triplet energy of the ligand is predicted using the corresponding TD-DFT calculation results of Na2L, which gives an energy level of ∼480 nm/20 800 cm−1. It is known that, in order to effectively activate the Tb(III)/Eu(III) ions,

Figure 4. Emission spectral changes of Tb1 (top) and Eu1 (bottom) with temperature in methanol recorded under nitrogen (λex = 365 nm). D


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Inorganic Chemistry

lanthanide is a key requirement to make the complexes’ emission sensitive to viscosity.54 In this study, the viscosity of the testing solution was tuned by changing the volume ratio between methanol and glycerol solvents. With increasing viscosity of the solution, the emission intensity of the complex gradually increased (see Table S4). However, the emission intensity ratio between metal center and the ligand as well as the decay lifetimes remain consistent, indicating that there is no pronounced BET effect in Eu1 emission. The temperature-dependent decay lifetimes of Eu1 and Tb1 were also examined under a nitrogen atmosphere, and the results are shown in Figure 6. It is clear that, within the given

below 220 K, reaching its maximum intensity at 180 K (the freezing point of methanol). In the case of Eu1, the luminescence profile does not significantly change with variable temperature, and can even be observed at 338 K. To further explore the influence of temperature on the [LnL3]3− emission, changes with temperature in the emission intensity ratios between ligand based fluorescence and metal ion emission (Iligand/ILn), and lanthanide emission itself (ILn/ ILn‑maxium) are plotted. As shown in Figure 5, in the range of

Figure 6. Decay lifetime change of Eu1 and Tb1 with temperature in methanol recorded under nitrogen.

temperature range, the decay lifetimes of both complexes decrease with increasing temperature, but at different rates. In the case of Eu1, the decay lifetime changes from 1538(1) μs at 180 K to 681(1) μs at 338 K, at an averaged rate of 5 μs/°C. For Tb1, the averaged rate of decay lifetime decrease is 17 μs/ °C. This observation is also consistent with the above emission intensity change. Photoisomerization Study. To investigate the optical modulation on the [LnL3]3− complexes, the photochromic activity of ligand H2L was first examined. To exclude the effect of hydrogen bonding on the photochromic properties, compound C (ester of H2L) was employed as the model compound. In degassed acetonitrile solution, compound C undergoes photoisomerization upon irradiation at 365 nm, similar to the parent compound B(ppy)Mes2,33 forming isomer compound C′. This process was monitored by NMR, UV−vis, and fluorescence tracking. As shown in Figure 7, the UV−vis spectrum of compound C shows the typical “dark isomer” absorption band at λmax ≈ 600 nm, along with the luminescence quenching at 530 nm. The photoisomerization quantum yield of compound C at 298 K was determined to be 27% with excitation at 365 nm. The lanthanide complexes also display spectral changes upon UV irradiation, similar to those observed for compound C, producing the corresponding dark isomers. Considering the fact that, at room temperature, ligand H2L can only sensitize the Eu(III) ion emission, we focused the study on the Eu1 complex. Upon 365 nm irradiation, both the ligand based and Eu(III) emission decreased drastically as shown in Figure 8B. The luminescence intensity of Eu1 at 615 nm was quenched almost completely and the emission quantum yield decreased from 12% to 0.1% within 120 s (Figure 8C). This drastic

Figure 5. Plots of emission intensity ratio (I/I0) of the lanthanide ion emission peak (615 nm for Eu1 and 545 nm for Tb1) versus T, and relative intensity ratio of ligand emission (525 nm for Eu1 and 515 nm for Tb1) to the Ln(III) ion emission (615 nm for Eu1 and 545 nm for Tb1) versus T.

180−240 K where the ligand can sensitize both of the Ln ions, Eu1 is less sensitive toward temperature alteration, while the Tb(III) ion emission decreases drastically with increasing temperature. These observations indicate that the ligand to Tb(III) energy transfer plays a key role in Tb1 emission. Due to the small energy gap between the CT state of the ligand and the Tb(III) accepting state, temperature can effectively adjust the efficiency of the Tb(III) ion emission through CT state tuning. It should be noted here that, during this process, the back energy transfer (BET) mechanism cannot be ruled out.52,53 In the case of Eu1, the large energy gap significantly weakens the influence of temperature, making thermal quenching a possible mechanism for emission intensity loss, which is consistent with our previous study.53 Additionally, the BET effect is also studied in Eu1 emission by viscositydependent emission measurements, since Bui and co-workers have reported that the reversibility of energy transfer to the E


