Triboluminescence of Centrosymmetric Lanthanide β-Diketonate Complexes with Aggregation-Induced Emission

Abstract

Triboluminescence (TL) is a form of light emission induced upon mechanical forces on the material. However, our understanding of this phenomenon is still unclear and more examples are therefore needed in order to elucidate its mechanism. In this work, two types of TL complexes, [Eu(pp-dbm-Cl₂)₃phen] and [Eu(mm-dbm-Cl₂)₃phen], which also displays aggregation-induced emission (AIE) were synthesized and investigated for its photo-physical and crystal structural properties. These complexes were crystallized in a centro-symmetric space group P2₁/n, and remarkably, displayed TL upon grinding that may be due to the presence of extensive π···π, C-H···π and C-H···Cl-C interactions in the close molecular packing of its structure. This rare example deviates from the widely accepted mechanism of TL, hence widening the scope of our understanding in the area.

1. Introduction

Triboluminescence (TL) is a form of luminescence induced by applying mechanical force to crystalline materials [1]. It has attracted considerable attention because of its potential application such as pressure sensing [2]. Even though this optical phenomenon has been discovered more than a century ago, the understanding of its mechanism is still elusive. It has been widely accepted that the piezoelectric effect of non-centrosymmetric crystals causes TL because piezoelectric charges can excite the molecules. However, some examples of structures with centrosymmetric space groups such as [Eu(tta)₃bipy], [Eu(tta)₃₍dmbp)].H₂O, dmpy[Eu(tta)₄], mlq[Eu(tta)₄], apy[Eu(tta)₄], and [Tb(AP)₆]I₃ displaying vibrant TL have refuted this correlation [3,4,5,6,7], in fact, there are many influential factors such as ionic charge, crystal defects, gases environment, and pressure [8]. To study this phenomenon, lanthanide(III) chelates are ideal candidates due to their large Stoke shift that can eliminate inherent reabsorption problems, especially associated to organic compounds [9,10].

Molecular packing is also known to be a crucial factor in affecting various optical phenomena and is often neglected. Only in recent years has there been more studies directed in this area with examples of organic luminogens showing aggregation induced emission (AIE) and TL. Such work has brought insight into the area, where the influence of the crystal packing also has a dominant role in parallel with the electronic structure of the molecule [11,12]. For example, it has also been shown that the para or meta position of phenanthrol [9,10-d]imidazole (PI) group on tetraphenylethene (TPE) can lead to a difference in TL. While both crystals of mm-TPE(PI)₂ and pp-TPE(PI)₂ are centrosymmetric, the former, with stronger intermolecular interactions and higher dipole moment, is TL-active, but the latter is not [13]. As the mechanism of TL of different materials can vary, the study of an AIE-active compound with a centrosymmetric structure can bring insight into developing structural and optical correlations to TL, especially by examining the intermolecular interactions in the molecular packing.

However, designing AIE-active lanthanide(III) chelates are still challenging due to the controversial mechanism of AIE [14]. The key factor is the restriction of intramolecular rotation and intermolecular π-π stacking. Exceptional cases have also been explained by molecular planarization, intramolecular charge transfer, twisted intramolecular charge transfer, E-Z isomerization, and J-aggregate formation. Lately, there have been reports of AIE-active transition metal complexes caused by the presence and formation of halogen bonds (XB) in the structure [15].

In this regard, in our study, we specifically incorporated halogens in our complex design to study the influence of halogens on both TL and AIE. Two chloride-containing ligands, pp-dbm-Cl₂ and mm-dbm-Cl₂ were employed for lanthanide(III) complexation, and to rule out the effect of ionic charge on TL, an auxiliary ligand 1,10-phenanthroline (phen) was used to complete the lanthanide coordination sphere, forming neutral Ln^III complexes (Figure 1). As Eu^III ions are known to be efficiently sensitized by β-diketonates, [Eu(pp-dbm-Cl₂)phen] and [Eu(mm-dbm-Cl₂)phen] are designed, synthesized, and their photo-physical and structural study are discussed herein.

