Orientation of (Hetero)aromatic Anions in the LEuH Interlayer and

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C: Physical Processes in Nanomaterials and Nanostructures

Orientation of (Hetero)aromatic Anions in the LEuH Interlayer and Enhanced Photoluminescence Baiyi Shao, Pingping Feng, Xinying Wang, Fangming Cui, and Xiaojing Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00888 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 10, 2019

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The Journal of Physical Chemistry

Orientation of (Hetero)aromatic Anions in the LEuH Interlayer and Enhanced Photoluminescence

Baiyi Shao,a Pingping Feng,a,† Xinying Wang,a Fangming Cui*,b and Xiaojing Yang*,a a

Beijing Key Laboratory of Energy Conversion and Storage Materials, College of

Chemistry, Beijing Normal University, 19-Xinjiekouwai Street, Haidian District, Beijing 100875, China. b Qian Xuesen Laboratory of Space Technology, China Academy of Space Technology,

104-Youyi Road, Haidian District, Beijing 100094, China. † Institute of Molecular Medicine, Renji Hospital, School of Medicine, Shanghai Jiaotong University, 160-Pujian Road, Shanghai, China. *Corresponding authors. Email: [email protected], [email protected]

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Eight kinds of (hetero)aromatic anions terminated with –COOˉ were intercalated into the interlayer of layered europium hydroxide (LEuH) as sensitizers for enhancing Eu3+ photoluminescence (PL) via an ion exchange with NO3ˉ. Orientations of the anions in the interlayer were found to be vertical, tilted, or horizontal relative to the LEuH layer. The –COOˉ group was directly bonded to Eu3+ in the mono- or di-dentate coordination in the vertical and tilted cases, whereas no directly coordination was detected as the anions in the horizontal orientation. There are two distinctive factors that determine the orientation of these anions in the interlayer. First, the –COOˉ group is anchored to the positively-charged layer through the electrostatic interaction. For the reason, 1,3,5-benzenetricarboxylic acid (BTA) could be vertical as it was deprotonated to BTA2ˉ while horizontal as to BTA3ˉ. The second important aspect is the key role of the weak interactions between the heteroatoms of the rings and OHˉ/ Eu3+ of the LEuH layer, including mainly hydrogen bonds, the ionogenic-characterized interactions and salt bridges. The stronger interactions resulted in more tilted guests. The hydrogen bonds between N–H in pyrrole-2-carboxylic acid and the OHˉ of the layer were strong enough to make the guest horizontally orientating. The organic anion-intercalated LEuHs with vertical and tilted sensitizers showed enhanced PL intensity due to the energy transition effect by the intercalated sensitizers with respect to that of the LEuHs with horizontally oriented sensitizers.


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INTRODUCTION Intercalation chemistry1 has been attracting attention due to the advantages of being able to control and modify the layers without changing the layered structure and to flexibly regulate the properties of the layered materials. Analogous to layered double hydroxides (LDHs) structurally, layered rare-earth hydroxides (LRHs) have positivelycharged layers and interlayer anions, with the general formula of R2(OH)6–mAm·nH2O (0.5 ≤ m ≤ 2.0, R is a trivalent lanthanide and A denotes anion). But differently, the high coordination number (CN) 8 and/or 9 of R3+ with OHˉ/H2O and even directly with A (e.g. as m = 2)2 and readily changeable CN (e.g. from type m = 1 to type m = 2)3 make LRHs more interesting in the view of layered structures. The rare-earth ions usually presented good photoluminescence (PL) properties, but the PL of R3+ in LRHs is quenched owing to the coordinated water molecules and hydroxyl groups.4 For this reason, LRHs are usually used as raw materials for preparing their corresponding oxides,5 topological transforming to fluorides,6,7 and the ideal host matrix for up-conversion PL materials, such as β-NaYF4. Although energy transfers between R3+ ions in the intralayer can hardly occur due to the quenching effect, the recent results indicated it is possible between the adjacent layers.8 This fact exhibits that the quenching effect of OHˉ loses efficacy in the interlayer and deepens our understanding of that the intercalated organic sensitizers could enhance the PL property of LRHs.9-11 So far, the sensitizers of benzoic acids/derivatives12-16 and coumarin acids/derivatives,17 characterized with a conjugated structure, were extensively 3



