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. 2024 Aug 20;4(10):3813–3822. doi: 10.1021/jacsau.4c00468

Unearthing the Real-Time Excited State Dynamics from Antenna to Rare Earth Ions Using Ultrafast Transient Absorption

Waygen Thor †,‡,∥, Hei-Yui Kai †, Yik-Hoi Yeung †, Yue Wu ∥, Tsz-Lam Cheung ∥, Leo K B Tam †, Yonghong Zhang ∥,▽, Loïc J Charbonnière ‡,*, Peter A Tanner †,*, Ka-Leung Wong †,*
PMCID: PMC11522919  PMID: 39483220

Abstract

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The conventional energy transfer pathway in organic lanthanide complexes is purported to be from the excited singlet state of the chromophore to the triplet state and subsequently directly to the emitting state of the trivalent lanthanide ion. In this work, we found that the energy transfer occurs from the triplet state to the nearest energy level, instead of directly to the emitting state of the lanthanide ion. The triplet decay rate for different lanthanide ions follows an energy gap law from the triplet level to the receiving level of the lanthanide ion. Three different categories of complexes were synthesized and inspected using different techniques, demonstrating the universality of our findings. This work renews the insights to conventional findings, highlighting the importance of the energy gap between the triplet state and the nearest lanthanide energy level in optimization of light harvesting. The rationale of ligand design of chromophores should be reconsidered, leading to various applications of lanthanide complexes with enhanced quantum yield and brightness.

Keywords: Lanthanide luminescence, Excited state dynamics, Photosensitization mechanism, Time-resolved spectroscopy, Energy transfer, Transient absorption

Introduction

In 1942, Weissman demonstrated strong Eu3+ luminescence from organic coordinated complexes, and this method of harvesting light from organic chromophores is widely recognized as the antenna effect.1 With the aid of the antenna effect, luminescence from trivalent lanthanide ions can be harvested despite the weak absorption coefficient of Ln3+.2,3 This discovery expands the possibilities of applying organic lanthanide complexes in various fields of application, including bioimaging,4−6 up-conversion,7−9 and functional materials.10−12 Understanding the energy transfer (ET) mechanism is crucial for optimizing the optical properties of organic lanthanide complexes. The nonradiative processes from the organic chromophore to the Ln3+ are typically responsible for determining the quantum yield13 or brightness14 of a chemical system. This ET from the antenna is generally said to follow the sensitization pathway from singlet (S1) → triplet (T1) → Ln3+ (Figure 1a).15−19 Often, the optimization of the ligand focuses upon the transfer from the triplet state to the lanthanide ion.20,21 However, the energy can also be transferred directly to the lanthanide singlet state,22 or from charge transfer (CT) states, which form electronically conjugated donors and acceptors in the same system.23,24 The main focus herein is on the conventional decay pathway from S1 to T1 and subsequently to Ln3+ since there is no spectral evidence that CT states are involved in the systems investigated in this work, and we do not discuss this further.25

Figure 1.

Figure 1

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(a) Illustration of the Jablonski diagram showing the conventional ET mechanism and the proposed ET mechanism occurring from the triplet state to the nearest energy state, as investigated in this work, instead of directly to the luminescence state of the trivalent lanthanide ions. This process can be monitored by employing femtosecond transient absorption spectroscopy, which monitors the change in the absorption at different time delay as illustrated. (b) The chemical structures of Ln(TTA)3·2H2O, Lnphen(TTA)3, LnLa, LnLb, and LnLc, which can be categorized into β-diketone-based, cyclen-based, and peptide-based ligands, to demonstrate the universality of the proposed mechanism.

