Structures and manifestation of ortho-, meta-, and para-NH2-substitution in the optical spectra of europium and terbium aminobenzoates

Journal of Photochemistry and Photobiology A: Chemistry 285 (2014) 52–61

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Structures and manifestation of ortho-, meta-, and para-NH2 -substitution in the optical spectra of europium and terbium aminobenzoates V. Tsaryuk a,∗ , A. Vologzhanina b , K. Zhuravlev a , V. Kudryashova a , R. Szostak c , V. Zolin a a

V.A. Kotelnikov Institute of Radioengineering and Electronics, Russian Academy of Sciences, 1 Vvedenskii sq., Fryazino Moscow reg. 141190, Russia A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov st., Moscow 119991, Russia c Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie str., Wrocław 50-383, Poland b

a r t i c l e

i n f o

Article history: Received 5 December 2013 Received in revised form 12 April 2014 Accepted 17 April 2014 Available online 26 April 2014 Keywords: Eu3+ Tb3+ Aminobenzoate Luminescence X-ray crystal structure LMCT state

a b s t r a c t Four pairs of isostructural europium and terbium ortho-, meta, and para-aminobenzoates ([Ln(2ABenz)3 (H2 O)], [Ln(3-ABenz)3 (H2 O)3 ], [Ln(3-ABenz)3 (H2 O)3 ]·3H2 O, [Ln(4-ABenz)3 (H2 O)], (Ln = Eu, Tb; ABenz – aminobenzoate anion)) were investigated using X-ray diffraction and optical spectroscopy (luminescence and luminescence excitation spectra, as well as vibrational IR and Raman spectra). The crystal structures of [Tb(2-ABenz)3 (H2 O)], [Eu(3-ABenz)3 (H2 O)3 ], [Eu(3-ABenz)3 (H2 O)3 ]·3H2 O, and [Eu(4-ABenz)3 (H2 O)] were determined by single crystal X-ray analysis. The [Eu(3-ABenz)3 (H2 O)3 ] structure constitutes a new type of lanthanide m-aminobenzoate in the R3 space group. The Ln3+ coordination polyhedron formed by three bidentate chelate carboxylic groups and three terminal water molecules in both structures of m-aminobenzoate can be described as a distorted three-capped trigonal prism. The influence of the incorporation of three solvate outer-sphere H2 O molecules in the crystal lattice of m-aminobenzoate on the Eu3+ luminescence center was analyzed. Restructuring of the LnO9 coordination polyhedron results in a decrease in the distortions of the crystal field that appears as a loss of extra splitting of the Ln3+ electronic levels in [Ln(3-ABenz)3 (H2 O)3 ]·3H2 O. The effect of the electrondonating NH2 group located in different positions on the benzene ring on process of the excitation energy transfer to Ln3+ ion is examined. The hypsochromic shift of an intense interligand charge transfer (ILCT) band in the Tb3+ excitation spectra of the sequence of o-, m-, and p-NH2 -substituted compounds is observed. This shift is the consequence of the different electron density distribution depending on the NH2 position in the ligands. The predominant role of low-energy ligand–metal charge transfer (LMCT) states in the quenching of the luminescence of the europium o- and p-aminobenzoates in contrast with the europium m-aminobenzoates is discussed. Compounds including a supplementary Cl substituent on the benzene ring [Ln(2-A-5-Cl-Benz)3 (H2 O)n ] (Ln = Eu, Tb) and [Eu(4-A-2-Cl-Benz)3 (H2 O)n ] are also under consideration. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Lanthanide aminobenzoates with a high quantum yield of luminescence are of considerable interest for application in various molecular electronics devices such as organic light-emitting diodes, switches, and plastic lasers. Lanthanide aminobenzoates can be used in sensors for chemical and biological species, in lighttransforming materials, solar cells, etc. The connection between the photophysical properties and the structures of this group of

∗ Corresponding author. Tel.: +7 4965652577; fax: +7 4957029572. E-mail addresses: [email protected], [email protected] (V. Tsaryuk). http://dx.doi.org/10.1016/j.jphotochem.2014.04.014 1010-6030/© 2014 Elsevier B.V. All rights reserved.

complexes has been studied in a number of papers [1–8]. The area of the application of these compounds can be extended by incorporating the compounds into porous matrices, sol–gel networks or polymers. Aromatic amino acids can serve as building blocks in a polymeric synthesis, including lanthanide-based hybrid materials. The molecules of these acids are capable of forming polymers or being linked to polymer molecules through the NH2 groups while also being coordinated by the Ln3+ ion through the COO− groups [9–13]. A new Tb3+ -containing porous material based on m-aminobenzoic acid bound to a siloxane matrix was elaborated in the literature [9]. In another paper [10], the use of lanthanide p-aminobenzoates for the preparation of optically functional polyurethane composites was suggested. The authors

V. Tsaryuk et al. / Journal of Photochemistry and Photobiology A: Chemistry 285 (2014) 52–61

of the publication [11] tested the terbium-containing hybrid material based on o-aminobenzoic (anthranilic) acid as a luminescence sensor. Modified aminobenzoate ligands as the components of lanthanide hybrid materials can serve as effective antennae for the excitation of intense Ln3+ luminescence. The aminobenzoic acids and their salts with alkali metals play an important role in biology and pharmaceuticals. The properties of these types of compounds depend in many respects on the position of the NH2 group on the aromatic ring. The effect of the position of the amino group on the electronic structure and aromaticity of the acids and alkali metal salts was investigated in a series of papers [14–17]. In the present work, several pairs of isostructural europium and terbium ortho-, meta, and para-aminobenzoates were synthesized and studied with the help of optical spectroscopy and X-ray diffraction. An effect of the strong electron-donating NH2 group located in the o-, m-, or p-positions of the benzene ring on the processes of the luminescence excitation and on the structure of Ln3+ coordination centers was investigated. Recently, the analogous problem of the excitation energy transfer to Eu3+ and Tb3+ ions depending on the position of weakly electron-donating methoxy and strong electron-withdrawing nitro groups in the benzoate ligand has been examined [18,19]. The luminescence efficiency of terbium aminobenzoates is known to be higher by a factor of 100 than the corresponding europium compounds [4]. The excitation and luminescence efficiencies of europium and terbium benzoates are significantly dependent on substitution effects. A change of position of the amino group in the ligand influences the relative location of the ligand singlet and triplet electronic states, including the interligand charge transfer (ILCT), the ligand–metal charge transfer (LMCT) states, and the emitting states of the Ln3+ ions, caused mainly by a different interaction of substituents with the benzene framework and by a different distribution of the electron density in the ligands. To see the deeper specific features of the luminescence excitation spectra and to investigate the effect of distortion of the Ln coordination polyhedra on the luminescence spectra, the crystal structures of four compounds were determined by the single crystal X-ray method. The known structural data for lanthanide aminobenzoates [20] were also analyzed.