2. Results and Discussion

2.1. Synthesis and Characterization

The ligands pp-dbm-Cl₂ and mm-dbm-Cl₂ were synthesized by a classical Claisen condensation reaction between the corresponding ketone and methyl ester using NaH as the base. Both compounds were purified by recrystallization using ethanol, with product yields of 47% and 27% for pp-dbm-Cl₂ and mm-dbm-Cl₂, respectively. These compounds were characterized by ¹H-NMR, ¹³C-NMR, and CHN elemental analysis.

The corresponding complexes were synthesized by addition of europium(III) chloride to a mixture of auxiliary ligands and the deprotonated dbm-Cl₂ by triethylamine at room temperature. Purification was performed by recrystallization with a solution mixture of THF and MeCN. Single crystals for X-ray diffraction experiments were obtained by slow evaporation of the above solvent mixture. These complexes were characterized by ¹H-NMR, CHN elemental analysis and SCXRD.

The successful complexation can also be evidenced by ¹H-NMR spectra in Figure 2. The spectra of the complexes show shifted dbm-Cl₂ and phen peaks caused by the paramagnetic nature of Eu^III ions. Additionally, the 3:1 ratio of dbm-Cl₂ and phen indicates the formation of [Eu(dbm-Cl₂)₃phen].

The molecular structures of the Eu^III complexes were revealed by X-ray diffraction measurement (Figure 3 and Table S1). For complex [Eu(pp-dbm-Cl₂)₃phen], the metal centres are surrounded by three pp-dbm-Cl₂ and one phen with a coordination number of eight. The coordination geometry can be examined by the use of a Shape Analysis program developed by Raymond’s group [16,17]. The structures can be best described as having a bicapped trigonal prism (C_2v) geometry. The bond lengths of Eu–O range from 2.326 to 2.356 Å, and those of Eu-N are 2.605 and 2.617 Å. The average O-Eu-O and N-Eu-N bond angles are 70.8 ± 0.1 and 62.89°, respectively.

For [Eu(mm-dbm-Cl₂)₃phen], the coordination is similar to [Eu(pp-dbm-Cl₂)₃phen] with three mm-dbm-Cl₂ and one phen. Shape analysis indicates the coordination geometry fits the bicapped trigonal prism as well. The bond lengths of Eu-O range from 2.318 to 2.382 Å and those of Eu-N are 2.582 and 2.645 Å. The average O-Eu-O and N-Eu-N bond angles are 70.5 ± 0.6 and 62.68°.

Compared to the similar structure of [Eu(dbm)₃phen] reported previously [4], these two complexes show normal bond lengths and angles, but the coordination geometry is, however, different. [Eu(dbm)₃phen] was best fitted to a square antiprism (D_4d) geometry. We hypothesize that this difference arises due to the presence of C-H···Cl-C and π···Cl interactions in both the [Eu(pp-dbm-Cl₂)₃phen] and [Eu(mm-dbm-Cl₂)₃phen] structures.

2.2. Luminescent Properties

The UV-vis absorption spectra of the ligands, pp-dbm-Cl₂ and mm-dbm-Cl₂ and their corresponding Eu complexes are recorded in THF solution and shown in Figure 4. For pp-dbm-Cl₂, the peak maximum is at 350 nm which is attributed to the π → π* transition. For mm-dbm-Cl₂, this is at 347 nm, which is slightly more blue-shifted than that of pp-dbm-Cl₂. This is attributed to the less symmetrical structure of mm-dbm-Cl₂. Upon complexation, [Eu(pp-dbm-Cl₂)₃phen] shows absorption at 360 nm, which is red-shifted 10 nm compared to pp-dbm-Cl₂. The complexation with Eu^III ion lowers the energy of the ligand. The peak at 270 nm is due to the π → π* transition of phen. [Eu(mm-dbm-Cl₂)₃phen] exhibits the same trend with an absorption peak maximum at 357 nm.