Gaining mechanistic understanding of lanthanide luminescence is challenging, and each ET step is crucial, including the intersystem crossing (ISC) from S1 ⇝ T1. It has been argued that the ISC rate is the rate-determining step to sensitize the luminescence of Ln3+.26,27 Despite the fact that ISC from S1 ⇝ T1 is spin-forbidden to first order, the mixing of singlet and triplet states is facilitated by a nearby heavy atom.28 The efficiency of ISC plays an important role in the ET model, as proven by calculation and experiment. Arppe et al. reported that an enhanced ISC rate can lead to the faster depopulation rate of the triplet state, including phosphorescence rate and ET rate to the lanthanide ion.29 Similar phenomena demonstrating the importance of ISC have been observed in the case of a Tb-coordinated complex using a multiconfigurational calculation.30 Introducing a Ag+ or Cu+ metal cation into a Tb-coordinated peptide loop decreases the rise time of Tb3+ due to the enhanced ISC by the heavy atom effect.31 By incorporating a heavier atom in the meta position of 1-phenyl-3-pyridin-3-yl-propane-1,3-dione (PPPD) and investigating the system using modeling, Wu et al. found that the ligand-centered ISC rate increased, followed by the rise rate of the 5D0 → 7F2 emission. In addition, a model with a good spin quantum number of the combined metal–ligand system was introduced to gain insight into the underlying ET.19

The role of ISC is more significant in the case of nonluminescent lanthanide complexes, where ligand-centered transitions are solely observed. Tobita et al.32 reported that phosphorescence lifetimes were found to be similar for closed shell La3+ (0.13 s) and Lu3+ (0.12 s) complexes but much smaller for Gd3+ complexes (2.4 ms), due to enhanced T1 ⇝ S0 nonradiative relaxation of the triplet state in the presence of Gd3+.

In addition to ISC, the ET efficiency from the triplet to Ln3+ is critical in obtaining a high luminescent quantum yield.33 The location of the triplet energy level is crucial in the sensitization of the lanthanide, for which the energy gap is said to range between 2500 and 5000 cm–1 above the luminescent state of the Ln3+.20,34 The downhill energy cascade then follows to promote the forward ET rate from the triplet state to Ln3+ and at the same time prevent the back ET. A significant correlation has been identified between the luminescence quantum yield and the ET rates.27,35 Despite challenges in discerning the ET rate from the triplet state to the Ln3+, employing Eu3+ complexes as a demonstration, it has been estimated using theory and experiment.19,36

To elucidate the excited dynamics in lanthanide complexes, which occur within the ultrafast time scale of picoseconds to nanoseconds, a spectroscopic technique capable of investigating this range is necessary. Femtosecond transient absorption spectroscopy (fs-TAS) can be employed to investigate the excited state properties of organic lanthanide complexes.37−39 Mara et al. demonstrated that the depopulation of the triplet state to the 5D1 state of Eu3+ occurs in the sub-picosecond scale, while that of the Gd-analog falls within the nanosecond range.26 Through an analysis of NIR-emitting Ln3+ complexes, Chong et al. concluded that the kinetic profiles obtained using fs-TAS can be essentially quantitative, correlating with the luminescence quantum yield.40 Instead of fs-TAS, nanosecond-TAS (ns-TAS) was utilized to examine the depopulation rate of the triplet state in analogous Gd3+ and Yb3+ complexes, which exhibited slower depopulation rates compared to the luminescent Er3+ analog.41 By calculating the difference between the ground state and excited state absorption at a certain wavelength of the complex at different time delays, the change in optical density, ΔOD(λ, t) can be obtained from fs-TAS, which can be defined as

graphic file with name au4c00468_m001.jpg 1

where T(λ, t) and T0(λ, t) are respectively the transmittance with pump light (exciting ray) and without pump light; Ipump and Iunpump are the transmitted light intensity of the probe (detection ray) with pump and without pump, respectively.

