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Review  |  Open Access  |  28 Jan 2024

Energy transfer process, luminescence optimizing and various applications of lanthanide complexes

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Chem Synth 2024;4:12.
10.20517/cs.2023.35 |  © The Author(s) 2024.
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Abstract

Modulating the spectroscopic overlap between the emission bands of donors and the absorption spectra of acceptors by various simulations, it is possible to systematically investigate the emission behaviors of lanthanide complexes under different conditions. To establish the relationships between emission behaviors and various external simulations, it is necessary to study the energy transfer rate and efficiency between the donor and acceptor under different conditions to clarify the luminescent mechanism of the complexes, providing a theoretical basis for high-performance smart materials. This review focuses on the recent progress of luminescence performance of lanthanide complexes, including energy transfer mechanisms, emission color modulation, the strategies for optimizing lanthanide luminescence, and finally, various applications based on luminescence performance of lanthanide complexes and lanthanide metal-organic frameworks.

Keywords

Lanthanide complexes, downshifting luminescence, upconversion luminescence, molecular motor, pH-responsive delivery, dual-modal imaging

INTRODUCTION

The special optical, especially their characteristic sharp bands of downshifting luminescence (DL) and upconversion luminescence (UCL) of lanthanide complexes enable them wildly in numerous applications and technologies[1], for instances, multiple signal detections[2], barcoded materials[3], and bioimaging nano-probes[4], and even multiple detections for luminescent imaging in bioprobes[5], among others. Due to the Laporte rule, the excited energy of lanthanide ions (LnIII) is usually efficiently populated by their surroundings[6], which transfer their absorbed energies into the excited states of lanthanide, lighting up typical LnIII-based luminescence[7]. Since the triplet excited energies of photosensitizers are susceptible to external stimuli, their excited energies dependent on external simulations variously overlap the absorption spectra of given LnIII. Benefiting from inert transitions of LnIII for shielding effect, the changes of luminescence performance of lanthanide complexes smartly reflect their peripheral environment. Thus, the principles of the design of smart lanthanide complexes depend on modulation of energy levels between photosensitizers by different means and LnIII, adjusting their energy transfer rates and efficiencies[8].

This review dominantly focuses on the recent progress of luminescence performance of lanthanide complexes mainly reported in our group. Three sections were involved. One was the energy transfer mechanism, including intermolecular sensitization for DL and UCL. Another was modulation of emission color and strategies for optimizing lanthanide luminescence through vibrational modes and rotational and trigger modes. The last one included various applications, such as water-soluble and luminescent imaging of lanthanide functionalized molecular motors and pH-responsive delivery and dual-modal imaging of lanthanide metal-organic frameworks (Ln-MOFs). Also, we point out the confusion on UCL of lanthanide complexes in solution, especially for bio-imaging and therapy.

INTERMOLECULAR OR INTRAMOLECULAR ENERGY

Through the antenna effect, efficient lanthanide luminescence is usually achieved by employing aromatic ligands or d-block chromophores adjacent to LnIII as sensitizers[9]. That is, population of the typical lanthanide luminescence is operative through intramolecular or intermolecular energy transfer from adjacent sensitizers. The former approach requires significant synthetic efforts to functionalize bridging connectors directly bounding to the lanthanide emitters by chelation or linking[10], promoting superposition of the emission bands of the sensitizers and the absorption spectra of the emitter mainly by the Dexter mechanism, also including the Förster mechanism. Alternatively, to avoid complicated and tedious synthesis, intermolecular sensitization is simply operative by closing both of them through van der Waals/Coulomb forces, such as ion-associated sensitization within ionic pair, noncovalent ligand-to-ligand interactions, and even cation-π interactions in a bioinspired scaffold. As intermolecular energy transfer is operative through-space, besides the absorption coefficient of sensitizers, the energy transfer efficiency (h) through the Förster mechanism strongly depends on their spatial separations; thus, high concentrations closing them to promote h are needed [Table 1].

Table 1

Comparison between intermolecular and intramolecular energy transfer in discrete lanthanide complexes

Intermolecular energy transferIntramolecular energy transfer
SynthesisSimpleComplicate
InteractionVan der Waals/Coulomb forcesChelation/linking
Energy transfer efficiencyLowHigh
ConcentrationHighLow

Ion-associated sensitization for DL

It is desirable to employ transition metal luminophores as the sensitizers, which easily red-shift the ligand-to-metal charge transfer (3LMCT) or metal-to-ligand charge transfer (3MLCT) absorption bands into a visible region[10], facilitating efficient d→f energy transfer to energize adjacent LnIII[11]. Xu et al. reported the first example affording near-infrared (NIR) lanthanide luminescence by intermolecular d→f sensitization across supramolecular interactions within the ionic d-f heterometallic architecture of Ru-Er[12].

In the crystal structure, every [Ru(bpy)2(dbim)]2+ (2,2’-bipyridine, bpy; 2,2’-dibenzimidazole, dbim) was adjacent to three [Er(hfac)4]- moieties by C-H···F interactions with the shortest RuII···ErIII distance ca. 8.71 Å [Figure 1], within the Förster sphere for energy transfer. They had demonstrated NIR ErIII-based luminescence was indeed lightened through an ion-associated sensitizer of Ru-subunit when irradiated within its 3MLCT absorption band. However, the typical ErIII-based luminescence could not be observed in solutions since a large number of solvent molecules dispersed the RuII···ErIII distances out of the effective diameter of the Förster energy transfer.

Energy transfer process, luminescence optimizing and various applications of lanthanide complexes

Figure 1. (A) Interactions among [Ru(bpy)2(dbim)]2+/[Er(hfac)4]-; (B) luminescence spectra of Ru-Er [λex = 420 nm, ErIII (red); residual RuII (black)] in solid state. This figure is used with permission from the Royal Society of Chemistry[12].

