Near-Infrared Luminescence and Inner Filter Effects of Lanthanide

Article pubs.acs.org/IC

Near-Infrared Luminescence and Inner Filter Effects of Lanthanide Coordination Polymers with 1,2-Di(4-pyridyl)ethylene Nicole Dannenbauer,† Philipp R. Matthes,† Thomas P. Scheller,† Jörn Nitsch,† Sven H. Zottnick,† Markus S. Gernert,† Andreas Steffen,† Christoph Lambert,‡ and Klaus Müller-Buschbaum*,† †

Institut für Anorganische Chemie and ‡Institut für Organische Chemie, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany S Supporting Information *

ABSTRACT: A series of 12 lanthanide coordination polymers was synthesized from anhydrous LnCl3 and 1,2-di(4-pyridyl)ethylene (dpe) under solvothermal conditions in either thiazole (thz) or pyridine (py). The reactions yielded 1∞[Ln2Cl6(dpe)2(thz)4]·dpe with Ln = Ce (1), Nd (2), 1 1 ∞[LnCl3 (dpe)(py)2]·(dpe/py) with Ln = Gd (3), Er (4), and ∞[LnCl3(dpe) (thz)2](dpe/thz) with Ln = Sm (5), Gd (6), Tb (7), Dy (8), Er (9), Yb (10), as well as 1∞[HoCl3(dpe)(thz)2]·thz (11) and 2∞[La2Cl6(dpe)3(py)2]·dpe (12). One-dimensional coordination polymers (CPs) and a two-dimensional network of five different constitutions are formed by connection of LnCl3 units via dpe molecules. As free ligand, dpe shows an excimer effect that is reduced in the coordination polymers. In addition, dipyridylethylene proves to be a suitable sensitizer for the photoluminescence of lanthanides in the near-infrared region (NIR) only. Thereby, dpe differs from the related ligand 1,2-di(4-pyridyl)ethane. For the compounds presented, four different luminescence effects were detected: luminescence based on fluorescence of the linker dpe is observed in the visible region, whereas ligand-sensitized 4f−4f NIR emission is dominating for trivalent Nd, Er, and Yb. The Er-containing CPs show an inner-filter effect of Er3+, which is based on reabsorption of emission of dpe triggering the erbium NIR emission.

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near-infrared (NIR) light emission.15 Furthermore, the successful formation of coordination polymers containing the dpe ligand as network building linker is already known from main group and transition metal chemistry. Various compounds are known such as [Sn2Cl2(dpe)Ph6],16 1∞[ZnCl2(dpe)],17 2 and porous networks such as ∞ [CuCl(dpe)]·2H2O18 or 19 2 However, compounds with direct Ln−dpe ∞[(Ag2Cl2(dpe)]. coordination including linkage via Ln−dpe−Ln bridges have not yet been reported. This can be correlated to the high oxophilic character of the lanthanide ions, hampering the formation of an acid−base adduct with nitrogen-containing ligands. As a result, dpe was used instead as template for the formation of cocrystals with lanthanide complexes.20 Only a few molecular examples with dpe coordinating Ln centers are known, such as the diketonates [Eu(dpe)(tta)3(MeOH)]21 (tta = thenoyltrifluoroacetone) or [Ln(dpe)(btfa)3)(MeOH)] (btfa = 4,4,4-trifluoro1-phenyl-1,3-butanedione) with Ln = Eu, Gd22 that contain the additional strong electron-withdrawing ligands tta and btfa, as well as the complexes [LnX3(dpe)n(H2O)m] with X = NO3−, ClO4−, NCS−, n = 2−3, m = 0, 623 and [Zn2Nd2L(dpe)]· 2H2O24 (L = N,N′-bis(3-methoxysalicyl-idene)ethylene-1,2diamine). In addition, UV light induces the [2 + 2] cycloaddition reaction of two dpe giving tetrakis(4-pyridyl)cyclobutane

INTRODUCTION The development of coordination polymers and metal−organic frameworks (MOFs) has become a rapidly growing field of research in recent years.1 These hybrid materials exhibit a wide variety of physical properties,2 with photoluminescence3 and sensor applications being a current focus.4 Furthermore, the variety of the available physical forms of these materials (such as nanoparticles5 or thin films6) and possible postsynthetic modification7 increase the options to influence their properties. Especially lanthanide-containing coordination polymers8 and MOFs show interesting photoluminescence properties due to the spectroscopic features of their metal ions and ligands.3b,9 Effective Ln3+ luminescence is achieved through bypassing the low light absorption coefficients of the 4f−4f transitions by combination with organic light harvesting molecules that have high absorption coefficients. The so-called “antenna effect”10 allows the sensitization of the lanthanide ions via a suitable organic linker by transferring excitation energy from the ligand to the metal center. An effective sensitization depends on the energetic position of the singlet and triplet states11 of the ligand in the coordination polymer.12 The linker 1,2-di(4-pyridyl)ethylene (dpe) exhibits both structural and electronic properties for the formation of coordination polymers with effective photoluminescence. In principle, the energetic positions of the singlet13 and triplet levels14 of the free ligand are in the range for possible sensitization of the lanthanide ions for visible or © XXXX American Chemical Society

Received: February 23, 2016

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DOI: 10.1021/acs.inorgchem.6b00447 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 1. Crystallographic Data for 1∞[Nd2Cl6(dpe)2(thz)4]·(dpe) (2), 1∞[GdCl3(dpe)(py)2]·(dpe/py) (3), 1∞[LnCl3(dpe)(thz)2]· (dpe/thz) Ln = Sm (5), Dy (8), Yb (10), 1∞[HoCl3(dpe)(thz)2]·thz (11), and 2∞[La2Cl6(dpe)3(py)2]·dpe (12) Nd (2) formula fw, g mol−1 crystal system space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z ρcalc, g/cm3 μ, cm−1 F(000) T, K data range X-ray radiation collected reflections no. of unique reflections R(int) no. of parameters R1 for reflections F0 > 2σ (F0)]a R1 (all)a wR2 (all)b S res electron density (e/Å3) a

Gd (3)

Sm (5)

Dy (8)

Yb (10)

Ho (11)

La (12)

C24H21Cl3NdN5S2 C61H55Cl6Gd2N11 694.19 1469.40 monoclinic triclinic C2/m P1̅ 12.426(3) 9.4170(10) 14.443(3) 12.6180(13) 16.443(3) 14.1070(14) 90 101.270(2) 111.44(3) 102.360(2) 90 100.700(2) 2746.9(11) 1559.9(3) 4 1 1.679 1.567 2.357 2.412 1372 730.0 100(3) 167(3) 2.66 ≤ 2Θ ≤ 60.10° 3.96 ≤ 2Θ ≤ 54.10° Mo Kα, λ = 0.710 73 Å 20 736 19 309

C51H45Cl6Sm2N11S5 1485.76 triclinic P1̅ 8.996(2) 12.616(3) 14.145(3) 97.33(3) 107.53(3) 99.84(3) 1480.6(7) 1 1.6628 2.456 732.82 167(3) 4.08 ≤ 2Θ ≤ 53.72°

C51H45Cl6Dy2N11S5 1510.04 triclinic P1̅ 8.946(2) 12.566(3) 14.023(3) 97.65(3) 107.16(3) 99.81(3) 1455.8(7) 1 1.7189 3.047 740.49 167(3) 6.58 ≤ 2Θ ≤ 54.04°

C51H45Cl6Yb2N11S5 1531.12 triclinic P1̅ 8.903(3) 12.526(4) 13.906(4) 97.829(8) 107.184(8) 99.906(8) 1429.0(1) 1 1.775 3.760 748.67 100(3) 3.12 ≤ 2Θ ≤ 56.68°

C21H19Cl3HoN5S3 708.90 orthorhombic Pbca 16.709(3) 11.365(2) 28.255(6) 90 90 90 5365.7(2) 8 1.7575 3.505 2778.9 100(3) 2.88 ≤ 2Θ ≤ 60.10°

