A Versatile Tripodal Ligand for Sensitizing Lanthanide (Ln^III) Ions and Color Tuning

Abstract

Lanthanide (Ln^III) ions were successfully chelated and sensitized with a tripodal ligand. The absolute Ln^III-centered emission efficiencies were ~3% for both the europium(III) (Eu^III) and terbium (Tb^III) complexes and up to 54% for the cerium(III) (Ce^III) complex. The differences in emission quantum yields for the early lanthanides (Ce^III) and the mid lanthanides (Eu^III and Tb^III) were attributed to their d–f and f–f nature, respectively. Despite the low quantum yield of the Eu^III complex, the combination of the residual ligand fluorescence and the red Eu^III emission resulted in a bluish-white material with the Commission Internationale de l’Eclairage (CIE) coordinates (0.258, 0.242). Thus, metal complexes of the ligand could be used in the generation of single-component white-light-emitting materials.

1. Introduction

The unique properties of trivalent lanthanide, Ln^III, ions have resulted in their applications in imaging, sensing, and telecommunications [1,2,3,4,5]. The emission spectra of Ln^III ions are sharp as the 4f orbitals of Ln^III ions are shielded from the external environment by the filled 5s and 5p orbitals [4]. These emission spectra arise from f–f transitions; thus, they are parity forbidden by the Laporte rule [4]. Consequently, Ln^III ions have long emission lifetimes which makes them ideal probes for imaging of biological tissue, as their long emission lifetimes can be discriminated from the autofluorescence of biological tissue [4]. Another consequence of the parity-forbidden nature of their emission is that Ln^III ions have low molar absorptivities (ε = 1–10 M⁻¹cm⁻¹); thus, their direct excitation is inefficient [4,6]. Therefore, Ln^III ions are often coordinated to organic ligands with well-matched triplet, ³T, excited energy levels that upon excitation can transfer the energy from their excited energy levels (singlet, ¹S, and triplet, ³T) to the excited emissive energy levels of the coordinated Ln^III ions [4,7,8,9,10,11,12]. As a result, extensive research has been devoted to the design and synthesis of organic ligands to sensitize the luminescence of Ln^III ions [11,13,14,15,16].

However, most efforts are focused on monodentate ligands and often require several synthetic steps that are tedious and reagent consuming. Polybenzimidazole ligands are a less explored class of chelating ligands for sensitizing Ln^III ions due to their limited solubility [15,17,18,19,20,21,22,23,24,25]. This class of ligands provides an opportunity for easy functionalization to yield versatile ligands with improved solubility. Therefore, there remains a need for multifunctional ligands with well-matched ³T excited states for Ln^III sensitization that can be synthesized in a few steps and are soluble in a variety of solvents. Efficient sensitization of Ln^III ions can be achieved by using ligands with high molar extinction coefficients and significant spectral overlap between the emission of the ligand and the absorption of the Ln^III ion [9]. In this work, the efficiency of energy transfer from a triethyl 2,2′,2″(2,2′,2″nitrilotris(methylene)tris(1H-benzimidazole-2,1-diyl)) triethanoate (BimOEt₃) ligand to Ln^III ions was investigated using Ln^III-complexes of the tripodal ligand. The tripodal BimOEt₃ ligand was chosen due to its ease of synthesis, high molar extinction coefficient, and high solubility in protic solvents [20,21]. The photophysical properties, along with the emission lifetimes and quantum yields of the three molecular complexes, are discussed.

2. Results and Discussion

2.1. Synthesis and Characterization of BimOEt₃

The BimOEt₃ ligand was synthesized following literature reports [20,21,26,27] and the successful synthesis was confirmed by a combination of ¹H and ¹³C NMR spectroscopy (Figures S1–S3). Investigation of the photophysics of BimOEt₃ in ethanol via UV–Vis absorption and fluorescence spectroscopy showed three absorption bands (253, 277, and 285 nm) and emission maxima at 322 nm that were attributed to singlet ¹π→¹π* and ¹π*→¹π transitions of the benzimidazole groups, respectively (Figure 1) [28]. The emission spectrum of BimOEt₃ ranges from 300 to 500 nm with a maximum at 322 nm when excited at 280 nm (Figure 1). The measured absolute fluorescence quantum yield, φ_L–Ln, and the lifetime, τ, of BimOEt₃ (ethanol, 298 K) were 8.1 ± 2.0% and 1.1 ± 0.1 ns, respectively.