2. Experimental

53

either of the methods. The 3 and 4 complexes of m-aminobenzoic acid were synthesized by the first procedure, the 5 and 6 by the second procedure. The terbium m-aminobenzoate obtained by the first procedure can sometimes contain six H2 O molecules. In the process of obtaining the [Ln(4-ABenz)3 (H2 O)] compounds, the pH value of the reaction mixture was adjusted to 4–5. All of the reagents used for syntheses were purchased from Sigma–Aldrich and were analytical grade. All solvents were purified by standard techniques. The samples synthesized are single-phase, as proven by powder X-ray diffraction (Figs. S2–S6). The crystals of the compounds that were used for the X-ray study were grown by the slow evaporation of the solvent over 2–4 weeks at room temperature. crystal structures of the aminobenzoates The [Tb(2-ABenz)3 (H2 O)] (2), [Eu(3-ABenz)3 (H2 O)3 ] (3), [Eu(3ABenz)3 (H2 O)3 ]·3H2 O (5) and [Eu(4-ABenz)3 (H2 O)] (7) were determined by single crystal X-ray diffraction analysis. The first aminobenzoate (2) is isostructural with [Yb(2-ABenz)3 (H2 O)] [22] coded as DEDQIN in the Cambridge structural database [20]. The third aminobenzoate (5) is isostructural with the [RE(3ABenz)3 (H2 O)3 ]·3H2 O series, where RE = Sm (KOZDAE01) [23], Dy (KOZDUY01) [23], Er (KOZBUW) [24], Lu (KOZCAD) [24] and Y (KOZBIK) [24], and is isotypical to the [Nd(2-ABenz)3 (H2 O)3 ]·3H2 O (DEGPAH) [22]. The fourth aminobenzoate (7) is isostructural with the [Ln(4-ABenz)3 (H2 O)] family, where Ln = La (NADYAT) [5], Ce (NADXIA) [5], Sm (NADYOH01) [25], Gd (ADETEJ01) [10], Tb (NADXAS01) [5], Dy (NADXOG) [5], Er (NADXUM and NADXUM01) [5,25]. The [Eu(3-ABenz)3 (H2 O)3 ] (3) corresponds to a new type of lanthanide m-aminobenzoate in the R3 space group, isotypical to [Sm(3,5-ABenz)3 (H2 O)3 ] [26] coded as YENHOO [20]. Judging from the optical spectra as well as from the X-ray data, the pairs of europium and terbium compounds (1, 2), (3, 4), (5, 6) and (7, 8) are isostructural. In particular, the X-ray diffraction powder patterns given in Figs. S2, S4 and S5 indicate the isostructurality of the pairs of compounds (1, 2) and (5, 6). The Raman spectra presented in Fig. S7 indicate the isostructurality of the pairs of compounds (3, 4) and (7, 8). Compounds including a supplementary Cl substituent (9–11) were also under consideration. The crystal structures of these compounds are unknown. The IR spectra (Fig. S8) clearly show that the europium and terbium compounds [Ln(2-A-5-Cl-Benz)3 (H2 O)n ] (9 and 10) are isostructural.

2.1. Compounds 2.2. X-ray crystallography The lanthanide o-, m-, and p-aminobenzoates [Ln(2(1, 2), [Ln(3-ABenz)3 (H2 O)3 ] (3, 4), ABenz)3 (H2 O)] [Ln(3-ABenz)3 (H2 O)3 ]·3H2 O (5, 6) and [Ln(4-ABenz)3 (H2 O)] (7, 8) (Ln = Eu, Tb; ABenz – aminobenzoate anion) have been synthesized and investigated. A list of the compounds and the chemical formulae of the ligands are given in Table 1 and in Fig. S1 in the Supplementary Information file. Compounds 5, 6 and 7, 8 have been synthesized before [5,21]. Lanthanide aminobenzoates were obtained by two methods. The first method involved preparing a water solution of a mixture of aminobenzoic acid and newly prepared hydroxide Eu(OH)3 in a molar ratio of 3:1 and heating it in a water bath (at 70–90 ◦ C) for approximately 1–2 h. Then, a precipitated powder of the complex was filtered. The second method involved preparing a water solution of a mixture of the sodium salt of aminobenzoic acid and LnCl3 ·6H2 O in a molar ratio of 3:1 and heating it in a water bath (at 70 ◦ C) for approximately 1 h. The mixture was left to stand for 2–4 weeks at room temperature, whereupon the mixture was filtered. In both cases, an extracted product was washed with water and isopropyl alcohol and dried under room conditions. The lanthanide complexes of o- and p-aminobenzoic acids could be obtained by

The single-crystal X-ray diffraction data for compounds 2, 3, 5, and 7 were collected on a Bruker APEX II Duo diffractome˚ or Mo K␣ (3 ter at 100 K, using Cu K␣ (2 and 7,  = 1.54178 A) ˚ radiation [27]. The absorption correction was and 5,  = 0.71073 A) performed with the SADABS program [28], using multiple measurements of equivalent reflections. The structures were solved by direct methods and refined by the full-matrix least squares technique against F2 of all data. Nonhydrogen atoms were located from the Fourier density synthesis and refined in the anisotropic approximation. The positions of the H(C) atoms were calculated, and the H(N) and H(O) atoms were located from the Fourier synthesis and ˚ All hydrogen atoms were included normalized to 0.90 and 0.85 A. in the refinement in the isotropic approximation with the riding model with the Uiso (H) = 1.5Ueq (O) and 1.2Ueq (X), where Ueq (X) is the equivalent thermal parameter of the parent atom. All calculations were performed with the SHELXTL and OLEX2 software packages [29]. Details of the crystal data collection and refinement parameters for the crystals investigated are listed in Table 2. The powder X-ray diffraction patterns were obtained on a Bruker D8 Advance diffractometer at 295 K.

