A General Model of Sensitized Luminescence in Lanthanide-Based

Article pubs.acs.org/IC

A General Model of Sensitized Luminescence in Lanthanide-Based Coordination Polymers and Metal−Organic Framework Materials Jeffrey D. Einkauf, Jessica M. Clark, Alec Paulive, Garrett P. Tanner, and Daniel T. de Lill* Department of Chemistry & Biochemistry, Florida Atlantic University, 777 Glades Road, Boca Raton, Florida 33431, United States S Supporting Information *

ABSTRACT: Luminescent lanthanides containing coordination polymers and metal− organic frameworks hold great potential in many applications due to their distinctive spectroscopic properties. While the ability to design coordination polymers for specific functions is often mentioned as a major benefit bestowed on these compounds, the lack of a meaningful understanding of the luminescence in lanthanide coordination polymers remains a significant challenge toward functional design. Currently, the study of these compounds is based on the antenna effect as derived from molecular systems, where organic antennae are used to facilitate lanthanide-centered luminescence. This molecular-based approach does not take into account the unique features of extended network solids, particularly the formation of band structure. While guidelines for the antenna effect are well established, they require modification before being applied to coordination polymers. A series of nine coordination polymers with varying topologies and organic linkers were studied to investigate the accuracy of the antenna effect in coordination polymer systems. By comparing a molecular-based approach to a band-based one, it was determined that the band structure that occurs in aggregated organic solids needs to be considered when evaluating the luminescence of lanthanide coordination polymers.

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INTRODUCTION The study of coordination polymers and metal−organic frameworks (collectively, CPs) ranges from fundamental topological studies to evaluating their use in a variety of applications ranging from gas sorption to catalysis.1−8 These multidimensional network solids are formed from the selfassembly of organic “linker” moieties with metal-based clusters into 1-, 2-, or 3-dimensional materials through covalent or ionic bonding. Coordination polymers constructed from trivalent lanthanide (Ln) ions have practical utility as sensors, new display technologies, and bioimaging agents as a result of their unique spectroscopic properties.9−13 The luminescence of trivalent Ln-containing materials (where Ln = [Xe]4fn (n = 0−14) electron configuration) is the result of f−f transitions, a formally forbidden process that results in long lifetimes, sharp emission peaks with color-pure emission, and large Stokes shifts.9 Additionally, the contraction of the valence f orbitals toward the nucleus shields the ions from significant ligand field effects by radially extended filled 5s and 5p orbitals. As a result, these ions have been studied extensively for a myriad of applications. These CP compounds sit at the intersection between discrete, 0-dimensional molecular complexes and traditional inorganic network solids, with new properties and features that arise from the unique combination of these two different kinds of compounds. Considering this, new models to explain the properties of CPs may be necessary. While Ln ions have various attractive features regarding luminescence, direct excitation of the ion is not readily efficient. Within coordination complexes, it was found that certain organic ligands (largely, aromatic compounds) could absorb © 2017 American Chemical Society

incident radiation and transfer this energy directly into the f excited state manifold, commonly referred to as the antenna effect, and were able to produce very efficient Ln luminescence in certain instances.9,14−17 Decades of research into these compounds has resulted in the establishment of a series of guidelines concerning the design of suitable organic antennae for various Ln ions.6,12−16 One of the primary parameters for efficient sensitization is based on the singlet and triplet state energies of the organic antenna and their role in the energy transfer process.16−19 Another guideline that is often used to assess nonradiative deactivation for emissive electronic states is the energy gap law (EGL).20 This considers that the energy gap between emissive state and the highest ground state level of a Ln ion can be quenched by vibrational modes of OH and other high-energy oscillators coordinated to or in close proximity to the Ln center.21 Until now, the study of Ln CP compounds has been based on what is known from studying molecular Ln complexes with no regard to the extended network structure of CPs. The lack of an appropriate model to explain the luminescence of Ln CPs and MOFs is a major limitation to designing functional luminescent materials. Since Ln CPs are not molecular entities, but rather network solids,22,23 a new framework built upon the foundation of the antenna effect to explain Ln luminescence in CPs has been developed (Figure 1). The periodic arrangement of the metal centers with organic linkers results in the formation of band features similar to insulators and semiconductors.24−26 When Received: November 30, 2016 Published: May 2, 2017 5544

DOI: 10.1021/acs.inorgchem.6b02838 Inorg. Chem. 2017, 56, 5544−5552

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Inorganic Chemistry

[Ln2(OBA)3(H2O)5.5]·0.5H2O (6),31 as well as three CPs previously reported by our group, [Ln(BPDC)3(H2O)3] (7),22 [Ln(PYDC)(HPYDC)(H2O)] (8),32 and [Ln(FDC)(HFDC)(H2O)] (9)33 (BPDC = 4,4′-benzophenonedicarboxylic acid, PYDC = 2,3-pyridinedicarboxylic acid, and FDC = 3,4furandicarboxylic acid), were also chosen. The linkers for each CP studied had the energy of their singlet and triplet states determined from molecular complexes as well as the singlet and triplet bands of the CPs. Quantum yields and lifetimes were obtained for each CP (both Eu and Tb) and used to compare and contrast the two models. The luminescence of these compounds was easily rationalized using the proposed band model, whereas the molecular approach failed to adequately explain all systems.

