Combining tetraphenylethene (TPE) derivative cations with Eu³⁺-β-diketone complex anions for tunable luminescence

Abstract

Tetraphenylethene (TPE) derivative cations (TPE⁺) and Eu³⁺-β-diketone complex anions (Eu(ABM)₄⁻) were combined to construct a novel dual energy transfer system (TPE⁺ to Eu³⁺ and ABM to Eu³⁺). Our system exhibits tunable luminescence in DMF/water mixtures under different f_w conditions owing to the AIE and ACQ properties of TPE⁺ and ABM, respectively. Its luminescence can be also regulated by adding P-containing oxysalts or polyacrylic acids.

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Propeller-like tetraphenylethene (TPE) derivatives, which show unique aggregation induced emission (AIE) behavior, are undergoing rapid development due to their widespread applications in light emission, bio-imaging and chemo/bio-sensing.^1–5 Most typically, the construction of an electron-donating/accepting system is an effective approach to change the fluorescence that is attributed to the twisted conformation of TPE derivatives. As a result, the TPE backbones are usually decorated with different electron withdrawing substituent groups.^6–9 However, these molecules require tedious synthesis methods and their fluorescence can barely be regulated conveniently, because even a small change in substituent group means that the synthesis route needs to be redesigned. Inspired by molecular assemblies via non-covalent interaction, the electron donating and accepting parts have been separated into cationic TPE derivatives and counteranions, respectively, resulting in the establishment of a novel flexible TPE structure with isolated donor–acceptor (D–A) motifs. There is no doubt that the counteranions have a very significant effect on the cationic TPE molecular packing upon aggregation as well as on the fluorescence, where the counteranions normally induce a change in the solubility.¹⁰ Thus, according to previous research, there is no need to take into account the luminescence of the anions. Moreover, the luminescence of these ionic compounds has normally been limited to the emission that originates from the TPE motif. However, when exploring excellent fine-tuning luminescent systems, the luminescence of the counteranions should not be neglected. Therefore, looking for suitable counteranions to interact with the TPE fluorophore is exciting yet challenging, wherein the energy transfer between the TPE cations and counteranions is an essential prerequisite. To the best of our knowledge, very few studies have focused on such cationic TPE/anion materials that exhibit combined electronic, structural and luminescent properties. Encouragingly, the unusual negatively charged complex that is formed by lanthanide and β-diketone is selected as the counteranion in this work.¹¹ 4,4,4-Trifluoro-1-phenyl-1,3-butanedione (ABM) is a typical type of β-diketone with aggregation-caused quenching (ACQ) characteristics, which can transfer energy to Ln³⁺.^12–14 In addition, the phenyl ring of the TPE-motif is locked through an aggregated state via electrostatic interaction with the negatively charged Ln³⁺–β-diketone complex, so the AIE-active TPE cation can also transfer energy to Ln³⁺.^15,16 Thus, this dual-ligand–Ln³⁺ energy transfer system can exhibit a stepwise pathway to sensitize the luminescence of Ln³⁺, which provides a novel approach for designing tunable fluorescent molecules. More strikingly, the luminescence regulation that is related to the dual energy transfer utilizing AIE/ACQ properties in distinct mixed solvents was investigated. To gain insight into the dual energy transfer process, a controllable approach for fine-tuning the luminescence was also studied.