Ion-associated sensitization for UCL

The upconversion (UC) of NIR to visible (vis) photons is attracted in the fields of display technology and energy conversion. Due to the pronounced nonradiative excited energy exhaustion of -XH (X = O, N, C) in organic ligands[13], the UC processes are usually confined to solids and nanoparticles[14,15]. Considering surface quenching effect and reproducibility[16], it is necessary to reduce the UC system to the molecular scale[17]. Although confronted with difficulties, the Ion-associated sensitization for UCL of discrete lanthanide-organic complexes in solution is fancy.

Dye-to-Er intermolecular energy transfer for UCL

Besides DL by intermolecular energy transfer, Hyppänen et al. realized molecular photon UCL even through intermolecular dye-to-Er energy transfer in Er(tta)4(IR-806) (tta = 2-thenoyltrifluoroacetonate, IR-806 = NIR-emitting cyanine dye) in anhydrous solution under ambient atmosphere [Figure 2][18]. In this setup, the IR-806 dye showed an absorption band centered at 808 nm and emission spectra at 827 nm. Upon irradiation at 808 nm, dual luminescences at ca. 510-540 nm (2H11/24I15/2) and 540-565 nm (2S3/24I15/2) of ErIII appeared in Er(tta)4(IR-806) rather than in Er(tta)4K or IR-806 dye. These facts suggested it was indeed by intermolecular energy transfer from IR-806 dye to Er3+ that triggered UCL at 510-565 nm by 808 nm irradiation. Also, the UCL was second-order dependent on the density of excitation power (P), suggesting an intermolecular dye-sensitized energy transfer upconversion (ETU) mechanism.

Energy transfer process, luminescence optimizing and various applications of lanthanide complexes

Figure 2. Chemical structure of ion pair of Er(TTA)4(IR-806). This figure is used with permission from the American Chemical Society[18]. ET: Energy transfer.

Yb-to-rubrene collisional energy transfer for UCL

Although triplet-triplet annihilation (TTA) is considered as an efficient approach for UCL under low P[19], it is still confronted with fundamental limitations by current strategies for NIR-vis UC. Howard and Turshatov employed Yb sensitizers to generate TTA-UCL by intermolecular energy transfer to the triplet state of the rubrene annihilator through S0 → T1 sensitization[20]. Due to the absence of covalent or supramolecular interactions between Yb subunits and rubrene, they proposed that the UC process was operative through collisional energy transfer from a Yb(β-diketonate)3 to rubrene, emitting yellow UCL at 559 nm under laser irradiation at 980 nm [Figure 3].

Energy transfer process, luminescence optimizing and various applications of lanthanide complexes

Figure 3. Scheme of TTA-UC within Yb(β-diketonate)3/rubrene. This figure is used with permission from the American Chemical Society[20].

Cr-to-Yb energy transfer for NIR UCL

According to energy levels between Cr3+ (2E/2T1)[21] and Yb3+ (2F5/2), Cr3+/Yb3+-architectures could operate via cooperatively sensitized upconversion (CSU), in which two Yb3+ (2F5/2) cooperatively transfer their excited energies into an excited state 4T2/4T1 of Cr3+, populating UCL at 755 nm, which corresponds to the transitions of CrIII from 2E state to the ground state (4A2) [Figure 4]. However, due to possible 2E (Cr3+) → 2F5/2 (Yb3+) transitions in Cr3+/Yb3+-architectures, it seems unsuitable for UC processes[22].

Energy transfer process, luminescence optimizing and various applications of lanthanide complexes

Figure 4. (A) Chemical structure of the Cr-Yb; (B) Schematic partial energy-level diagram of Yb3+ and Cr3+ relevant for CSU. This figure is used with permission from Wiley-VCH GmbH[21]. ISC: Intersystem crossing; CET: cooperative energy transfer; CSU: cooperatively sensitized upconversion.

To efficiently prevent downshifting energy transfer from 2E (Cr3+) to 2F5/2 (Yb3+), Wang et al. constructed [Cr3+][Yb3-] ion pairs (named Cr-Yb) using [Yb(dpa)3]3- (2,6-pyridine-dicarboxylate, dpa) as sensitizers and octahedral [Cr(ddpd)2]3+ (N,N’-dimethyl-N,N’-dipyridine-2-ylpyridine-2,6-diamine, ddpd) as an activator for UCL, by Förster mechanism through spatial dispersion of metal centers. In the crystal structure, every Cr3+ activator was encompassed with five Yb3+ sensitizers within the nearest distances (8.75 Å < rCr-Yb < 9.07 Å), falling the diameter of Förster energy transfer. Upon excitation at 435 nm into the 4A24T2 band of [Cr(ddpd)2]3+[23], not only the expected CrIII-based phosphorescence around 780 nm but also the presence of a Yb3+ luminescence at around 1,000 nm appeared. As Na3[Yb(dpa)3]·6H2O was non-emissive, these facts clearly indicated undesired Cr → Yb energy transfer was still present in Cr-Yb. Unexpectedly, there was no UCL observed for Cr-Lu, but intensive UCL of the activator Cr3+ (lem = 780 nm) by excitation at 976 nm at P = 67 W·cm-2 in Cr-Yb appeared. Both excitation and UC emissions falling into the NIR region favor bioimaging.