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Inorganic Chemistry

of La1 to La1′. However, this conversion is not fully thermally reversible. After heating at 353 K for 2 h, some signals corresponding to impurities appear, which is consistent with the fatigue resistance study of the Eu1 film. It has been demonstrated that, in transition metal41 or organic moiety linked π-conjugated polyboryl compounds,55 only one boron chromophore undergoes isomerization due to the low lying energy state of the dark isomer which quenches any further isomerization of the molecule. Surprisingly, based on NMR integration analysis, the isomerization conversion of the ligands in La1 is nearly 100%, indicating that all three photochromic units are capable of converting to their dark isomers. This result may be explained from the viewpoint of the interaction between the lanthanide ion and ligands. Unlike transition metal or organic moiety linked π-conjugated polyboryl compounds which are connected by covalent bonds, the bonding between the lanthanide ion and boron containing DPA ligands is more ionic in nature, which may better insulate the individual boron units from one another and thus allow complete conversion of each ligand to their dark form. The isomerization properties of La1 and Tb1 were also studied by UV−vis and fluorescence spectroscopy, and the results are given in Figures S7 and S8. Both of the complexes display similar spectral changes upon UV irradiation. Since there is no characteristic Ln(III) emission for these two complexes, only the ligand based emission quenching along with the corresponding low energy absorption band enhancement are observed. Under the same conditions, all the Ln(III) complexes show similar isomerization quantum efficiencies which were determined to be 24%, 19%, and 16% for Eu1, La1, and Tb1, respectively.

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CONCLUSIONS A novel organoboron based photochromic ligand H2L and its corresponding Ln(III) complexes (Eu1, Tb1, and La1) have been accomplished. The photophysical studies demonstrate that, at room temperature, this ligand can effectively sensitize the Eu(III) emission. By decreasing the temperature, the CT state of the ligand can be tuned, leading to the sensitization of Tb(III) emission at low temperature. The photochromic behavior of the complexes has been unambiguously established by NMR, absorption, and emission spectroscopies under UV irradiation. The luminescence of the Eu(III) emission can be efficiently photomodulated. And the isomerization conversion is determined to be 100% through NMR analysis, which may be due to the ionic nature of the bond between the lanthanide ion and the boron containing DPA ligands.

Figure 7. Absorption (top) and emission (bottom) spectral changes of compound C in degassed CH3CN (8.8 × 10−6 M) upon 365 nm irradiation.

change in intensity can be explained by the efficient energy transfer to the dark isomer once it is formed. As shown in Figure 8D, when the dark isomer is generated, the energy of the antenna ligands at the excited state will be transferred to the low T1 energy of the dark isomer, and thus shut down the sensitization of the Eu(III) ion through this alternative energy transfer pathway. The data of luminescence intensity versus time are fitted according to an exponential relationship, which agrees well with previous reports.20,22 The reversibility of this modulating effect was also studied. After heating the solution at 80 °C for 1 h, the absorption and emission spectra revert back to their initial state of Eu1′. Considering that the isomerization behavior of this type of compound is retained in the solid state,33 the fatigue resistance of Eu1 was examined by monitoring the absorption spectral changes of its corresponding PMMA doped film (10%, w/w) with repeated Eu1 → Eu1′ → Eu1 transformation cycles. As shown in Figure S5, after the 7th cycle, there is a visible loss of intensity, indicating some decomposition of Eu1 after repeated photothermal switching. In order to monitor this photoisomerization process and investigate the isomerization conversion ratio by NMR spectroscopy, the analogue compound La1 was synthesized. As shown in Figure 9, upon UV irradiation, characteristic chemical shifts of the pyridyl and mesityl protons emerge in the 1H NMR spectrum and are in good agreement with the previously reported “dark isomer”, confirming the conversion

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EXPERIMENTAL SECTION

General Information. Starting materials were purchased from either Energy Chemical Co. or J&K Scientific Ltd. and used as received. All reactions were performed under dry N2 with standard Schlenk techniques unless otherwise noted. Solvents were freshly distilled over appropriate drying reagents using standard procedures prior to use. NMR spectra were recorded on Bruker Advance 400 or 700 MHz spectrometers, and deuterated solvents (Cambridge Isotopes) were used as received. High-resolution mass spectra (HRMS) were obtained from an Agilent Q-TOF 6520 LC-MS spectrometer, and methanol was used for sample testing. UV−vis Spectra were recorded on an Agilent Cary 300 Scan UV−vis absorption spectrophotometer. Emission spectra and phosphorescent decay lifetimes were obtained on an Edinburgh Instruments FLS980 spectrophotometer that was equipped with a xenon flash lamp. All solutions for photophysical data measurements were degassed under F