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reported among the organic intercalates (Table S1, Supporting Information). These organic sensitizers exhibited PL enhancing effect of the rare-earth ions in LRHs, maybe due to energy level matching, the same as found in rare-earth metal complexes.18 Other sensitizers, as biphenylcarboxylate derivatives,19-21 naphthalene-1,5-disulfonate and aphthalene-2,6-dicarboxylate,22 were reported to enhance the PL properties of Eu3+ by intercalating them into Eu3+-doped LDHs. Inversely, phenylalanine and tyrosine23 were explored to quench PL of Eu3+, which was used to detect amino acids in biological systems. To understand why all the reported organic sensitizers, regardless of their configuration, adopted the same up-right orientation in the LRH interlayer, it is necessary to investigate intensively the influence factors on the orientations of different organic sensitizers in the interlayer and furthermore the influence of orientations on PL behaviors of LRHs. In the present study, several (hetero)aromatic compounds including cinnamic acid (CA), 1-naphthoic acid (NA), 1,3,5-benzenetricarboxylic acid (BTA), 2-picolinic acid (PLA),

pyrrole-2-carboxylic

acid

(PRA),

2-furoic

acid

(FRA)

and

2-

thiophenecarboxylic acid (TPA) were chosen as organic sensitizers of LEuHs. The deprotonated (hetero)aromatic compounds were intercalated into the interlayers of NO3ˉ-type LEuHs through ion exchange reaction. And our results showed that orientations of the (hetero)aromatic anions could be regulated and their sensitization effect on PL of LEuHs depended on the orientations.

4


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EXPERIMENTAL SECTION Preparation of NO3ˉ-LEuHs. Homogeneous precipitation24 was the way to synthesize the NO3ˉ-LEuHs. A mixture of Eu(NO3)3·6H2O (1 mmol), NaNO3 (13 mmol), HMT (1 mmol) and distilled water (80 mL) in a Teflon-autoclave was kept at 90 ˚C for 12 h. The autoclave was then cooled to room temperature. The desired NO3ˉ-LEuHs were centrifuged and washed with distilled water and ethanol, and finally vacuum dried at 40 ˚C. Intercalation of (hetero)aromatic anions into interlayer of LEuHs. The intercalation of (hetero)aromatic anions into interlayer of LEuHs was carried out through the ion exchange process. Firstly, 3.6 mmol an organic compound of CA, NA, BTA, PLA, PRA, FRA or TPA were added into distilled water (70 ml), and then, to deproton the organic compound, excess sodium hydroxide was added into the solution and stirred for 1 h. It is worth noted that a BTA molecule can be deprotonated to the different extent by adjusting NaOH/BTA molar ratios. Since the BTA molecule has 3 carboxyl terminals, theoretically, the ratio should be 3 to deproton all the terminals. But this could be achieved experimentally as the ratio was larger than 3 in the present experimental condition. Similarly, the ratio of 2 could give the deprotonation degrees between 1/3 and 2/3. This was confirmed by the chemical analysis shown below. After the deprotonation, 0.3 mmol NO3ˉ-LEuHs were separately added into the above target solutions for the intercalation of anions into interlayer of LEuH, and stirred in a beaker at the room temperature for 24 h, except for the case of CAˉ solution heated in Teflon-autoclave at 90 °C for 24 h. The anion-LEuH samples were separated and obtained by centrifugation, washing and drying at 40 °C for 12 h in a vacuum oven. Characterizations. We collected the X-ray diffraction (XRD) patterns of the samples by a Philips X’Pert Pro MPD diffractmeter with Cu-Kɑ radiation at 20 °C with the 2θ 5



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ranging from 4.5 to 70˚. The X-ray generator was set at 40 kV and 40 mA. And the scanning step size of wide angle XRD and small angle XRD were 0.0165˚ and 0.007˚. Fourier-transform infrared spectra (FT-IR) were obtained by a Nicolet 380 FT-IR spectrometer with the KBr method. Excitation and emission spectra of photoluminescence

(PL)

were

measured

by

a

Shimadzu

RF-5301PC

Spectrofluorophotometer. The rare earth Eu content was determined by Inductively Coupled Plasma (ICP) atomic emission spectroscopy though dissolving the samples in the HNO3 aqueous solution. The contents of C, H and N of the samples were determined via an Elementar vario EL elemental analyzer. We calculated the chemical formula of the samples based on the results of ICP and CHN results. Scanning electron microscope (SEM) observations were carried out by using a Hitachi S-4800 microscope at 5.0 KV.