The photoluminescence properties of the Eu^III complexes in THF solution were studied (Figure 5). The observed emission spectra shows a typical fingerprint Eu^III emission profile with ⁵D₀ → ⁷F_J (J = 0–4) transitions upon excitation at 360 nm. Excitation spectra for the ⁵D₀ → ⁷F₂ peak (612 nm) shows a broad band with peak maximum at 360 nm, which is similar to their absorption spectrum, indicating that the luminescence is mainly originated from the absorption of pp-dbm-Cl₂ or mm-dbm-Cl₂ (Figures S7 and S8). The symmetry of the Eu^III centre can be evaluated by calculating the asymmetric ratio, which is the intensity ratio between the J = 2 and J = 1 transition. The values are 19.4 and 17.2, indicating a low symmetry in the Eu^III site, which is in agreement with the crystal structure. Furthermore, the luminescence decay of the ⁵D₀ → ⁷F₂ transition were best fitted to be biexponential (Figures S9 and S10), which indicates the presence of some coordinating solvent molecules. The resulting lifetimes are 25.1 (93%) and 115.8 (7%) µs for [Eu(pp-dbm-Cl₂)₃phen], and 11.7 (75%) and 65.4 (25%) µs for [Eu(mm-dbm-Cl₂)₃phen]. Their quantum yield is only 0.4% and 0.03% in THF solution at 10 µM for [Eu(pp-dbm-Cl₂)₃phen] and [Eu(mm-dbm-Cl₂)₃phen], respectively. Such low values are seemingly attributed to inefficient energy transfer from pp-dbm-Cl₂ or mm-dbm-Cl₂ to the Eu^III centre, but this is not supported by the triplet values obtained from the low temperature phosphorescence measurements of [Gd(pp-dbm-Cl₂)₃phen] and [Gd(mm-dbm-Cl₂)₃phen], which are both 20,202 cm⁻¹ (Figures S11 and S12). These values are slightly higher than the lowest excited state of Eu^III ions ⁵D₀ at 17,200 cm⁻¹, which should promote the efficient energy transfer process [9]. The weak luminescence may be due to quenching by solvent molecules because at solid state, both Eu^III complexes are luminescent, visible by the naked eye. This is also suggested from the biexponential lifetimes obtained.

The solid-state emission spectra of [Eu(pp-dbm-Cl₂)₃phen] and [Eu(mm-dbm-Cl₂)₃phen] were recorded (Figure 6). Since the intensity at solid state is higher than that in the solution state, the emission profiles show finer splitting of the Eu^III profile. [Eu(pp-dbm-Cl₂)₃phen] contains five peaks at the ⁵D₀ → ⁷F₂ transition, whereas [Eu(mm-dbm-Cl₂)₃phen] has four peaks, revealing low symmetry of the Eu^III site (C_2v and C_s, respectively). Unlike the solution state, the luminescence decay of the ⁵D₀ → ⁷F₂ transition at solid state is fitted to a monoexponential decay and the resulting lifetimes are much longer, 455 and 317 µs for [Eu(pp-dbm-Cl₂)₃phen] and [Eu(mm-dbm-Cl₂)₃phen], respectively (Figures S13 and S14). The quantum yields are measured to be 73% and 38% for [Eu(pp-dbm-Cl₂)₃phen] and [Eu(mm-dbm-Cl₂)₃phen]. Such an increase is probably due to the rigidity of the solid that minimizes molecular motion, as well as eliminating the factor of solvent quenching to the inner and outer coordination sphere.