2.2. Photophysical Properties of the Ln^III-BimOEt₃ Complex

Ln^III complexes were prepared by mixing 1:2 Ln^III-to-BimOEt₃ stoichiometric amounts in ethanol/acetonitrile solutions due to the tetradentate nature of BimOEt₃ and the photophysics of the resulting complexes investigated (details in Supporting Information). The coordination of the metal ions was determined through infrared spectroscopy. The infrared spectra showed that the carbonyl C=O stretch shifted from 1736 cm⁻¹ to 1745 cm⁻¹ following complexation of Eu^III and Tb^III ions by the ligand (Figure 2) [1,2,3,4,13,15,19,29,30]. In contrast, the C=O stretch of the Ce^III complex remained at 1736 cm⁻¹. This was initially surprising and prompted further investigation. Thus, as a proof-of-concept, single crystals of the Ce^III complex were grown by slow diffusion in an ethanol/acetonitrile/ether solvent mixture and analyzed via single-crystal X-ray diffraction. The solution to the structure revealed that the Ce^III ions were coordinated to the ligand via the central nitrogen and the imidazole nitrogen atoms of BimOEt₃ (Figure S4). This finding suggested that BimOEt₃ can selectively bind early (Ce^III) and mid (Eu^III and Tb^III) Ln^III ions via its nitrogen and carbonyl groups, respectively. However, further studies by a combination of ¹H and ¹³C NMR spectral analysis of the metal complexes indicated that the Eu^III complex is unstable in solution since the proton resonances of the Eu^III complex are not shifted compared to the free ligand (Figures S1–S3). In contrast, the yttrium(III) (Y^III) and Ce^III complexes were stable in solution as suggested by the shifts in the proton resonances of the central methylene groups. The three absorption transitions of BimOEt₃ remain at 253, 276, and 284 nm but with subtle changes in their intensities (Figure 1 and Figure S5).

Low-temperature measurements using the analogous Gd^III-BimOEt₃ complex indicated that the excited singlet, ¹S, and triplet, ³T, energy levels of BimOEt₃ were 27,800 and 24,500 cm⁻¹, respectively (

Table 1. The excited singlet (¹S) and triplet (³T) energy levels, emission quantum yields (φ_L–Ln), intrinsic quantum yields (φ_Ln–Ln), and lifetimes (τ) of the 1:2 lanthanide (Ln^III)-to-ligand complexes in air-saturated ethanol at 298 K.

Complex	1S (cm−1)	3T (cm−1)	φL–Ln (%)	φLn–Ln (%)	τ (ms)
BimOEt3			7.5 ± 1.7		(1.06 ± 0.06) × 10−6
GdIII	27,800 ± 500	24,500 ± 500	20.0 ± 1.8		(5.38 ± 0.07) × 10−6
EuIII			3.2 ± 0.1	6.0 ± 0.9	0.08 ± 0.01 0.37 ± 0.02
TbIII			2.8 ± 0.1	4.4 ± 1.0	0.77 ± 0.01 2.57 ± 0.02
CeIII			17.6 ± 3.0	53.6 ± 1.1	(5.50 ± 0.02) × 10−5

BimOEt₃ is triethyl 2,2′,2″(2,2′,2″nitrilotris(methylene)tris(1H-benzimidazole-2,1-diyl)) triethanoate.