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Table 1 Lanthanide aminobenzoates under investigation. No. 1 2 3 4 5 6 7 8 9 10 11

Compound

C.N.

Ligand coordination

Ligand coordination in terms of [30]

[Eu(2-ABenz)3 (H2 O)] [Tb(2-ABenz)3 (H2 O)] [Eu(3-ABenz)3 (H2 O)3 ] [Tb(3-ABenz)3 (H2 O)3 ] [Eu(3-ABenz)3 (H2 O)3 ]·3H2 O [Tb(3-ABenz)3 (H2 O)3 ]·3H2 O [Eu(4-ABenz)3 (H2 O)]

7 7 9 9 9 9 8

6 COO− bridge, 1 H2 O 6 COO− bridge, 1 H2 O 3 COO− chelate, 3H2 O 3 COO− chelate, 3H2 O 3 COO− chelate, 3H2 O 3 COO− chelate, 3H2 O 4 COO− bridge, 1 chelate, 1H2 O, 1 NH2 4 COO− bridge, 1 chelate, 1H2 O, 1 NH2

AB2 3 M1 AB2 3 M1 AB01 3 M1 3 AB01 3 M1 3 AB01 3 M1 3 AB01 3 M1 3 AT3 B2 B01 M1

[Tb(4-ABenz)3 (H2 O)]

8

3

2

01

1

AT B B M

Dimensionality

Space group

Ref. structure

Chain Discrete

C2/c R3

This work This work

Discrete

R3c

This work

Layer

P21 /n

This work

Layer

P21 /n

[5,20]

[Eu(2-A-5-Cl-Benz)3 (H2 O)n ] [Tb(2-A-5-Cl-Benz)3 (H2 O)n ] [Eu(4-A-2-Cl-Benz)3 (H2 O)n ]

Unknown Unknown Unknown

2.3. Optical spectroscopy

3. Results and discussion

Luminescence and luminescence excitation spectra of the powders or crystals of the europium and terbium compounds were measured on a LOMO ISP-51 spectrometer with the recording system MORS-2 from the Institute of Spectroscopy RAS and on a SLM Aminco SPF 500 spectrofluorimeter at 77 and 295 K. With the first setup, a region at 300–380 nm of the spectrum of a DRSh-250 highpressure mercury lamp including the intense Hg 365 nm line was isolated by UV filters for luminescence excitation. The registration of excitation spectra was accomplished with the emission monitored at the most intense line of 5 D0 –7 F2 (Eu3+ ) and 5 D4 –7 F5 (Tb3+ ) transitions. The luminescence spectra were recorded at 8 cm−1 resolution. The IR spectra were obtained on a Nicolet Magna 750 FTIR spectrophotometer using the KBr pellet technique. The Raman spectra were recorded on a Nicolet FT Raman module coupled to a Nicolet Magna 860 spectrometer using a Nd:VO4 laser as an excitation source, CaF2 beamsplitter and InGaAs detector. Scattered radiation was collected by the 180◦ reflective optics. The interferograms were averaged over 500 scans, Hap-Genzel apodized and Fourier transformed to yield spectra at 2 cm−1 resolution.

3.1. Crystal structures The asymmetric unit and the environment of Ln atoms in the structures of [Tb(2-ABenz)3 (H2 O)] (2), [Eu(3-ABenz)3 (H2 O)3 ] (3) and [Eu(4-ABenz)3 (H2 O)] (7) are drawn in Fig. 1. In the structures of compounds under discussion, o-aminobenzoate acts as a bidentate bridge ligand (B2 in terms of Serezhkin’s notation [30]), and m-aminobenzoate acts as a bidentate chelate ligand (B01 ) through oxygen atoms of carboxylic group. Water molecules are terminal (M1 ) for all complex compounds. p-Aminobenzoate demonstrates not only B2 and B01 coordination modes but also the tridentate bridge (T3 ) one (the two oxygen atoms of carboxylic group and a nitrogen atom are involved in coordination bonding). As a result, complex units in the structures of [Tb(2-ABenz)3 (H2 O)] (2), [Eu(3-ABenz)3 (H2 O)3 ] (3), [Eu(3-ABenz)3 (H2 O)3 ]·3H2 O (5) and [Eu(4-ABenz)3 (H2 O)] (7) form, respectively, chains, discrete groups, discrete groups and layers, which belong to the AB2 3 M1 , AB01 3 M1 3 , AB01 3 M1 3 and AT3 B2 B01 M1 crystal-chemical groups. The coordination numbers (C.N.s) of lanthanides are equal to 7, 9, 9

Table 2 Crystallographic data and experimental details for the lanthanide aminobenzoates 2, 3, 5 and 7.

Empirical formula Fw Color, habit Crystal size (mm3 ) a (Å) b (Å) c (Å) ˛ (◦ ) ˇ (◦ )  (◦ ) V (Å3 ) Z Crystal system Space group dcalc (g cm−3 )  (mm−1 ) 2 max (◦ ) Independent reflections (Rint ) Obs.refl./restraints/parameters R,a % [I > 2(I)] Rw ,b % F(0 0 0) GOFc a

R = ||Fo | − |Fc ||/|Fo |.

b

Rw =

c

GOF =

2

3

5

7

[Tb(2-ABenz)3 (H2 O)] C21 H20 N3 O7 Tb 585.32 Colorless, needle 0.16 × 0.03 × 0.03 31.463(3) 9.0761(7) 15.7005(10) 90 109.058(3) 90 4237.7(5) 8 Monoclinic C2/c 1.835 16.848 130 3556 (0.057) 2971/0/289 0.039 0.100 2304 1.01

[Eu(3-ABenz)3 (H2 O)3 ] C21 H24 EuN3 O9 614.39 Colorless, needle 0.30 × 0.06 × 0.06 18.536(3) 18.536(3) 5.959(1) 90 90 120 1773.1(5) 3 Trigonal R3 1.726 2.708 54 1601 (0.034) 1601/1/103 0.018 0.036 918 1.01