Often, the rise time of the lanthanide luminescence is scrutinized to monitor the dynamics of the triplet state. However, the rise lifetime of the luminescent state may be unrelated to the triplet decay lifetime.42 This could indicate that the ET from the antenna to the metal ion does not occur directly to the luminescent Ln3+ state. Furthermore, a previous study concerning the Sm3+ complex suggested that the receiving state of Sm3+ is slightly higher than the luminescent state.43 The transfer from the triplet state to the relevant energy state of a lanthanide ion can be studied by employing fs-TAS at the picosecond scale. Therefore, we hypothesized that ET occurs from the triplet state to the nearest energy state of the Ln3+ (as shown in Figure 1a) if there are intermediate states, rather than directly to the luminescent state of the Ln3+, as conventionally posited. To demonstrate the validity of our hypothesis, it is crucial to investigate the impact of different Ln3+ ions on (i) the ISC rate, by observing the triplet rise lifetime; and (ii) the ET rate to the relevant Ln3+ ion, by observing the T1 decay rate, with the aid of fs-TAS. In this work, we utilize spectroscopic studies on five different systems, Ln(TTA)3·2H2O, Lnphen(TTA)3, LnLa, LnLb, and LnLc (Figure 1b) to resolve these questions and investigate the photophysical processes more deeply. These systems can be separated into three different categories, which are β-diketone-based, cyclen-based, and peptide-based ligands, to demonstrate the universality of the proposed mechanism. We opted to include β-diketone-based complexes in our study because they have been extensively investigated for mechanistic studies.27,44,45 In addition, we chose cyclen-based and peptide-based complexes,6,46−48 as they are commonly used in biological applications due to their effectiveness in biological systems.

Results and Discussion

Lanthanide complexes typically luminesce in both the visible and near-infrared (NIR) regions, depending upon the particular metal ion. Inspecting the luminescent properties for the Ln(TTA)3·2H2O series, we find that visible emission is observed for Ln = Sm, Eu, and Tb (Figure S12), whereas NIR emission is observed for Nd, Sm, Yb, and Er (Figure S13). Ligand singlet emission is observed for Ln = Lu, Gd, and Tb at room temperature (RT) (Figure S12).

As previously highlighted, the fs-TAS can be employed to explore the dynamics of excited states that occur within the picosecond time scale. The RT fs-TAS of the three sets of complexes across the Ln3+ series show a similar profile shape for the same complexes (Figures 3, S18–S24, S50–S54, S56–S58, S60, S61, S63, S64) since they are essentially ligand-based. As demonstrated using Ln(TTA)3·2H2O, Ln = Sm, Eu, the spectral shape remains invariant despite the solvent environment (Figures S39–S42, S45–S47).

Figure 3.

Figure 3

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3D plot of the fs-TAS of 100 μM Ln(TTA)3·2H2O, LnLb, and LnLc in CHCl3 from 0.1 ps to 5 ns, separated in different categories. The time delay is plotted against wavelength, while the ΔOD is plotted as a color bar from positive change (red) to negative change (blue) as indicated. Across all categories, the +ΔOD at RIII (as labeled) undergoes a quicker return to zero in Sm3+ as compared to Eu3+, despite the similar energy of the luminescent state of Eu3+ and Sm3+. This indicates that the transfer from the triplet state is to the higher lying energy states of the Ln3+.

The analysis of TAS spectra indicates that a positive change in optical density (+ΔOD) can be attributed to an excited state absorption, while a negative change in optical density (−ΔOD) can correspond to emissions from the sample. As seen in Figure 2, we can classify the spectra into three different regions for Ln(TTA)3·2H2O: (i) RI, with +ΔOD at ca. 390 nm; (ii) RII, with −ΔOD at 425–500 nm; and (iii) RIII, with maximum +ΔOD at ca. 600 nm. We associate region RI with the ligand’s singlet–singlet absorption band from the first singlet state, S1 → Sn; RII to the stimulated (and spontaneous) emission from the ligand; and RIII to triplet–triplet (T1 → Tn) absorption (Figure 2). The assignments of these bands are supported by the similar fs-TAS shape of Eu(DBM)3·H2O.49 Region RII is overlapped by RI and RIII, and this distorts its time profile. The effective lifetime of stimulated emission depends upon the radiation density and differs from that of spontaneous emission. A minor red-shift in the region RIII and a minor blue-shift in the region RI are observed for all complexes. This indicates the possibility of (i) a shift in the energy gap due to vibronic or relaxation processes or (ii) another excited state being involved.

Figure 2.