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RESULTS AND DISCUSSION Intercalation of (hetero)aromatic anions into LEuHs. Figure 1 depicts the XRD patterns of the NO3ˉ-LEuH precursor and the (hetero)aromatic anion-intercalated LEuHs. The pattern of the NO3ˉ-LEuH precursor (Figure 1(a)) can be indexed to monoclinic symmetry with lattice parameters of a = 12.9540(2) Å, b = 7.347(2) Å, c = 16.7906(2) Å, and β = 94.5941(1)˚, agreeing with the result reported for the type m = 1 LRHs,25 referring to the formula of R2(OH)6–mAm·nH2O mentioned above. This implies that the host layer is constituted by two kinds of the polyhedrons of OHˉ/H2Ocoordinated Eu3+ with CN = 8 and 9. The basal spacing (𝑑𝑏𝑎𝑠𝑎𝑙) of 0.83 nm indicates that the interlayer species are mainly NO3ˉ and water. The analyzed chemical formula (Table 1) is close to R2(OH)5A·nH2O of type m = 1, agreeing with the XRD result. The SEM images of the precursor indicate the plate-shaped crystals form an orbicular secondary particle (Figure 2). Comparison with that of the precursor, enlarged 𝑑𝑏𝑎𝑠𝑎𝑙 values from 2.21 to 1.24 nm (Figure 1(b)-(f) and 1(h)) and shrunk value of 0.76 nm and 0.75 nm (Figure 1(g) and 1(i) ) are observed for the other samples, indicating a successful intercalation of the (hetero)aromatic anions. The PLAˉ-LEuH presents two 𝑑𝑏𝑎𝑠𝑎𝑙 values of 1.62 and 1.50 nm, corresponding to the diffraction peaks at 1.62, 0.81, and 0.54 nm, and those at 1.50, 0.75, and 0.50 nm, respectively (Figure 1(e)). Two series of the diffraction peaks have, correspondingly, close heights (Figure 1(e)), indicating that the two phases are close in amount. Similarly, the TPAˉ-LEuH has a relatively small amount of a phase with 𝑑𝑏𝑎𝑠𝑎𝑙 = 1.58 nm beside the main with 𝑑𝑏𝑎𝑠𝑎𝑙 = 1.62 nm (Figure 1 (d)). In all the XRD patterns (Figure 1(b)-(i)) of the organic anion-intercalated samples, a peak at d = 0.31 nm is observable, corresponding to (220) plane of the LEuH precursor (Figure 1(a)), implying that the distance between adjacent Eu3+ ions is hardly changed after the ion-exchange.26 7



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The coordination state between –COOˉ of the anions and Eu3+ was investigated by FT-IR spectroscopy. As shown in Figure 3, the NO3ˉ-LEuH precursor presents two absorption peaks at 1637 and 1384 cm-1, corresponding to the vibration absorption of H2O and NO3ˉ. The (hetero)aromatic anions mainly present two absorption bands approximately ranged from 1500 to 1560 cm-1 and 1360 to 1440 cm-1, assigned to the asymmetric 𝜈𝑎𝑠 and symmetrical 𝜈𝑠 stretching vibrations of –COOˉ groups,27 respectively. The two vibrations can be still seen in the spectra of the LEuH samples but along with slight shifts in wavenumber. A parameter of 𝛥𝜈 = 𝜈𝑎𝑠  𝜈𝑠 is employed to deduce the coordination states of -COOˉ with the central atom, Eu3+ in the present case, similar to the treatment in studying metal coordination complex.27 The 𝛥𝜈 values for –COOˉ in LEuHs (𝛥𝜈𝐿𝐸𝑢𝐻) and in organic anions alone (𝛥𝜈𝑓𝑟𝑒𝑒) are measured from the spectra and listed in Table 2. As the relative magnitude, D = 𝛥𝜈𝐿𝐸𝑈𝐻 ― 𝛥𝜈𝑓𝑟𝑒𝑒 is greater than 0, mono-dentate occurs whereas bi-dentate occurs. And the –COOˉ is in no coordination as the relative magnitude D is equal to 0.27.28