[Eu(3-ABenz)3 (H2 O)3 ]·3H2 O C21 H30 EuN3 O12 668.44 Colorless, needle 0.24 × 0.03 × 0.03 18.837(2) 18.837(2) 11.809(1) 90 90 120 3628.9(10) 6 Trigonal R3c 1.835 2.663 60 2358 (0.053) 1741/1/112 0.026 0.057 2016 0.99

[Eu(4-ABenz)3 (H2 O)] C21 H20 EuN3 O7 578.36 Colorless, needle 0.25 × 0.04 × 0.04 9.6889(3) 22.5756(5) 9.8309(3) 90 99.552(2) 90 2120.5(1) 4 Monoclinic P21 /n 1.812 21.607 135 3648 (0.031) 3092/0/289 0.024 0.064 1144 1.00

 2 2 2  2 1/2 (w(Fo − Fc ) )/ (w(Fo )) .  2 2 2 1/2 w(Fo − Fc ) /(Nobs − Nparam )

.

V. Tsaryuk et al. / Journal of Photochemistry and Photobiology A: Chemistry 285 (2014) 52–61

55

Table 3 Selected geometric parameters of the coordination polyhedron in 2, 3, 5, 7 and 8 compounds (Å).

Ln Ln Ln Ln

O (H2 O) O (COO, B2 ) O (COO, B01 ) N

2

3

5

7

8 [20]

2.349(4) 2.269(4)–2.373(4)

2.388(2)

2.422(4)

2.507(2)–2.510(2)

2.475(4)–2.485 (4)

2.509(2) 2.317(2)–2.367(2) 2.462(2)–2.464(2) 2.660(2)

2.499(3) 2.286(3)–2.335(3) 2.446(3)–2.452(3) 2.689(3)

and 8, respectively, for 2, 3, 5 and 7. Fig. 1 shows that the coordination polyhedra for seven-, nine- and eight -coordinated metal atoms are a distorted pentagonal bipyramid, a tricapped trigonal prism and a bicapped trigonal prism. Selected geometric parameters of the coordination polyhedra are given in Table 3. In the case of a pentagonal bipyramid, the axial Tb O bonds are shorter than the equatorial bonds (r(Tb1 O6) = 2.269(4), ˚ ˚ r(Tb1 O5) = 2.272(4) A, r(Tb Oeq ) = 2.324(4)–2.373(4) A), the Oax Tb1 Oax angle is equal to 177.0(1)◦ , and the Oax Tb1 Oeq angles vary from 75.8(1) to 99.1(2)◦ . A comparison of 2 and 8, as well as 3, 5 and 7 shows that the Ln O distances for the bridge carboxylic group are shorter on average than the lengths of the chelate groups, as was also observed for the series of the dimeric

lanthanide carboxylates [31]. A Ln O(H2 O) bond is the most labile. The length of the Ln O(H2 O) bond varies over a broad range of distances. The influence of the presence of a solvate molecule (and the reorganization of H-bonds) on features of the coordination polyhedron of the central atom, crystal structure and spectra can be demonstrated in the example of the 3 and 5 compounds. Fig. 1 shows that the [Eu(3-ABenz)3 (H2 O)3 ] complex can occur in two enantiomeric forms. Although both enantiomers should be present in the reaction mixture, the crystal structure of 3 contains only one form, and both forms can be found in the structure of 5 (Fig. 2B and C). The structures are similar: piles of complex units parallel with the crystallographic c-axis are formed by means of H-bonds between the water molecules of one complex and the COO− group of the other and the parameters of a trigonal cell are close to each other (with the exception of the c parameter, which is approximately doubled in the case of 5). Moreover, the presence of noncoordinated water molecules (the closest is situated at 4.533 A˚ from the Eu atom) causes the elongation of the Eu· · ·Eu distances from 10.885–11.415 A˚ in 3 to 11.052–11.566 A˚ in 5. The piles are further packed through H-bonds to form the ilc H-bonded net (the three-letter code is given in terms of RCSR symbols [32]). In the structure of 3, the ilc net can be obtained by direct simplification of the compound, while in the case of 5, the neighboring rods are connected through solvate water molecules. The parameters of the H-bonds for these crystals are listed in Table 4, and the parameters for the H-bonds of 2 and 7 are given in Table S1. As Tables 3 and 4 show, the Eu O (carboxyl) bond distances in the structure of 5 ˚ is involved in are alternated, the O1 atom (r(Eu1 O1) = 2.475(4) A) ˚ a strong H-bonding, while the O2 atom (r(Eu1 O2) = 2.485(4) A) takes part in a weak O4-H· · ·O2 interaction. In the structure of 3, both O1 and O2 take part in a strong H-bond, and the Eu O (carboxyl) distances are close. 3.2. Vibrational IR and Raman spectra The vibrational spectra of the lanthanide aminobenzoates (Figs. S9 and S10) agree with specific features of the structures of complexes and reflect the manner of coordination of the ligands. The assignments of the bands in the spectra are given following the results of the experimental and theoretical study of aminobenzoic acids and their complexes with alkali metals [14–16,33–35]. The broad band and several narrow lines in the 2900–3600 cm−1 region of the IR spectra (Fig. S9) can be attributed to symmetric

Table 4 Hydrogen bonds in the structures of 3 and 5 (Å,◦ ).

Fig. 1. Complex units [Tb(2-ABenz)3 (H2 O)], [Eu(3-ABenz)3 (H2 O)3 ] and [Eu(4ABenz)3 (H2 O)] in the structures of 2 (A), 3 (B) and 7 (C), respectively, depicted in thermal ellipsoids drawn at 50% probability level. Only atoms of the asymmetric unit are labeled.

Interactiona

r(D· · ·A)

r(H· · ·A)

D H· · ·A

3

O3–H1O· · ·O1#a N1–H1N· · ·O2#b

2.678(3) 2.850(4)

1.833 2.076

173 150

5

O3–H1· · ·O4#c O3–H2· · ·O1#d O4–H3· · ·N1#e O4–H4· · ·O2#f N1–H1A· · ·O4

2.682(6) 2.738(6) 2.940(6) 3.079(6) 3.027(6)

2.024 1.896 2.148 2.373 2.263

169 171 155 141 145

a Symmetry transformations used to generate equivalent atoms #a x, y, z + 1, #b −y + 2/3, x − y + 1/3, z − 2/3, #c −x + y + 5/3, −x + 4/3, z − 2/3; #d −y + 1, −x + 1, z − 1/2; #e −y + 2/3, x − y − 2/3, z + 1/3; #f −x + y + 4/3, −x + 2/3, z + 2/3.