Figure 1. A modified Jabłoński diagram illustrating the antenna effect in molecular complexes. A = absorption, F = fluorescence, P = phosphorescence, L = luminescence, NRD = nonradiative decay, 1S = singlet state, 3T = triplet state, ISC = intersystem crossing, ET = energy transfer, BT = back transfer. Also depicted is NR deactivation of excited Ln ions by proximal high-energy oscillators (here, OH).

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Ln ions are incorporated into inorganic semiconducting materials, their luminescence is often sensitized by charge carrier (exciton) recombination since there exist no organic antennae within these systems. Recently we reported a study that indicated that a similar mechanism occurs in Ln CP systems, and that modifying the antenna effect guidelines for these systems is necessary.22 This new approach applies known characteristics of aggregated organic solids with band structure to the antenna effect as derived from molecular systems. Herein, nine coordination polymers have been synthesized from the literature with Eu(III), Gd(III), and Tb(III) ions to test the validity of this modified approach to the antenna effect for luminescent trivalent lanthanide (Ln = [Xe]4fn, n = 0−14) CPs and MOFs (Figure 2). Four CP systems have been

EXPERIMENTAL SECTION

Literature methods were used to synthesize compounds 1 through 9 with Eu(III), Gd(III), and Tb(III) ions. See the Supporting Information for synthesis details. Photophysical Measurements. Quantum yields were determined from the following equation:

Φx =

1 − R ST I × x × ΦST 1 − Rx IST

(1)

where R is the diffuse reflectance value at the excitation wavelength of 313 nm and I is the value obtained after integrating the emission spectra, ST is a standard pyrene (Φ = 61% at 313 nm) with known quantum yield, and x is the sample of interest. The standard, the sample, and the matrix material PMMA (poly(methyl methacrylate), approximate molecular weight 15,000 by GPC) were all measured using a PerkinElmer LS55 fluorescence spectrometer at room temperature. Due to the precarious nature of quantum yield measurements, the quantum yield of anthracene (Φ = 22%) was also measured during each trial as a quality control measure. Trials that produced an anthracene quantum yield outside the range of 22 ± 5% were discarded. PMMA was ground with each sample to reduce the possibility of self-absorbance of emitted light and to avoid refractive index issues in the photophysical calculations. Solid-state reflectance values were obtained using a 60 mm integrating sphere on a PerkinElmer Lamba 850 UV/vis spectrometer. The I values are found from the integrated emission profile of pyrene, anthracene, and the sample. Lifetime measurements were collected using the BioLight Fluorescence Application software. Quantum yield and lifetime measurement data were analyzed using Origin Pro 8.1. Emission spectra were collected of the singlet and triplet states of the organic linkers using the LS 55 fluorescence spectrometer at 77 K, with time delays of 0 to 0.05 ms, outlined by Crosby.34 Band energy determinations for the Gd CPs were collected at room temperature with identical delays according to our previously reported method.22 Spectra were deconvoluted using Origin Pro 8.1. The structures of 1−9 were confirmed by powder X-ray diffraction using an Olympus BTX II Benchtop XRD. PXRD diffractograms were compared to those calculated from single-crystal data to confirm compound identity.

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RESULTS AND DISCUSSION Compounds 1−922,27−33,35 were chosen due to their varying topologies, number of coordinated OH oscillators, and linker composition. The target structures were synthesized with Eu(III), Gd(III), and Tb(III) ions, and structures were confirmed with powder X-ray diffraction. The first set of compounds (1−4) use the 1,4-BDC linker. Compound 1 is made of two-dimensional sheets, held together by extensive hydrogen bonding to form a 3D network. This is structurally similar to compounds previously studied in our lab (8, 9) and has an OH oscillator from one bound ethanol. Compound 2

Figure 2. Linkers used in compounds 1−9.

