The Aggregations and Strong Emissions of d8 and d10 Metal−8

CRYSTAL GROWTH & DESIGN

The Aggregations and Strong Emissions of d8 and d10 Metal-8-Hydroxyquinaldine Complexes Chengyang Yue, Feilong Jiang, Ying Xu, Daqiang Yuan, Lian Chen, Chunfeng Yan, and Maochun Hong*

2008 VOL. 8, NO. 8 2721–2728

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, Fuzhou, Fujian, 350002, China ReceiVed August 20, 2007; ReVised Manuscript ReceiVed March 19, 2008

ABSTRACT: Five new d8 and d10 metal complexes, Pd(8-hq)2 (1), [Ag(8-Hhq)2]NO3 (2), Ag(8-hq)(8-Hhq) (3), [(8-H2hq)2PdCl4] · 2H2O (4), and Pt(8-hq)(tht)Cl (5) (tht ) tetrahydrothiophene), all with a fluorogenic chelating agent, 8-hydroxyquinaldine (8-Hhq), have been synthesized and characterized. The aggregations of their photoactive metal centers through π-π stacking and hydrogen bonding interactions construct the interesting frameworks. The strong emissions of 1-3 and 5 in solid states with orange, cyan, and green luminescences originating from metal · · · metal and π-π interactions are assigned as metal to ligand charge transfer/metal–metal to ligand charge transfer/intraligand emissions. The solvent effects on their photophysical properties are examined, which exhibit a wide range of strong luminescences in the different organic solvents. In addition, time-dependent density functional theory calculations using the B3LYP functional on 1 are in good agreement with the experiments. Thermogravimetric analyses and UV/vis spectra with composition of certain simulated results are also reported. Introduction 8

10

In recent years, luminescent d and d metal complexes have attracted much attention because of their potential applications in sensors and photochemical and electroluminescent devices.1–3 In addition, research on luminescence of their complexes has achieved much success.4 These complexes show a wide range of luminescence properties that depend strongly on the structural and electronic characteristics of their ligands, as well as on the temperature. The surrounding medium of the luminescent molecules, such as the solvent, counterions, and neighboring complexes also plays a part in their luminiscence.5 Furthermore, in the research of d8 and d10 metal complexes, rich and diverse low-energy excited states, including IL (intraligand: π-π*), MLCT (metal-to-ligand charge transfer) [dσ*-pσ], and MMLCT (metal-metal-to-ligand charge transfer) [dσ*-π*], have been observed. These enrich their luminescent research and practical applications.6 Noncovalent inter- and intramolecular forces, such as hydrogen bonds, π-π stacking, and C-H · · · π interactions have received intense interest since they often play key roles in the crystal and molecular structures of various compounds,7 organic synthesis,8 biochemistry,9 macromolecular chemistry,10 and so on. The aryl-aryl (π-π stacking) noncovalent interactions might offer an efficient means of controlling the assembly of photoactive metal centers. Moreover, such interactions have proven to be a powerful tool for linking organic molecules into crystal-engineered structures. As combining aromatic and pyridyl rings, benzopyridine derivatives can provide great π-π stacking interactions to construct the extended structures and to further have a great effect on the potential metal-metal interactions.11 In our research, a benzopyridine ligand compound, 8-hydroxyquinaldine (8-Hhq), has been employed, on the one hand, to introduce the possible aryl-aryl interactions and to influence the arrangement of the metal centers without causing the solubility problems generally found in larger aryl systems and covalent polymers. On the other hand, the optically * To whom correspondence should be addressed. E-mail: [email protected]; fax: +86-59-3714946.