Yb-to-Eu intermolecular energy transfer for UCL

Recently, Sun et al. reported [YbEu] co-crystals constructed by discrete complexes Yb(DBM)3Bpy (sensitizer) and Eu(DBM)3Bpy (DBM = dibenzoylmethane) as an activator through noncovalent interactions, with the closest distance between Yb3+ and Eu3+ of 9.1 Å, for UCL through cooperative sensitization [Figure 5][24]. Without resonance between the virtual state of Yb3+ and the excited states of Eu3+, they considered it was the simultaneous energy transfer from two Yb3+ ions that populated Eu3+ through cooperative sensitization UC. Moreover, the optimized Yb3+:Eu3+ molar ratio of 1:1 for the most intense UCL with a quantum yield (ФUC) up to 0.67% at 2.1 W/cm2, it was considered that the rigid polyhedral structure formed by the ligands effectively protected both Yb3+ and Eu3+ from the quenching competitors, resulting in effective intermolecular energy transfer from Yb3+ to Eu3+.

Energy transfer process, luminescence optimizing and various applications of lanthanide complexes

Figure 5. Mechanism of [YbEu] assembly for DL (λex = 365 nm) and UCL (λex = 980 nm). This figure is used with permission from Wiley-VCH GmbH[24]. ISC: Intersystem crossing; DL: downshifting luminescence; UCL: upconversion luminescence.

In brief, achieving both DL and UCL by intermolecular energy transfer not only provides a simpler way of avoiding tedious synthesis but also broadens the approaches for modulation of luminescence performance of lanthanide complexes. However, it is also confronted with low energy transfer efficiency from a donor to an acceptor, even operative in higher concentration solutions for UCL, which inevitably suffers from the “aggregation-caused quenching” (ACQ) effect[25]. Moreover, regulating the excited photon transition pathway remains challenging.

LUMINESCENCE REGULATION

Emission color modulation

Traditionally, high extinction coefficient organic bridging ligands are usually employed as light harvesters to enhance lanthanide emission through the antennal effect. Thus, it is practicable to increase the rigidity of the bridging ligands for diverse structures[26] and also tune the emissive color by combining the residual emission of the sensitizers and the typical LnIII-based luminescence[27]. Xu et al. employed similar length but different conjugacy bridging ligands, bis(diphenylphosphine oxide) 1,4-butane (PBuPO2), bis(diphenylphosphine oxide)-1,4-benzene (PBePO2), and bis(diphenylphosphine oxide)-9,10-anthracene (PAnPO2) [Figure 6A] to react with Sm(hfac)3(H2O)2 (hfac- = hexafluoroacetylacetonato), affording one-dimensional ribbon {Sm(hfac)3(PBuPO2)} (Sm-PBuPO2), two-dimensional layer {Sm2(hfac)5(PBePO2)6} (Sm-PBePO2), and zero-dimensional cyclic {Sm(hfac)3(PAnPO2)}2 (Sm-PAnPO2), respectively [Figure 6B][28]. The configuration of the SmIII-based subunits changed from trans- in Sm-PBuPO2 and Sm-PBePO2 to cis- in Sm-PAnPO2. Additionally, the driving force of π···π stacking interactions transferred from the intermolecular patterns between benzene rings in Sm-PBuPO2 to the intramolecular one between anthracenyl rings in Sm-PAnPO2.

Energy transfer process, luminescence optimizing and various applications of lanthanide complexes

Figure 6. (A) Similar lengths but different levels of conjugacy of bridging ligands; (B) crystal structures of lanthanide complexes; (C) their emissions and corresponding colors depending on the bridging ligand in solid state; (D) spectroscopic function diagram of lanthanide complexes with similar lengths but different conjugated bridging ligands. This figure is used with permission from the Royal Society of Chemistry[28].

Interestingly, it is not only to control the structural dimensionality but also to regulate the emissive color of the complexes by increasing the conjugation of the bridging ligands. Due to the intermolecular π···π stacking interactions between sensitizers, their emission bands gradually red-shift to cover the fixed SmIII-based ones, producing red (CIE = 0.72, 0.28), white (0.36, 0.35), and green color emissions (0.31, 0.68) for Sm-PBuPO2, Sm-PBePO2, and Sm-PAnPO2, respectively [Figure 6C]. This strategy provided a practicable way to regulate the structure performance of lanthanide luminescent materials [Figure 6D] and also afforded a feasible method to optimize lanthanide emission through modulating the spectroscopic overlap between the emission bands of the sensitizers and the absorptions of LnIII through various simulations.

Shifting emissive colors through UC at the molecular level holds more significant meaning compared to controlling emissive colors through down-conversion. Gálico et al. had successfully designed and synthesized a series of UC molecular clusters (MCAs) containing 15 LnIII[29]. These clusters exhibited highly tunable compositions, particularly demonstrating excellent capabilities in adjusting emissive colors. For instance, {Er2Yb10}, {Y10Er1Yb4}, and {Er10Yb5} MCAs showed significant changes in emissive colors at different laser powers. At low laser power, the emitted light tended toward red, while with increasing power, it shifted toward green. This outstanding tunability in emissive colors was attributed to the molecular structure of MCAs. This structure not only increased the probability of energy transfer from YbIII to ErIII but also enhanced the intensity of upconverted emissions, achieving an extremely high UC quantum yield (UCQY) of up to 10-3%.