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V. Tsaryuk et al. / Journal of Photochemistry and Photobiology A: Chemistry 285 (2014) 52–61

Fig. 2. The ilc net (A) as a structural type of H-bonded net in the structures of 3 (B and D) and 5 (C and E). View of the crystal structures along the [0 0 1] (B and C) and [1 1 0] (D and E) crystallographic directions. H-bonds are depicted with dashed lines.

and anti-symmetric stretching vibrations of the H2 O molecules participating in the formation of the hydrogen bonds. The stretching vibrations of the NH2 group at ∼3380–3400 cm−1 and the C H stretching vibrations at ∼3000–3100 cm−1 also fall into this region. The broad H2 O band is the most intense in compounds 3 and 6, which have the largest water contents. Two lines at 3390 and 3585 cm−1 in the spectrum of 6 are related to the weakly bonded outer-sphere H2 O molecules. The broad band overlapping with narrow lines in the 400–850 cm−1 region is mainly formed by the external vibrations (i.e., librations) of H2 O molecules and NH2 groups. The strongest IR bands at 1500–1540 and 1400–1410 cm−1 are related to the vas and vs stretching vibrations of the COO− group, respectively. The small difference  = vas − vs = 100–120 cm−1 points to an equivalence of effective charges on the oxygen atoms of the COO− groups and to a small polarization of these anions [36,37] that agree with the structures of aminobenzoates, in which only the weakly distorted bridge and chelate COO− groups are present. Judging from the small difference in the vas and vs stretching vibrations of COO− in the IR spectra of Cl-substituted aminobenzoates (Fig. S8), these compounds may be assumed to also contain the bridge and chelate COO− groups. The dependence of the  difference on the coordination function of the carboxylate anion is not universal, as the  range also correlates with the nature and donor–acceptor ability of the substituents in the benzoate ligand characterized by the Hammett constants [38,39]. For example, the frequency of vas (COO− ) for metal nitrobenzoates [38,39] including lanthanide compounds [19], is higher by 40–50 cm−1 than aminobenzoates, while the frequency of vs (COO− ) changes slightly. The complex vibrations (C C) + ı(CH) + ı(NH2 ) with various contributions of normal modes are located in the 1300–1600 cm−1

region of the IR spectra in addition to the stretching v(COO− ) vibrations. The bands in the 1570–1630 cm−1 region are attributed to the bending ı(H2 O) and ı(NH2 ) vibrations as well as to (C C) + ı(NH2 ). The Raman spectra of the aminobenzoates are presented in Fig. S10. The strongest line at 1000 cm−1 in the spectra of maminobenzoates 3 and 6 is related to the “breathing” vibration of the aromatic ring. Most of the lines in the 1400–1600 cm−1 region of the Raman spectra can be attributed to vibrations with a considerable contribution of (C C) stretching of the aromatic ring. In the case of the p-NH2 substitution, a conjugation effect involves NH2 , the COO− group and the benzene framework in contrast to the case of m-substitution, in which the effect of the conjugation of the amino group with the ring is very small. The following differences in the change of polarizability at the (C C) stretching vibrations are reflected in the intensities of the Raman lines at ∼1400–1470 and ∼1600 cm−1 .

3.3. Luminescence spectra. Features of the trigonal Eu3+ luminescence center in meta-aminobenzoates The luminescence spectra of europium and terbium aminobenzoates are presented in Figs. 3 and 4 and S11–S13. A similarity of the Stark splitting of the electronic transitions in Eu3+ and Tb3+ spectra that can be seen especially well in the 5 D0 –7 F1,2 (Eu3+ ) and 5 D –7 F 3+ 4 1,2 (Tb ) regions of the spectra confirms the identity of the crystal structures of the europium and terbium compounds. The 5 D –7 F transition in the spectrum of 1 has a very strong intensity 0 4 that is rarely found in the europium compounds. A high symmetry of charge distribution in the nearest surroundings of the Eu3+ ion is a property of m-aminobenzoates. A smaller number of Stark

7

7

5

5

D4- F1

7

5

D4- F0

D4- F3

7

7 5

D4- F2

D0- F4

c

820

5

927

7

5

Intensity (a.u.)

7

D0- F2

5

7

D0- F1

5

1545

d

d

550

57

5

1545

630 685

7

7

5

5

7

D0- F3

D1- F3

D1- F2

5

Intensity (a.u.)

265

7

D1- F1

5

7

D4- F4

D0- F0

V. Tsaryuk et al. / Journal of Photochemistry and Photobiology A: Chemistry 285 (2014) 52–61

c

600

650

b

700

Wavelength (nm)

a 600

620

640

660

680

7

D0- F3

Wavelength (nm)

5

Fig. 4. Luminescence spectra of [Tb(2-ABenz)3 (H2 O)] (2) (a), [Tb(3-ABenz)3 (H2 O)3 ] (4) (b), [Tb(3-ABenz)3 (H2 O)3 ]·3H2 O (6) (c), [Tb(4-ABenz)3 (H2 O)] (8) (d) at 77 K.

1546

679

b

7

D0- F2

7 5

D1- F3

7

D1- F2

5

D0- F1

7

7

D0- F4

827

5

934

5

5

5

5

D1- F0

7

Intensity (a.u.)