Due to such dramatic differences in the luminescence properties, we postulated that these compounds might exhibit some aggregation-induced emission (AIE) properties. To illustrate this, we demonstrated the luminescence behavior of [Eu(pp-dbm-Cl₂)₃phen] in THF-water mixtures with various water fractions (f_w) at 10 µM (Figure S15). By naked eye, the solution turns turbid at 80% water content, indicating the formation of aggregates. This can also be proved by the UV-vis absorption spectra because at 80% f_w, a level-off tail appears, and the light transmission decreases due to Mie scattering, indicating aggregation. (Figure S16). The absorption band also becomes broader, resembling the absorption in the solid state (Figure S17), and the red-shifting indicates formation of J-aggregates. The luminescence of the aggregates are recorded, and their spectra are shown in Figure 7 and Figure S18 (with higher water content). From 0% to 70% f_w, the emission intensity drops, as the aggregates have not yet been formed, and the quenching arises from the presence of the O-H oscillating phonons from the water, which quenches the Eu^III luminescence. When f_w increases from 80% to 95%, the luminescence intensity increases, and the quantum yield rises from 0.56% to 41%. This is due to the formation of a higher content of smaller hydrophobic aggregates that protect the Eu^III center from water. However, when the water content increases to 99%, the luminescence intensity drops (Ф = 24% at 99%) because the aggregate size becomes smaller (95% = 131 nm and 99% = 88 nm), lowering the number of [Eu(pp-dbm-Cl₂)₃phen] encapsulated in the aggregate. This phenomenon is similar to the study by Zhang et al. [18].

2.3. Tribolumimescence and Molecular Packing

From the findings of the AIE property, [Eu(pp-dbm-Cl₂)₃phen] is highly emissive with decent quantum yield in the solid state, and hence is ideal for triboluminescence study. Together with [Eu(mm-dbm-Cl₂)₃phen], their dried crystals were crashed by a glass rod and were both found to be TL-active, evident by the TL spectra shown in Figure 8. In fact, these TL lights are very difficult to be seen by naked eye because these TL are very weak compared to their PL; therefore, the TL spectra contains much noise. Nevertheless, the TL spectra are identical to their corresponding PL spectra, indicating that the symmetry of the complexes did not change upon fracture of the crystal structure. If so, the splitting of the hypertensive peaks would have been altered [19].

To correlate triboluminescence with its molecular structure, the crystal structures of [Eu(pp-dbm-Cl₂)₃phen] and [Eu(mm-dbm-Cl₂)₃phen] were studied. Both compounds share the same symmetry with a centrosymmetric space group P2₁/n. According to Zink et al., non-centrosymmetric crystals are likely to exhibit TL because of the piezoelectric effect [20]. The weak TL of [Eu(pp-dbm-Cl₂)₃phen] and [Eu(mm-dbm-Cl₂)₃phen] may be due to the lack of piezoelectric charge generated due to their centrosymmetricity. This may explain why their TL is only detected when the intact crystal is fractured. When the crystal is crushed into much smaller grains, no TL signal can be detected. It is because, when fracture occurs in large crystals, the newly formed cracked surfaces have larger surface area in which more charge can be accumulated than for the smaller crystals. However, for the non-centrosymmetric structures such as [Eu(dbm)₄TMP] reported previously [4], even when it is grinded to a crystalline powder, there is still detectable TL generated upon mechanical action.

We also observed that the molecular packing within the crystal structure also has an influence on the TL. With a change in the Cl position, the packing mode was found to differ slightly for the two complexes (Figure 9). For the Cl at the para position, [Eu(pp-dbm-Cl₂)₃phen], one complex is connected to five others by different intermolecular interaction. There are π···π interactions (3.607, 3.660, 3.734 Å), C-H···π interactions (2.625 Å), C-H···O interactions (2.441 Å) and C-H···Cl-C (2.788 Å). For [Eu(mm-dbm-Cl₂)₃phen], each complex is surrounded by eight more complexes, connected through Van der Vaals forces. There are π···π interactions, C-H···O interactions (2.496 Å) and C-H···Cl-C (2.861, 2.883 Å) and C-Cl···π (3.448 Å). When Cl is at the meta position, all the C-H···π interactions are changed to a weaker C-Cl···π interactions (longer distance). This may decrease the rigidity of the crystal causing more slippage upon mechanical forces, hence accounting for the poorer luminescence quantum yield and much weaker TL observed for [Eu(mm-dbm-Cl₂)₃phen] than [Eu(pp-dbm-Cl₂)₃phen].