Figure 2

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Selected transient absorption spectra at specific time scales for Sm(TTA)3·2H2O. The spectra are separated into the three regions: RI with +ΔOD (singlet–singlet absorption), RII with −ΔOD (spontaneous and stimulated emission), and RIII with +ΔOD (triplet–triplet absorption). The positive or negative ΔOD in the TAS spectra can correspond to different transitions, as illustrated in the Jablonski diagram. λexc = 355 nm.

It is worth noting that despite different systems, the depopulation rate of the RIII region is always faster in the case of Sm3+, compared to that of Eu3+, as demonstrated in Figure 3. Monitoring at the RIII region (peak at ca. 600 nm) in Figure 3, the ΔOD goes back to zero at least 1 order of magnitude faster in the case of Sm3+ compared with that for Eu3+. This is unexpected since the luminescent energy states of Sm3+ (4G5/2) and of Eu3+ (5D0) are at similar energies, located at 17 742 and 17 208 cm–1, respectively. This indicates that the ET does not occur directly to the luminescent state of the Ln3+, as discussed in the latter part of this manuscript.

The RIII kinetics profiles for the Ln(TTA)3·2H2O series of complexes are shown in Figures 4a, S29 and Table 1. The triplet rise lifetimes, representing ISC, are short and similar (between 0.4 and 2.5 ps) except for much longer rise lifetimes for Ln = Gd and Lu. Similar results are also observed for the Lnphen(TTA)3 series (Table S3, Figures S48, S49). Interestingly, the introduction of the −phen moiety into the complex does not have a substantial impact on the behavior of the excited state dynamics at the ultrafast time scale. The singlet state in these cases has a smaller proportion of triplet character mixed into it. Monitoring the wavelength at region RI, on the other hand, shows a similar fast population within ∼0.2 ps after excitation (Figure 4b). The decay lifetime at region RI is about 0.1–1 ps, similar to the triplet rise time at RIII. There appears to be a longer lifetime component also, accompanied by a shift of the maximum by about 4 nm to shorter wavelength. We have simulated this behavior by the involvement of higher energy singlet states (denoted by Sn) (SI, Figure S71). Our excited state time-dependent density functional theory (TD-DFT) calculation demonstrates that there are six lower lying triplet states below S1 and three higher singlet states close in energy for Lu(TTA)3·2H2O. The orbital transitions for the excited singlet state demonstrate inter-TTA ET, while a transition within the same TTA moiety is observed for the triplet states (Figures 4c, S68). The possibilities of ET from the higher energy singlet states to the triplet states cannot be ruled out. Despite the presence of a correlation between the decay of RI and the rise of RIII, no clear relationship has been found between the depopulation rate of RII (ca. 1 ps across the series, Table 1) and the depopulation of either the singlet or triplet state. This indicates that the ligand emission lifetime comprises contributions not only from spontaneous emission but also from stimulated emission. It is worth noting that while similar fs-TAS spectral shapes can be observed for different solvents as discussed earlier, there is a significant variation in the kinetics, depending on the solvent polarity (Table S2). The triplet depopulation rate is observed to be longest in chloroform and fastest in methanol, which highlights the influence of the solvent on the excited state dynamics, which is not uncommon for lanthanide complexes.50

Figure 4.

Figure 4

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Kinetics monitored at region (a) RIII and (b) RI on a logarithmic time scale, corresponding to (a) the population of the triplet state and (b) depopulation of the singlet state of 100 μM Ln(TTA)3·2H2O in CHCl3, across the Ln series. The rise lifetime of region RIII and the decay lifetime of region RI for different lanthanides are included in the figures. λexc = 355 nm. (c) The calculated energy levels and selected orbital transitions of Lu(TTA)3·2H2O. Green: decrease in electron density; yellow: increase in electron density. Isovalue: 0.001.

Table 1. Rise and Decay Lifetimes of 100 μM Ln(TTA)3·2H2O in CHCl3 Monitored at Regions RI and RIII of the fs-TAS, λexc = 355 nm.