active 8-Hhq derivatives are promising fluorogenic chelating agents in luminescence investigation, and may possess the capacity to contribute excellent emission when it works with the coordinated metal ions.12 Employing 8-Hhq as the fluorogenic chelating agent, and d8 or d10 transition metal ions as the photoactive metal centers, we have successfully obtained five new complexes with interesting stacking structures and strong photoluminescent emissions: Pd(8-hq)2 (1), [Ag(8-Hhq)2]NO3 (2), Ag(8-hq)(8Hhq) (3), [(8-H2hq)2PdCl4] · 2H2O (4), and Pt(8-hq)(tht)Cl (5) (tht ) tetrahydrothiophene). Experimental Section Materials and Analyses. All chemicals were used as received without further purification except for (tht)PtCl2. The applied material (tht)PtCl2 was prepared by the following method: the commercial K2PtCl6 was reduced to K2PtCl4 by hydrazine hydrochloride in water, as a cherry-red solution, to which tetrahydrothiophene was added. After filtration the expected product, powder precipitate (tht)PtCl2, was obtained. Fluorescent spectra were measured on an Edinburgh Instruments analyzer model FLS920 with 450W xenon light. IR spectra were recorded with KBr disks on a Perkin-Elmer Spectrum One FT-IR spectrometer. UV spectra were taken on a Perkin-Elmer Lambda-35 UV/vis spectrometer in the range of 190-1100 nm at ambient temperature. Thermal gravimetric analyses (TGA) were performed with a heating rate of 10 °C · min-1 using a NETZSCH STA 449C simultaneous TG-DSC instrument. Elemental analyses were carried out on an Elementar Vario EL III microanalyzer. All calculations were carried out with Gaussian0313 via density functional theory (DFT) using the hybrid functional B3LYP.14 The basis set 3-21G** was used for all elements. X-ray powder diffraction patterns were collected on an X’Pert-Pro diffractometer using Cu KR radiation (λ ) 1.5406 Å) in the 2θ range of 5-85°. The generator voltage is 45 kV and the tube current is 40 mA. Preparation of Pd(8-hq)2 (1). The red solution of Na2PdCl4 (59 mg, 0.2 mmol) in 30 mL of THF turned cloudy after 8-Hhq (64 mg, 0.4 mmol) and sodium methoxide (NaOMe, 22 mg, 0.4 mmol) were added to it during stirring. The reaction mixture was kept stirring for six hours and filtered out, and the resulting filtrate was allowed to slowly evaporate at ambient atmosphere for five days. Red crystals suitable for single crystal X-ray analysis were collected, washed by acetone,

10.1021/cg700785h CCC: $40.75  2008 American Chemical Society Published on Web 07/01/2008

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Table 1. Crystallographic Data for 1-5 complex formula fw crystal system space group unit cell dimensions

volume (Å3) Z crystal size (mm) final R, wR indices (I > 2σ(I))

1

2

3

4

5

C60H48Pd3N6O6 1268.24 monoclinic P21/n a ) 9.5205 Å b ) 15.5500 Å c ) 16.7196 Å R ) 90.00° β ) 94.493° γ ) 90.00° 2467.6 2 0.20 × 0.13 × 0.13 0.0406, 0.0813

C20H18AgN3O5 488.24 orthorhombic Pbcn a ) 14.655 Å b ) 7.612 Å c ) 17.194 Å R ) 90.00° β ) 90.00° γ ) 90.00° 1918.1 4 0.60 × 0.20 × 0.10 0.0500, 0.1174

C40H34Ag2N4O4 850.45 triclinic P1j a ) 8.158 Å b ) 10.921 Å c ) 20.073 Å R ) 83.06° β ) 87.46° γ ) 73.67° 1703.58 2 0.25 × 0.20 × 0.20 0.0322, 0.0850

C20H24Cl4N2O4Pd 604.61 triclinic P1j a ) 7.0330 Å b ) 8.1804 Å c ) 10.1362 Å R ) 87.805° β ) 88.231° γ ) 89.157° 582.40 1 0.35 × 0.10 × 0.05 0.0306, 0.0611