synthesized from the linker 1,4-benzenedicarboxylic acid (1,4BDC), namely, [Ln2(1,4-BDC)3(DEF)2(EtOH)2]·2(DEF) (1)27 (DEF = diethylformamide; EtOH = ethanol), [Ln2(1,4BDC)3(H2O)4] (2),28 [Ln2(OH)4(1,4-BDC)(H2O)2] (3),29 and [Ln2(OH)4(1,4-BDC)] (4).29 Another CP system was synthesized with a 1,3-benzenedicarboxylic acid (1,3-BDC), [Ln2(1,3-BDC)3(H2O)2] (5).30 A system with a highly efficient sensitizing linker with two aromatic rings bridged by an ether oxygen, 4,4′-oxybis(benzoic acid) (OBA), 5545


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molecular complexes. With an energy difference of more than 5,000 cm−1 to the emissive levels of Eu and Tb in the BDC systems (1−4), 1,4-BDC should display poor luminescence with a triplet state of 25,410 cm−1. However, the energy difference between the 5D1 manifold of Eu and the triplet state of 1,4-BDC does indeed fall within this ideal energy range at 3,200 cm−1, which could potentially explain the ∼10% quantum yield observed in these four systems. Here, energy transfer to the 5D1 manifold in Eu would occur, followed by internal conversion to the 5D0 manifold to produce Eu emission. The higher quantum yield of Tb (26−35%), however, is not explicable by this triplet state value. The energy difference between the 3T of 1,4-BDC and 5D4 emitting manifold is too large at an energy difference of ∼4,980 cm−1, and is too low in energy to resonate with the 5D3 level. This should result in a minimal quantum yield in the Tb system, opposite of what is seen. Since the singlet and triplet states of 1,3-BDC is effectively identical to 1,4-BDC, with similar quantum yields, the same analysis holds for 5. In the OBA system (6), the triplet state of 27,840 cm−1 potentially allows for population of the excited state manifold of the Eu ion with an energy difference of 6,395 cm−1 from the 5 D2 level, though it can potentially resonate with 5D3 (ΔE = ∼3,470 cm−1) and/or 5L6 (ΔE = ∼2,660 cm−1). However, these manifolds are much higher than Eu’s emissive level, which could adequately explain the reported QY of 3.9%. In the Tb system, the quantum yield of 66% is surprising, as the OBA’s triplet state is likely too close in energy to the 5D3 level (ΔE = ∼1,570 cm−1), facilitating potential back transfer. Furthermore, the ET range of 7,410 cm−1 from the triplet state to Tb’s 5D4 emissive level is too large to allow for efficient ET. Combined, these factors should result in minimal sensitization of Tb emission instead of the high QY seen. In the BPDC system (7), the triplet level of 23,420 cm−1 is too high to efficiently populate Eu’s 5D1 and 5D0 energy manifolds. Also, the 5D2 energy manifold is only 1,975 cm−1 from the triplet state of BPDC, which could reduce sensitization by a back transfer process. Despite this, a 35.5% quantum yield was instead recorded. For the Tb system, however, the energy difference of 2,990 cm−1 should be ideal for efficient energy transfer to Tb’s 5D4 emitting level, but the quantum yield of 6% does not reflect this. The triplet state of the PYDC linker (8) at 25,970 cm−1 follows a similar trend as OBA, such that it is close in energy with higher levels (5D3, 5L6) of the Eu(III) ion, yet too far from 5 D2 to resonate with this level. This would result in no appropriately suited energy manifolds in Eu for efficient ET, explaining the low quantum yield. In the Tb system, the energy gap between 5D4 and the linker’s triplet state is greater than 5,000 cm−1, which can also effectively explain the poor QY of 4.6%. The FDC linker’s (9) triplet state of 23,190 cm−1 is most likely too high to promote efficient energy transfer to the 5D1 level of the Eu(III) ion with an energy gap of 4,170 cm−1, just outside the optimum ET range and too close to 5D2 (ΔE = ∼1,700 cm−1), resulting in the observed QY of 1.1%. In the Tb system, the only energy level that could be occupied is the 5D4 emissive level with an energy gap of 2,760 cm−1, which is within the range of optimum energy transfer and should promote decent quantum yields, but an efficiency of 3.3% was instead observed. At the time, the low Tb quantum yield was justified through nonradiative deactivation of the 5D4 manifold by the bound aqua ligand.30