7

D1- F1

5

827

7

D0- F0

580

550

600

a 650

700

Wavelength (nm)

The patterns of the 5 D0 –7 F4 transition in both of the compounds having a lower number of components testify, most likely, to an effect of the apparent increase in the effective symmetry of the crystal field for the 7 F4 state, analogous to another study [41]. At the same time, the general Stark splitting of the 5 D0,1 –7 FJ transitions of Eu3+ ion in 5 and the 5 D4 –7 FJ transitions of Tb3+ ion in 6 is significantly smaller than in 3 and 4 (Figs. 3 and 4). To all appearances, one should expect lower distortions of the Ln3+ luminescence centers in 5 and 6 in comparison with 3 and 4. One can compare distortions of the Ln3+ coordination polyhedron in the structures of m-aminobenzoates having three and six water

Fig. 3. Luminescence spectra of [Eu(3-ABenz)3 (H2 O)3 ] (3) (a and b) and [Eu(3ABenz)3 (H2 O)3 ]·3H2 O (5) (c and d) at 77 K. The most intense vibronic lines are marked.

components in comparison with the maximum possible number are observed in the Eu3+ electronic transitions of 3 and 5. We compared the Eu3+ luminescence spectra of two types of m-aminobenzoates having the trigonal nearest surroundings of Ln3+ ion (Fig. 3) and tried to connect their differences with specific features of the crystal structures. The Stark structure of the Eu3+ state in m-aminobenzoates [Eu(3-ABenz)3 (H2 O)3 ] (3) and [Eu(3-ABenz)3 (H2 O)3 ]·3H2 O (5) relating to R3 (C3 4 ) and R3c (C3v 6 ) space groups, respectively, corresponds closely to the three-capped trigonal prismatic coordination geometry of the luminescence center. The base of the prism is formed by the three oxygens (O3) of the three H2 O molecules, and the head is formed by the three oxygens (O1) of the three chelate COO− groups (Fig. 5). The other three (O2) oxygens of the chelate COO− groups are capping atoms. The C3 point group should be chosen as the Eu3+ site symmetry for both of these compounds [40]. In this case, the 7 F1 level must have two Stark components including one degenerate, the 7 F2 level must have three components including two degenerate, and the 7 F4 level must have six components including three degenerate. The 5 D1 level, like the 7 F1 level, also has two Stark components, which follows from the sufficiently intense 5 D1 –7 F1 electronic transition of the Eu3+ ion in 3 and 5.

Fig. 5. View of the Eu coordination polyhedron of [Eu(3-ABenz)3 (H2 O)3 ] (3) (a) and [Eu(3-ABenz)3 (H2 O)3 ]·3H2 O (5) (b) along the threefold axis. Schematic representation of the Eu polyhedron distortions.

58

V. Tsaryuk et al. / Journal of Photochemistry and Photobiology A: Chemistry 285 (2014) 52–61 Eu3H2O Sm6H2O Eu6H2O Dy6H2O Y6H2O Er6H2O Lu6H2O

Ln-O3(H2O)

Ln-O3(H2O)

d

Ln-O1(COO-) Ln-O2(COO-) Ln-O1(COO-)

2,34

2,36

2,38

2,40

2,42

2,44

2,46

2,48

2,50

7

7

Ln-O2(COO-)

5

5

Ln-O1(COO-)

F6- D4

F6- D3

Ln-O2(COO-)

Intensity (a.u.)

Ln-ligand bond

Ln-O3(H2O)

c

b

2,52

r(Ln-O), A Fig. 6. Visualization of the distribution of “metal-oxygen” bonds in the metal polyhedron for [Eu(3-ABenz)3 (H2 O)3 ] and [RE(3-ABenz)3 (H2 O)3 ]·3H2 O (RE = Sm, Eu, Dy, Y, Er, Lu).

a 250

molecules for one Ln3+ ion. The visual distributions of Ln–O distances including Y–O and of O–Ln–O angles in the three-capped trigonal prism for the series of m-aminobenzoates taken from X-ray data are given in Figs. 6 and S14. The main distortions of the Ln3+ coordination polyhedron are caused by a difference in the Ln O(COO− ) and Ln O(H2 O) bond lengths and by a rotation of the trigonal prism base and head relative to one another. In the structure of [Eu(3-ABenz)3 (H2 O)3 ] (3), the Eu O(H2 O) bond lengths are shorter by 0.12 A˚ than the Eu O(COO− ) bond lengths (Table 3). In the structure of [Eu(3-ABenz)3 (H2 O)3 ]·3H2 O (5), this ˚ In addition, two Eu O bonds of each of difference is only 0.057 A. the COO− groups in 5 become somewhat nonequivalent. At the same time, the angles of rotation ϕ of the trigonal prism base (plane O3O3O3) relative to the prism head (plane O1O1O1) (Fig. 5), being equal to 28.6 and −10.5◦ in 3 and 5, respectively, differ significantly from each other. The angles of rotation of the capping atom plane O2O2O2 relative to the head plane being equal to 76 and 64◦ in 3 and 5, respectively, differ weakly from each other. So, a decrease of distortions of the LnO9 coordination polyhedron in transition from 3 to 5 is caused: (i) by closing in the Ln O(COO− ) and Ln O(H2 O) bond lengths, i.e., by the approach of the base and the head of the prism; (ii) by closing in the projections (along the threefold axis) of the base and the head, mainly due to the rotation of the prism base formed by three water molecules. The restructuring of the LnO9 coordination polyhedron accompanied by a reorganization of the hydrogen bonds results in the lowering of the potential energy of the luminescence center and in the apparent raising of the crystal field symmetry in 5. The raising of the symmetry appears as a minimal splitting of some Ln3+ electronic levels, in particular the 7 F1 and 5 D1 levels of the Eu3+ ion, and as a reduction of the general Stark splitting of the 7 FJ states. Moreover, a redistribution of the electron density occurs in the aromatic rings of 5 and that manifests itself through the changes in the 1250–1500 cm−1 region of the vibrational spectra and the changes in the excitation spectra. In the isomorphous [RE(3-ABenz)3 (H2 O)3 ]·3H2 O (RE = Ln, Y) series [20,23,24], increasing the RE3+ ionic radius (Sm) in comparison with the Eu3+ radius, as usual, causes the RE-ligand bonds to lengthen but decreasing the RE3+ radius (Dy, Y, Er, Lu) causes these bonds to shorten (Fig. 6). An average difference in bond lengths of the lanthanide ion with the water molecules and the carboxylic groups is ∼0.033 A˚ for Sm, ∼0.05–0.06 A˚ for Dy, Er, Lu, and ∼0.12 A˚ for Y. The non-equivalence of the two distances Ln–O1 and Ln–O2 relating to the carboxylic group increases from Sm to Lu and that, evidently, can also promote changes in the