STRATEGY ON OPTIMIZING LANTHANIDE LUMINESCENCE

Vibrational mode - walkable dual emissions

Regarding a molecule containing two types of luminophores, one is stationary, such as the lanthanide subunits, which is not sensitive to external stimulations. The other is a walker (conjugate aromatic ligands), and its emission behavior is susceptible to the outside environment. If the two chromophores are constructed into a square structure, the parallel walkers would readily experience π···π stacking interactions, enabling the emission spectra of the walker back and forth compared to those of the stationary. Xu et al. provided a cyclic structure of {PAnPO2Eu(hfac)3}2 (PAnPO2 = 9,10-bis(diphenylphosphino)anthracenedioxide)[30], in which the two coplanar anthracenes within 3.4 Å suffered from strong π···π stacking interactions [Figure 7A]. Since such configuration possessed shorter distances between the two walkers in the aggregated state by π···π stacking interactions than in the isolated state, the central emissive bands of PAnPO2 (the walker) red-shifted to close the EuIII-based luminescence (the stationary) by increasing the concentrations [Figure 7B] or decreasing the measurement temperature [Figure 7C]. In turn, reversely diluting the solution concentrations, the emission bands of the walker leave that of the stationary stepwise. Further, using TbIII instead of EuIII, the PAnPO2-based emissions red-shifted to cross the fixed TbIII-based emissions rather than shield almost the EuIII ones [Figure 7D]. Such non-destructive physical cycles enable these phenomena to be completely reversible and reliable. The walker shows bathochromic and hypsochromic behavior dependent on temperature and concentration, while the stator remains static throughout; the former can cross and leave the latter, depending on experimental environments. Thus, by precisely controlling the separations of adjacent walkers, the cyclic dimeric lanthanide would be promising in information gateways, providing a simple way to spread the information from one to another.

Energy transfer process, luminescence optimizing and various applications of lanthanide complexes

Figure 7. (A) Ring structures of {PAnPO2Eu(hfac)3}2 with vibrational modes; (B) normalized concentration-dependent luminescence (λex = 350 nm); (C) normalized temperature-dependent emission (λex = 350 nm) in CH2Cl2 (1.5 × 10-5 M); (D) normalized emission spectra of {PAnPO2Tb(hfac)3}2 (λex = 330 nm) in CH2Cl2 (1.5 × 10-5 M, black) and solid state (red) under room temperature. This figure is used with permission from Springer Nature[30].

ROTATIONAL MODE

AIPE-activity of discrete NIR lanthanide architectures

Considering NIR materials for powders or thin films, it is necessary to broaden “aggregation-induced phosphorescence enhancement” (AIPE) family from transition metals to discrete NIR lanthanide compounds. As the excited state of organometallic luminogens and “aggregation-induced emission” (AIE) ligands are sensitive to external stimuli, it is theoretically challenged to afford the AIPE activity of metallic compounds through energy transfer[31], although the AIPE on organic compounds has been reported for decades[32]. As far as lanthanide complexes are concerned, the shielding effect and the inner f-f’ electronic transitions enable their excited energies to be unsusceptible to the outside environment; the excited energies of ligands induced by aggregation could efficiently energize the lanthanide(III)-subunit, possibly affording AIPE-active lanthanide complexes.

Ideal sensitizers suitable for sensitizing NIR LnIII-based luminescence have been demonstrated to mainly possess three combining features, including matched energy to the given NIR lanthanide ion, effectively reducing nonradiative dissipation, and low-energy shifting the excitation window [Figure 8][33]. Herein, we afforded two NIR lanthanide complexes by employing AIE-active tetraphenylethylene (TPE) derivatives as sensitizers[34]; the crystal structure shows that the NdIII chelates two ligands to furnish a ten-coordinate environment, leading to formation of Nd(TPE2-BPY)2 and Nd(TPE-BPY)2. The corresponding signals of Electrospray ion mass spectra (ESI-MS) under positive patterns confirmed their stabilities in solutions. Both TPE2-BPY and TPE-BPY had matched energy to NdIII, effectively preventing nonradiative processes by aggregation, even red-shifting the excitation wavelength through twisted intramolecular charge transfer. Moreover, the excited energies of the AIE-active sensitizers were significantly enhanced by structural rigidification, which energized the excited state of inert NdIII through the antenna effect.

Energy transfer process, luminescence optimizing and various applications of lanthanide complexes

Figure 8. Strategy on optimizing NIR lanthanide luminescence performance. This figure is used with permission from the American Chemical Society[33]. AIE: Aggregation-induced emission; NIR: near-infrared.

Since TPE2-BPY was equipped with more TPE subunits than TPE-BPY, the luminescence enhancement in 90% n-hexane for the former (18 times) was more obvious than that of the latter (11 times) respect to those in no n-hexane, suggesting the AIE-activity of TPE2-BPY was superior to that of TPE-BPY. Consequently, ηL→Nd of Nd(TPE2-BPY)2 were superior to those of Nd(TPE-BPY)2. Additionally, the intensity amplification in 95% n-hexane of Nd(TPE2-BPY)2 (about 15 times) was more than that of Nd(TPE-BPY)2 (1.8 times) with respect to that without n-hexane. It was reasonable to draw the conclusion that the ηL→Nd dependent on the number of TPE in sensitizers by structural rigidification, induces the crystal of Nd(TPE2-BPY)2 with one of the longest lifetimes (9.69 μs) among NdIII-based complexes containing C−H bonds.

Switchable sensitizers stepwise activated to lighten LnIII-based emissions

Ideal bioprobes are considered to have consistent signals with high sensitivity and resolution in any concentrations so that they could always be captured in complicated bioassay systems. However, it is hard to accomplish this aim for the common ACQ effect, hampering the detection of single luminescent molecules in high concentration solution[35]. Herein, the lanthanide complex [TPE-TPY-Eu(hfac)3] was reported to be equipped with switchable sensitizers [Figure 9A]. One was a “bladed” structure, TPE-TPY (4'-(4-(1,2,2-triphenylvinyl)phenyl)-2,2':6',2''- terpyridine) with AIE-activity, while the other was ACQ-active planar luminogenic hfac-, stepwise activating for consistent lanthanide emission triggered by variable concentrations [Figure 9B][36].