resembles the stereotypical 3D, porous metal−organic framework structure with two bound water molecules per Ln center, which are extended in three dimensions by the linker. Compounds 3 and 4 are similar in composition and consist of dense Ln-hydroxide sheets. These two-dimensional sheets are stacked by the organic linker into a three-dimensional compound akin to a pillared clay, more resembling a traditional inorganic network solid than the other compounds in this study, with two and one bound water molecule(s) per Ln ion, respectively. Compound 5 is composed of 1,3-BDC and forms one-dimensional helical chains with two coordinated aqua ligands. Compound 6 is constructed with the OBA linker, which has an sp3 ether oxygen atom connecting two benzoate groups. The CP is compact and three-dimensional, with 3 coordinated aqua ligands. Compound 7 is composed of a rigid, aromatic BPDC linker that forms a two-dimensional compound with three aqua ligands coordinated to each Ln center. The SBUs consist of Ln dimers, and each Ln has one bound aqua ligand. Similar to 1 and 7, compounds 8 and 9 consist of twodimensional sheets composed of Ln dimers extended by the linker, which acts to hold the sheets together through hydrogen bonding. Instead of a central benzene core as in 1, however, the linkers consist of a pyridine (8) ring and a furan (9) ring. The triplet state of the molecular organic antenna is commonly used as a major contributing factor in the efficiency of the antenna effect in complexes, but it cannot be used to explain sensitized luminescence in Ln CP systems. Arguably the most followed tenet when designing a sensitizer states that the energy difference between the sensitizer’s triplet state and that of Eu’s 5D0 state should be approximately 2,500 to 3,500 cm−1 and approximately 2,500 to 4,000 cm−1 for Tb’s 5D4 energy level for maximized sensitization efficiency (i.e., high Φ), assuming efficient ISC from 1S to 3T.18,36 This is a very general rule, with exceptions and intricacies to it. Low temperature luminescence studies were performed on each linker in an acetonitrile solution with Gd(III) to obtain the organic sensitizer’s triplet state value (Table 1).34 When these values Table 1. Singlet and Triplet State/Band Values (cm−1) of Linkers and Gd CPs Used in 1−9 molecular states 1

−1

3

CP states −1

1

−1

3

T (cm−1)

compd

linker

1 2 3 4 5 6 7 8 9

1,4-BDC

29,850

25,410

30,960

22,220

1,3-BDC OBA BPDC PYDC FDC

32,790 34,680 30,580 32,790 31,620a

25,060 27,840 23,420 25,970 23,190a

31,250 26,670 23,040 26,180 30,030

22,220 22,420 20,410 21,050 21,830

a

S (cm )

T (cm )

S (cm )

Hydrolyzed linker calculated with Gaussian.33

were used to find a similar correlation between the triplet state of the molecule and quantum yield of the CPs, this general tenet began to fail as more systems were studied. It was thus hypothesized that treating CPs and MOFs in a manner akin to molecular complexes was an invalid approach. Instead, the nature of MOFs and CPs extended network structures needed to be considered. Evaluation of Molecular-Based Approach. First, 1−9 were studied using triplet state values determined from 5546


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Inorganic Chemistry Evaluation of Band-Based Approach. After the analysis of 1−9 using the commonly used molecular triplet state approach, inexplicable deviations in nearly every system arose. Taking the extended network nature of CPs coupled with the unique chemistry of Ln ions, it was hypothesized that the organic linkers would behave more like aggregated organic compounds with band structure rather than discrete molecular antennae.24−26 As such, the band nature of network solids would need to be considered when studying sensitized luminescence. The composition of a CP’s conduction band is largely derived from the organic linker as shown by several computational studies.37−39 It has even been noted that lower energy band gaps are observed for linkers with larger numbers of sp2 carbon atoms.39 In such instances, an electron is being excited from the material’s valence band to the conduction band. Charge carrier recombination eventually occurs, resulting in an energy transfer that permits population of the f* state to promote sensitized emission. To investigate this further, compounds 1−9 were synthesized with Gd(III) ions to form isostructural analogues from which the triplet band energy levels of these compounds were determined. The triplet band energies were then used to justify the photoluminescent behavior of 1−9. The triplet state of the band in BDC-based systems (1−5), at 22,220 cm−1, more accurately explains the observed quantum yields (∼10% in Eu, ∼33% in Tb) (Figure 3, Table 3). In the