The

accordingly deduced coordination states of the anions with Eu3+ shown in Table 2 indicate that the anions except for PRAˉ and BTA3ˉ are directly coordinated with Eu3+ ion. Such a direct coordination of the guest organic anions to the rare-earth ions of LRH layers was also found in the cases of sodium dodecyl sulfate (SDS). This direct coordination easily occurs regardless of how DSˉ is introduced, in the synthesis of DSˉLRHs,3 the intercalation of DSˉ into LRHs,29 and in the stacking process of LRH 2D crystals.26 In the case of inorganic sulfate-LRHs, such direct coordinations make of LRH becoming to type m = 2.11 As shown in Table 1, the calculated content of each element is close to the found one, indicating the chemical formula could represent the composition of the samples. N element could not be detected by the CNH analysis for those samples in Table 1, 8


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indicating that NO3ˉ has been entirely removed by the organic anions. This is consistent with the result of IR spectra. Thus, the organic anions directly coordinated to Eu3+ (Table 2) take the role of making the LEuH layer itself electrically neutral,26 different from PRAˉ and BTA3ˉ, which are only the interlayer species. Since the layered structure is retained and furthermore the distance between adjacent Eu3+ ions is hardly changed (Figure 1), such direct coordination should be accompanied by some substitution for the H2O molecule or OHˉ anion of the layer. Because in the Eu3+centered polyhedrons of layer, water molecule can directly be coordinated with the central ion, the substitution could not be reflected by the Eu3+/OHˉ ratio changes. Note that the chemical formula of the BTA2–- and BTA3–-intercalated samples confirm that deprotonation degree of BTA molecules. Orientations of the (hetero)aromatic anions between LEuH layers. The area per unit charge23 (𝑆𝑐ℎ𝑎𝑟𝑔𝑒) is employed to determine the arrangement of the guests. Dimension of the anions are listed in Table 1 for calculating 𝑆𝑐ℎ𝑎𝑟𝑔𝑒 values. Bilayer or monolayer arrangement is speculated as an 𝑆𝑐ℎ𝑎𝑟𝑔𝑒 value of guest ion is larger or smaller than that of the host LEuH layer (0.22~0.24 nm2/charge),30 respectively. For BTA3ˉ- and PRAˉ-LEuHs, since Figures 1(g), (i) and (a) indicate their 𝑑𝑏𝑎𝑠𝑎𝑙 is less than that of NO3ˉ-LEuH that has a horizontal monolayer-arranged NO3ˉ layer (Scheme 1c),12 the organic anions would orientate in the same arrangement (Scheme 1(a) and (b)). The less 𝑆𝑐ℎ𝑎𝑟𝑔𝑒 values of 0.10 and 0.20 nm2/charge (Table 1), respectively, agree with the arrangement. For those samples, with larger 𝑑𝑏𝑎𝑠𝑎𝑙, by assuming that the anions are vertical to the LEuH layer, the 𝑆𝑐ℎ𝑎𝑟𝑔𝑒 values are less for BTA2ˉ -LEuH than that of the layer, but larger for all others (Table 1). The calculated 𝑑𝑏𝑎𝑠𝑎𝑙 value by adding the height of BTA2ˉ to the thickness of LEuH layer (0.65 nm)3 coincides well with the observed value. Similar results are obtained for the aromatic anion-intercalated 9



samples by considering double of the height of CAˉ or NAˉ. However, in the cases of the heteroaromatic anions, the result of calculated 𝑑𝑏𝑎𝑠𝑎𝑙 values larger than the observed ones signifies that bilayer-arranged anions might be (1) an alternating antiparallel arrangement with partial overlaps of the heteroaromatic rings (vertical)19 or (2) tilted with angles relative to the layer (tilted). The latter orientation would be reasonably preferred if the interactions of the heteroatom rings with the host layer (To be discussed below). The orientations of all the organic anions are proposed and schematically shown in Scheme 1. The ionic interaction of the –COOˉ with the LEuH layer with the positive charge is the primary factors responsible for the arrangements of the guests. Due to the interaction, the BTA2– ion is anchored to two adjacent, presenting a vertical monolayer arrangement (Scheme 1(f)). Different distribution of –COOˉ in BTA3– from BTA2– results in horizontal orientation (Scheme 1(a)), as is attributed to the anchors at three positions. This strong interaction makes the anions with one –COOˉ terminal to be vertical or tilted in the LEuH interlayer (Scheme 1(d)-(i)). However, the fact that PRAˉ adopts the horizontal orientation (Scheme 1(b)) reveals that the “intermolecular” weak interactions between the (hetero)aromatic rings and the LEuH layer are also influence factors on the orientations. Detailly, the interactions may occur between Eu3+ and/or OHˉ of the layer with the rings, and therefore involve, as described especially by the organometallic chemistry and supramolecular chemistry, hydrogen bonds (including C–H···O), agostic interaction, cation–π interaction, salt bridge (a combination of two non-covalent interactions: hydrogen bonding and ionic bonding), etc. The active N–H in the pyrrole group has a strong hydrogen bond31 interaction with the OHˉ of the layer in PRAˉ-LEuHs, such an interaction and the ionic bond of –COOˉ with the layers cause PRAˉ bonding to the 10