The parallelogram-like TPE-based onium salt 1-(4-(1,2,2-triphenylvinyl)benzyl)pyridin-1-ium bromide (denoted as TPE⁺-Br⁻) was synthesized first, which was reacted by adding 1-(bromomethyl)-4-(1,2,2-triphenylethenyl)benzene to pyridine in toluene at 45 °C (Fig. 1 and Fig. S1–S3, ESI†).^17–19 When the TPE⁺-Br⁻ salt was dissolved in a mixture of THF/n-hexane, it showed typical aggregation-induced emission enhancement (AIEE) activity before the volume fraction of n-hexane (f_hex) reached 70% (Fig. S4, ESI†). It was almost non-emissive when f_hex increased from 0% to 30%, then strong aggregated-induced emission at 435 nm was observed when f_hex was increased from 40 to 70% with a maximum 362 times enhancement (f_hex = 70%). However, it exhibited a drastic decline in emission intensity at a higher n-hexane fraction (>70%), revealing the formation of amorphous molecular aggregates. To combine the typical AIE fluorophore with a lanthanide-based complex, the bromine ion of the TPE derivative was removed using a negatively charged lanthanide complex through adding the lanthanide salt and ABM ligand (mole ratio 1 : 4) with stirring at room temperature. As a result, the bar-shaped TPE⁺–EuL₄⁻ compound was formed (Fig. 1 and Fig. S5, ESI†). XPS results suggest that the material consists of C, N, O, F and Eu (Fig. S6, ESI†). The results of XRD patterns reveal that the as-synthesized sample contains both TPE⁺ and EuL₄⁻ parts (Fig. S7, ESI†). The vibrational frequency of v(C=O) and v(CF₃) shifts towards lower frequency compared with ABM in the IR spectrum, indicating that ABM has coordinated with the Eu³⁺ ions (Fig. S8 and Table S1, ESI†). In addition, the contents of C, H, O, N and Eu are consistent with the calculated results, which demonstrates the pure phase (Table S2, ESI†). The typical AIE emission of TPE⁺–EuL₄⁻ (about 467 nm) was observed, but TPE⁺-Br⁻ was almost non-emissive when dissolved in the DMF/H₂O (f_w = 90%), demonstrating restriction of the intramolecular rotation of the TPE moiety owing to the electrostatic interaction between the TPE-motif and the anionic lanthanide complex (Fig. S9, ESI†). Most excitingly, besides the TPE-motif fluorescence, TPE⁺–EuL₄⁻ displayed clear sharp peaks from 550 nm to 650 nm, which belong to the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions of typical Eu³⁺ luminescence,^20–23 indicating the energy transfer process from the dual ligands to Eu³⁺. To elucidate the mechanism (Fig. S10–S12, ESI†), the Gd-based compound was synthesized, in which Gd³⁺ ions could not accept any energy from the excited state of most organic ligands due to the higher ⁶P_7/2 energy level (Gd³⁺: 32 500 cm⁻¹).^24,25 The UV-vis absorption and phosphorescence of the Gd-based compound TPE⁺–GdL′₄⁻ (L′ = citric acid) without ABM were detected to determine the singlet state (S1) (32 680 cm⁻¹) and the triplet energy level (T1) (20 202 cm⁻¹) of the TPE ligand, respectively. Similarly, S1 (30 864 cm⁻¹) and T1 (21 413 cm⁻¹) for ABM were calculated according to the UV-vis absorption and phosphorescence of (C₂H₅)₃HN⁺–GdL₄⁻ (L = ABM) in the absence of TPE⁺. The energy gap ΔE₁ between S1 and T1 is 12 478 cm⁻¹ and 9451 cm⁻¹ for TPE-based ligand and ABM, respectively, implying an effective intersystem crossing process. According to the antenna effect,²⁶ ΔE₂ between T1 of the ligand and the excited state of the Ln³⁺ ions must be higher than 3500 cm⁻¹ to achieve the irreversible energy transfer. In this system, ΔE₂ is 2916 cm⁻¹ (TPE-motif to Eu³⁺) and 4127 cm⁻¹ (ABM to Eu³⁺). These results indicate that both ligands can contribute to transferring energy to Ln³⁺, but ABM can sensitize the luminescence more effectively.

Fig. 1 Synthetic strategy for TPE⁺–EuL₄⁻ materials.