and Figure 6a,b). Thus, the excited ³T energy level of BimOEt₃ is well positioned to transfer energy to the ⁵D₀ and ⁵D₄ emitting levels of Eu^III and Tb^III ions, respectively [9,31,32]. The excitation of the Eu^III and Tb^III complexes at 280 nm resulted in typical red ⁵D₀→⁷F_J (0–4) and green ⁵D₄→⁷F_J (6–3) transitions of Eu^III and Tb^III, respectively (Figure 3a,b). However, a broad emission band in the range 350–600 nm was observed in both complexes but was more intense in the Eu^III complex. This residual ligand emission suggests inefficient energy transfer from the ligand to the metal ions. The measured absolute emission quantum yields for both the Eu^III and Tb^III complexes were 3% following excitation at 280 nm (Table 1). However, directly exciting the Eu^III and Tb^III complexes at 394 and 484 nm, respectively, resulted in intrinsic Ln^III emission efficiencies of 6% and 4%, respectively. The low efficiencies of the Eu^III and Tb^III complexes are comparable to values (1–13%) reported for analogous polybenzimidazole-type ligands and are attributed to non-radiative deactivation caused by the rotational freedom via the ester groups of the tripodal ligand [28,33]. Evidence of fluorescence quenching by the ester groups was observed following functionalization of the BimH₃ ligand to BimOEt₃ (Scheme S1). Using the measured excited energy levels of BimOEt₃, the sensitization mechanism of the Ln^III ions by the ligand can be described by the Jablonski diagram below (Figure 4).

Despite the low emission quantum yields, a bluish-white-emitting material was obtained through a combination of the residual blue ligand fluorescence and weak-red emission of the Eu^III complex with the CIE coordinates (0.258, 0.242). This suggests that single-component white-light-emitting devices (wLEDs) could be generated using metal complexes of BimOEt₃ (Figure 5) [10,34,35,36].

In contrast to the Eu^III and Tb^III complexes, excitation of the Ce^III complex at 280 nm resulted in two broad emission bands that are attributed to the ¹π*→¹π transitions of the ligand and the ²D_3/2→²F_J (J = 5/2–7/2) transitions of the Ce^III ion (Figure 6a,b). In addition, another transition at 489 nm was observed in the Ce^III emission spectrum and can be attributed to ligand perturbation of the Ce^III ion [37]. Further analysis of the ²D_3/2→²F_J (J = 5/2–7/2) transitions of the Ce^III emission at 77 K revealed two distinct bands at 434 nm (23,041 cm⁻¹) and 472 nm (21,186 cm⁻¹) with a resulting energy gap of ~1900 cm⁻¹, which is in agreement with previous reports (Figure 6b) [19,21,37].

The emission lifetimes of the Eu^III and Tb^III complexes were fit to bi-exponential function. The observed lifetimes do not arise from the ligand states by comparison to the Gd^III data and, therefore, are assigned as arising from the presence of two Ln^III coordination environments in solution (Table 1 and Figure S7a,b). This was assigned to contributions arising from the lack of stability of the Eu^III and Tb^III complexes in solution.

In contrast, the measured emission lifetime of the Ce^III complex was fit to a mono-exponential function following excitation at 280 or 377 nm (Table 1 and Figure S8a,b). In contrast to the Eu^III and Tb^III emission lifetimes, the Ce^III emission lifetime was short lived (55.0 ± 0.2 ns), consistent with a lifetime expected for a parity allowed 5d–4f transition (Table 1) [21]. Excitation of the Ce^III complex at 280 and 377 nm results in emission quantum yields of 17.6 ± 2.0% and 53.6 ± 1.3%, respectively. These values are similar to reports by Zheng et al. [21] and Harada et al. [19] for analogous Ce^III-polybenzimidazole complexes. Using the measured emission quantum yields, the efficiency of energy transfer, ${\phi }_{\mathrm{ET}}$, from BimOEt₃ to Ce^III was determined to be 33% using the equation ${\phi }_{\mathrm{L}-\mathrm{Ln}}={\phi }_{ET}{\phi }_{\mathrm{Ln}-\mathrm{Ln}}$, where the subscripts L−Ln and the Ln−Ln denote the overall emission and the 5d–4f Ce^III emission quantum yields, respectively. These results are in agreement with reports by Harada et al. [19] and Zheng et al. [21].

3. Materials and Methods

3.1. Materials

All the chemicals used were reagent grade and used as received.