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layers at two positions, similar to BTA2–; but the distribution of the positions in PRAˉ causes it horizontal like BTA3–. Similarly, with OHˉ of the layer, the hydrogen bonds of the heteroatoms O and N respectively in the furan ring of FRAˉ and pyridine ring of PLAˉ (O−H···:O and O−H···:N) are also considerable. On the other hand, in the rings, N, O, S with high electronegativity should have an interaction with Eu3+. This is expected by the hardsoft Lewis acid-base (HSAB) theory.32 According to HSAB, O, N, S, and OHˉ are hard, intermediate-hard, soft-intermediate and soft bases, respectively; while Eu3+ is hard acid, and the hard-hard interaction has an ionogenic character. Therefore the stability of the interactions with Eu3+ (referred as to Eu3+−:heteroatom) decreases in the sequence of O, N, S and OHˉ. The ionogenic character of the bond between the heteroatoms and Eu3+ combines with the hydrogen bond, showing a salt bridge character. Thus, it is reasonable to affirm that such interactions between the heteroaromatic rings and the LEuH layer leads to the titled orientation of FRAˉ and PLAˉ (Scheme 1(h) and (e)). Since FRAˉ and PLAˉ as well as TPAˉ have almost the same dimensions, the smaller the tilted angle shown in Scheme 1(g) and (e) as well as (d) the smaller the 𝑑𝑏𝑎𝑠𝑎𝑙 values. Because (1) the close amount of the two phases of 𝑑𝑏𝑎𝑠𝑎𝑙 = 1.62 and 1.50 nm in PLAˉ-LEuHs (Figure 1(e)) are probably the result that one interaction dominates (O−H···:N and Eu3+−:N), and (2) only one 𝑑𝑏𝑎𝑠𝑎𝑙 (1.44 nm) observed in FRAˉ-LEuHs (Figure 1(f)) is the balanced state of the two interactions (O−H···:O and Eu3+−:O) and the 𝑑𝑏𝑎𝑠𝑎𝑙 value between the two 𝑑𝑏𝑎𝑠𝑎𝑙 values of PLAˉ-LEuHs, the two non-covalent interactions, therefore, might be similar in strength, For TPAˉ, heteroatom S in the ring is not considered to have a hydrogen bond with the layer, but the interaction of Eu3+−:S is weaker than those of O an N atoms. A large proportion of the TPAˉ anions is vertical in the interlayer, whereas a small amount of 11



them titled in a titled angle of 65.8˚ exhibits the weak interaction of Eu3+−:S (Figure 1d and Scheme 1d). NAˉ and CAˉ are vertical-bilayer-arrangement in the interlayer (Scheme 1(h) and (i)), the same as these for other aromatic anions reported.12-14 No marked agostic interaction between meal and a C−H bond is observed, might indicating that the transition metal is well coordinatively-saturated. All the orientations are summarized in Table 2 too for comparison. As shown, the anions in vertical or tilted orientations are directly coordinated to Eu3+ through monoor di-dentate of –COOˉ other than those in the horizontal orientation. Although the ligands with –COOˉ (or –SO42ˉ, mentioned above) easily substitute for OHˉ/H2O to coordinate to the big rare-earth ions of LRHs, but the current result indicates that the orientation and configuration of the organics should be taken into account. In other words, it is unnecessary for the carboxylated (and, maybe, sulfated) organics in the interlayer to directly coordinate to the central rare earth of LRHs. PL enhancement of LEuHs with vertical or tilted sensitizers. Figure 4 shows the excitation and emission PL spectra of NO3ˉ-LEuH and sensitizer anion-intercalated LEuHs. The original NO3ˉ-LEuHs present a series of PL peaks in the excitation spectra (Figure 4(A)) as a result of the intra-4 f 6 electric transitions of Eu3+, among which the peak at 394 nm is the strongest and corresponding to 7F0-5L6 transition of Eu3+. And a series of the PL peaks of the NO3ˉ-LEuH (Figure 4(D)) at 580, 595, 614, 652 and 700 nm are observed in the emission spectra under the excitation of 394 nm laser, which are attributed to the characteristic 5D0→7FJ (J = 0, 1, 2, 3, and 4) transitions of Eu3+ in LEuHs. 33-35 The CAˉ- and NAˉ-LEuH samples separately show two distinctly broad PL peaks at 321 and 336 nm in the excitation spectra (detected at 614 nm) compared with that of 12