  RI RIII
Ln τdecay (ps) τrise (ps) τdecay (ps)
Nd 0.7 1.1 13.3
Sm 0.7 1.5 24.8
Eu 0.4 0.5 300
Gd 1.3 5.9 22 000a
Tb 0.6 2.2 102
16 700
Er 2.3 2.5 53.1
Yb 1.3 1.8 6320
Lu 0.6 53.4 –a
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a

The long RIII decay lifetime of Gd(TTA)3·2H2O is due to the absence of a receiving state, while that for Lu(TTA)3·2H2O is outside the detection range.

The deconvolution of the TAS spectra can be represented by the evolution associated spectra (EAS) and the species associated spectra (SAS). With the aid of global fitting employing the spectrotemporal model,51 the sequential EAS represent the evolution of the spectra, with the rise and decay rate constants.

The population of the species that are associated with the spectral evolution can then be plotted in the SAS. The TAS spectra recorded are well fitted with three (Figures S30–S36) rather than two components (Figures S37, S38). It is not appropriate to fit the data with two components because then both species would not be simultaneously populated at time zero (Figures S37, S38). Considering the fitting with three components, we suggest that the first EAS component is associated with the population of the singlet states, exhibiting a fast decay time constant. The second component, comprising a mixture of regions RI and RIII, arises from the combination of processes from the slower decay of neighboring singlet states and nonradiative cascade through triplet states above T1 (Figure 4c, Table S4). A shift in the maximum ΔOD is evident in both regions. The third component, with time constants between 13 and 300 ps for different luminescent lanthanide ions (and unlimited ∼22 ns for Ln = Gd, where there is no receiving lanthanide state, Table 1), corresponds to the triplet decay from T1.

The shortest rise lifetime for population of the triplet state for Ln(TTA)3·2H2O occurs for Ln = Eu (Figure 4a). A similar observation can be seen from a recent fs-TAS study of NaYF4@NaGdF4 nanoparticles with a surface coating of a carbazole derivative,52 where the shortest rise lifetime (9.3 ps) also occurs for Eu3+. The ISC rate is sensitive to the admixture of singlet and triplet states, which depends upon the nature of the perturbing state and its energy separation from S1 and T1. The fast ISC rate also agrees with the calculated rate, found to be in the picosecond range, which has been associated with the involvement of the Eu3+7F1 state.19,36 Hence, the involvement of f-electrons is likely to influence the ISC rate. A model that takes into account the influence of f-electrons across the Ln3+ series is currently being developed for a systematic explanation of the observed results.

The prevailing research studies on Eu3+ complexes suggest that the ET from an antenna takes place to the 5D1 energy level.26,36,53 Our previous calculations on the system Eu(TTA)3·2H2O showed that the pathway for singlet ET is feasible, but we were unable to calculate the triplet ET rate due to the exchange mechanism.44 The ET rates from the triplet state by multipolar mechanisms were found to be of minor importance, so that if ET does occur from the triplet state, it must be by the exchange mechanism. It turns out that we underestimated the ISC and triplet decay rates in our simulations by several orders of magnitude. Herein, we observe the experimental triplet decay lifetime of 0.27 ± 0.03 ns for Eu3+, in both polar and nonpolar solvents (Figures 4a, S43, S44). We therefore present revised kinetic simulations of the antenna-metal ET process which occurs through the triplet state. The revised kinetics for the triplet and 5D1 states are displayed in Figure 5a, and the other graphs are included in the Supporting Information.

Figure 5.

Figure 5

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(a) Calculated rise and decay curves for T1 and 5D1 states of Eu(TTA)3·2H2O using the rate equation parameters in Table S5. (b) The relevant energy levels of Ln3+ with labeled selected states. The triplet state of Ln(TTA)3·2H2O is labeled as a purple dash-dotted line located at 19886 cm–1. (c) The RT decay rate of the triplet state plotted against the absolute energy difference between the triplet state and the nearest level of the respective Ln3+ in logarithmic scale.