As ABM has been demonstrated to be one of the best sensitizers for Eu³⁺ emission in dilute DMF solutions and water is a poor solute for the organic ligand,¹³ the DMF/H₂O system was selected and the fluorescence was investigated under different f_w conditions (Fig. 2). Notably, the TPE-motif was almost non-emissive when f_w increased from 0 to 60%, after which the fluorescence intensity at about 467 nm that is associated with the AIE properties enhances steadily when f_w exceeded 70%. Meanwhile, the characteristic emission intensity of Eu³⁺ (⁵D₀ → ⁷F₂: 614 nm) declined gradually at first, reaching a minimum at 60%, and finally increased sharply when f_w raised from 70 to 90%. When f_w was below 60%, since the rotation of the TPE-based ligand was not totally restricted, the energy absorbed from the UV region was consumed by rotation of the benzene rings and an ineffective energy transfer process (TPE ligand to Eu³⁺) existed. Meanwhile, the energy transfer process from the ABM ligand to Eu³⁺ dominated,^27,28 but the ACQ effect of ABM decreased its performance as a sensitizer, which was confirmed by the decline in luminescence intensity associated with Eu³⁺ when f_w was raised from 0 to 60%. Interestingly, when f_w exceeded 60%, the emission performance of Eu³⁺ was activated, in which the intensity of 614 nm soared to 888.20 (f_w = 90%); this was because the restriction of the intramolecular motion of the TPE-motif blocked the non-radiative relaxation channels, and this restricted ligand began to transfer energy to Eu³⁺. However, ABM exhibited the ACQ effect under such a high-water ratio, so the TPE ligand acted as the main sensitizer for Eu³⁺. When f_w reached 90%, the fluorescence lifetime detected at 467 nm associated with the TPE motif in TPE⁺–EuL₄⁻ (4.56 μs) declined compared with TPE⁺–GdL′₄⁻ (L′ = citric acid) (6.01 μs) (Fig. S13, ESI†), also implying that the TPE motif transferred energy to Eu³⁺. The energy-transfer efficiency of the TPE motif to Eu³⁺ could be calculated via E = 1 − τ/τ₀, where τ₀ and τ are the lifetime values detected at 467 nm in the absence and presence of the Eu³⁺ containing complex, respectively. The energy-transfer efficiency was lower (24.13%), which agrees with the lower ΔE₂ of 2916 cm⁻¹ (from the T1 of TPE⁺ to Eu³⁺). The lifetime detected at 614 nm (the ⁵D₀ → ⁷F₂ transition of Eu³⁺) for TPE⁺–EuL₄⁻ (503.5 μs) was longer compared with (C₂H₅)₃HN⁺–EuL₄⁻ (249.8 μs) (Fig. S14, ESI†). Moreover, as shown in Fig. S15 (ESI†), the broad band located at 300–380 nm (attributed to organic ligands) was observed, indicating energy migration from the ligands to Eu³⁺. Notably, the intensity of the excitation peak that ranged from 300 to 350 nm was raised significantly (f_w > 60%), which demonstrates that the TPE motif behaved as the main donor when the restriction of intramolecular rotation was enhanced (Fig. S15 and 16, ESI†). The quantum efficiency (QE) of TPE⁺–EuL₄⁻ detected in the DMF/water mixture (f_w = 90%) was 10.95%, but the value of this solid climbed to 26.23%, implying that the restriction of intramolecular rotation in the TPE moiety can improve the QE of the material. In essence, the arrangement of the compound was the switch that controls the dual energy transfer process and luminescence when changing the ratio of solvents.

Fig. 2 Emission spectra of TPE⁺–EuL₄⁻ materials dissolved in DMF/water mixtures with different water fractions (λ_ex = 385 nm) (a); variation of I_467 nm and I_614 nm with different f_w (b); and 1931 CIE chromaticity diagram (c).