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Inorganic Chemistry

Figure 8. (A) Absorption, (B) emission spectral, and (C) emission intensity changes at 615 nm of Eu1 in degassed CH3CN upon 365 nm irradiation. (D) A diagram showing the relative energy levels of the photoactive triplet state (Tp) on the ligand and Eu(III) ion before and after irradiation. intensity of the corrected Eu(III) emission spectrum to the integrated intensity of the magnetic dipole 5D0 → 7F1 transition. Eu Q Eu = τobs/τR

ij I yz 1 = AMD,0 × n3 × jjj tot zzz j IMD z τR k {

(1)

(2)

Synthesis of Ester C. A mixture of 4-azidopyridine-2,6dicarboxylate (A, 243 mg, 1.0 mmol, 1.1 equiv), compound B (400 mg, 0.9 mmol, 1.0 equiv), diisopropylethylamine (DIPEA, 0.3 mL), tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine (TBTA, 15 mg, 0.028 mmol, 0.03 equiv), and [Cu(CH3CN)4][PF6] (9 mg, 0.028 mmol, 0.03 equiv) was dissolved in 10 mL of dichloromethane. The mixture was stirred in the dark at room temperature for 18 h. Then NH4Cl (aq) and water were used to wash the reaction mixture. The organic layer was dried over MgSO4 and further purified by column chromatography (20:1 CH2Cl2/ethyl acetate) to obtain compound C as a pale yellow powder (yield 87%). 1H NMR (400 MHz, CDCl3, ppm): δ = 9.13 (d, J = 1.1 Hz, 1H), 8.67 (s, 2H), 8.42 (dd, J = 8.6, 1.6 Hz, 1H), 8.29 (s, 1H), 8.02 (d, J = 8.4 Hz, 1H), 7.81 (d, J = 7.6 Hz, 1H), 7.75 (d, J = 7.6 Hz, 1H), 7.27 (t, J = 6.8 Hz, 1H), 7.21 (t, J = 7.2 Hz, 1H), 6.60 (s, 4H), 4.02 (s, 6H), 2.10 (s, 6H), 1.76 (s, 12H); 13 C{1H} NMR (175 MHz, CDCl3, ppm): δ = 164.06, 159.26, 150.65, 145.24, 145.06, 144.87, 143.47, 140.04, 137.51, 134.36, 134.06, 131.87, 131.40, 129.94, 125.54, 123.72, 121.76, 118.15, 118.04, 117.61, 117.51, 53.73, 25.09, 20.71. HRMS calcd for C40H39BN5O4 (M + H+) 664.3090; found, 664.3100. Synthesis of H2L. Compound C (540 mg, 0.81 mmol, 1.0 equiv) and NaOH (652 mg, 16.3 mmol, 20.0 equiv) were added to 8.0 mL of THF/water (1:1) mixed solvent and stirred at 50 °C for 2 h. After cooled down to room temperature, the pH of the solution was adjusted to 5 by dropwise addition of HCl (1 M). The product was precipitated out, which was subsequently filtered off and dried under

Figure 9. 1H NMR spectral changes of La1 in MeOD upon irradiation by UV light (365 nm) and after heating at 353 K for 2 h. The representative signals of dark isomer La1′ are highlighted in yellow.

N2 prior to use unless otherwise noted. The absolute luminescence quantum yields were determined on a Hamamatsu QY C11347-11 spectrometer. Precursor compounds A43 and B55 were synthesized following literature procedures. Photoisomerization quantum yield was determined according to the literature method.56 The intrinsic quantum yield of Eu1 (QEu Eu)was calculated using the following equations according to the literature methods.57 Herein, τobs is the observed luminescence decay. And the radiative lifetime (τR) is calculated by using eq 2, where n is the refractive index, AMD,0 is the spontaneous emission probability for the 5D0 → 7F1 transition in vacuo (14.65 s−1), and Itot/IMD is the ratio of the total integrated G