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NO3ˉ-LEuH, as shown in Figure 4(A), indicating a strong absorption of CAˉ and NAˉ at ~ 321 and 336 nm that can be ascribed to the transitions of CAˉ and NAˉ from ground state S0 (π) to excited state S1 (1π*), and resulting in the strong PL emission of Eu3+ at 614 nm. The PL emission spectra of CAˉ- and NAˉ- LEuHs, excited by 321 and 336 nm laser, show enhanced PL peaks compared with that of NO3ˉ-LEuH, which can be attributed to the sensitization enhancement by CAˉ and NAˉ and energy transfer from CAˉ and NAˉ to Eu3+, as seen Figure 4(D). PLAˉ-, PRAˉ-, FRAˉ- and TPAˉ-LEuHs present almost the same PL peaks both in excitation and emission spectra, as seen Figure 4(B) and (E), with enhanced PL intensity except for that of PRAˉ-LEuH. The BTA2–-LEuH sample show a distinct wide PL peak at 298 nm in the excitation spectra (detected at 614 nm) compared with those of NO3ˉ-LEuH and BTA3–-LEuH, which can be ascribed to the transitions of BTA2– from ground state S0(π) to excited state S1(1π*), indicating the absorption of BTA2– at 298 nm and resulting in the strong PL emission of Eu3+ at 614 nm. The emission spectrum of BTA2–-LEuH, excited by 298 nm laser, show obviously enhanced PL peaks compared with those of NO3ˉ-LEuH and BTA3–-LEuH, which can be attributed to the sensitization enhancement of BTA2– on PL, as seen Figure 4(C) and (F). Therefore the aromatic anions with vertical orientation between the LEuH layers, CAˉ, NAˉ, and BTA2–, can absorb energy from the incident ultraviolet light and jump from S0(π) to S1(1π*) then translate to the lowest excited triplet state (3π*) through intersystem crossing,17 followed with energy transfer to the 5D0 excited state of Eu3+ and enhanced PL of Eu3+. This sensitization mechanism36-38 for PL enhancement of LEuHs is schematically shown in Scheme 2(C). For PLAˉ-, FRAˉ-and TPAˉ-LEuH with vertical or tilted orientation, no apparent excitation peaks of organic anions are observable (Figure 4(B)), which means that the energy levels are mismatched between 13



between 3π* and 5D0. however, the PL is also enhanced compared with NO3ˉ-LEuH (Figure 4(E)), which may be due to coordination of carboxyl groups with rare earth ions. Based on the discussion above, the organic anions-LEuHs with vertical or tilted oriented sensitizers show enhanced PL intensity, but the LEuHs with horizontal oriented sensitizers do not. Meanwhile all the LEuHs with vertical oriented sensitizers have bi- or mono-dentate coordination of -COOˉ with the Eu3+, and the LEuHs with horizontal oriented sensitizers have no coordination of -COOˉ with the Eu3+, as seen Table 2. Therefore, the enhanced PL of LEuHs would be related with the orientation and coordination of the sensitizers between the LEuH layers. Whether the energy levels match or not with, the coordinated sensitizers vertical and tilted orientations between LEuH layers may absorb the optical energy from the incident light and transfer the energy to Eu3+ through the coordination bands (Scheme 2(A)). In addition, the coordinated anions between LEuH layers may stabilize Eu3+ at excited state and improve the number of the excited Eu3+, finally resulting in enhancement in PL intensity both in excitation and emission spectra. On the other hand, the non-coordinated anions with horizontal orientations between the LEuH layers could not transfer energy to Eu3+ and sensitize the PL center Eu3+, as seen Scheme 2(B), even though the optical absorption of the non-coordinated aromatic anions maybe occur during the radiation of the incident light. Therefore, there no apparently enhanced PL peaks appeared both in the excitation and emission spectra of these LEuHs.