Eu system, energy can transfer from BDC’s triplet band to the D1 level (ΔE = 3,200 cm−1), followed by IC to 5D0, which more readily rationalizes its quantum yield. In the Tb system, the triplet band is 1,790 cm−1 from the emissive 5D4 level of the Tb(III) ion. This is outside the ideal range proposed from the classic model for complexes, usually resulting in ineffective population of f* due to back transfer, yet moderately decent quantum yields are observed. This could be due to the nature CP structures compared to discrete complexes, where aggregation and intermolecular interactions are strong and static. While this ideal range for ET in complexes was empirically determined from decades of research, it is likely that some modifications to this may be required for CPs, but can only be estimated once significantly more data have been collected. The OBA linker in 6, with too high a molecular triplet state, does not convincingly explicate the observed luminescence in this system. In the triplet band model, however, a triplet energy band of 22,420 cm−1 is 1,990 cm−1 from Tb’s 5D3 emissive level, explaining the high quantum yields. The triplet band energy is in resonance with Eu’s 5D2 energy manifold, allowing for potential back transfer and subsequent luminescence quenching and/or energy loss to IC processes to populate Eu’s 5D0 emitting level, explaining the poor quantum yield of 3.9% for the Eu system. In the BPDC system 7, the triplet band energy of 20,410 cm−1 is essentially identical to Tb’s 5D4, too close to its emissive manifold, explaining the low quantum yield of 6.0%. In the Eu system, the energy can be effectively transferred to the 5D0 state of the Eu ion, producing a quantum yield of 35%. In the PYDC system 8 the triplet band energy transfer to the 5 D0 level of the Eu(III) ion is favorable at 3,800 cm−1, although a close interaction range of 2,030 cm−1 to Eu’s 5D1 manifold may facilitate back transfer, effectively predicting the outcome of poor Eu luminescence in this system. In the Tb system, the only manifold that could potentially be occupied is the 5D4 emissive manifold, which is too close in energy from PYDC’s triplet band (ΔE = 620 cm−1) to allow for efficient energy transfer, explaining the poor quantum yield value of 4.6%. There also exist multiple strong π−π interactions within this system, which may influence the ET mechanism, as discussed later. The triplet band of system 9 has an energy of 21,830 cm−1. This energy is too close in resonance for the 5D2 energy level of the Eu(III) ion, which may deactivate luminescence, resulting in the poor QYs observed. Similarly, the triplet band is in close resonance with the 5D4 level of the Tb(III) ion, producing the 5

Figure 3. Modified Dieke diagrams of Eu, Gd, and Tb ions with experimentally obtained energy levels of the molecular triplet states (left, red) and triplet bands (right, blue) of the linkers and CPs used in this study. Since 1,3-BDC and 1,4-BDC have the same triplet band energies, they have been combined in the band figure for clarity.

Table 2. Quantum Yield (%), Lifetime (ms), Number of OH Quanta per Ln Center, and Overall Dimensionality (Dim) of Eu and Tb Analogues of 1−9 compd 1 2 3 4 5 6a 7a 8a 9a a

Eu QY (%) 12.6 9.5 10.6 9.8 10.4 3.9 35.5 2.6 1.1

± ± ± ± ±

2.1 0.8 1.6 5.4 0.8

± 4.9 ± 0.3 ± 0.3

Eu τ (ms) 0.560 0.264 0.240 0.358 0.476 0.28 0.265 0.530 0.387

± ± ± ± ±

0.012 0.006 0.017 0.071 0.005

± 0.016 ± 0.010 ± 0.0001

Tb QY (%)

Tb τ (ms)

no. of OH

Dim

± ± ± ± ±

1.260 ± 0.173 0.724 ± 0.025 0.700 ± 0.045 0.731 ± 0.053 1.020 ± 0.040 0.79 0.280 ± 0.023 1.21 ± 0.10 0.769 ± 0.001

1 4 4 2 4 ∼6 6 2 2

2 3 3 3 1 1 2 2 2

26.1 37.0 36.3 35.0 31.9 66 6.0 4.6 3.3

3.8 2.4 3.5 12.3 3.0

± 0.3 ± 0.2 ± 0.8

As reported.22,31−33 5547


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Inorganic Chemistry Table 3. Difference from Energy Levels of Linker to Energy States of Ln(III) Ions triplet state for molecule

triplet band for CP

Eu(III) systema 5

D0

1,4-BDC 1,3-BDC OBA BPDC PYDC FDC a5

8,140 7,790 10,570 6,150 8,700 5,920

5

D1

6,380 6,030 8,810 4,390 6,940 4,130

Tb(III) systemb 5

D2

3,910 3,560 6,340 1,920 4,470 1,690

D0: 17,270 cm−1. 5D1: 19,030 cm−1. 5D2: 21,500 cm−1.