Energy transfer process, luminescence optimizing and various applications of lanthanide complexes

Figure 9. (A) Diagram of switchable TPE-TPY-Eu(hfac)3; (B) its emission spectra with different concentrations in CH2Cl2; (C) its excitation spectra (λem = 613 nm) in different concentrations; (D) emission dependent on different concentrations in CH2Cl2 at 298 K. This figure is used with permission from Springer Nature[36]. ACQ: Aggregation-caused quenching; AIE: aggregation-induced emission.

The excitation spectra of [TPE-TPY-Eu(hfac)3] (10-5 M) only resembled those of Eu(hfac)3·2H2O, not [TPY-Eu(NO3)3], suggesting it was the main hfac- instead of TPY transferring the excited energy into the Eu3+ with η = 42.5% in very dilute solutions (≤ 10-5 M). Additionally, the much lower emission intensity (λex = 306 nm) of [TPE-TPY-Eu(NO3)3] than that of [TPY-Eu(NO3)3] indicated it was the intramolecular rotations of TPE that consumed the excited energies of TPY, markedly reducing ηTPY→Eu. All these facts confirmed it was the hfac- that sensitized the EuIII-based emission at low concentrations.

As shown in Figure 9C, the maximal excitation wavelengths red-shifted to 426 nm from 310 nm by increasing the concentrations, suggesting different sensitizers in variable concentrations were stepwise activated to lighten lanthanide emission. Thus, different types (AIE and ACQ effects) of sensitizers were stepwise activated to energize EuIII-based emissions. Also, the excitation wavelength extended from ca. 310 to 426 nm, and Φem dropped from 3.8% to 0.16%, then up to 0.95% by increasing the concentrations [Figure 9D]. Moreover, a good linear relationship between lexvs. {lg(M)} indicated that the concentration of [TPE-TPY-Eu(hfac)3] could be monitored digitally by lex, realizing consistent EuIII-based emission within large-scale concentrations by switching the ACQ/AIE-active sensitizers, suitable in complicated bioassay systems.

Trigger mode

Excluding statistical crystallization[37], the control of the synthesis of heterometallic complexes containing more than two different LnIII is confusing due to similar ionic radius and coordination preferences[38]. Making use of versatile ligating µ2,3-fashion of unsubstituted Hq (8-hydroxyquinoline), {Lnq2(hfac)}2[(μ3-OH)]2 [Ln*q(hfac)]2 (Ln, Ln* = Er, Gd, Yb, named [Ln]2[Ln*]2) with two types of lanthanide atoms were constructed [Figure 10A][39]. Since the nonradiative O−H vibration from the direct linkage of μ3-OH- to the LnIII severely quenched the LnIII-based luminescence, the introduction of 2 equivalents of fluoride to generate strong O−H···F interactions successfully prevented the vibrational nonradiative processes, inducing two-fold intensity enhancement of YbIII-based luminescence, rather than the hoped-for UCL signals of [Yb]2[Er]2 [Figure 10B].

Energy transfer process, luminescence optimizing and various applications of lanthanide complexes

Figure 10. (A) Heteronuclear lanthanide complexes by metallic blocks orientational assembly; (B) their emission intensity depending on fluoride in CH2Cl2. This figure is used with permission from Springer Nature[39].

Till now, both homonuclear erbium and discrete heteronuclear lanthanide architectures[40] exhibit weak molecular UCL with a low quantum yield (ФUC < 0.1%), even under extreme conditions[41]. It has been demonstrated that UC processes in discrete heteronuclear lanthanide compounds through mixed mechanisms may afford highly intensive UCL in an aqueous solution[42,43]. Instead of traditional supramolecular chemical self-assembly for construction of heteronuclear lanthanide compositions[44], Wang et al. employed tridentate H2hmq (2-(hydroxymethyl)quinolin-8-ol) replacing bidentate Hq, affording {[Ln2Ln*(Hhmq)3(OAc)3(hfac)2]+ [Ln*(hfac)3(OAc)(MeOH)]-} (Ln, Ln* = Er, Gd, Yb)[45]. The presence of corresponding signals of ESI-MS in positive models confirmed the consistency of solid-state/solution structures of the stable [Yb2Er]+/[Er2Yb]+ core in solution [Figure 11A]. The μ2- and μ3-bridged two and three Ln atoms, with close Ln···Ln* distances (< 3.7 Å), favor energy transfer from sensitizer (S) to activator (A). Also, O–H···F interactions are employed to prevent nonradiative processes from the undehydrogenated hydroxymethyl group of Hhmq-[46].

Energy transfer process, luminescence optimizing and various applications of lanthanide complexes

Figure 11. (A) Crystal structure of [Yb2Er]+ and the positive mode of ESI-MS spectra of [Yb2Er]+/[Er2Yb]+ in MeOH; (B) DL spectra (λex = 980 and 400 nm) of [Yb2Er]+ before and after addition of 2 equiv of fluoride in solid state; (C) The fluoride-dependent UCL of [Yb2Er]+ (2 × 10-4 M) under 2 W/cm2 in CH2Cl2; (D) The P-dependent UCL of [Yb2Er]+ (2 × 10-4 M) with 3.5 molar equivalent [Bu4N]F. This figure is used with permission from Wiley-VCH GmbH[45]. UCL: Upconversion luminescence; ESI-MS: electrospray ion mass spectra; DL: downshifting luminescence.