3. Experimental

3.1. Material Preparation

Unless otherwise stated, all reagents for synthesis were obtained commercially and used without further purification. Methyl 4-chlorobenzoate, 4′-chloroacetophenone, methyl 3-chlorobenzoate, and 3′-chloroacetophenone were purchased from Meryer (Shanghai, China). Sodium hydride, 1,10-phenanthroline, europium(III) chloride hexahydrate (99.99%), gadolinium(III) chloride hexahydrate (99.999%) were purchased from Sigma (St. Louis, MO, USA). Anhydrous tetrahydrofuran was purchased from Acros (Waltham, MA, USA). The ligand synthesis was performed using standard Schlenk line techniques. Glassware was dried at 130 °C prior use for water-free reaction.

3.2. Synthesis of Ligands and Complexes

1,3-Bis(4-chlorophenyl)propane-1,3-dione (pp-dbm-Cl₂). To a 500 mL two-neck round bottom flask were added sodium hydride (60% in mineral oil, 5.0 g, 125 mmol) and then anhydrous THF (200 mL). This mixture was cooled to 0 °C, and then methyl 4-chlorobenzoate (55 mmol) and 4’-chloroacetophenone (50 mmol) were added. The suspension was heated to reflux under nitrogen for 48 h. After cooled down to room temperature, the mixture was filtered through Celite and washed once with ethanol. The filtrate was treated with a mixture of ether and dilute hydrochloric acid. The ether phase was separated and washed three times with brine and dried on magnesium sulfate. After removal of the ether, the residue was recrystallized by ethanol to give pure products as white or light-yellow solids. Yield 47%. ¹H-NMR (400 MHz, CDCl₃) δ 16.75 (s, 1H), 7.91 (d, J = 12 Hz, 4H), 7.46 (d, J = 8 Hz, 4H), 6.76 (s, 1H). ¹³C-NMR (100 MHz, CDCl₃) δ 184.60, 138.92, 133.76, 129.04, 128.53, and 92.88. Anal. (%) Calcd for C₁₅H₁₀Cl₂O₂: C, 61.46; and H, 3.44. Found: C, 61.48 and H, 3.36.

1,3-bis(3-chlorophenyl)propane-1,3-dione (mm-dbm-Cl₂). The procedure is the same as pp-dbm-Cl₂ by using reagents 3-chlorobenzoate and 3′-chloroacetophenone instead. Yield 27%. ¹H-NMR (400 MHz, CDCl₃) δ 16.61 (s, 1H), 7.95 (s, 2H), 7.86 (d, J = 7.8 Hz, 2H), 7.53 (d, J = 8.8 Hz, 2H), 7.44 (t, J = 7.9 Hz, 2H), 6.77 (s, 1H). ¹³C-NMR (100 MHz, CDCl₃) δ 184.49, 137.04, 135.02, 132.54, 130.03, 127.33, 125.30, and 93.45. Anal. (%) Calcd for C₁₅H₁₀Cl₂O₂: C, 61.46; and H, 3.44. Found: C, 61.47 and H, 3.32.

[Eu(pp-dbm-Cl₂)₃phen]. To a 10 mL reaction tube with pp-dbm-Cl₂ (0.3 mmol) and 1,10-phenanthroline (0.1 mmol) was added 1 mL THF, triethylamine (0.3 mmol) and 1 mL of 0.1 M europium(III) chloride hexahydrate ethanolic solution. The mixture was stirred overnight. The solvent was evaporated, and the solid was washed with water and MeCN. Recrystallization of the solid with THF/MeCN mixture yielded a yellow crystalline solid. The same approach was applied for all the Ln^III complexes in this study. Yield: 75%. ¹H-NMR (400 MHz, CDCl₃) δ 10.91 (d, J = 7.7 Hz, 2H), 10.57 (s, 2H), 10.44 (s, 2H), 8.97 (d, J = 7.5 Hz, 2H), 6.75 (d, J = 7.5 Hz, 12H), 5.87 (d, J = 7.7 Hz, 12H), and 2.60 (s, 3H). Anal. (%) Calcd for C₅₇H₃₅Cl₆N₂O₆Eu: C, 56.65; H, 2.92; and N, 2.32. Found: C, 56.67; H, 2.85; and N, 2.25.