Adenosine 5'-triphosphate disodium salt (ATP disodium salt) is a central component of energy storage as well as an important endogenous signaling molecule in immunity and inflammation, so its detection strategy has been widely investigated.²⁹ As shown in Fig. 3, the luminescence was quenched with an increasing concentration of ATP²⁻. Due to the characteristic three adjacent phosphate groups and the heterocyclic ring that contains both N and O coordinated atoms, ATP²⁻ has a very strong coordinating ability with Eu³⁺, in which the adjacent phosphate groups can link to Eu³⁺ through several coordination modes. Since the excellent coordination ability of ATP²⁻ helps to elevate the competitive capacity, ABM was substituted by ATP²⁻ and dissolved in solution, which would further indicate that the new coordination interaction between Eu³⁺ and ATP²⁻ existed. It was confirmed by the UV absorption spectrum of TPE⁺–EuL₄⁻ (1.0 × 10⁻⁴ mol L⁻¹) in DMF/H₂O (f_w = 50%) upon addition of ATP²⁻ (Fig. S17, ESI†). The intensity of the peak located at about 320 nm dropped, whereas the intensity of the relatively narrow peak (about 256 nm) was elevated. According to the UV absorption spectra of (C₂H₅)₃HN⁺–EuL₄⁻ and TPE⁺–GdL′₄⁻ mentioned above (Fig. S10, ESI†), it can be deduced that both ABM and TPE⁺ contribute to the absorption peak at about 323 nm in TPE⁺–EuL₄⁻. Notably, there is keto–enol tautomeric equilibrium in ABM, in which the absorption located at 225–265 nm denotes the keto-form, and the peak red-shifted to over 300 nm is the enol-form.³⁰ In this system, the amount of TPE⁺ was not removed from the system, so the decline of the intensity was mainly induced by the decreasing enol-form of ABM. Although the intensity associated with the keto-form increased sharply, the absorption of ATP²⁻ was also located around 256 nm (Fig. S18, ESI†). As a result, the absorption at >300 nm was mainly considered to measure ABM with the enol-form. The decline of the absorption at about 323 nm indicates that ABM (enol-form) dissociated from the surface of Eu³⁺ and then turned to the keto-form. Meanwhile, because the structure of Eu(ABM)₄⁻ was destroyed, TPE⁺ was no longer immobilized and it was attracted by the negative ATP²⁻ instead (Fig. 3). The compound associated with ATP²⁻ and TPE⁺ had a certain solubility in water owing to the hydrophilic groups of ATP²⁻, and TPE⁺ no longer displayed typical AIE behavior, which was confirmed by the non-emissive properties of the mixture of ATP²⁻ and TPE⁺ (Fig. S19, ESI†). Moreover, when P₂O₇⁴⁻ and H₂PO₄⁻ with decreasing numbers of coordinated sites were added, the luminescence of the peaks located at 467 nm and 614 nm also decreased, but were not quenched completely. However, H₂PO₂⁻ and HPO₃²⁻ cannot induce luminescence quenching due to their weak coordination ability (Fig. 4a and Fig. S20–S24, ESI†).

Fig. 3 Emission spectra of TPE⁺–EuL₄⁻ (1.0 × 10⁻⁴ mol L⁻¹) in DMF/H₂O (f_w = 90%) upon addition of increasing concentrations of ATP²⁻ (λ_ex = 385 nm) and PAA-1 (M_w = 1 450 000) (λ_ex = 320 nm), respectively; emission intensity of the peaks (467 nm and 614 nm) varies with amount of PAA-1; possible mechanism of the different luminescence responses by adding ATP²⁻ and PAA-1, respectively.

Fig. 4 Emission spectra of TPE⁺–EuL₄⁻ (1.0 × 10⁻⁴ mol L⁻¹) in DMF/H₂O (f_w = 90%) in the presence of aqueous solutions of different various salts (2.0 × 10⁻⁴ mol L⁻¹) (λ_ex = 385 nm) (a); emission spectra of TPE⁺–EuL₄⁻ (1.0 × 10⁻⁴ mol L⁻¹) in DMF/H₂O (f_w = 90%) in the presence of PAA-1 (M_w = 1 450 000), PAA-2 (M_w = 3000), and AA (0.2 mg) (λ_ex = 320 nm) (b). Insets in (a) and (b) show digital photos under 365 nm UV light.