As demonstrated in Figure 3, the depopulation rate of the triplet state is always faster for Sm3+ compared to Eu3+. Considering Ln3+ across the series in the same complex system, as shown in Figure 4a, the depopulation rate of the triplet state of Ln(TTA)3·2H2O follows the trend of Nd > Sm > Er > Eu > Yb, Tb. Hence, the depopulation rate at RT can be rationalized by the difference in the energy gap between the first triplet state and the nearest Ln3+ receiving state. As demonstrated in Figure 5b, the near resonance between the donor (purple line, triplet energy level located at 19 886 cm–1)44 and acceptor energy levels is expected to lead to rapid ET in the cases of Nd3+, Sm3+, or Er3+. Figure 5c shows a slower decay rate for Ln3+ with receiving states that are more distant from the triplet state energy. Similar plots are observed for the Lnphen(TTA)3 complexes, where the depopulation rate follows Nd > Sm > Eu > Yb (Figures S55). This graph is similar to that for the well-known energy gap law of Riseberg et al.:54,55

graphic file with name au4c00468_m002.jpg 2

where W is the nonradiative rate; C and α are constants; and ΔE is the energy gap between levels. However, that scenario refers to the intraconfigurational multiphonon relaxation as determined by phonons of maximum energy. The present case, Figure 5c, refers to organometallic systems with much higher vibrational frequencies and much smaller energy gaps. The T1 → Ln3+ rate is faster when the gap to the nearest receiving state is smaller.

It is significant to highlight that since the depopulation rate of the triplet is heavily influenced here, the energy gap plays an important role in determining this ET step. As mentioned before, the participation of f-electrons is one of the reasons causing the fastest ISC observed for Eu complexes, which, to some extent, suggests that the possibility of overlapping spin populations between the lanthanide ion and organic chromophore cannot be ruled out.56

In light of Eu complexes, Faustino et al.57 measured the kinetics of 5D1 population and decay at RT for solid-state Eu(TTA)3·2H2O. The time to maximum emission intensity (tmax) was 84 ns, and the 5D1 decay lifetime (τA) was 0.4 μs. To calculate the triplet decay lifetime, the formula comprising a donor (D) and acceptor (A) can be written as

graphic file with name au4c00468_m003.jpg 3

where the triplet decay lifetime, τD, is calculated to be 30 ns. This is 2 orders of magnitude slower than in our present solution study and presumably indicates a much greater nonradiative contribution in solution. Our measurement for the triplet decay lifetime is not unusual, with the value 0.32 ns recorded for [Eu(DPA)3]3– in solution58 and 0.88 ns for Eu3+ in solid-state nanoparticles.52 The rise lifetime of the subsequent luminescent state, 5D0, is in the sub-millisecond regime.42 The major point here is that the rise lifetime of the luminescent state is unrelated to the triplet decay lifetime if there are intermediate states. This is the case in literature reports of microsecond rise lifetimes for luminescent states of Nd3+,59,60 Sm3+,61 Er3+,62 and Yb3+.59 Consequently, ET occurs from the triplet state to the nearest energy state of the Ln3+, as shown in Figure 1a, in situations where intermediates are present, instead of directly to the luminescence state of the Ln3+ as commonly suggested. It is also worth emphasizing that the depopulation of the triplet state outpaces the quenching rate (∼107 s–1 time scale) by oxygen in solution;63,64 hence the emission intensity of Eu3+ remains largely unaffected by the oxygen content.

Conclusions

Through the use of ultrafast TAS, TD-DFT calculations, and a rate equation model, we have gained a deeper insight into the ET, where the ET occurs from the triplet state to the intermediate states, if they exist, rather than directly to the luminescence state of the Ln3+. An energy gap dependence has been found for the triplet decay rate by matching the energy gap with the nearest receiving state of the Ln3+. The diverse structures used in this work support the universality of this proposition. However, it is significant to underscore that ISC plays a pivotal role in determining the luminescence of the Ln3+. The ISC rate is fastest for Eu3+, and a deeper understanding of the role of 4f electrons is required to explain this. Nevertheless, our discovery sheds light on the ET from the triplet state to the Ln3+. The tuning of the energy level of the triplet state should correspond with the nearest receiving state, rather than the luminescence state of the trivalent lanthanide ions, so as to optimize the light harvesting from the antenna to enhance the luminescence of the complex for use in diverse applications.