[Eu(mm-dbm-Cl₂)₃phen]. Yield: 74%. ¹H-NMR (400 MHz, CDCl₃) δ 11.27 (s, 2H), 10.80 (d, J = 7.7 Hz, 2H), 10.23 (s, 2H), 9.02 (d, J = 7.7 Hz, 2H), 6.82 (d, J = 7.9 Hz, 6H), 6.70 (t, J = 7.7 Hz, 6H), 6.01 (d, J = 7.3 Hz, 6H), 5.85 (s, 6H), and 2.85 (s, 3H). Anal. (%) Calcd for C₅₇H₃₅Cl₆N₂O₆Eu: C, 56.65; H, 2.92; and N, 2.32. Found: C, 56.69; H, 2.86; and N, 2.24.

3.3. Characterization Measurement

NMR spectra were recorded on a Bruker Ultrashield Advance Pro 400 spectormeter (Billerica, MA, USA), and the chemical shifts were referenced internally to tetramethylsilane or the corresponding solvents residue in parts per million (ppm). FT-IR spectra were recorded on a Nicolet iS 50 FT-IR spectrometer (Waltham, MA, USA) with a KBr pellet. Elemental analysis was performed on an Elementar Vario Micro Cube elemental analyzer (Langenselbold, Germany). Dynamic light scattering experiment to measure the size distributions of the aggregates were on the Zetasizer Nano ZS (Malvern, UK).

3.4. Single Crystal Structure Determination

Single crystals of the Eu^III complexes were grown by slow evaporation of THF/MeCN mixture. A suitable crystal was picked and mounted using the “oil-drop mounting” technique and its data were collected using either the Bruker Smart Apex II or Bruker D8-Venture single-crystal diffractometer (Madison, WI, USA). Multiscan absorption correction was then applied to the data using the Bruker SAINT (version 8.34A)/SADABS (ver. 2014/5) software (Billerica, MA, USA). The SHELX program suite (SHELX97 or SHELX 2014/7) was used to calculate the initial structural solution through either the direct or the Patterson method, and was then refined by full matrix least-squares on F2. CCDC 1890158, 1890142.

3.5. Photophysical Measurement

UV-vis absorption spectra were measured on an Agilent HP UV-8453 spectrophotometer (Santa Clara, CA, USA). Steady-state room temperature photoluminescence measurements were performed on an Edinburgh Instrument (Livingston, UK) FLSP920 spectrophotometer equipped with a Xe900 continuous xenon lamp, mF920 microsecond flashlamp and a single photon counting photomultiplier tube. Spectra were corrected with the bundled F900 software. Reflectance and absolute quantum yield were recorded by using a corrected integrating sphere provided by Edinburgh Instrument. Triboluminescence spectra were recorded on an Ocean Optics QEPro multichannel spectrometer (Largo, FL, USA) [4].

4. Conclusions

In summary, two novel Eu^III complexes were developed. The complexes are typical AIE material exhibiting strong luminescence in the solid state but being non-emissive in the solution state. Due to the centrosymmetric space group, these compounds are weakly TL-active, and hence the intensity of these compounds are not comparable with those of a non-centrosymmetric structure due to the lack of piezoelectric charge. With the introduction of a Cl group at the para position, [Eu(pp-dbm-Cl₂)₃phen] has higher quantum yield and displays a slightly more intense TL than [Eu(mm-dbm-Cl₂)₃phen] due to a better and more rigid packing that reduces non-radiative dissipation.