C14H16ClNOPtS 476.88 orthorhombic Pbca a ) 10.9725 Å b ) 9.1655 Å c ) 29.1477 Å R ) 90.00° β ) 90.00° γ ) 90.00° 2931.3 8 0.25 × 0.25 × 0.15 0.0355, 0.0737

and air-dried. Yield: 81.6% (69 mg) based on Pd. Anal. for C60H48N6O6Pd3 (%), Calcd: H 3.81, C 56.82, N 6.63; Found: H 3.80, C 56.65, N 6.59. IR (KBr pellet, cm-1): 1562 (s), 1506 (m), 1465 (s), 1430 (m), 1330 (s), 1286 (m), 1115 (m), 823 (m), 760 (m). Preparation of [Ag(8-Hhq)2]NO3 (2). The colorless solution of AgNO3 (34 mg, 0.2 mmol) in 30 mL of methanol turned into a yellow suspension after adding 8-Hhq (64 mg, 0.4 mmol) and stirring for 6 h. After filtration, the filtrate was allowed to stand at ambient temperatures for about five days, and the resulting light brown crystals suitable for single crystal X-ray analysis were isolated, washed with ethanol and ether, and air-dried. Yield: 44.0% (43 mg) based on Ag. Anal. for C20H18N3O5Ag (%), Calcd: H 3.72, C 49.20, N 8.61; Found: H3.69, C49.80, N.8.25. IR (KBr pellet, cm-1): 1572 (m), 1508 (w), 1383 (s), 832 (m), 738 (w). Preparation of Ag(8-hq)(8-Hhq) (3). The light-brown crystals of complex 3 were obtained from the diffusion of ethanol solution (6 mL) of 8-Hhq (160 mg, 1 mmol) to the aqueous ammonia solution (2.5 mL) of AgCl (72 mg, 0.5 mmol), and then washed with ethanol and ether, and air-dried. Yield: 85.4% (182 mg) based on Ag. Anal. for C40H34Ag2N4O4 (%), Calcd: H 4.03, C 56.49, N 6.59; Found: H 4.39, C 54.51, N 6.49. IR (KBr pellet, cm-1): 1571 (s), 1508 (s), 1430 (m), 1374 (m), 1245 (m), 831 (vs), 749 (m). Preparation of [(8-H2hq)2PdCl4] · 2H2O (4). To the solution of Na2PdCl4 (16 mg, 0.1 mmol) in 30 mL of THF, 8-Hhq (32 mg, 0.2 mmol) was added with stirring. The reaction mixture was kept stirring for four hours and filtered out. The filter residue was resolved in MeOH, after 5 days’ evaporation, and the red crystals of complex 4 suitable for single crystal X-ray analysis were isolated and air-dried. Yield: 8.2% (6 mg) based on Pd. Anal. for C20H24Cl4N2O4Pd (%), Calcd: H 4.00, C 39.73, N 4.63; Found: H 3.98, C 40.35, N 4.72. IR (KBr pellet, cm-1): 1562(w), 1507(m), 1384(s), 1328(m), 823(m), 738(m), 668(s). Preparation of Pt(8-hq)(tht)Cl (5). (tht)PtCl2 (44 mg, 0.1 mmol) and 8-Hhq (16 mg, 0.1 mmol) were solved in acetone and ethanol, respectively. The two light yellow solutions were mixed then, and the system immediately got cloudy. By 6 h of continual stirring, the solution turned orange-red. The solution was allowed to evaporate slowly at ambient atmosphere for seven days, and red block crystals suitable for single crystal X-ray analysis were collected. Yield: 88.0% (42 mg) based on Pt. Anal. for C14H16ClNOPtS (%), Calcd: H 3.38, C 35.26, N 2.94; Found: H 3.34, C 35.09, N 3.02. IR (KBr pellet, cm-1): 1564(s), 1506(m), 1462(s), 1431(s), 1324(s),1284(s), 1112(s), 843(m), 819(m), 761(m),741(m), 652(m). Crystal Structural Determination. Intensity data for complexes 1-5 were measured on a Rigaku Mercury CCD diffractometer with graphite-monochromated Mo KR radiation (λ ) 0.71073 Å) at 293 K. The structures were solved by direct methods and refined on F2 by full-matrix least-squares procedures using the SHELXTL software suite. All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were treated as idealized contributions except those on water molecules and the shared ones. A summary of the crystallographic data of complexes 1-5 are listed in Table 1, and their selected bond lengths and angles are listed in Tables 2–6, respectively. Crystal Structures. Pd(8-hq)2 (1). Three molecules of Pd(8-hq)2 exist as a unit, and parallel to each other in the crystal structure of complex 1. This is different from Pd(bpy)(bdt), Pt(bpy)(bdt), Pt(bpy)(mnt), Pt(bpy)(edt),16e 4,7-diphenyl-1,10-phenanthroline-(tetrachloroorthoquinone-O,O′)-palladium dimethylformamide solvate,16f and (2,9-