5

D4

4,950 4,600 7,380 2,960 5,510 2,730

Eu(III) systema

5

5

D3

5

D0

−860 −1,210 1,570 −2,850 −300 −3,080

4,950 4,950 5,150 3,140 3,780 4,560

D1

3,190 3,190 3,390 1,380 2,020 2,800

Tb(III) systemb 5

D2

720 720 920 −1,090 −450 330

5

D4

1,760 1,760 1,960 −50 590 1,370

5

D3

−4,050 −4,050 −3,850 −5,860 −5,220 −4,440

D4: 20,460 cm−1. 5D3: 26,270 cm−1.40

b5

same results. Similar to 8, the strong interactions between the π systems of the organic linkers may also play a role in the photophysical processes involved. Further Considerations of the Band Nature of Organic Solids. In molecular complexes, the absorption and emission of UV/vis/NIR radiation are dictated by two selection rules: Laporte and spin selection rules. In the antenna effect, what this generally means is that singlet state emission, being an allowed transition, is short-lived. It decays faster than energy transfer to a Ln can generally occur, but ISC to the triplet state can happen. Once the triplet state is populated, its longer lifetime caused by its forbidden return to the ground state provides enough time for energy transfer to occur, exciting the Ln ion followed by emission. In network solids where band formation occurs, an additional selection rule concerning the absorption and emission of phonons (Δk, crystal momentum) is required if the band transition is indirect (i.e., forbidden) in nature. This means that if the Ln CP under study has an indirect band gap, decay from the singlet band is forbidden, possibly being longlived enough to be a contributor to Ln sensitization. Thus, singlet sensitization of Ln ions in Ln CPs is a potentially more common occurrence than in complexes, and needs to be considered when evaluating the luminescence of Ln CPs and MOFs. While most linkers currently in use in the construction of Ln CPs have large band gaps with high singlet band energies, in linkers with lower band gap energies (closer to those found in semiconductors instead of insulators), the singlet sensitization pathway could be significant. Of course, this is a simplistic evaluation, as band structure and band transitions in organic semiconductors are fundamentally more complicated than inorganic semiconductors.41−46 Triplet versus Spectral Overlap Approaches. When luminescent lanthanide-hybrid systems are analyzed, it is very common to see the discussion restricted to a cursory description based largely on the triplet state. This often provides a simplified view of the sensitization process, and while convenient, it does not always produce a complete picture. In molecular systems, the triplet discussion is arguably sufficient in that any population of higher energy states on the Ln by higher energy levels of the ligand would likely be insignificant compared to the dynamics involved in ligand exchange, solvent interactions, etc. Ln CP systems, however, are static. If a higher energy level on the Ln is populated, there are fewer competing mechanisms for nonradiative deactivation. Aside from loss during internal conversion, the population of these higher energy manifolds in Ln CPs may be more significant. For a more accurate and precise view of the energy transfer processes in Ln CPs, the overlap between donor (linker) emission and acceptor (Ln) absorption spectra may

need to be considered instead of just the singlet or triplet band energies, where better overlap generally produces better luminescent behavior.22 Influence of High-Energy Vibrational Quanta. Vibrational quanta such as high-energy CH, NH, and OH oscillators provide a pathway for nonradiative decay, reducing efficiencies in these systems. When constructing luminescent lanthanide complexes, the sensitizing organic ligand is designed to reduce the presence of these oscillators, especially those from bound aqua ligands.47 Three to four OH quanta are sufficient to fill the energy gap between Eu and Tb’s excited and ground states.48,49 Within CPs, bound aqua ligands do not appear to be nearly as detrimental, likely due to the presence of extensive, static hydrogen bonding.19 The systems selected for this study contain 1 to 6 OH oscillators bound to the Ln ion, and it is expected that a decrease in emission efficiency should be seen as more aqua ligands coordinate to the Ln center. Instead, the quantum yields for both the Eu and Tb systems are statistically nearly identical in cases for the 1,4-BDC systems, even though the Ln centers were coordinated to an ethanol (1), 1 aqua ligand (4), and 2 aqua ligands (2, 3). While the presence of increasing oscillators more or less decreases the excited state lifetimes as expected (Table 2), their presence does not appear to congruently influence emission quantum yields in a similar manner. Since it is well-known and has been reported that these high-energy oscillators do impact emission quantum yields, it is evident that decently emissive lanthanide CPs can be constructed even with such oscillators present. It is believed that the strong hydrogen bonding networks reduce their overall impact, and that similar hydrogen bonding motifs in the 1,4BDC systems result in similar quantum yields despite the presence of varying numbers of O−H oscillators. Interestingly, there is a strong linear correlation between lifetimes and number of O−H oscillators in all but 8, possibly due to the influence of the Ln−N bond from the pyridine moiety of the linker.19 It is possible that a coordination sphere largely dominated by aqua ligands and/or a weak hydrogen-bonding network would negatively impact the sensitization efficiency more than observed within these systems. There have been studies highlighting the influence of such high-energy oscillators in CP systems.22,50 In some cases, such as changing H2O to D2O, there is a direct correlation.51,52 In other systems, such as changing C−H to C−F in 1,4-BDC, the correlation may be less straightforward.50,53 In such instances, the electronic effects of changing substituents on the linkers need to likewise be considered. This change can impact singlet/ triplet band energies, as well as overall energy transfer dynamics due to electron-withdrawing or -donating effects. As this model is further developed with additional research, such differences 5548