Methods

Syntheses

Ln(TTA)3·2H2O

2-Thenoyltrifluoroacetone (TTA, 3 equiv., 3 mmol, 0.666 g) was first dissolved in 10 mL of EtOH, and 3 mL of 1.0 M NaOH solution was added to adjust the pH of the solution. The solution was heated to 60 °C, and an aqueous solution of LnCl3 (Ln = Nd, Sm, Eu, Gd, Tb, Er, Yb, Lu, 1 equiv., 1 mmol) was added dropwise. The solution was allowed to react for 2 h, and the contents were dried in a rotary evaporator. The solid was filtered and washed with hexane and cold water and dried in a desiccator.

Lnphen(TTA)3

The Lnphen(TTA)3 samples were synthesized by the literature method.65 Typically, TTA (3 mmol) and 1,10-phenanthroline (phen) (1 mmol) were dissolved in 15 mL of ethanol in a flask, with stirring at room temperature. Then, the pH of the solution was adjusted to 7.0 by the addition of NaOH solution (1.0 M). After that, 1 mmol of LnCl3 solution in 5.0 mL of deionized water was added into the above mixture at 60 °C with vigorous stirring for 1.0 h to ensure a complete precipitation. The precipitate was finally filtered, washed repeatedly with ethanol and water, and dried overnight under vacuum. The yields and characterizations are included in a previous publication.66

LnLa

The synthetic route is outlined in Figure S1. Briefly, the azide-containing antenna (S7) was constructed via multistep reactions, which include two Sonogashira couplings and direct amination of the methyl ester with an azide-containing linker (S6). The antenna was then installed onto the cyclen core (tBu-DO3A, S8), and the corresponding lanthanide complexes (Ln-N3) were obtained after deprotection of the tert-butyl ester and coordination with the lanthanide ion. The yields and characterizations are included in a previous publication.6

LnLb

The synthetic route is outlined in Figure S2. The S3 was obtained by the same procedure as shown in Figure S1. The S3 was further converted to S11 by conducting Sonogashira coupling with S10. The antenna S11 was then installed onto the cyclen core (tris-tBu-DO3AM, S12) to give S13, and the corresponding lanthanide complexes (LnLb) were obtained after coordination with the lanthanide ion. The yield and characterization is included in a previous publication.67

LnLc

The synthetic route is outlined in Figure S3. Briefly, the solid-phase peptide synthesis (SPPS) was conducted on Rink AM resin to give a N-capped Lys(Mtt)-Arg(Pbf)-Gly-Asp(tBu)-Lys(Mtt) (S14). The Mtt protecting groups on two Lys side chains were removed to release two free amine groups, and bromoacetic acid was further installed to give S16. The bis-Alloc-protected cyclen building block S17 was then incorporated onto the peptide to give S18. And the two Alloc protecting groups were removed to release two amine groups for installing antenna S19. The global cleavage and deprotecting were conducted to give desired cyclen-embedded cyclic pepetide S21, and the corresponding lanthanide complexes (LnLc) were obtained after coordination with the lanthanide ion. The yield and characterization are included in a previous publication.46

Nuclear Magnetic Resonance Spectroscopy

The NMR spectra reported were obtained from a Bruker UltraShield 400 Plus NMR spectrometer. The 1H chemical shifts were referenced to corresponding solvent peaks of MeOD-d4 at 3.31 ppm. The 13C chemical shifts were referenced to corresponding solvent peaks of MeOD-d4 at 49.00 ppm.

Steady State Photophysical Characterization

FT-IR spectra were recorded by a PerkinElmer FT-IR Spectrum Two instrument. The ultraviolet–visible absorption spectra were measured in solution in the range 200–800 nm by a PerkinElmer LAMBDA 1050+ UV/vis/NIR double beam spectrophotometer. The emission, excitation spectra, and luminescence decay curves were recorded using a Horiba Fluorolog-3 instrument with a 450 W xenon lamp. The signal was detected by a Hamamatsu R928 photomultiplier and corrected with excitation and emission correction factors to eliminate response characteristics. A custom-made liquid nitrogen cryostat with an NMR tube sample holder was employed for 77 K studies. A 10 mm path length cuvette was used for all room-temperature measurements except where specified.