Table 2. Selected Bond Lengths (Å) and Angles (°) for 1a Pd(1)-O(1) 1.986(2) Pd(1)-O(2) 1.995(2) Pd(1)-N(2) 2.058(2) Pd(1)-N(1) 2.063(3) Pd(2)-O(3)#1 1.993(2) Pd(2)-O(3) 1.993(2) Pd(2)-N(3) 2.066(3) Pd(2)-N(3)#1 2.066(3) O(1)-Pd(1)-O(2) 177.33(11) O(1)-Pd(1)-N(2) 97.46(10) O(2)-Pd(1)-N(2) 82.36(10) O(1)-Pd(1)-N(1) 82.28(10) O(2)-Pd(1)-N(1) 97.82(10) N(2)-Pd(1)-N(1) 178.33(10) O(3)#1-Pd(2)-O(3) 180.000(1) O(3)#1-Pd(2)-N(3) 98.20(10) O(3)-Pd(2)-N(3) 81.80(10) O(3)#1-Pd(2)-N(3)#1 81.80(10) O(3)-Pd(2)-N(3)#1 98.20(10) N(3)-Pd(2)-N(3)#1 180.00(10) a Symmetry transformations used to generate equivalent atoms: #1 -x + 1, -y + 1, -z + 1.

Figure 1. The coordination environment (a) and stacking structure (b) of complex 1. dimethyl-1,10-phenanthroline-N,N′)-(phenanthrene-9,10-diolato-O,O′)palladium16g with a bigger conjugated system, in which two molecules are found in one unit. There are two kinds of palladium atoms in each group, two Pd1 on the sides and one Pd2 in the middle (Figure 1a), both taking a square planar coordination. Each Pd(II) atom is coordinated by two N atoms and two O atoms from two different 8-hqligands which are deprotoned for charge balance (Pd1: O1, O2, N1 and N2; Pd2: O3 and N3). The Pd-O, Pd-N distances fall in the range of 1.986(2)-1.995(2) Å and 2.058(2)-2.066(3) Å, respectively, which are comparable with those reported.15a,b Because of π-π interactions (face-to-face distance: 3.354 Å) of the parallel quinaldine rings, weak

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Figure 3. (a) The coordination environments and stacking structure of complex 3. (b) View of 2D network from the parallel zigzag chains in 3.

Figure 2. (a) The coordination environments and stacking structure of [Ag(8-Hhq)2]+ in complex 2. (b) View of hydrogen bonding in 2. (c) The view of 3D network in 2. metal-metal interactions appear between Pd1 and Pd2 with Pd1 · · · Pd2 separation of 3.344 Å, which is much shorter than those found in reported Pd(II) complexes with similar coordination modes (3.473, 4.77, and 4.934 Å).15c–e As seen in Figure 1b, the unit of three molecules is perpendicular to its neighbor units in plane [1, 0, 0]. In addition, the molecules extend along the axis shoulder to shoulder. It is interesting to compare the structure of complex 1 with a reported complex 8-Hydroxyquinolinato palladium(II).15e Both of them feature Pd(II) atoms with square planar coordinated by two ligand molecules, but with a different stacking manner. Similar structures can also be found in this type of Pt(II) complex named bis(8-quinolinolato-N,O)platinum(II) with the same coordination modes.15f [Ag(8-Hhq)2]NO3 (2). In complex 2, Ag1 is four-coordinated, bonded to two O1s and two N1s from the 8-Hhq ligands (Figure 2). The Ag1-O1 and Ag1-N1 bonds are 2.508(3) and 2.197(2) Å, respectively. In the asymmetry unit, there is a disordered nitrate anion, so the hydroxyl groups are not deprotoned. The two 8-Hhq ligands