Based on the results above, the related negative phosphates do not have the ability to fix TPE⁺. To immobilize the TPE-motif, polyacrylic acid (PAA: M_w = 1 450 000, denoted as PAA-1) with long flexible carbon chains and multiple –COOH groups was introduced in the system (Fig. 3). The UV absorption peak located at 256 nm was only assigned to the keto-form of ABM, due to there being no PAA-1 absorption peak in this range (200–400 nm) (Fig. S18, ESI†). Similar to the condition with the addition of ATP²⁻, the dissociated ABM with the enol-form detached from the Eu–ABM complex and transformed to the keto-form, which was confirmed by the downward trend of I_323 nm/I_256 nm (Fig. S25, ESI†). However, a small amount of PAA-1 could only induce some ABM molecules to leave the complex, because the value of I_323 nm/I_256 nm only changed slightly when 0.025 mg of PAA-1 was added. As a result, PAA-1 behaved as a bridge to connect the donor (TPE⁺ and ABM) and acceptor (Eu³⁺). TPE⁺ was surrounded by polymer coils and the rotation was further restricted, leading to the promoted energy transfer process from TPE⁺ to Eu³⁺. This was confirmed by the tiny increase in the intensity of the peaks located at about 467 nm that belong to the TPE-motif and at 614 nm, which is associated with Eu³⁺ (Fig. 3). The insignificant extent of increase is consistent with the fact that TPE cannot sensitize the luminescence effectively as mentioned above. When more PAA-1 was added, nearly all the ABM dissolved, and the ACQ effects of ABM in the aqueous system prevented any further occurrence of energy transfer from ABM to Eu³⁺, even if the dissociative ABM existed. The intermolecular rotation of TPE⁺ was restricted owing to the twined carbon chains of PAA-1. However, when the carbon chains were intertwined with each other to a greater extent, the increased steric hindrance resulted in blocked energy transfer (TPE⁺ to Eu³⁺). Therefore, although the peaks assigned to the TPE-motif increased obviously, the luminescence intensity associated with Eu³⁺ fell rapidly and was quenched completely. When the amount of PAA-1 reached above 0.05 mg, both TPE⁺ and Eu³⁺ were wrapped around a large amount of PAA-1 with multiple –COOH groups, leading to increasing hydrophilicity and a decreased intensity of the peaks (467 nm). When 0.1 mg of PAA-1 was added, nearly all the PAA-1 substituted the ABM, because the value of I_323 nm/I_256 nm remained basically unchanged when more PAA-1 was added (0.2 mg) (Fig. S25, ESI†). To verify the fact that steric hindrance was unfavorable for energy transfer from TPE⁺ to Eu³⁺, the same weight (0.05 mg) of PAA (M_w = 3000, denoted as PAA-2), PAA-1 and acrylic acid (AA) were added, respectively (Fig. 4b and Fig. S26, S27, ESI†). Because they have the same amount of –COOH groups, the change in energy transfer caused by the disassociation of ABM in the acidic system was not considered. For the intensity associated with TPE⁺, the sequence I_{467 nm-PAA-1} > I_{467 nm-PAA-2} > I_467 nm-AA indicated that the ability to restrict the rotation of TPE⁺ was weakened from AA to PAA-1, which further demonstrated that the steric hindrance caused by the wrapped acids decreased from PAA-1, PAA-2 to AA. The intensity sequence located at about 614 nm (I_{614 nm-PAA-1} < I_{614 nm-PAA-2} < I_614 nm-AA) also verified that the stronger steric hindrance prohibited energy transfer from TPE⁺ to Eu³⁺ more seriously.

In summary, a novel lanthanide-based ionic compound was synthesized by combining TPE⁺ derivatives with AIE properties and the EuL₄⁻ (L = ABM) complex built using a ligand with ACQ behavior. The compound exhibited a stepwise energy transfer pathway from dual AIE/ACQ ligands to lanthanide in the DMF/H₂O system. The addition of ATP²⁻ and PAA had a significant effect on the dual energy transfer process and the tunable luminescence, respectively. This discovery provides new insight into the tunable fluorescence of the compounds combined with AIE ligands, ACQ ligands and lanthanide ions.

This work was financially supported by the National Natural Science Foundation of China (21601107) and the Shandong Provincial Natural Science Foundation, China (ZR2021MB052).

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The detailed experimental procedure and related spectroscopic data. See DOI: https://doi.org/10.1039/d2cc03903f

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