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CONCLUSIONS The (hetero)aromatic anions with –COOˉ are easily intercalated into NO3ˉ-LEuHs via an ion exchange. The organic anions can orientate vertical, tilted and horizontal relative to the LEuH layer in monolayer and bilayer arrangements. It is unnecessary for the –COOˉ terminal in the interlayer to directly coordinate to Eu3+. The direct coordination depends on the orientation of the anions: it occurs as the anion are vertical or tilted to the layer rather than horizontal to the layer. The orientations of the organic anions depend on (1) the ionic bonding between –COOˉ and the layer and (2) the interactions between the heteroatoms of the rings and Eu3+ and/or OHˉ of the layer, mainly involving hydrogen bonding, Eu3+−:heteroatom with the ionogenic character, and salt bridge interaction that combines the former two interactions. In the cases of the aromatic anions, consideration of the interaction (1) can explain their arrangements. But for the heteroaromatic anions, the interaction (2) should be also taken into account. The organic anions in vertical and tilted orientations enhance PL intensity in both of the excitation and emission spectra, comparison with that of the LEuHs with horizontally-oriented anions, due to their sensitization improving effect.

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ASSOCIATED CONTENT Supporting Information Summary of organic guests intercalated LRHs and lanthanide-LDHs AUTHOR INFORMATION *Corresponding Authors Xiaojing Yang, PhD, Professor Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, China. Email: [email protected] Fangming Cui, PhD, Professor Qian Xuesen Laboratory of Space Technology, China Academy of Space Technology, Beijing 100094, China. Email: [email protected] Conflicts of interests The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Grants. 51572031 and 51272030) and Programs for Changjiang Scholars and Innovative Research Team in University.

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FIGURE CAPTIONS Figure 1. XRD patterns of (a) NO3ˉ-LEuH and (b) CAˉ, (c) NAˉ, (d) TPAˉ, (e) PLAˉ, (f) FRAˉ, (g) PRAˉ, (h) BTA2ˉ and (i) BTA3ˉ intercalated LEuHs. Figure 2. SEM images of the NO3ˉ-LEuH precursor. Figure 3. FT-IR transmittance spectra of the LEuH samples intercalated with different anions (solid line), (a) NO3ˉ, (b) CAˉ, (c) NAˉ, (d) TPAˉ, (e) PLAˉ, (f) FRAˉ,(g) PRAˉ, (h) BTA2– and (i) BTA3–, compared with those of the corresponding anions alone (dotted lines). (Note: (1) νas(COOˉ); (2) νs(COOˉ); (3) ν(C=C); (4) ν(SV); (5) ν(C=N); (6) δ(H O); (7) ν(NO ˉ).) 2

3

Scheme 1. Schematic diagram of the different sensitizer orientations between the LEuH layers with respective 𝑑𝑏𝑎𝑠𝑎𝑙 (in nanometer). Figure 4. PL excitation and emission spectra of (a) NO3ˉ-LEuH and LEuHs intercalated with (b) CAˉ, (c) NAˉ, (d) PLAˉ, (e) PRAˉ, (f) FRAˉ, (g) TPAˉ, (h) BTA2– and (i) BTA3–. Scheme 2. Schematic diagram of sensitization mechanism of the (hetero)aromatic anions for PL performance of LEuHs.

23



Figure 1. XRD patterns of (a) NO3ˉ-LEuH and (b) CAˉ, (c) NAˉ, (d) TPAˉ, (e) PLAˉ, (f) FRAˉ, (g) PRAˉ, (h) BTA2ˉ and (i) BTA3ˉ intercalated LEuHs.

Figure 2. SEM images of the NO3ˉ-LEuH precursor. 24


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Figure 3. FT-IR transmittance spectra of the LEuH samples intercalated with different anions (solid line), (a) NO3ˉ, (b) CAˉ, (c) NAˉ, (d) TPAˉ, (e) PLAˉ, (f) FRAˉ,(g) PRAˉ, (h) BTA2– and (i) BTA3–, compared with those of the corresponding anions alone (dotted lines). (Note: (1) νas(COOˉ); (2) νs(COOˉ); (3) ν(C=C); (4) ν(SV); (5) ν(C=N); (6) δ(H2O); (7) ν(NO3ˉ).)

25



Scheme 1. Schematic diagram of the different sensitizer orientations between the LEuH layers with respective 𝒅𝒃𝒂𝒔𝒂𝒍 (in nanometer).