C52H45Cl6La2N9 1286.52 triclinic P1̅ 13.734(2) 14.594(2) 14.888(2) 98.734(3) 98.193(3) 95.796(3) 2896.4(7) 2 1.580 1.779 1368.0 167(3) 2.80 ≤ 2Θ ≤ 54.60°

14 888

17 808

21 313

80 904

33 895

3878

6825

6184

6367

7081

7747

12 800

0.0959 229

0.0314 361

0.0236 415

0.0221 351

0.0298 353

0.0381 307

0.0810 685

0.0441 0.0784 0.0979 1.051 1.62/−1.85

0.0293 0.0348 0.0634 1.076 1.01/−0.50

0.0432 0.0461 0.0952 1.036 1.70/−1.67

0.0285 0.0315 0.0724 1.047 1.65/−0.89

0.0262 0.0295 0.0622 1.031 1.94/−1.23

0.0216 0.0354 0.0787 1.017 1.02/−0.86

0.0662 0.1124 0.1246 1.045 2.45/−0.92

R1 = ∑[|Fo| − |Fc|]/∑|Fo|. bR2 = [∑w(|Fo|2 − |Fc|2)2/∑w(|Fo|2)2]1/2.35

and [LnCl3(thz)4]·0.5(thz), Ln = Sm, Eu, Gd, Tb, Dy29 for thiazole. With increasing reaction temperature and increasing excess of the connecting linker, substitution of the terminal pyridine or thiazole ligands with dpe molecules can be achieved. This results in the formation of new coordination polymers in which nonchelating nitrogen-containing aromatic ligands can be utilized as linkers. In addition, remarkable influence of ligandbased photophysical properties are available, as shown for 4,4′bipyridine (bipy) and anhydrous LnCl3 for the MOF materials 30 3 and 2∞[Ln2Cl6(bipy)3]·2(bipy) with ∞[La2Cl6(bipy)5]·4(bipy) 31 Ln = Pr, Nd, Sm−Tb and the complexes [Ln2Cl6(bipy)(py)6] with Ln = Pr, Nd, Sm−Yb.32 To elaborate beyond the typical 4f−4f luminescent lanthanide ions such as Tb 3+ , we concentrated on the dipyridylethylene-sensitized NIR emission of the lanthanide ions Nd3+, Er3+, and Yb3+ in this work, as dpe proved to be an excellent sensitizer for these emitters. Crystal Structures. Crystal Structure of the DoubleStrand Coordination Polymer 1∞[Ln2Cl6(dpe)2(thz)4]·dpe (1,2). The double-strand coordination polymers with Ce and Nd crystallize isotypic in the monoclinic space group C2/m, as shown for single crystalline 1∞[Nd2Cl6(dpe)2(thz)4]·(dpe) (2). The corresponding crystallographic data are given in Tables 1 and S1. The trivalent Nd ion exhibits a distorted double-capped trigonal prismatic coordination polyhedron of four nitrogen atoms of two dpe-ligands and two thiazole ligands as well as four chlorine atoms (Figure 1). Two of the chlorine atoms are in a terminal position, whereas two other chlorides are linking

(TCBP), which can result in single-crystal to single-crystal (SCSC) transformation processes.20a TCBP has been used as a ligand for the formation of 2∞[PrCl3(TCPB)(H2O)(MeOH)]· 2(MeOH)·H2O.25 With the 12 compounds presented here, we now add coordination polymers based on LnCl3 and bridging dpe linkers together with the coligands thiazole (thz) and pyridine (py). Investigations of the photoluminescence and thermal properties provide insights into their properties and especially the excellent sensitization possibilities of NIR emission of trivalent lanthanide ions.

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RESULTS AND DISCUSSION General Considerations. The reported coordination polymers 1−12 (1∞[Ln2Cl6(dpe)2(thz)4]·dpe, Ln = Ce (1), Nd (2), 1∞[LnCl3(dpe)(py)2]·(dpe/py), Ln = Gd (3), Er (4), 1 ∞[LnCl3(dpe)(thz)2]·(dpe/thz), Ln = Sm (5), Gd (6), Tb (7), Dy (8), Er (9), Yb (10), 1∞[HoCl3(dpe)(thz)2]·thz (11), and 2 ∞[La2Cl6(dpe)3(py)2]·dpe (12)) were synthesized via a solvothermal reaction route alternatively in thiazole or pyridine. Hereby, anhydrous lanthanide chlorides were treated with the linker molecule dpe in the solvents pyridine or thiazole. The compounds show the typical colors of the corresponding lanthanide ions26 and are air- and moisture-sensitive, but stable to dry oxygen (vide infra). The solvents react as Lewis bases that break up the lanthanide chloride structures, forming luminescent solvate complexes of the type [LnCl3(py)4] with Ln = Y, La, Er, Yb27 or [La2(μ2-Cl)2Cl4(py)8]28 for pyridine B


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Inorganic Chemistry

Nd2Cl6 units are formed as connecting nodes that are interconnected to two other dimers along the b-axis by two parallel aligned dpe molecules giving a ladderlike polymer structure. Furthermore, an intercalated dpe molecule is located between two double strands along the a-axis (Figure S1). Topology investigations via TOPOS36 revealed a four-edge circulation via a lanthanide double-chloride μ-bridge and dpe, which is illustrated in Figure 1. Furthermore, the ladder structure gives a uninodal 3-c network with SP-1 net topology type, which can, for example, also be observed in [Fe2L(dpa)2]·x(MeOH) (L = tetraethyl-[2,2′,2″,2‴]-[1,2,4,5-phenylene-tetra-(iminomethylidyne)]tetra(3-oxobutanato)]).37 1 Crystal Structures of ∞ [GdCl3(dpe)(py)2]·(dpe/py) (3), 1 [LnCl (dpe)(thz) ]·(dpe/thz) with Ln = Sm (5), Dy (8), Yb ∞ 3 2 (10), and 1∞[HoCl3(dpe)(thz)2]·(thz) (11). In the following section, a one-dimensional strand-type structure with different end-on ligands and intercalations is discussed that is found for several lanthanides together with dpe, thz, and py. The coordination polymers 1∞[GdCl3(dpe)(py)2]·(dpe/py) (3) and 1 ∞[LnCl3(dpe)(thz)2]·(dpe/thz), with Ln = Sm (5), Dy (8), and Yb (10), are all crystallizing in the triclinic space group P1̅, whereas the strand 1∞[HoCl3(dpe)(thz)2]·(thz) (11) crystallizes in the orthorhombic space group Pbca (see Table 1 for crystallographic data of 3, 5, 8, 10, and 11). Each lanthanide ion exhibits a distorted pentagonal-bipyramidal coordination polyhedron. Ln is surrounded by four nitrogen atoms from two pyridine/thiazole molecules and of two dpe molecules as well as three terminal chlorine atoms in a T-shaped arrangement, two of the latter occupying the axial positions and one positioned in the equatorial plane (Figure 2). Because of the different interatomic distances found for Ln−Cl and Ln−N, the structures deviate significantly from an ideal polyhedron (see, e.g., the axial angles Cl−Ln−Cl of 172.93°(3) in Gd (3), 171.82°(5) in Sm (5), 172.90°(3) in Dy (8), 173.89°(2) in Yb (10), and 169.23°(2) in Ho (11)). A comparison of the interatomic distances within 3, 5, 8, 10, and 11 to compounds, which are structurally related, are corresponding well with Ln−Cl distances and the expected values for the trivalent state of the lanthanides. For example, the range of Sm−Cl distances in the single strand (2.643(2)−2.702(2) Å) match with [Sm(ntb)Cl3] (ntb = tris(benzimidazol-2-ylmethyl)amine, Sm−Cl 2.674−2.713 Å).38 For the heavier lanthanides, such as Yb, the Yb−Cl distances are between 2.5630(10) and 2.6305(10) Å, which is in good accordance to the distances in the monomeric complex YbCl3(py)4 (Yb−Cl = 2.559−2.571 Å).27b This accordance is also found for the interatomic distances Ln−N of 3, 5, 8, 10, and 11.27b,38 The trend of the interatomic distances follows the direction of the lanthanide contraction from lighter to heavier atoms, ranging from Sm−N with 2.566(4)−2.618(4) Å to Yb−N with 2.479(3) to 2.530(3) Å. Further interatomic distances are listed in Table S1. Distorted pentagonal-bipyramidal coordination polyhedra are frequent for Ln−Cl−pyridyl compounds, for example, being present in the monomer [LnCl3(py)4]·0.5(py)27 or in the dinuclear complexes [Ln2Cl6(bipy)(py)6] with Ln = Y, Pr, Nd, and Sm−Yb.32 In 1 1 ∞ [GdCl 3 (dpe)(py) 2]·(dpe/py) (3), ∞ [LnCl 3 (dpe)(thz) 2 ]· (dpe/thz) with Ln = Sm (5), Dy (8), and Yb (10), and 1 ∞[HoCl3(dpe)(thz)2]·(thz) (11), [LnCl3(py/thz)] units are connected to one-dimensional strands by the ligand dpe within the equatorial plane built by one chlorine atom, two nitrogen atoms of pyridine/thiazole, and two nitrogen atoms of two dpe molecules. Within the ab-plane the strands are alternately stacked with a rotation angle of 180° and shifted by half of the