Femtosecond Transient Absorption

Room-temperature femtosecond transient absorption measurements were performed using an amplified laser system (Spitfire ACE, Spectra Physics) as the excitation source, delivering ca. 100 fs 800 nm laser pulses at a 1 kHz repetition rate. The Maitai laser system is centered with a wavelength of 800 nm and 6 mJ/pulse energy with a pulse time width of 85 fs. For the beam, approximately 0.1 mJ of the output from the laser excitation source was focused onto a 3 mm CaF2 window to generate a white light continuum probe pulse in the visible region from ca. 350 to 650 nm. For the pump, the remainder of the laser output is used to pump into the Optical Parametric Amplifier (OPA) to tune to the desired excitation wavelength of the sample, with a pump fluence of roughly 2 mJ/cm2 (∼20 μJ). The absorption is collected using an MS260i automated spectrograph. In the spectrometer, we used a 1000 ms integration time and an average of 3 spectra at each delay position. The samples were stirred using a magnetic stir bar throughout the measurement. The resulting time traces were analyzed globally using GloTarAn68 and the R package TIMP.69 A 2 mm path length cuvette was used for measurements.

Computational Modeling

The optimization of all structures by DFT calculations, both in gas and in solvent conditions, was performed using the Gaussian16 package.70 The nature of each stationary point was characterized using frequency calculations. Molecules were built using Avogadro software.71 Excited state calculations were performed using the ORCA 4.2 suite of programs.72,73 The integration grid was set to Lebedev 590 points with the final grid of Lebedev 770 points. Geometry optimizations and free-energy calculations were performed at the PBE0-D3/def2-TZVP level of theory, with the Stuttgart in-core large core relative effective core potentials (LCRECP)74,75 and basis sets for the lanthanide ions. TD-DFT calculations were performed with the same level of theory using the Pople solver. However, a small core relative effective core potential (MWB28)76 was employed for TD-DFT calculations to take into account the participation of f-electrons. Spin–orbit coupling was considered. The simulated absorption spectra employed the bandwidth of 3000 cm–1. UCSF Chimera (version 1.14)77 was used for the visualization of geometries and orbitals. Maple 2021 was employed for computations involving the rate equation model.

Acknowledgments

K.-L.W. acknowledges financial assistance from the Hong Kong Research Grants Council No. 12300021, NSFC/RGC Joint Research Scheme (N_PolyU209/21), and the Centre for Medical Engineering of Molecular and Biological Probes (AoE/M 401/20).

Glossary

Abbreviations

fs-TAS

femtosecond transient absorption spectroscopy

ET

energy transfer

NIR

near-infrared

EAS

evolution associated spectra

SAS

species associated spectra

ISC

intersystem crossing

RT

room temperature

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.4c00468.

  • Synthesis and methodologies, structural and photophysical characterizations for complexes, cryogenic photoluminescence spectra and lifetime analysis, additional TAS data with kinetics analysis, additional TD-DFT calculation data and kinetics model for the ET process (PDF)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. CRediT: Waygen Thor conceptualization, data curation, formal analysis, writing-original draft, writing-review & editing; Hei-Yui Kai data curation, formal analysis; Yik-Hoi Yeung data curation; Yue Wu data curation; Tsz-Lam Cheung data curation; Leo K. B. Tam data curation; Yonghong Zhang data curation; Loïc J. Charbonnière supervision, writing-review & editing; Peter A. Tanner formal analysis, supervision, writing-original draft, writing-review & editing; Ka-Leung Wong conceptualization, funding acquisition, supervision, writing-review & editing.

The authors declare no competing financial interest.

Supplementary Material

au4c00468_si_001.pdf (13.2MB, pdf)

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