With the help of diamagnetic La3+, the predictable hydrogen nuclear magnetic resonance (1HNMR) spectra of EuIII, and Density functional theory (DFT) calculations, scrambling possibility of different LnIII in the structure could be excluded. As only the cationic motif is emissive, by merely exchanging the precursors, [Yb2Er]+ and [Er2Yb]+ were afforded with different ratios of S to A. Doublet emission bands from both YbIII (1,010 nm) and ErIII (1,535 nm) were observed in both solid state [Figure 11B] and CH2Cl2 solution at ambient atmosphere by irradiation within the absorption bands of YbIII (980 nm) and Hhmq-/hfac-(250 nm < λex < 420 nm). Due to strong nonradiative competitors exhausting excited energies, the hoped-for UCL could not appear. However, after introduction of 1.5 molar equivalent [Bu4N]F into the solutions, a competitive UCL by a 980 nm laser irradiation, even with P = 0.288 W/cm2, was observed, and 7.3-time emission enhancement with the optimized 3.5 molar equivalent [Bu4N]F was present [Figure 11C]. Correspondingly, powder X-ray diffraction (PXRD), multistage mass spectra (MS), and the 1H NMR of [Eu2La][La] were employed to confirm the stability of [Yb2Er]+ under fluoride intervention. Since the enhancement of DL was negligible with respect to that of UCL, it is practicable to switch on UCL by fluoride association. Moreover, the competition between linear decay and UC on depletion of intermediate excited states is more intense under fluoride introduction [Scheme 1], and ηDL (0.30) was slightly higher than ηUC (0.28).

Energy transfer process, luminescence optimizing and various applications of lanthanide complexes

Scheme 1. Yb2Er-centered energy-level diagrams derived from the observed emissions associated with 3.5 equiv of fluoride and possible mechanisms highlighted. This figure is used with permission from Wiley-VCH GmbH[45]. DL: Downshifting luminescence; UCL: upconversion luminescence.

By increasing P amplifies the competition between DL and UCL [Scheme 1], the slope lower to 1.25, suggesting a saturated intermediate state. Moreover, the long risetimes, non-P-dependent UCL (P > 2 W/cm2), and matched energy of two sensitizers for one activator suggested both ETU and CSU mechanisms operative in [Yb2Er]+ [Scheme 1]. Beneficial from more sensitizers energizing fewer activators within short distances (3.7 Å), [Yb2Er]+ exhibited higher ФUC and longer decays than [Er2Yb]+. More importantly, its relative ФUC is at least twenty times stronger than that in the previous results.

As discussed above, strategies for optimizing lanthanide luminescence include making use of vibrational modes for walkable dual emissions and rotational modes for AIPE-active lanthanide complexes, even triggering the ACQ/AIE for consistent lanthanide emission, and using fluoride to prevent nonradiative processes triggering enhanced DL and even achieving high ФUC of lanthanide complexes in solutions [Table 2]. All these strategies are based on modulation of energy levels between the photosensitizer in external simulations and the inner LnIII, corresponding to their various energy transfer rates and efficiencies. The changes of luminescence performance of lanthanide complexes smartly reflect their peripheral environment.

Table 2

Differences between these three modes in discrete lanthanide complexes

Vibrational modeRotational modeTrigger mode
SensitizerConjugate aromatic sensitizer in square structureAIE-active sensitizerContaining nonradiative competitor
Mechanismπ···π stacking interactions red-shifting and shielding the fixed lanthanide emissionThe enhanced excited energy of sensitizer by structural rigidification energizing the inert LnIII excited state through the antenna effectPreventing nonradiative processes by hydrogen bonding

VARIOUS APPLICATIONS

Water-soluble and luminescent tracking lanthanide-complexed motors

Beneficial from noninvasive controls, light-driven molecular motors are promising for smart materials, even in bionanotechnology[47]. However, due to stilbene core structure, they are inherently hydrophobic[48]. Although it has been demonstrated that metal-complexed motors disturb the photochemical isomerization of free motors under visible light and even experience triplet energy transfer proceeds[49,50], most of them retain their performances in nonpolar solvents rather than aqueous mediums[51,52]. Moreover, the rotary speed in thermal helix inversion (THI) steps remains unclear, dependent on the efficiency of triplet energy transfer.

In the reaction of molecular motors based on overcrowded alkenes (Lm) with Ln(NO3)3, Li et al. reported the first water-soluble ionic lanthanide-complexed motors [(Lm)2-Ln(NO3)2(H2O)2]∙NO3 (Ln(Lm)2)[53]. In the crystal packing [Figure 12A], LnLm(R)2 and LnLm(S)2 alternate stacking with each other, inducing almost no signals of the circular dichroism (CD) signals in methanol. Confirmed by only one set of 1HNMR signals of -CH3, the possible conformer of LnLm(R)Lm(S) in solutions could be reasonably excluded. As shown in the crystal structure, the Ln-Lm interactions not only shorten N1–N2 distances but also planarize the rotator, promoting charge transfer from ligand to metal (3LMCT). Moreover, in comparison with the decay lifetimes of phosphorescence at 530 nm of Lm, the residual ones in Ln(Lm)2 (Ln = Eu, Ho, Er) are obviously shorter than that in La(Lm)2 by irradiation within 3LMCT absorption bands, sensitizing typical lanthanide-based luminescence. Thus, by employing Lm as a sensitizer, it was able to variously induce different triplet ηL→Ln by replacing LnIII (Ln = Eu, Ho, Er), correspondingly decreasing the intensity of low-energy 3LMCT absorption in the order: Eu(Lm)2 < Er(Lm)2 < Ho(Lm)2 < La(Lm)2. Furthermore, the 3LMCT sensitization of EuIII-based emission of Eu(Lm)2 [Figure 12B] favors tracing its movement by delayed emission spectra [Figure 12C], and the temperature-dependent luminescence makes it as possible as luminescent thermometers.