Figure 1. Extended Nd3+ coordination spheres of 1∞[Nd2Cl6(dpe)2(thz)4]· dpe (2) with thermal ellipsoids depicting 50% of the probability level of the atoms. Symmetry operations: Ix,1 + y, z; IIx, −y, z, IIIx, −y − 1, z IV −x, y, 1 − z, V−x, −y − 1, 1 − z, VI−x, y, 2 − z, VII−x, −y, 1 − z, −x, VIII 1 + y, 1 − z (top). Crystal structure of Nd (2) (center). Depiction of the SP-1 topology type; Nd−Cl double chloride bridges are marked in red, Nd-dpe bridges are marked in blue (bottom).

adjacent lanthanide centers, which generates a dinuclear unit. The interatomic distances of the coordinating ligands are in the range of 2.724(2)−2.793(2) Å for Nd−Cl(terminal), 2.866(2)−2.937(2) Å for Nd−Cl(bridging), and 2.608(5)− 2.777(6) Å for Nd−N. This is in good agreement with the interatomic distances found in NdCl3(H2O) (terpy) (terpy = 2,2′:6′,2″-tertpyridine),33 (Nd−Cl(terminal): 2.746−2.787 Å, Nd−Cl(bridging): 2.917−2.929 Å, Nd−N: 2.592−2.615 Å), or in NdCl3(phen)2, phen = phenanthroline34 (Nd−Cl(terminal): 2.741−2.742 Å, Nd−Cl(bridging): 2.859−2.884 Å, Nd−N: 2.634 up to 2.661 Å). C


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Inorganic Chemistry

Figure 2. Depiction of the lanthanide coordination sphere in 1 I ∞[SmCl3(dpe)(thz)2]·(dpe/thz) (5), symmetry operations: x, y, z − 1; IIx, y, z + 1; III−x − 1, −y, 2 − z; IV−x − 1, 1 − y, 3 − z (top). One-dimensional chain structure of 1∞[GdCl3(dpe)(py)2]·(dpe/py) (3) (center). Parallel arranged one-dimensional strands of 1 ∞[HoCl3(dpe)(thz)2]·thz (11) extending along the c-axis (bottom). All thermal ellipsoids show 50% of the probability level of the atoms. Figure 3. La3+ coordination-spheres of 2∞[La2Cl6(dpe)3(py)2]·(dpe) (12), symmetry operations: Ix, y − 1, z; II1 − x, 1 − y, −z − 1; III−x, −y, 1 − z (top). Depiction of the two-dimensional sheet network of 2 ∞[La2Cl6(dpe)3(py)2]·(dpe), consisting of La2Cl6 dimers connected by dpe molecules and cavities filled with intercalated dpe molecules. Depiction of the sql topology type of 2∞[La2Cl6(dpe)3(py)2]·(dpe). La−Cl double chloride bridges are marked in red, La−dpe bridges are marked in blue (bottom). All thermal ellipsoids show 50% of the probability level of the atoms.

c-axis along [001]. Contrary to Gd (3), Sm (5), Dy (8), and Yb (10), the strand for Ho (11) shows a zigzag chain along the c-axis. The strands are shifted by the length of a dpe molecule along [001]. Additionally, the crystal structures of Gd (3) (Figures 2 and S2−S4), Sm (5), Dy (8), and Yb (10) (Figures 2 and S5−S7) contain one noncoordinating dpe molecule and 1 equiv of either pyridine or thiazole, whereas 1∞[HoCl3(dpe) (thz)2]·thz (11) incorporates only noncoordinating thiazole (Figures 2 and S8−S10). 2 Crystal Structure of ∞ [La 2 Cl 6 (dpe) 3 (py) 2 ]·dpe (12). 2 ∞[La2Cl6(dpe)3(py)2]·dpe (12) crystallizes in the triclinic space group P1.̅ Additional crystallographic data are listed in Table 1 and Table S1. The trivalent lanthanum ions are each coordinated by four chloride ions and four nitrogen atoms of coordinating pyridyl rings. The coordination number of La3+ is eight, and the coordination sphere is built by a distorted double-capped trigonal prism. Two coordination spheres are edge-connected via a double-chloride bridge forming a La2Cl6 dimeric unit (see Figure 3). The interatomic distances are in the range of 2.773(2)−2.823(2) Å for La−Cl (terminal), for La−Cl (bridging) 2.913(2)−3.000(2) Å, and for La−N in the range of 2.643(5)−2.866(5) Å. This is in good accordance, for example, with 3∞[La2Cl6(bipy)5]·4(bipy)30 with La−Cl 2.760−2.964 Å and La−N 2.750−2.818 Å or the complex [La2(μ-Cl)2Cl4(py)8]28 with La−Cl 2.778−2.929 Å and La−N 2.722−2.789 Å. The dimeric unit La2Cl6 is interconnected to four other dimers by six bridging dpe molecules. Hereby, two

dimers are double-connected by two parallel aligned dpe molecules along the a-axis (Figure 3). The complete connection of all dimers leads to the formation of a two-dimensional sheet along the (201) plane. The La-coordination spheres are completed by a nitrogen atom of a terminal pyridine ligand. Within the horizontal sheet, two different cavities with rhomboid shape are found. One cavity is filled by four terminal coordinating pyridine molecules, the other by parallel aligned intercalated dpe molecules. Both cavities protrude along the b-axis. The two-dimensional network sheets are shifted from another one-half of the b-axis along [001] (Figures 3, S11, and S12). Investigation of the topology of the network via TOPOS36 revealed a net of the sql type (Figure 3), which can, for example, also be observed in the networks 2∞[M(tris(nicotinoylN-oxide))(cyclotriguaiacylene)2(dmf)2]·2(ClO4)8(dmf) with dmf = dimethylformamide, M = Cd, Cu,39 and 2∞[Ln2Cl6(bipy)3]· 2(bipy) with Ln = Pr, Nd, Sm−Tb.31 D