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Inorganic Chemistry will need to be carefully distinguished between and evaluated separately. Topological Impact on Sensitized Luminescence. The 1,4-BDC and 1,3-BDC systems, with essentially identical quantum yields and singlet/triplet band energies, can be used to show how topological diversity does not seem to have a drastic impact on sensitized luminescence in these Ln CPs. With varying overall dimensionality (1−3), varying SBU dimensionality (0−2), and different general overall structural motifs (2D CP → 3D MOF → clay-mimetic), it would appear that the general CP/MOF structure has minimal influence on luminescence. Other structural effects related to NRD mechanisms (above) and organic aggregation effects (below) seem more important than the nature of the inorganic unit. While this study aimed to show diversity in both structural topology and organic linker energetics, the number of structures is limited, and this correlation cannot be definitively stated. One aspect of these structural motifs that can impact emission is the SBU, in that it can promote concentration quenching effects. By diluting the emissive Ln centers in a “nonemissive” Ln matrix such as La, Ce, or Gd, optimal doping concentrations for maximum efficiencies can be determined. This is also used to promote Ln−Ln′ sensitized emission, which can tailor emission profiles and is commonly used to produce white-light emitting materials.54,55 The potential for aggregation can arise in these materials, which is not necessary to consider in isolated molecular complexes, but can impact concentrated solutions and solid phases of such complexes. Compounds 6 (OBA), 7 (BPDC), 8 (PYDC), and 9 (FDC) do have significant π−π interactions within the 3.0 to 4.9 Å range, which may provide a pathway for deactivation of luminescence.56,57 In the OBA system (6), the close interacting centroid systems are stacked at unfavorable angles (less than 45°), unlikely allowing effective aggregationbased reduction of emission. The unusual channels within the structure of 7 consisting of head to tail interactions of the BPDC linkers could lead to possible aggregation-based behavior.58 These distances around 2.6 Å between linkers in the same geometric plane and π−π interactions distances around 3.5 Å could lead to reduced emission, but this impact on luminescence is currently uncertain, as Eu luminescence was remarkably high. Other systems, with π−π, C−H···π, and S···π, interactions show significant increases (1.5- to 7-fold increases) upon introduction of various stacking motifs into organic crystals.59 The result of these effects on emission and the fundamental luminescence of Ln CPs is largely underdeveloped, but should be taken into consideration when evaluating luminescent Ln CPs. Research into these effects is currently underway in our lab. Quantitative Assessment of Eu Emission. The emission profile of Eu complexes has long been used to estimate several quantitative factors impacting sensitized emission. These include the determination of radiative and nonradiative decay rates, intrinsic Eu quantum yield, and sensitization efficiency of the organic linker. Please see the Supporting Information for the equations used to determine these values. When similar studies are applied to Ln CPs, the necessity to treat these compounds differently from complexes is further demonstrated (Table 4). From this table, it is evident that radiative rates do not vary much. This supports the premise that the SBU structure has minimal impact on sensitized emission. Most strikingly, however, is the possibility of obtaining quantum yields that surpass the intrinsic Eu quantum yield, leading to

Table 4. Luminescence Parameters for Compounds 1−9 compd

ηsens, %

ΦEu, %

τrad, s

krad, s−1

knonrad, s−1

1 2 3 4 5 6 7 8 9

114 192 244 143 102 49 697 25 15

11.1 4.9 4.3 6.9 10.1 7.9 5.1 10.3 7.5

0.0051 0.0053 0.0055 0.0052 0.0047 0.0035 0.0052 0.0056 0.0053

196.08 188.68 181.82 192.31 212.77 285.71 193.67 178.57 188.68

1589.6 3599.2 3984.8 2600.9 1888.1 3285.7 3579.9 1708.2 2395.3

linker sensitization efficiencies greater than 100%. In previous work,22 such an increase was potentially attributed to a coupling of the high-energy O−H oscillators with internal conversion processes that acted to support the Eu sensitization pathway. Since the values in Table 4 are determined indirectly from experimental data rather than directly calculated, the nonphysical values of ηsens ≫ 100% are a reflection of this. The experimental excited-state lifetimes (5D0 → 7F1 transition of the Eu compounds) used in these calculations are only a measure of NRD pathways and do not account for any assistance these oscillators may contribute to the sensitization process. Thus, ηsens may appear to be greater than 100% in systems where this assistance is occurring. It is further speculated that the strong hydrogen-bonding interactions may also be partially responsible for this observation.19 Regardless, it is clear that using methods and techniques derived from complexes is not fully applicable in CPs. From the systems studied, there are several criteria to consider for the band model of sensitized luminescence in Ln CPs (Figure 4): First, the triplet band of the organic must resonate with any of the f excited states within the Ln ion manifold, but the higher the energy of the manifold, the more energy losses from internal conversion are expected. In this case Ln ions act as a