Energy transfer process, luminescence optimizing and various applications of lanthanide complexes

Figure 12. (A) Crystal structure of La(Lm)2; (B) emission spectra of Eu(Lm)2 and Eu-based lifetimes (λem = 616 nm, inset); (C) delayed emission spectra of Eu(Lm)2 in MeOH; (D) CD spectra of La(Lm)2 [LaLm(S)2, black, LaLm(R)2 red; calculated one, blue]; photochemistry (E) and THI processes (F) of Eu(Lm)2 in water. This figure is used with permission from Wiley-VCH GmbH[53]. CD: Circular dichroism; THI: thermal helix inversion.

Since the Ln-Lm interactions contract the stator to decrease the steric hindrance on the “fjord region”, accelerating the rotation of Lm[54]. From the crystal structure, “lanthanide contraction effect”, and DFT calculation [Figure 12D], the rotation followed the order: Er(Lm)2 > Eu(Lm)2 > La(Lm)2. Since the heavy atom effect induced by the introduced LnIII favors the percentage of triplet states of Lm[55], and the excited state of Ln-complexed Lm is easily quenched by intramolecular rotations, the higher ηL→Ln for triplet states, the lower energy barrier ($$ \Delta^\neq G $$). We thus conclude that both allostery and 3LMCT sensitization cooperatively modulate their rotations as: La(Lm)2 < Ho(Lm)2 < Eu(Lm)2 < Er(Lm)2. On the other hand, hydrogen bonds around LnIII favor their rotations in different aqueous mediums, inducing the rotary speed in methanol to be slower than that in water [Figure 12E]. Besides solvent effects[56], hydrogen bonds surrounding Ln-subunits in water more and stronger than those in methanol may account for the faster rotations in H2O than those in MeOH, favoring the THI process [Figure 12F][57]. Therefore, this work puts up a novel strategy for water-soluble, visible-light-driven, controllable rotation, and luminescence tracing lanthanide functionalized ionic motors in an aqueous medium, pushing forward the progress of their principles and applications in biological fields.

pH-responsive delivery and dual-modal imaging of Gd/Tm-MOFs

Beneficial from linear luminescence, large specific surface areas, and porous structures, Ln-MOFs have been widely applied to catalysis[58], bioimaging[59], besides sensing[60]. Due to minimal auto-fluorescence interference, deeper NIR light penetration, and slight photo-damage, Ln-MOFs with UCL offer huge potential for vivo optical imaging and bioassay[61]. It has been demonstrated that Gd3+/Tm3+ co-doped metal-organic frameworks (MOFs) possess superior UCL and can even potentially be used as UCL/magnetic resonance image (MRI) dual-mode imaging. Herein, through a facile one-step hydrothermal method, we designed Gd/Tm-MOFs ([(Tm, Gd)(BTC)·(H2O)·DMF])[62], which exhibited strong UCL with the lifetime of 379 ± 2 μs and the quantum yield of 0.76 % under the Tm3+ content at 6%, along with superior positive MRI. Further employing uniform mesoporous silica (mSiO2) shells and folic acid (FA) to functionalize Gd/Tm-MOFs enabled its drug loading of doxorubicin hydrochloride (DOX) from 10.6 up to 41.5 mg/g. Correspondingly, the drug release was also climbed up from 12% to 64% by precise regulation of the pH to 7.4 from 5.8 [Figure 13].

Energy transfer process, luminescence optimizing and various applications of lanthanide complexes

Figure 13. The schematic diagram of Gd/Tm-MOFs@mSiO2-FA on anti-tumor drug loading and UCL/MRI dual-mode imaging. This figure is used with permission from the Royal Society of Chemistry[62]. UCL: Upconversion luminescence; MRI: magnetic resonance image.

Both Tm3+ and Gd3+ in the crystal structure of Gd/Tm-MOFs exhibited a seven-coordination mode, which facilitated further interactions between the empty orbits of Gd3+ and H2O and also provided more opportunities for large positive magnetic susceptibility anisotropy of Tm3+ to combine with H2O or -OH on the surface of mSiO2 shells. All these facts improved the longitudinal relativity (r1) value of 225.86 mM-1·s-1 up to the second rank among the reported gadolinium complexes. Consequently, Gd/Tm-MOFs@mSiO2-FA was considered as one of the promising drug carriers/releases and enabled UCL/MRI dual-mode imaging, advancing the development of accurate cancer treatment in the near future.

Temperature sensing

In lanthanide complexes, the presence of organic chromophores allows nonradiative excited energy dissipation through molecular vibrations. Decreasing the measurement temperature prevents nonradiative energy dissipation, increasing the luminescent intensity and lifetime. Thus, studying the correlation between luminescent intensity and temperature enables the development of temperature sensors[63]. To develop non-contact thermometers with superior sensitivity and an adjustable response range, optimizing energy transfer efficiency between different energy-matched LnIII by altering their molar ratios is more practicable[64]. Recently, an easy-to-use luminescent thermometry by coating lanthanide complexes onto surfaces with an airbrush was developed[65]. By optimizing the film compositions of Tb3+ and Eu3+ complexes, a thermometric parameter based on the intensity ratio of green and red channels by UV-excitation was afforded, which exhibited high sensitivity (1.7%-3.6% K-1) in the 278-312 K range, making it a practical for real-time thermal monitoring of surfaces. For more information, please refer to[66,67].