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Inorganic Chemistry Powder X-ray Diffraction Investigations. Powder X-ray diffraction (PXRD) investigations were utilized to prove three issues: verification of phase purity of the bulk products, accordance of observed powder patterns with simulated diffraction patterns of the corresponding single-crystal structures, as well as possible isotypic/isostructural character of isoconstitutional compounds. The observed PXRD data of the bulk products of all one-dimensional coordination polymers 1−11 match well with the patterns simulated from the single-crystal X-ray data without any nonindexed reflections. Accordingly, the compounds were obtained as phasepure products. Among the different structure types observed, 1 cerium-containing ∞ [Ce2Cl6(dpe)2(thz)4]·(dpe) (1) was found to adopt the double-strand type of structure being isotypic to Nd in 2 (Figure 4). The coordination polymer

to be isotypic to the series of single-crystalline polymers with Ln = Sm (5), Dy (8), and Yb (10). Photoluminescence and Vibrational Spectroscopy. The coordination polymers presented show a variety of luminescence properties upon excitation with UV light. Because of their structural resemblance, all compounds can be reasonably compared. Photoluminescence (PL) spectroscopy was used to record excitation and emission spectra of compounds 2−4, 7, and 9−11 and dpe in the solid state at room temperature. Simultaneous blue fluorescence and weak yellow phosphorescence of dpe as well as the energy levels of the singlet and triplet excited states have been reported for 77 K before.13,14 Because we recently observed excimer formation for the closely related ligand 1,2-di(4-pyridyl)ethane (bpe),40 we reinvestigated the PL properties of dpe, too. These investigations include excitation and emission in the solid state from UV to NIR and the lifetimes of the processes. Excitation and emission spectra for dpe dissolved in iPrOH were also recorded (Figure 5 and S13) for a comprehensive overview. The ligand

Figure 5. Emission spectra of free dpe in the solid state and in iPrOH and of the compounds 2−4, 7, and 9−11 at room temperature. The spectra of 4 and 9 were recorded with and without edge filter to identify the emission at λ = 900−1300 nm as higher-order scattering of the dpe ligand emission.

dpe can be excited by UV light (λexc = 250−365 nm, the energetically lowest excitation band corresponding to the S1 level at 30.770 cm−1 13) and exhibits a fluorescence band in the range of λ = 330−430 nm with a vibrational progression of ca. 1430 cm−1 and with an emission lifetime of τav = 3.2 ns. An additional emission band can be observed in the solid state and in concentrated iPrOH solution in the range of λ = 460− 700 nm with λmax = 532 nm (corresponding to the energetic position of 17.800 cm−1), which, according to its lifetime of τav = 4.5 ns, also appears to be fluorescence. In accordance with the observations for bpe, this emission is a result of aggregation in the solid state, most likely due to excimer formation, as the vibrational progression of the high-energy fluorescence is

Figure 4. Comparison of the observed PXRD patterns of 1−11 at room temperature with the simulated diffraction pattern of the singlecrystal X-ray structure determination of Nd (2), Gd (3), Dy (8), and Ho (11) at 100 K. 1 ∞[ErCl3(dpe)(py)2]·(dpe/py)

(4) was found to be isotypic to the Gd-containing compound 3, and 1∞[LnCl3(dpe)(thz)2]· (dpe/thz), Ln = Gd (6), Tb (7), and Er (9), were all identified E


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Inorganic Chemistry

The NIR emission of 2 and 10 occurs with lifetimes of 3.1 and 9.3 μs, respectively. Again, the excitation spectra of the Nd (2) and Yb (10) compounds resemble the dpe ligand. Ligand sensitization of Ln3+ ions emitting in the NIR range is known for coordination polymers and MOFs.44 However, in our case the above-mentioned participation of the dpe fluorescence indicates the antenna effect in 2 and 10 occurs via the singlet excited state S1 of dpe, as further indicated by its reduced lifetime upon coordination (Table S2), and not via the T1 state, as is often assumed.44 We note that for the Nd compound 2, a slow intermediary pathway appears to be involved in the sensitization process, as the emission of the 4f transitions occurs with a rise time of ca. 1.0 μs. It can be shown by an investigation of the lifetimes upon contact with atmospheric gases such as dry air and oxygen that the compounds are stable versus these gases, as they are not influenced by oxygen, because the lifetime values (decay) are identical under air and under argon (vide supra). In comparison, contact with moisture quenches the luminescence completely. For Er3+ in 4 and 9 the respective 4f transition 4I13/2 → 4I15/2 (λ = 1540 nm) is found as weak emission together with intentions of 4f transtitions in the ligand-based emission. In contrast to the other lanthanide ions, the metal-centered luminescence of the Er-containing compounds (4 and 9) is triggered by an inner-filter effect of Er3+,45 indicating the presence of an incomplete energy transfer between the dpe ligand and the Er ions.43 Hereby, the emitting state of the ligand is the donor state, and emitted energy is absorbed by the Er3+ ions. This is different from the coordination polymers with Nd3+ (2) and Yb3+ ions (10) that show an energy transfer leading to 4f−4f emission triggered by allowed organic light absorption bypassing the parityforbidden direct excitation of the corresponding lanthanide ions. Both the fluorescence and the excimer band of the dpe ligand are partially reabsorbed by direct excitation of the 4f−4f transitions 4 I15/2 → 2H3/2 (λ = 410 nm), 4F7/2 (λ = 493 nm), 4S3/2 (λ = 524 nm), 2H11/2 (λ = 546 nm), 4F9/2 (λ = 656 nm), 4I11/2 (λ = 1046 nm) of Er3+ within the same compound (Figure 5). The respective transitions can be observed as indentions within the dpe emission of 4 and 9. Apart from this minor perturbation, the emission of the ligand remains unaltered, and thus dpe acts as an independent chromophore in the coordination polymers 4 and 9 despite its coordination to and sensitization of the Er3+ ion, which emits simultanously. The inner-filter effect within one compound was just recently reported for Ln-triazolate coordination polymers and the related Ln-bpe coordination polymers.40,46 In addition, also the excitation and emission spectra of the coordination polymers 7, 10, and 11 were recorded for the otherwise typical VIS 4f-emitting trivalent ions Sm, Tb, and Dy. However, the spectra resemble the Gd3+-compound 3 showing ligand participation, only. The typical emissions based on 4f−4f transitions with the respective colors for Sm3+ (salmon-red), Tb3+ (green), and Dy3+ (yellow) as well as 5d−4f centered emission of Ce3+ cannot be observed (Figure 5). Hereby, we assume the lack of sensitization of the corresponding lanthanide ions via an antenna effect3b,10 of the ligand to be a result of the absence of populated T1 states of dpe. Furthermore, linelike excitation bands based on direct Ln 4f−4f excitation cannot be detected either. Presence and origin of short-lived aggregationinduced bands of dpe in the coordination polymers remain unknown, as we want to avoid speculation. In contrast, complexes with the ligands pyridine and thiazole [LnCl3(py)4]· 0.5(py) with Ln = Eu, Tb or [LnCl3(thz)4]·0.5(thz) with