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Inorganic Chemistry vacuum. Yield: 494 mg (95%). 1H NMR (400 MHz, MeOD, ppm): δ = 9.40 (s, 1H), 9.32 (s, 1H), 8.85 (s, 2H), 8.61 (dd, J = 8.6, 1.7 Hz, 1H), 8.36 (d, J = 8.6 Hz, 1H), 8.05 (m, 1H), 7.72 (m, 1H), 7.30 (m, 2H), 6.60 (s, 4H), 2.13 (s, 6H), 1.81 (s, 12H). Due to the poor solubility, the 13C NMR spectrum was not obtained for this compound. HRMS calcd for C38H35BN5O4 (M + H+):636.2782; found, 636.2777. Synthesis of Ln(III) Complexes. All Ln(III) complexes were prepared according to a literature procedure14 as described below. Free ligand H2L was suspended in 15 mL of water. Then [NBu4]OH (40 wt % in MeOH) was added dropwise and the resulting mixture was stirred in the dark for 30 min. Then Ln(NO3)3· 6H2O was added into the solution, and the resulting mixture was stirred for 2 h in the dark at room temperature. The crude product was precipitated gradually, which was subsequently filtered off and rinsed with water completely to remove impurities. The resulting pure compound was obtained as a pale yellow powder and further dried under vacuum. [NBu4]3[EuL3] (Eu1). H2L (101 mg, 0.16 mmol, 3.05 equiv), Eu(NO3)3·6H2O (23 mg, 0.05 mmol, 1 equiv), and NBu4OH (40 wt % in methanol, 0.15 mL, 6 equiv) were reacted following the procedure described above to synthesize Eu1. Yield: 97 mg (70%). HRMS: calcd for C130H132N16O12B3Eu1 (M − 2NBu4+), 1147.4845; found, 1147.4863. [NBu4]3[TbL3] (Tb1). H2L (102 mg, 0.16 mmol, 3.05 equiv), Tb(NO3)3·6H2O (24 mg, 0.05 mmol, 1 equiv), and NBu4OH (40 wt % in methanol, 0.15 mL, 6 equiv) were reacted following the procedure described above to synthesize Tb1. Yield: 90 mg (65%). HRMS: calcd for C130H132N16O12B3Tb1 (M − 2NBu4+), 1150.4872; found, 1150.4880. [NBu4]3[LaL3] (La1). H2L (92 mg, 0.15 mmol, 3.05 equiv), La(NO3)3·6H2O (20 mg, 0.05 mmol, 1 equiv), and NBu4OH (40 wt % in methanol, 0.15 mL, 6 equiv) were reacted following the procedure described above to synthesize La1. Yield: 94 mg (68%). 1H NMR (400 MHz, MeOD, ppm): δ = 9.26 (br, s, 3H), 9.23 (br, s, 3H), 8.54 (s, 9H), 8.25 (d, J = 7.9 Hz, 3H), 7.94 (br, s, 3H), 7.61 (br, s, 3H), 7.19 (br, s, 6H), 6.51 (s, 12H), 3.12 (br, s, 24H), 2.03 (s, 18H), 1.72 (s, 36H), 1.55 (br, s, 24H), 1.30 (br, s, 24H), 0.90 (br, s, 36H). HRMS: calcd for C162H205N18O12B3La1 (M + H), 2766.5327; found, 2766.5378. X-ray Crystallographic Analysis. By slow solvent evaporation at room temperature, single crystals of ligand H2L suitable for X-ray diffraction analysis were obtained from methanol solution and mounted on glass fibers. A Bruker D8-Venture diffractometer with Mo target (λ = 0.71073 Å) was used to collected diffraction data, which were processed on a PC with the aid of the Bruker SHELXTL software package58 and corrected for absorption effects. All nonhydrogen atoms were refined anisotropically. The crystal structural data of H2L are given in the Supporting Information and deposited to the Cambridge Crystallographic Data Center [CCDC No. 1829934]. Theoretical Calculations. The computational calculations were performed with the Gaussian 0959 program package. The ground-state geometries were fully optimized at the B3LYP60−62 level by using the 6-31G(d) basis set.63,64 The initial geometric parameters of ligand H2L were employed from crystal-structure data and used for geometry optimizations. The vertical singlet and triplet excitation energies were obtained based on the time-dependent density functional theory (TDDFT) calculations.

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Accession Codes

CCDC 1829934 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nan Wang: 0000-0001-5973-4496 Author Contributions

All authors have approved the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (21501011, 21571017, 21701011). N.W. is thankful for the financial support from the Opening Project of Key Laboratory of Optoelectronic Chemical Materials and Devices (Jianghan University), Ministry of Education (JDGD201504).

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REFERENCES

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Inorganic Chemistry

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Lanthanide Complexes with Photochromic Organoboron Ligand

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