Biological imaging

Not only responding to diverse environmental stimuli by structural modulation based on a molecular platform but also superior NIR emission, stability, and longer lifetimes enable lanthanide-coordinated compounds ideal for fluorescence lifetime imaging (FLIM)[68,69]. Highly stable and luminescent β-fluorinated Yb3+ complexes, which exhibited strong NIR luminescence (Ф = 23%) and extended decay lifetimes (249 μs), were suitable for both steady-state and time-resolved FLIM within the NIR range[70]. Interestingly, these complexes showed specific luminescent signals, precisely revealing intracellular lifetime distributions (100-200 μs) as FLIM. These results highlighted the potential of these NIR lanthanide complexes as efficient tools for biological imaging, offering high sensitivity and discrimination capabilities. Further to realize targeted bioimaging, a reactive chemical group, which allowed coupling to biological vectors, was employed to enhance the functionality of lanthanide luminescent bioprobes (LLBs)[71]. For instance, incorporating a primary amine on the para-position of the macrocyclic pyridine unit as LLBs involved them coupling to biological vectors (proteins, peptides, antibodies) for deep targeted in vivo bioimaging.

Organic light-emitting diode

Lanthanide complexes with organic ligands also show great potential for applications in optoelectronic devices, particularly in organic light-emitting diodes (OLEDs)[72]. A novel class of EuII complexes relying on d-f transitions demonstrate high photoluminescence quantum yield (PLQY) and remarkable stability in ambient air conditions [Figure 14A][73]. They were capable of achieving 100% exciton utilization efficiency and mitigating efficiency roll-off in OLEDs. To afford blue OLEDs, triazolyl borate with controllable substituents on the ligands of Ce(III) complexes with PLQYs > 95% for the blue 5d-4f transitions in both solid powder and diluted dichloromethane solutions and lifetimes in tens of nanoseconds range were afforded as emitting layer material [Figure 14B][74], achieving a maximum external quantum efficiency of 14.1% and a maximum luminance of 33,160 cd·m-2 for the blue OLEDs. These relative findings underscored the potential of lanthanide complexes in OLED applications. For more information, please refer to[72].

Energy transfer process, luminescence optimizing and various applications of lanthanide complexes

Figure 14. (A) Eu(II) complexes; (B) Ce(III) complexes, R1 = R3 = H or Me; R2 = H, Me, Br; R4 = Pz, nBu, iPr, H. The figure (A) is used with permission from the American Chemical Society[73]. The figure (B) is used with permission from Wiley-VCH GmbH[74].

CONCLUSION AND OUTLOOK

Unlike the intermolecular energy transfer from the sensitizers to the activators controlled by second-order kinetic rate laws in lanthanide-doped solids, the one in discrete lanthanide complexes is operative by first-order kinetic rate laws. Therefore, their luminescence performance is highly dependent on that of photosensitizers, the excited energies of which are sensitive to external stimuli, inducing versatile luminescence performance of lanthanide complexes smartly reflecting the peripheral environment. Thus, multifunctional lanthanide-based hybrid luminescent materials exhibit wide applications[75]. Moreover, smart lanthanide complexes with versatile emission behaviors possessing greater superiority than traditional materials can be developed as the next generation of intelligent materials. For example, (1) different luminescence behaviors under various external stimuli are more intelligent than those of conventional functional materials that simply repeat a single behavior[76]; (2) The smart bio-probes triggered by concentrations promote consistent luminescence signals in a wide range of concentrations, not only overcoming the false detection of the traditional probe but also red-shifting the excitation wavelength to the visible region, reducing the damage to biological organisms[77]; (3) Beneficial from UV to NIR emission spectra, lanthanide-based materials usually act as both downshifting and UC layers, conversing the solar spectra via luminescence to match the absorption of semiconductor devices, to improve the efficiency of solar cells[78] and so on.

Furthermore, benefiting from easy reproducibility, optical rationalization, and controlled speciation, molecular UCL, especially one of water-soluble discrete heteropolynuclear lanthanide assemblies, potentially brings new dimensional applications, especially in deep-seated lymphoma diagnosis and optogenetics therapy. Although great progress has been made on UCL of lanthanide complexes in solution, the achievement of a high ФUC molecular UC system under mild conditions by shifting the excitation window, regulating the excited photon transition pathway, and even in an aqueous solution suitable for bio-imaging and therapy is still on the way[79]. Thus, setting up a library of structure-property relations for functional lanthanide materials to bring new insights for a deeper understanding of future superior materials is necessary and urgent[80].

DECLARATIONS

Authors’ contributions

Prepared and corrected the manuscript: Fan W, Wang H, Huang X, Shi T, Du J, Xu HB

Availability of data and materials

Not applicable.

Financial support and sponsorship

This research was funded through the NSFC (22273019).

Conflicts of interest

All authors declared that there are no conflicts of interest

Ethical approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Copyright

© The Author(s) 2024.

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Cite This Article

OAE Style

Fan W, Wang H, Huang X, Shi T, Du J, Xu HB. Energy transfer process, luminescence optimizing and various applications of lanthanide complexes. Chem Synth 2024;4:12. http://dx.doi.org/10.20517/cs.2023.35

AMA Style

Fan W, Wang H, Huang X, Shi T, Du J, Xu HB. Energy transfer process, luminescence optimizing and various applications of lanthanide complexes. Chemical Synthesis. 2024; 4(1): 12. http://dx.doi.org/10.20517/cs.2023.35

Chicago/Turabian Style

Fan, Wei, Huijuan Wang, Xuejian Huang, Tingchang Shi, Jing Du, Hai-Bing Xu. 2024. "Energy transfer process, luminescence optimizing and various applications of lanthanide complexes" Chemical Synthesis. 4, no.1: 12. http://dx.doi.org/10.20517/cs.2023.35

ACS Style

Fan, W.; Wang H.; Huang X.; Shi T.; Du J.; Xu H.B. Energy transfer process, luminescence optimizing and various applications of lanthanide complexes. Chem. Synth. 2024, 4, 12. http://dx.doi.org/10.20517/cs.2023.35

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