not observed, and the excitation spectra of the two bands are identical. This assignment is further confirmed by concentration-dependent emission measurements of dpe in poly(methyl methacrylate) (PMMA) films showing an intensity decrease upon decreasing the concentration (see Figure S14). We note that this low-energy emission has not been observed before, presumably because the photophysical properties of dpe have only been studied in dilute solution. Coincidentally, the excimer emission occurs at a rather similar energy previously found for the triplet excited state at 77 K.14 However, the nanoseconds lifetimes unambiguously identify the nature of the emission as fluorescence (vide supra; see Supporting Information). In concentrated iPrOH solution the high-energy fluorescence of dpe undergoes a hypsochromic shift to the range of λ = 315−400 nm, while the excimer emission almost disappears. The lowest energetic maximum of the excitation also exhibits a hypsochromic shift upon dissolution in iPrOH (solid state: λmax = 360 nm; iPr−OH: λmax = 318 nm). Therefore, potentially populated triplet states at room temperature could not be confirmed. The origin of the excimer band, in particular, is also of considerable interest for the coordination polymer luminescence and potential ligand sensitizer effects of dpe (vide infra). The coordination polymers 2−4, 7, and 9−11 show fluorescence stemming from the dpe ligand (τav = 1.5 ns in 3 and 1.1 ns in 10; Figure 5). Interestingly, for some of the coordination polymers, the excimer-based weak emission band at ca. λ = 460−700 nm can also be found for purified products. We exclude dpe ligand-based triplet states according to the short decay times of τav < 5 ns. In addition, spectroscopic investigations of the Gd3+-compound 3 serve as an example for comparison of free dpe and dpe immobilized by the front- and back-side coordination to different lanthanide ions. The Gd3+ ion does not influence the luminescence by metal-based processes itself, as it has only one excited 4f state that does not contribute, here. It can potentially interact with the S1 state of dpe, but the latter is not observed in 3, although spin−orbit coupling of the heavy atom could potentially enhance S1 → T1 intersystem crossing.41 Instead, only a slight hypsochromic shift of the dpe fluorescence is observed, and the absence of any lower-energy emission shows that a radiative decay pathway via the triplet state of the dpe ligand is not favored (Figure 5). In comparison, in previous reports, defined population of triplet states for 4,4′-bipyridine was mentioned, for example, in the dinuclear complexes [Ln2Cl6(bipy)(py)6] with Ln = Y, Gd.32 It is intriguing that the additional ethylene group in 1,2-di(4pyridyl)ethylene (dpe) and ethane group in 1,2-di(4-pyridyl)ethane (bpe)40 led to a difference compared to bipy. Apparently, immobilized dpe separated by lanthanide metal nodes sensitizes the emission process of lanthanide-containing coordination polymers via its fluorescent S1 state (τobs = 7.5(2) ns), as the excitation spectra of 3 and free dpe strongly resemble one another (see Figure S13), indicating similar excitation pathways. Besides the fluorescence stemming from the dpe ligand and some weak excimer emission, the coordination polymers 2, 4, 9, and 10 exhibit emissive 4f transitions in the NIR region typical for the Ln3+ ions.42 They are particularly strong for Nd3+ in 2 and Yb3+ in 10. In the emission spectrum of 2, the Nd3+ transitions 4F3/2 → 4IJ with J = 9/2 (λ = 892 nm), 11/2 (λmax = 1074 nm), and 13/2 (λmax = 1355 nm) were detected, whereas 10 shows a Yb3+ transition 2F5/2 → 2F7/2 (λ = 993 nm). F


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Inorganic Chemistry Ln = Sm, Eu, Gd, Tb, Dy29 show the typical 4f transitions and sensitization by ligand triplet states. Therefore, the dpe ligand is no suitable sensitizer for Ln emission in the visible range and even suppresses Ln3+ luminescence potentially sensitized by the coligands thz and py in the coordination polymers with Sm (5), Tb (7), and Dy (8). Hence, the NIR-emitting and visibleemitting lanthanides exhibit a major difference also to 1,2-di(4pyridyl)ethylene regarding possible sensitization by the ligand. Dipyridylethylene (dpe) sensitizes Ln3+-NIR emission, only. The exact origin of this difference is yet unresolved. The compounds were also investigated by mid-infrared (MIR) vibrational spectroscopy. The corresponding vibrational bands in the MIR range for the ligands dpe, py, and thz can be observed for all compounds. The interpretation of results and the assignment of vibrational bands of the dpe ligand are supported by investigations of the complexes [LnX3(dpe)n(H2O)m] with X = NO3−, ClO4−, NCS−, n = 2−3, m = 0, 6.23 Coordination of the dpe ligand to Ln3+ ions leads to a shift to higher energy in the range of 10−15 cm−1 for the corresponding interplanar C−H bending vibration at 819 cm−1 and the aromatic C−C/C−N stretching vibration at 1594 cm−1 of free dpe. In addition, broadening and splitting of the dpe ligand vibration bands according to different coordination modes can be observed for 1∞[Ln2Cl6(dpe)2(thz)4]·(dpe), Ln = Ce (1), Nd (2), 1∞[LnCl3(dpe)(py)2]·0.5(dpe) 0.5 (py), Ln = Gd (3), Er (4), 1∞[LnCl3(dpe)(thz)2]·(dpe/thz), Ln = Sm (5), Gd (6), Tb (7), Dy (8), Er (9), Yb (10), and 1∞[HoCl3(dpe)(thz)2]· (thz) (11). The resulting barycenter of the broadened bands can be assigned to higher energies in the ranges of 828−833 and 1602−1608 cm−1. Noncoordinating pyridine can easily be identified with the intense band of the interplanar ω(C−H) vibration at 1439 cm−1, and the ligand thiazole can be identified with the band of the vibration δ (C−H) at 1041 cm−1. Coordination to metal ions leads to an energy shift to higher wavenumbers in the range of ∼5−7 cm−1, respectively. For the coordination polymers Gd (3) and Er (4) the energetical shift of coordinated pyridine increases to 1443−1444 cm−1; for the compounds 1−11 the shift of the thiazole band increases to 1045−1048 cm−1. Thermal Investigations. The thermal behavior was determined by simultaneous DTA/TG analyses exemplarily for the coordination polymers 1∞[Nd2Cl6(dpe)2(thz)4]·dpe (2), 1 1 ∞ [GdCl 3(dpe)(py) 2 ]·(dpe/py) (3), ∞ [DyCl3 (dpe)(thz)2 ]· 1 (dpe/thz) (8) and ∞[HoCl3(dpe)(thz)2]·thz (11). Knowledge of melting and boiling points of dpe (mp = 150−153 °C and bp = 334 °C),47 thiazole (mp = −31 °C, bp = 117−118 °C)48 and pyridine (bp = 114 °C)49 allows assignment of the differential thermal analysis (DTA)/thermogravimetric (TG) signals of the compounds. All investigated compounds show rather similar thermal behavior with three defined mass losses around 150, 340, and 500 °C, indicating related transformation and decomposition processes. Such similarities corroborate their structural and chemical resemblance. The double-strand coordination polymer 1∞[Nd2Cl6(dpe)2(thz)4]· dpe (2) indicates reformation of LnCl3 upon final decomposition upon a fourth endothermic heat-flow signal. The three other signals are related to mass losses (Figure 6). The first endothermic signal 1 can be assigned to the release of 4 equiv of coordinating thiazole (130 °C, theoretical mass: 24.4%, observed 22.0%) upon heating. The remaining product decomposes stepby-step, giving endothermic signals 2 (275 °C) and 3 (490 °C) with mass losses of 11 and 26.0%, which can be addressed to the release of 1 equiv of dpe, in the first step (theoretical mass

Figure 6. Depiction of the DTA/TG investigations on 1∞[Nd2Cl6(dpe)2(thz)4]dpe (2) (top) and 1∞[GdCl3(dpe)(py)2]·(dpe/py) (3) (bottom), performed with a heating rate of 10 °C/min and a flow rate of 20 mL min−1 Ar.

loss: 13.1%), and in the second step of 2 equiv of dpe (theoretical mass loss: 26.2%), releasing noncoordinated dpe first, followed by bound dpe. The remaining mass of 38.0% coincides with the theoretical value of NdCl3 (theoretical mass loss: 36.1%), as a low amount of carbonized material is also observed. The melting point of NdCl3 (745 °C, theoretical mp = 775 °C)50 can also be observed, leading to the conclusion of a reformation of crystalline NdCl3. 1 ∞[GdCl3(dpe)(py)2]·(dpe/py) (3) exhibits release of volatile pyridine, first. Signal 1 (Figure 6) at 145 °C correlates to the evaporation of intercalated and coordinated pyridine (theoretical mass loss: 27.0%, observed 29%). The double signal 2 beginning at 300 °C can be addressed to the evaporation of intercalated dpe (theoretical mass loss: 12.4%, observed 12.5) at the boiling point of dpe. Further mass loss derives from the decomposition of the strands by removal of coordinating dpe starting at 475 °C (signal 4). The theoretical combined mass fraction of the ligands dpe and py adds to 64.1% and corresponds to the complete mass loss of 64.5% observed at 900 °C (with no melting point of GdCl3 being observed, mp = 609 °C).51 Exemplarily for the single strands 1∞[LnCl3(dpe)(thz)2]· (dpe/thz) (5−10), DTA/TG investigations are discussed for Dy in 8, here (Figure S16). This type of coordination polymers also shows four endothermic signals, addressed to the release of thz (140 °C, 2.5 equiv of thiazole: theoretical mass loss: 28.3%, observed 27.5%), followed by release, melting, and evaporation of dpe (310 °C, 0.5 equiv of intercalated dpe: theoretical mass loss: 12.1%, observed 13.5%; 505 °C, coordinating dpe: theoretical mass: 24.1%, observed 23%) to 850 °C. G