3.2. General Procedures

The infrared spectra were acquired on a PerkinElmer spectrum 100 FT-IR spectrometer (PerkinElmer, Shelton, CT, USA) in the range 650–4000 cm⁻¹ with a 4.0 cm⁻¹ resolution and 8 scans per sample. The spectra were corrected for H₂O and CO₂ vibrations before data acquisition. The UV–Vis absorption spectra were acquired using a Cary Varian spectrophotometer operating at a medium scan speed in the range 800–200 nm. The emission spectra were acquired on a Horiba Fluoromax-4 spectrofluorimeter (Horiba Scientific, Piscataway, NJ, USA) equipped with a 150 W CW Ozone free xenon arc lamp, Czerny–Turner monochromators with excitation grating blazed at 330 nm (1200 groove/mm), and emission grating blazed at 500 nm (1200 grooves/mm). All spectra were corrected for the instrument response function and the intensity of the lamp. The emission lifetimes of the samples were acquired by exciting the samples with an Nd:YAG laser that was focused through a variable neutral density filter (Edinburg F-B01 laser mount) and a 2-mm diameter iris (Newport ID-1.0). The emission lifetimes of the Ce^III complex were acquired using a TCSPC system equipped with a 370-nm NanoLED.

3.3. Measurement of Ligand Excited States

The excited singlet (¹S) and triplet (³T) energy levels of BimOEt₃ (Florida State University, Tallahassee, FL, USA) were determined by measuring the emission spectra of the analogous gadolinium (Gd^III) complexes at 77 K [31]. 2-Methyltetrahydrofuran solutions (Alfa Aesar, Tewksbury, MA, USA) of the complexes were excited at 295 nm and the emission spectra collected in the range 300–700 nm. The emission spectra were deconvoluted into their Franck–Condon progression and the highest energy peak acquired at either the zero delay or gated emission taken as the excited ¹S and ³T energy levels, respectively. The delay time for the fluorescence spectrum was acquired at 0 ms delay while the phosphorescence spectrum was acquired at 0.5 ms delay.

3.4. X-ray Crystallography

The slow-diffusion of a 1:1 ethanol/acetonitrile (Florida State University, Tallahassee, FL, USA) mixture in diethyl ether vapor yielded X-ray-quality single-crystals of BimOEt₃ after a week. A crystal was mounted on a glass fiber of a Rigaku XtaLAB Synergy-DW dual wavelength X-ray diffractometer (CuKα = 1.54184 Å) at 298 K (Rigaku, Houston, TX, USA). Data reduction was performed using empirical absorption correction based on “multi-scan”. The structures were solved by the intrinsic phasing and least-square refinements using ShelXT and ShelXL (2014/2, Bruker AXS, Madison, WI, USA) of the Olex2 package [38,39,40]. The best crystal was selected for the data collection; however, the crystal still had quite a few type A and B alerts in the Cifcheck file due to a highly disordered structure caused by unstable solvent (trifluoromethane-sulfonate) in the unit cell.

4. Conclusions

The versatility of a tripodal ligand was demonstrated with successful sensitization of Ln^III ions in molecular complexes, leading to an intense blue emission with a quantum yield of 54% for the Ce^III complex. However, the measured emission quantum yields of the mid-lanthanide complexes (Eu^III and Tb^III) were 3%. Despite the low quantum yield of the Eu^III complex, the combination of the residual ligand fluorescence and the red Eu^III emission resulted in a bluish-white-emitting material with the CIE coordinates (0.258, 0.242). Thus, metal complexes of the ligand could be used in the generation of single-component-white-light-emitting materials. Although with improved solubility in polar solvents, the functionalization with ester groups leads to a rapid quenching of the ligand fluorescence. The coordination nature of the ligand to the metal ions was revealed through a combination of infrared and NMR spectroscopy, as well as single-crystal X-ray diffraction analysis of the metal complexes of BimOEt₃. The results serve as a starting point for further studies to better understand the stability of tripodal polybenzimidazole ligands for lanthanide separation/extraction and imaging applications.