Figure 4. Proposed band model diagram where an electron is promoted from the valence band to conduction band (or, other type of organic inter/intraband transition), charge carrier recombination occurs, and the energy is then transferred to the manifold of the Ln ion. The emissive state of the Ln ion is occupied through internal conversion, followed by luminescence. Here, ET = energy transfer, IC = internal conversion. 5549


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sensitized luminescence has been outlined, though there is more research needed to further establish concrete guidelines. It is expected that future work on luminescent Ln CPs and MOFs will be evaluated by the band model, allowing for this approach to be further developed. Detailed excitation, absorption, and computational studies will also surely serve to address these parameters to a greater extent.

dopant within an organic matrix with band structure, similar to Ln-doped inorganic semiconductors. This is due to the contracted nature of the f orbitals not significantly interacting with the organic linker, thus having minimal influence on the photodynamics of the organic matrix.11,60 Based on molecular systems, the energy transfer from the conduction band to an f* state should have an ideal range around 2,000 to 4,000 cm−1. While significant deviation from this guideline is not expected, further studies will need to be performed to empirically determine a range appropriate to CP systems. Congruent sensitization by both the singlet and triplet bands in Ln CPs is likely more common in CPs dependent upon if the band transition is direct (allowed) or indirect (forbidden).45,61,62 Next, high-energy oscillators may play either a positive or detrimental role for sensitized emission. High energy oscillators can be detrimental by offering nonradiative decay pathways through OH/OD or CH/CF oscillators.50 Although this may not be as detrimental as in molecular complexes since there is no dynamic ligand exchange and the framework is often stabilized by strong hydrogen bonding interactions, it is clear that their presence does not necessarily always result in poor luminescence. In other instances, it has been noted that these oscillators can sometimes be beneficial to the luminescence where emission is increased in their presence.22,63,64 When studying these oscillators, care should be taken to consider any potential electronic effects induced upon changing of sensitizing linker functionality (e.g., changing from C−H bonds to C− F bonds in BDC). All of these influences may be reflected in the quantitative assessment of the photodynamics of these systems, where linker sensitization efficiencies may appear larger than physically feasible. Finally, the nature of the secondary building unit and overall dimensionality of Ln CPs seems to be less important than anticipated (Table 2). In the case of Ln CPs, self-quenching is much more important than SBU, so dilute concentrations of the emissive Ln may need to be considered.65−67 Linker aggregation effects are likely to pose a significant contribution to sensitized emission in these materials.58 Using methodologies derived from molecular systems on Ln CPs needs to be done carefully, as argued herein. While the antenna effect is a valid way to promote sensitized emission in Ln CPs, guidelines established from molecular complexes need to be modified to account for the unique nature of Ln CPs. Comparing the newly proposed “band model” versus the antenna model illustrates that in many instances the antenna model guidelines fail to explain sensitized emission in Ln CPs. The antenna model emphasizes the triplet state energy of the organic linker being important for ET to the Ln ion, whereas in the band model the triplet band produced from the periodic array of the organic within these materials is a more accurate predictor of sensitization efficiency, though singlet sensitization and other effects may also contribute. The spectral overlap approach may also provide a more precise and accurate depiction of the energy transfer processes involved. Aromatic linkers are required for both models; in the antenna model they are required for their large molar absorptivities where in the band model they are additionally required to produce bands through the delocalization of sp2 orbitals. In both approaches, the presence of high-energy oscillators can reduce emission by nonradiative decay mechanisms, but seems to be less detrimental in Ln CP systems, and in some rare cases can even assist in the sensitization efficiency. Herein, the features of Ln CP systems that need to be considered when studying their

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CONCLUSION Nine CP systems have been constructed, each from Eu(III), Gd(III), and Tb(III) ions. The emission quantum yields within some of these systems are surprisingly high based on parameters commonly used to describe the antenna effect in molecular complexes. The linker’s triplet state may not always accurately rationalize emission quantum yields within lanthanide coordination polymers due to formation of band-like structure. Considering triplet band formation in these network solids provides a more accurate method of predicting sensitized emission within these systems. Additionally, explaining poor quantum yields based on the presence of high-energy oscillators as is common in coordination complexes may not always be suitable. Their negative impact may be reduced by hydrogen bonding, and/or nonradiative processes may assist in internal conversion processes. The rules put forth herein are not to discredit the antenna effect, but to modify it for considerations within luminescent coordination polymers. While all details of this band model have not been fully determined, the perspectives provided should guide future research endeavors in this field.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02838. Synthesis details, singlet and triplet state spectra, equations for data in Table 4, and tables of centroid distances and angles (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Daniel T. de Lill: 0000-0003-0891-7246 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Florida Atlantic University, especially the Department of Chemistry & Biochemistry, for funding this project.

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REFERENCES

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A General Model of Sensitized Luminescence in Lanthanide-Based

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