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Inorganic Chemistry

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were sealed into evacuated (1 × 10−3 mbar) glass ampules using Quickfit techniques and degassed by freezing with liquid nitrogen. The heating program for the compounds 1, 2, 5−11 consisted of four steps, starting with heating to 100 °C with a rate of 1.4 °C h−1, and then to 125 °C with 1 °C h−1. The temperature was maintained for 48 h and then lowered to room temperature at 2 °C h−1. The compounds 3, 4, and 12 were heated in six steps starting with heating to 50 °C with a heating rate of 10 °C h−1, to 100 °C at 1 °C h−1, and finally to 150 °C at 0.5 °C h−1. The temperature was maintained for 168 h, then reduced to 100 °C at 0.5 °C h−1, and reduced to room temperature at 1 °C h−1. For compound 6 the heating program was similar, except that a maximum temperature of 130 °C was reached with 1 °C h−1. All reactions yielded crystalline compounds within the saturated solvent. For purification, the compounds containing pyridine were washed with 2 × 1 mL of anhydrous pyridine, removing remaining non-reacted dpe, and dried. For the thiazole-containing compounds, the crystalline products were washed with 2 × 1 mL of anhydrous dichloromethane. All products are air- and moisture-sensitive. Synthesis. 1∞[Ce2Cl6(dpe)2(thz)4]·dpe (1). CeCl3 (0.3 mmol = 74 mg), 1,2-di(4-pyridyl)ethylene (C12H10N2, 0.5 mmol = 91 mg), and thiazole (C3H3NS, 4.4 mmol = 374 mg) were placed into an ampule, and synthesis was performed as described above. The reaction yielded a mixture of colorless crystals and microcrystalline powder. Yield: 132 mg = 64%. MIR (attenuated total reflectance (ATR)): (3047 w, 1609 vs, 1557 vw, 1504 vw, 1494 m, 1428 m, 1380 m, 1310 vw, 1247 vw, 1206 w, 1077 m, 1047 m, 1012 s, 982 m, 828 s, 725 m) cm−1. Anal. Calcd (%) for C48H42Cl6Ce2N10S4 (Mr = 1380.14 g mol−1): C, 41.77; N, 10.15; H, 3.07; S, 9.29. Found: C, 41.46; N, 9.66; H, 3.14; S, 8.69. 1 ∞[Nd2Cl6(dpe)2(thz)4]·dpe (2). NdCl3 (0.3 mmol = 75 mg), 1,2di(4-pyridyl)ethylene (C12H10N2, 0.5 mmol = 91 mg), and thiazole (C3H3NS, 4.4 mmol = 374 mg) were transferred to an ampule, and synthesis was performed as described above. The reaction yielded a mixture of slightly bluish crystals and microcrystalline powder. Yield: 153 mg = 73%. MIR (ATR): (3066 w, 1608 vs, 1556 vw, 1503 vw, 1493 m, 1427 m, 1380 m, 1309 vw, 1242 vw, 1207 w, 1074 m, 1047 m, 1009 s, 987 m, 829 s, 723 m) cm−1. Anal. Calcd (%) for C48H42Cl6Nd2N10S4 (Mr = 1388.39 g mol−1): C, 41.52; N, 10.09; H, 3.05; S, 9.24. Found: C, 41.09; N, 9.90; H, 2.99; S, 9.49. 1 ∞[GdCl3(dpe)(py)2]·(dpe/py) (3). GdCl3 (0.5 mmol = 132 mg), 1,2-di(4-pyridyl)ethylene (C12H10N2, 1.5 mmol = 273 mg), and pyridine (C5H5N, 6.2 mmol = 490 mg) were brought in an ampule, and synthesis was performed as described above. The reaction gained a mixture of colorless crystals and microcrystalline powder. Yield: 212 mg = 58%. MIR (KBr): (3034 m, 1603 vs, 1556 m, 1505 m, 1488 m, 1444 vs, 1432 vssh, 1354 vw, 1299 w, 1252 m, 1220 s, 1154 w, 1091 vw, 1069 s, 1038 m, 1010 vs, 989 msh, 970 msh, 884 w, 833 vs, 804 vw, 752 s, 702 vs, 624 m, 553 vs, 425 m) cm−1. Anal. Calcd (%) for C61H55Cl6Gd2N11 (Mr = 1469.40 g mol−1): C, 49.86; N, 10.49; H, 3.77. Found: C, 49.54; N, 10.58; H, 3.52. 1 ∞[ErCl3(dpe)(py)2]·(dpe/py) (4). ErCl3 (0.5 mmol = 137 mg), 1,2-di(4-pyridyl)ethylene (C12H10N2, 1.5 mmol = 273 mg), and pyridine (C5H5N, 6.2 mmol = 490 mg) were transferred to an ampule, and synthesis was performed as described above. A mixture of colorless crystals and microcrystalline powder was obtained after reaction. Yield: 190 mg = 52%. MIR (KBr): (3036 w, 1608 vs, 1556 vw, 1505 m, 1488 m, 1444 vs, 1433 vssh, 1354 vw, 1299 vw, 1251 vw, 1220 s, 1154 vw, 1069 s, 1038 m, 1010 s, 989 m, 971 msh, 886 vw, 832 s, 753 m, 702 vs, 624 m, 552 vs, 425 m) cm−1. Anal. Calcd (%) for C61H55Cl6Er2N11 (Mr = 1489.42 g mol−1): C, 49.19; N, 10.34; H, 3.72. Found: C, 48.77; N, 10.26; H, 3.66. 1 ∞[SmCl3(dpe)(thz)2]·(dpe/thz) (5). SmCl3 (0.3 mmol = 77 mg), 1,2-di(4-pyridyl)ethylene (C12H10N2, 0.6 mmol = 110 mg), and thiazole (C3H3NS, 0.9 mmol = 77 mg) were transferred to an ampule, and synthesis was performed as described above. The reaction yielded a mixture of colorless crystals and microcrystalline powder. Yield: 127 mg = 57%. MIR (KBr): (3051 w, 1605 vs, 1556 vw, 1504 m, 1427 m, 1204 w, 1048 m, 1010 s, 974 msh, 828 s, 550 vs) cm−1. Anal. Calcd (%) for C51H45Cl6Sm2N11S5 (Mr = 1485.76 g mol−1):

CONCLUSIONS A series of 12 lanthanide-coordination polymers with the linker 1,2-di(4-pyridyl)ethylene as potential sensitizer for metal-based luminescence was synthesized resulting in five different constitutions together with the coligands pyridine and thiazole: 1 ∞[Ln2Cl6(dpe)2(thz)4]·dpe with Ln = Ce (1), Nd (2), 1 ∞[LnCl3(dpe)(py)2]·(dpe/py) with Ln = Gd (3), Er (4), 1 ∞[LnCl3(dpe)(thz)2]·(dpe/thz) with Ln = Sm (5), Gd (6), Tb (7), Dy (8), Er (9), Yb (10), 1∞[HoCl3(dpe)(thz)2]·thz (11), and 2∞[La2Cl6(dpe)3(py)2]·dpe (12). To the best of our knowledge, they are the first lanthanide compounds with the linker dpe. The compounds exhibit variable PL properties, and the dpe linker proves a suitable sensitizer for various metalbased NIR emissions. The coordination polymers with the Ln3+ ions of Nd (2) and Yb (10) show strong luminescence in the NIR region due to 4f−4f transitions. The NIR emission is caused by an antenna effect triggered by dpe. For Er3+ in 4 and 9, weak 4f-NIR emission is caused by an inner-filter effect by reabsorption of dpe ligand-based emission. It is intriguing that the typical 4f−4f-centered emission of the Ln3+ ions in the visible could not be observed in the compounds with Gd (3), Sm (5), Tb (7), and Dy (8) but ligand-centered emission, only. No suitable antenna effect between the organic linker molecules and the Ln3+ ions is observed for these otherwise typical 4f-emitting metal ions. We assume the absence of significant triplet population of dpe to be the reason, as the weak yellow ligand emission at room temperature proves to be the result of an excimer effect of dpe, which is not present or substantially weakened according to concentration in the coordination polymers. It is not clear, yet, why only the NIR emission is sensitized. A direct influence of the coligands py and thz could not be observed. Altogether, we present various luminescence effects and remarkably strong NIR emission for the lanthanide ions Nd3+ and Yb3+ together with dpe in the rigid polymeric structures.