300

350

400

450

500

Wavelength (nm) Fig. 7. Luminescence excitation spectra of [Tb(2-ABenz)3 (H2 O)] (2) (a), [Tb(3ABenz)3 (H2 O)3 ] (4) (b), [Tb(3-ABenz)3 (H2 O)3 ]·3H2 O (6) (c), [Tb(4-ABenz)3 (H2 O)] (8) (d) at 77 K.

symmetry of the luminescence center. The slight reduction in distortions is shown as a minor decrease in the value of the splitting of the Stark components of the 7 F1 and 7 F4 states in the Eu3+ luminescence spectra of a row of the lanthanum, europium and yttrium compounds [RE(3-ABenz)3 (H2 O)3 ]·3H2 O (RE = La, Eu, Y), where the first and the third compounds are doped with 5% Eu (Fig. S15). Furthermore, a slight nephelauxetic effect is observed for this series of aminobenzoates. The energies of the Eu3+ 5 D0 –7 FJ transitions shift by only 5–10 cm−1 to the long-wavelength side with the host crystal matrix of this sequence of compounds. The small value of the changes is caused by a “pliability” of the crystal lattice of m-aminobenzoates constructed from discrete fragments. In the Eu3+ luminescence spectra of the 3 and 5 compounds, vibronic satellites accompanying the 5 D0 –7 F0 and 5 D0 –7 F2 transitions are observed (Fig. 3). These weak lines in the 630–830 cm−1 region, at ∼930 and 1545 cm−1 are related to several bending vibrations of the COO− group, to the stretching vibrations v(C C) and vas (COO− ), respectively. One can observe the resonant vibronic effect in the region of the 5 D0 –7 F2 electronic transition. The highfrequency Stark component of this transition “loans the intensity” to the neighboring vibronic satellites at 820, 827 and 927, 934 cm−1 associated with the 5 D0 –7 F0 transition. Vibronic satellites related to the vs and vas vibrations of the COO− group are also seen in the luminescence spectrum of europium p-aminobenzoate 7 (Fig. S11b). 3.4. Luminescence excitation spectra and features of the excitation energy transfer The influence of the position of the NH2 group in the aminobenzoates on the processes of the excitation energy transfer from the ligand to the Ln3+ ion can be partly clarified with the help of the luminescence excitation spectra, which are presented in Figs. 7–9. The efficiency of excitation of the Tb3+ ions through the bands of the aminobenzoate ligands is high in the terbium compounds with three acid isomers (Fig. 7) in contrast to the europium compounds (Fig. 8). A short-wavelength region, lower than 280 nm,

7

5

F1- D1

V. Tsaryuk et al. / Journal of Photochemistry and Photobiology A: Chemistry 285 (2014) 52–61

5

F0- D0

c

7

5

F0- G2,4,6

7

7

7

7

5

5

F0- D1

F0- D2

7

5

Intensity (a.u.)

F0- D4

5

F0- L6

d

b

a 300

350

400

450

500

550

Wavelength (nm) Fig. 8. Luminescence excitation spectra of [Eu(2-ABenz)3 (H2 O)] (1) (a), [Eu(3ABenz)3 (H2 O)3 ] (3) (b), [Eu(4-ABenz)3 (H2 O)] (7) (c and d). T = 77 K (a–c) and T = 295 K (d).

5

F0- D0

7

5

F0- D1

7

7

c

Intensity (a.u.)

7

5

F0- L6

5

F0- D2

of the broad ligand band in the excitation spectra is related to S–S* (␲–␲*) transitions of the aromatic framework of the ligand extended by the amino and carboxylic groups. A long-wavelength part of the ligand band belongs to ILCT (n–␲*) transitions caused by a shift of the electron density from the electron-donating NH2 substituent to the electron-withdrawing COO− group with a participation of the ␲-system of the aromatic nucleus. These two equally intense bands overlap one another, in contrast to the methoxybenzoates [18] and hydroxybenzoates (investigated by us) having the weaker donating OCH3 and OH groups and to the phenylacetates [42], where the two bands are well identified, due to the fact that the long-wavelength ILCT band has a lower intensity in comparison

b

a 300

350

400

450

500

550

Wavelength (nm) Fig. 9. Luminescence excitation spectra of [Eu(2-A-5-Cl-Benz)3 (H2 O)n ] (9) (a), [Tb(2-A-5-Cl-Benz)3 (H2 O)n ] (10) (b), [Eu(4-A-2-Cl-Benz)3 (H2 O)n ] (11) (c) at 77 K.