26


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Figure 4. PL excitation and emission spectra of (a) NO3ˉ-LEuH and LEuHs intercalated with (b) CAˉ, (c) NAˉ, (d) PLAˉ, (e) PRAˉ, (f) FRAˉ, (g) TPAˉ, (h) BTA2– and (i) BTA3–.

27



Scheme 2. Schematic diagram of sensitization mechanism of the (hetero)aromatic anions for PL performance of LEuHs.

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Table 1. Dimension of the Organic Anions and Chemical Analysis Results of Their Intercalated LEuHs interlayer anion

content calculated (foundc)) /wt% configuration

NO3ˉ -

O

O N+

O

COO-

COO-

TPAˉ

S

PLAˉ

N

FRAˉ

O

PRAˉ BTA3-

H N

-

COO-

COO-

COO-

COO-

COO-

OOC

COO

BTA2OOC

0.23×0.19×0.13

COOH

chemical formulae)

0.06

Eu 59.81 (59.25)

N 3.08 (3.05)

S 一

C 0.27 (0.27)

H 1.98 (1.98)

0.79×0.44×0.3

0.26

53.05 (53.12)

N/Ad)

一

19.22 (19.54)

2.78 Eu2(OH)4.98(C9H7O2)1.02·1.91H2O (2.81）

0.58×0.66×0.3

0.39

51.24 (50.08)

N/A

一

24.03 (23.91)

2.50 Eu2(OH)4.92(C11H7O2)1.08·1.16H2O (2.46）

0.52×0.34×0.45

47.63 (48.09)

N/A

8.22 (7.90)

15.40 (15.56)

2.36 Eu2(OH)4.36(C5H3SO2)1.64·2.88H2O (2.40）

54.47 (53.87)

2.65 (2.67)

一

13.67 (13.77)

2.41 Eu2(OH)4.94(C6H4NO2)1.06·2.26H2O (2.45）

0.49×0.42×0.3

0.27 0.22 0.25 0.31 0.23

56.91 (56.04)

N/A

一

12.35 (12.38)

2.07 (2.06)

Eu2(OH)4.90(C5H3O3)1.10·1.44H2O

0.49×0.41×0.3

0.20

56.18 (55.89)

3.37 (3.35)

一

14.42 (14.34)

2.09 (2.10)

Eu2(OH)4.70(C5H4NO2)1.30·0.56H2O

0.60×0.60×0.3

0.10

58.72 (58.68)

N/A

一

9.30 (9.32)

2.15 (2.34)

Eu2(OH)4.66(C9H3O6)0.44·2.34H2O

0.60×0.60×0.3

0.18

51.98 (51.95)

N/A

一

12.55 (12.56)

2.41 (2.66)

Eu2(OH)4.64(C9H4O6)0.68·3.36H2O

0.56×0.42×0.3

-

COO-

-

Schargeb)

-

CAˉ NAˉ

dimension /nma)

a)

Eu2(OH)4.64(NO3)1.02(CO3)0.12·2.72H2O

height × width × thickness, where height is defined from the COOˉ terminal to other side; b) calculated by the (the area projected onto the layer) ×2/charge; by ICP for Eu and CHN for the elements of N, C, and H; d) calculated by considering that Eu3+ is charge-balanced by the organic anion and OHˉ and neglecting the carbonate according to the analysis content; e) not detected by CHN. c) measured

29



Page 30 of 31

Table 2. Summary of Coordination State with Eu3+, Orientations in the Interlayer and PL Effects of the Organic Anions Organic anions

𝜟𝝂𝑳𝑬𝒖𝑯

𝜟𝝂𝒇𝒓𝒆𝒆

Coordination with Eu3+

Orientation

PL effect

CAˉ

133

144

bi-dentate

Vertical

Enhanced

TPAˉ

147

158

bi-dentate

vertical/tilted (65.8˚)

Enhanced

PLAˉ

138

154

bi-dentate

tilted (59.9˚/49.4˚)

Enhanced

FRAˉ

133

145

bi-dentate

tilted (59.2˚)

Enhanced

NAˉ

122

114

mono-dentate

vertical

Enhanced

BTA2ˉ

173

124

mono-dentate

vertical

Enhanced

PRAˉ

100

100

no

horizontal

no influence

BTA3ˉ

124

124

no

horizontal

no influence

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TOC Graphic

31


Orientation of (Hetero)aromatic Anions in the LEuH Interlayer and

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