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EXPERIMENTAL SECTION

All experiments were performed under inert conditions (argon atmosphere) using vacuum line, Schlenk, glovebox (MBraun, LabMaster SP, and Innovative technology, Pure Lab), and DURAN ampule techniques. Heating furnaces based on Al2O3 tubes with Kanthal wire resistance heating and NiCr/Ni temperature elements controlled by Eurotherm 2416 control units were used for synthesis in sealed glass ampules. Anhydrous rare earth chlorides were synthesized according to the ammonium halide route52 using the oxides Er2O3, Yb2O3 (99.9%, RC-Nukor), La2O3, Nd2O3, Sm2O3, Tb4O7 (99.9%, Auer-Remy), Dy2O3 (99.9%, Serva), Gd2O3 (99.9%, Koch Chemicals), Ho2O3 (99.9%, Strem), HCl solution (10 mol L−1, reagent grade), and NH4Cl (99.9%, Fluka). Anhydrous CeCl3 was prepared by a modified ammonium halide route. Hereby, high-temperature calcined CeO2 (99.9%, Auer-Remy) was dissolved by hours-long boiling in various mixtures of concentrated acids HCl (37%, fuming)/HNO3 (69%, fuming) and HCl (37%, fuming)/H2O2 (30%). Furthermore, colorless, pure Ce(OH)3 was obtained by alkaline precipitation with concentrated aqueous NaOH solution (37%). An alternative route to obtain cerium hydroxide is an alkaline treatment of an aqueous Ce(NO3)3·6(H2O) solution (99%, Riedel-deHaën). The corresponding precipitates were dissolved in HCl acid (10 mol L−1, reagent grade), and NH4Cl (99.9%, Fluka) was added. The intermediate trivalent lanthanide ammonium chlorides were decomposed and further purified by sublimation under vacuum. 1,2-Di(4-pyridyl)ethylene (dpe, 97%, Sigma-Aldrich) was dried under vacuum, anhydrous thiazole (thz, ≥99%, Sigma-Aldrich) was dried over molecular sieves, and anhydrous pyridine (py, 99%, Acros) was used as purchased. The respective lanthanide chlorides, dpe and pyridine/thiazole, H


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Inorganic Chemistry C, 41.23; N, 10.37; H, 3.05; S, 10.79. Found: C, 41.66; N, 10.28; H, 3.11; S, 10.80. 1 ∞[GdCl3(dpe)(thz)2]·(dpe/thz) (6). GdCl3 (0.3 mmol = 79 mg), 1,2-di(4-pyridyl)ethylene (C12H10N2, 0.6 mmol = 110 mg), and thiazole (C3H3NS, 0.9 mmol = 77 mg) were placed into an ampule, and synthesis was performed as described above. A mixture of colorless crystals and microcrystalline powder was obtained. Yield: 143 mg = 64%. MIR (ATR): (3051 w, 1603 vs, 1556 vw, 1495 m, 1427 m, 1382 m, 1311 vw, 1245 vw, 1072 m, 1046 m, 1014 s, 872 m, 828 s, 800 m, 738 m) cm−1. Anal. Calcd (%) for C51H45Cl6Gd2N11S5 (Mr = 1499.54 g mol−1): C, 40.85; N, 10.27; H, 3.02; S, 10.69. Found: C, 39.98; N, 9.79; H, 2.90; S, 10.71. 1 ∞[TbCl3(dpe)(thz)2]·(dpe/thz) (7). TbCl3 (0.3 mmol = 80 mg), 1,2-di(4-pyridyl)ethylene (C12H10N2, 0.6 mmol = 110 mg), and thiazole (C3H3NS, 0.9 mmol = 77 mg) were transferred to an ampule, and synthesis was performed as described above. The reaction yielded a mixture of colorless crystals and microcrystalline powder. Yield: 148 mg = 61%. MIR (ATR): (3047 w, 1609 vs, 1556 vw, 1495 m, 1427 m, 1380 m, 1310 vw, 1256 vw, 1206 w, 1077 m, 1046 m, 1014 s, 976 msh, 828 s, 800 m, 738 m) cm−1. Anal. Calcd (%) for C51H45Cl6Tb2N11S5 (Mr = 1502.89 g mol−1): C, 40.76; N, 10.25; H, 3.02; S, 10.67. Found: C, 40.29; N, 10.32; H, 3.00; S, 11.70. 1 ∞[DyCl3(dpe)(thz)2]·(dpe/thz) (8). DyCl3 (0.3 mmol = 81 mg), 1,2-di(4-pyridyl)ethylene (C12H10N2, 0.6 mmol = 110 mg), and thiazole (C3H3NS, 0.9 mmol = 77 mg) were brought in an ampule, and synthesis was performed as described above. The reaction yielded a mixture of colorless crystals and microcrystalline powder. Yield: 121 mg = 53%. MIR (KBr): (3053 w, 1607 vs, 1556 vw, 1503 m, 1427 m, 1300 vw, 1256 vw, 1204 w, 1066 m, 1045 m, 1013 s, 976 msh, 827 s, 550 vs) cm−1. Anal. Calcd (%) for C51H45Cl6Dy2N11S5 (Mr = 1510.04 g mol−1): C, 40.57; N, 10.20; H, 3.00; S, 10.62. Found: C, 40.35; N, 10.32; H, 2.99; S, 10.21. 1 ∞[ErCl3(dpe)(thz)2]·(dpe/thz) (9). ErCl3 (0.3 mmol = 82 mg), 1,2-di(4-pyridyl)ethylene (C12H10N2, 0.6 mmol = 110 mg), and thiazole (C3H3NS, 0.9 mmol = 77 mg) were placed into an ampule, and synthesis was performed as described above. A mixture of slightly pink crystals and microcrystalline powder was yielded after reaction. Yield: 117 mg = 51%. MIR (ATR): (3053 w, 1606 vs, 1557 vw, 1504 m, 1428 m, 1384 m, 1307 vw, 1221 vw, 1074 m, 1047 m, 1015 s, 872 m, 829 s, 801 m, 737 m) cm−1. Anal. Calcd (%) for C51H45Cl6Er2N11S5 (Mr = 1519.56 g mol−1): C, 40.31; N, 10.14; H, 2.98; S, 10.55. Found: C, 40.36; N, 9.77; H, 3.04; S, 9.91. 1 ∞[YbCl3(dpe)(thz)2]·(dpe/thz) (10). YbCl3 (0.3 mmol = 84 mg), 1,2-di(4-pyridyl)ethylene (C12H10N2, 0.6 mmol = 110 mg), and thiazole (C3H3NS, 0.9 mmol = 77 mg) were transferred to an ampule, and synthesis was performed as described above. The reaction yielded a mixture of colorless crystals and microcrystalline powder. Yield: 109 mg = 47%. MIR (ATR): (3050 w, 1609 vs, 1558 vw, 1501 m, 1495 m, 1426 m, 1383 m, 1309 vw, 1239 vw, 1072 m, 1047 m, 1014 s, 872 m, 829 s, 803 m, 736 m) cm−1. Anal. Calcd (%) for C51H45Cl6Yb2N11S5 (Mr = 1531.12 g mol−1): C, 40.01; N, 10.06; H, 2.96; S, 10.47. Found: C, 40.64; N, 10.13; H, 2.99; S, 10.06. 1 ∞ [HoCl3(dpe)(thz)2]·thz (11). HoCl3 (0.3 mmol = 81 mg), 1,2-di(4-pyridyl)ethylene (C12H10N2, 0.6 mmol = 110 mg), and thiazole (C3H3NS, 0.9 mmol = 77 mg) were brought in an ampule, and synthesis was performed as described above. The reaction yielded a mixture of slightly pink crystals and microcrystalline powder. Yield: 157 mg = 74%. MIR (KBr): (3101 w, 1607 vs, 1552 vw, 1497 m, 1429 m, 1383 m, 1308 vw, 1240 vw, 1071 m, 1045 m, 1014 s, 982 w, 872 m, 833 s, 808 m, 741 w) cm−1. Anal. Calcd (%) for C21H19Cl3HoN5S3 (Mr = 708.90 g mol−1): C, 35.58; N, 9.88; H, 2.70; S, 13.57. Found: C, 36.08; N, 9.67; H, 2.74; S, 12.21. 2 ∞[La2Cl6(dpe)3(py)2]·dpe (12). LaCl3 (0.5 mmol = 123 mg), 1,2-di(4-pyridyl)ethylene (C12H10N2, 1.5 mmol = 273 mg), and pyridine (C5H5N, 6.2 mmol = 490 mg) were transferred to an ampule, and the synthesis was performed as described above. The reaction yielded a mixture of colorless crystals and microcrystalline powder. The extraction of determinable single-crystals was possible, but the synthesis of bulk amounts of product was not successful. XRPD investigations revealed the presence of the extracted phase, as side