59

with the short-wavelength ␲–␲* band. The excitation spectra of terbium aminobenzoates conform satisfactorily to the absorption spectra of aminobenzoic acids [1,9,43–45] and the absorption spectra of the methanol solutions of lanthanide complexes with anthranilic acid [46]. In this study [1], the frontier orbitals, HOMO and LUMO, of the o-aminobenzoic acid were calculated. Judging from these results [1], the contributions of the COOH, NH2 and C6 H4 fragments of the anthranilic acid in HOMO and LUMO are 3.7 and 35.4%; 36 and 3.5%; 60.2 and 61.2%, respectively, which demonstrates the ILCT from NH2 to COOH. In another paper [44], a blue shift of the absorption bands of the aminobenzoic acids is observed in the row from o- to m- and then to p-aminobenzoic acid. Analogously, in the terbium aminobenzoates investigated, a longwavelength edge of the ILCT band shifts to the short-wavelength side in the transition from o- (2) to m- (4 and 6) and then to p-aminobenzoate (8) (Fig. 7). Such behavior of this band is conditioned by a different distribution of the electron density in the three types of aminobenzoate ligands, depending, first of all, on the position of the NH2 group in the aromatic ring. Specific features of the bonding of ligands in the crystal lattice, especially with the lanthanide ion, must also influence the location of the ILCT band. The hypsochromic shift of this band points to an increase in the value of E(HOMO − LUMO) in the sequence of terbium o-, m- and p- aminobenzoates. That increase can be caused by a lowering of the acceptor ability of the COO− as well as of the donor ability of the NH2 group, including a weakening of the localization of some NH2 groups in p-aminobenzoates due to their coordination with the Ln3+ ion. The distinction between the spectra of the two maminobenzoates 4 and 6 is explained by the differences in bonding the ligand in the crystal lattice, which were discussed above. Steric effects of the substituents on the energy of ILCT [44,47] should be insignificant in all the cases of the complexes investigated as, judging from the X-ray data, the structures of the coordinated ligands are nearly coplanar. Naturally, a short-wavelength shift of the longwavelength edge of the ILCT band is observed in the excitation spectrum of [Tb(2-A-5-Cl-Benz)3 (H2 O)n ] (10) at the introduction of a weak donating Cl substituent in the m-position of the oaminobenzoate (Fig. 9). The excitation spectra of the isostructural europium and terbium m-aminobenzoates [Ln(3-ABenz)3 (H2 O)3 ] (3 and 4) contain equally broad intense ligand bands with the long-wavelength edge at ∼350 nm. The spectra of the o- and p-aminobenzoates demonstrate cardinal differences in the excitation processes for europium and terbium compounds. The LMCT states can be identified in the europium o- and p-aminobenzoates. A role of the LMCT states in the excitation energy transfer to Ln3+ ion in complexes of the trivalent lanthanides has been studied in many publications [48–50]. An absence of both wide ligand bands and narrow lines of the Eu3+ ion in the wavelength region shorter than 360 nm in the excitation spectrum of [Eu(4-ABenz)3 (H2 O)] (7) in comparison with [Tb(4-ABenz)3 (H2 O)] (8) can be related to the existence of a low-energy LMCT state. Analogously, the excitation spectrum of [Eu(2-ABenz)3 (H2 O)] (1) indicates the LMCT state, which has a lower energy than in 7, though the spectrum of compound 1 was not satisfactorily recorded because of a low value emission signal. Only a weak band of the ligand at ∼285 cm−1 is seen in 1, in contrast to a broad band in the isostructural terbium compound 2. Most likely, a weak band with a maximum at ∼400 nm in [Eu(2-ABenz)3 (H2 O)] (1) may be related to the LMCT. Its position corresponds to the data on the absorption spectrum of the europium anthranilate solution [46]. The LMCT state in the europium complexes with maminobenzoic acid is not identified, as it has the highest energy. A great difference in the energy of the LMCT state in the maminobenzoate and the compounds with the other two isomers of aminobenzoic acid is connected with a well-known distinction of the polar effect in the case of m-substitution from the effect

60

V. Tsaryuk et al. / Journal of Photochemistry and Photobiology A: Chemistry 285 (2014) 52–61

in the cases of o- and p-substitutions [51]. Increasing the energy of the LMCT states in the sequence of europium o-, p- and maminobenzoates goes in line with weakening the Eu O(COO− ) bonds obtained from the X-ray data (Table 3). The presence of the weak electron-donating Cl substituent in the aminobenzoate ligands can change the energy of the LMCT state (Fig. 9). The introduction of Cl in the o-position of the p-aminobenzoate ligand ([Eu(4-A-2-Cl-Benz)3 (H2 O)n ] (11)) gives rise to a decrease in the LMCT state energy unlike a compound with Cl in the m-position ([Eu(2-A-5-Cl-Benz)3 (H2 O)n ] (9)). The lowering of the energy of the LMCT state can be caused by the stronger Eu-O bonds in the first compound. Thus, it is apparent from the results listed above that the LMCT states can form an intense quenching channel of the excitation energy in europium o- and p-aminobenzoates.

4. Conclusions The connection between the photophysical properties and the details of the structures of four pairs of europium and terbium ortho-, meta-, and para-aminobenzoates has been studied. The crystal structures of [Tb(2-ABenz)3 (H2 O)], [Eu(3-ABenz)3 (H2 O)3 ], [Eu(3-ABenz)3 (H2 O)3 ]·3H2 O, and [Eu(4-ABenz)3 (H2 O)] were determined. The [Eu(3-ABenz)3 (H2 O)3 ] structure is related to the new type of lanthanide m-aminobenzoates in the R3 space group. The structures of the Ln3+ luminescence centers (described by a C3 point group) in two trigonal m-aminobenzoates were analyzed. The Ln3+ coordination polyhedra in these compounds are formed by three chelate carboxylic groups and three water molecules, therefore, the local structure around the Ln3+ ion shows tricapped trigonal prismatic geometry. The main distortions of the coordination polyhedra are caused by a difference in the Ln-O(COO− ) and Ln O(H2 O) bond lengths and a rotation of the trigonal prism bases. Restructuring of the LnO9 polyhedron with the incorporation of three outer-sphere H2 O molecules in the H-bonded network in transition from [Ln(3-ABenz)3 (H2 O)3 ] to [Ln(3-ABenz)3 (H2 O)3 ]·3H2 O results in the apparent decrease in distortions of the crystal field. That decrease appears as a loss of the extra splitting of some Ln3+ electronic levels in the latter and a reduction of the general Stark splitting of the 7 FJ states. The position of the electron-donating NH2 group in the benzene ring clearly defines the efficiency of the Ln3+ luminescence excitation in europium and terbium aminobenzoates. The ILCT bands in all terbium aminobenzoates and in europium maminobenzoates can be used for excitation of the intense Ln3+ luminescence. An effect of the o-, m-, and p-NH2 -substitutions on the electron density distribution in the ligands is shown in the hypsochromic shift of the ILCT band in the Tb3+ excitation spectra. The low-energy LMCT states were identified in the europium o- and paminobenzoates. The weak sensitization of the Eu3+ luminescence in [Eu(2-ABenz)3 (H2 O)] and [Eu(4-ABenz)3 (H2 O)] compounds is caused by a dissipation of the excitation energy through the LMCT states. The introduction of the supplementary Cl substituent in the o-position of the p-aminobenzoate ligand in the europium complex was found to give rise to a decrease in the LMCT state energy.

Acknowledgements This work was supported by the Russian Foundation for Basic Research (Grants 12-03-31560, 12-03-33107 and 12-03-00878). The authors are indebted to their colleagues Prof. P. Gawryszewska, Dr. Z.S. Klemenkova, and Mrs. I.S. Pekareva for their help in the experiments.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jphotochem. 2014.04.014.

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