phase combined with unreacted LaCl3 and additional reflections of unknown crystalline phases. Therefore, elemental analysis proved unreliable. Single Crystal X-ray Diffraction. Suitable single crystals of 1 1 ∞ [Nd2Cl6(dpe)2(thz)4]·(dpe) (2), ∞ [GdCl3(dpe)(py)2]·(dpe/py) 1 (3), ∞[LnCl3(dpe)(py)2]·(dpe/py) with Ln = Sm (5), Dy (8), Yb 1 2 [HoCl3(dpe)(thz)2]·thz (11), and ∞ [La2Cl6(dpe)3(py)2]· (10), ∞ (dpe) (12) were selected for X-ray diffraction from the crystalline product in mother-liquor mixed with high-viscosity perfluorinated ether (99.9%, ABCR). Data collection for the compounds was performed on a Bruker AXS Smart Apex 1 diffractometer at 168 K equipped with graphite monochromators (Mo Kα radiation; λ = 0.7107 Å) and on a Bruker AXS Apex II diffractometer at 100 K with Helios mirror using the Bruker AXS Smart Software package or the Bruker AXS Apex Suite.53 Further data processing was done with XPREP.35 All structure solutions were performed with direct methods using SHELXS,35 and the crystal structures were refined by leastsquares techniques using SHELXL35 on the graphical platform XSEED54 and OLEX2.55 Integrity of symmetry was checked using PLATON.56 For all compounds, the non-hydrogen atoms were refined anisotropically by least-squares techniques, all hydrogen atoms with geometrical constraints regarding their positions. For the crystal structure of compound Nd (2), a systematic displacement of the connecting dpe molecule was refined. Additional crystallographic information is available in the Supporting Information. Powder X-ray Diffraction. The powder diffraction samples were ground and put into Lindemann glass capillaries (Ø 0.3 mm). Diffraction data were collected on a Bruker AXS D8 Discover powder X-ray diffractometer equipped with Lynx-Eye detector in transmission geometry. X-ray radiation (Cu Kα1; λ = 154.06 pm) was focused with a Goebel mirror, Cu Kα2 radiation was eliminated by the application of a Ni absorber. Diffraction patterns were recorded and analyzed using the Bruker AXS Diffrac-Suite. Diffraction data were collected in Debye−Scherrer geometry on a STOE Stadi P powder diffractometer with Ge(111)-monochromatized Cu Kα radiation (λ = 154.06 pm) using Win-X-Pow software.57 Photoluminescence and Vibrational Spectroscopy. Excitation and emission spectra were recorded with a HORIBA Jobin Yvon Spex Fluorolog 3 spectrometer equipped with a 450 W Xe lamp, doublegrated excitation and emission monochromators, and a photomultiplier tube (R928P) at room temperature, using the FluorEssence software. Excitation spectra were corrected for the spectral distribution of the lamp intensity using a photodiode reference detector. Additionally, both excitation and emission spectra were corrected for the spectral response of the monochromators and the detector using correction spectra provided by the manufacturer. All samples were investigated as solids in spectroscopically pure quartz cuvettes in front face mode at room temperature and 77 K using a quartz dewar. NIR investigations were performed on a Photon Technology International Quanta Master TM Model QM-2000−4 spectrometer with an InGaAsNIR detector and a 75 W xenon short arc lamp (UXL-75XE, Ushio). Additional filters for excitation (300 nm bandpass, Δ20 nm, OD 5, Edmund Optics), as well as a respective edge filter for emission (GG450, RG645, and RG830, Reichmann Optics) were used. Emissions were also recorded without edge filters to observe the influence of second-order phenomena of the measurement setup. The luminescence lifetimes were measured on a Edinburgh Instruments FLSP920 spectrometer, equipped with photomultipliers (PMT-R928, R5509−72) as detectors and double monochromators for the excitation and emission pathways, either via time-correlated single-photon counting (TCSPC) using a 375 nm pulsed picosecond laser diode (5 mW, pulse width 72.6 ps), or using a μF900 pulsed 60 W xenon microsecond flashlamp, with a repetition rate of 100 Hz, and a multichannel scaling module. For TCSPC mode, the instrument response function (IRF) was measured using a scattering sample and setting the monochromator at the emission wavelength of the excitation light source. The resulting intensity decay is a convolution of the luminescence decay with the IRF and iterative reconvolution of the IRF with a decay function, and nonlinear least-squares analysis was used to analyze the convoluted data. In case of biexponential decays, the I


Article

Inorganic Chemistry amplitude weighted average lifetime τav = (A1τ1 + A2τ2)/(A1+A2) was used. The absolute quantum yields of compounds 2 and 10 were determined with the aid of an Ulbricht sphere. First, the diffuse reflection of the sample was determined under excitation. Second, the emission was measured for this excitation wavelength. Integration over the reflected and emitted photons allows calculating the absolute quantum yield, which gave values of Φ = 1 ± 3% for 2 and 10, respectively. PMMA films of dpe in various concentrations were prepared for photophysical studies by dissolving PMMA with the respective amount of dpe in dichloromethane and spin-coating on a quartz glass substrate. IR spectra were recorded with a Thermo Nicolet 380 FT-IR spectrometer in transmission mode using OMNIC 32 software. Five milligrams of the compounds were mixed with 300 mg of anhydrous KBr and pressed to transparent pellets, or an ATR unit was used. Thermal and Elemental Analyses. Thermal investigations were performed by simultaneous DTA/TG (Netzsch STA 409, Proteus Software), using 15−30 mg of the compounds in an inert gas flow (50% Ar and 50% N2) and being heated from 20 to 900 °C with a heating rate of 10 °C h−1 and a constant gas flow of 20 mL min−1 Ar and 20 mL min−1 N2. Carbon, nitrogen, hydrogen, and sulfur elemental analyses were performed with a vario microcube (Elementar Analysensysteme GmbH).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00447. Further information was deposited at the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (e-mail: [email protected]) and may be requested by citing the deposition numbers CCDC Nd (2) 1414645, Gd (3) 1023748, Sm (5) 1414642, Dy (8) 1414641, Yb (10) 1414643, Ho (11) 1414644, and La (12) 1023749, or the names of the authors and the literature citation. Additional crystal structure information, figures, selected interatomic distances and angles, excitation spectra, luminescence decay times, additional spectroscopic and thermal investigations. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+)49-931-3184785. Phone: (+)49-931-3188724. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Deutsche Forschungsgemeinschaft for supporting this work through the project MU-1562/8-1 and the Evangelisches Studienwerk Villigst e.V. for Ph.D. fellowships for P.R.M. and S.H.Z.

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