coordinated to an Ag atom are not coplanar, with a dihedral angle of 47.3°. Each quinaldine ring is parallel to the one in the reverse side of the neighboring unit along the b axis, and π-π interaction exists between them with a face-to-face distance of 3.481 Å (Figure 2a). Through hydrogen bonds between oxygen atoms of nitrate and the hydroxyl groups of 8-Hhq (O1-H · · · O4: 2.6369 Å, O1-H · · · O3: 2.5354 Å), the anions and cations are further linked into a wave-like chain (Figure 2b). By the coactions of the two kinds of noncovalent interactions, the structure extends to a 3D network as shown in Figure 2c. This is similar to that found in [Ag(C21H18N2O2)](CF3O3S), in which the Ag cation units are linked through hydrogen bonding between oxygen atoms of sulfonic group and the hydroxyl groups of 8-Hhq.16b Ag(8-hq)(8-Hhq) (3). Both Ag1 and Ag2 centers in complex 3 have the coordination modes similar to those in 2, and each of Ag(I) ions in 3 is coordinated by two oxygen and two nitrogen atoms from 8-Hhq ligands (Ag1: O1, N1, O2, and N2; Ag2: O3, N3, O4, and N4). The Ag-O bond distances exist in the range of 2.3957(17) and 2.4584(18) Å, shorter than those in complex 2. The Ag-N bond lengths fall in the range of 2.2218(18) and 2.242(2) Å, longer than those in 2. The shortest metal · · · metal separation between molecules is 6.640 Å, and no direct metal-metal interaction is found. The quinaldine rings of two 8-Hhq bonded to the Ag(I) center are also not coplanar with a dihedral angle of 64.2°, implying it distorted in a higher degree than the recently reported complex (quinolin-8-ol-κ2N,O)(quinolin-8-olatoκ2N,O)-silver(I) (55.91°).16c However, complex 3 appears to be neutral, since its hydroxyl groups of coordinated 8-Hhq ligands are only partly deprotoned (Figure 3a). The two units of Ag1 and Ag2 share one hydrogen H5 via O2 and O3, lengthening the O-H bonds between them (O2-H5, 1.197Å; O3-H5, 1.232Å). Two O1s and two O4s, respectively, share H1 and H8 (O1-H1, 1.215 Å; O4-H8, 1.207 Å), and this phenomenon can also be found in other reports.16d For the reported complex (quinolin-8-ol-κ2N,O)(quinolin-8-olato-κ2N,O)silver(I),16c the hydrogen atom is merely located at one matrix. By means of these O-H · · · O hydrogen bonds, the units of Ag1 and Ag2 are linked into zigzag chains. Furthermore, π-π interactions appear between the quinaldine rings of adjacent chains with a face-to-face distance of 3.48 Å, expanding the parallel zigzag chains into a 2D network (Figure 3b). It should be noted that a variety of Ag(I) complexes with similar coordination modes have been reported, and the Ag units stack to form various 2D or 3D framework via tunable

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Figure 4. (a) The coordination environments and stacking structure of complex 4; (b) the network of complex 4 through hydrogen bonds and π-π interaction. hydrogen bonds or π-π interactions, such as (quinolin-8-ol-κ2N,O)(quinolin-8-olato-κ2N,O)-silver(I),16c which can also feature similar 2D zigzag chains through hydrogen bonds. (8-H2hq)2PdCl4 · 2H2O (4). In the structure of complex 4, there are two protonated 8-H2hq molecules, one PdCl42- ion, and two water molecules crystallized in it. Metal-metal interactions cannot found in complex 4 as the shortest Pd · · · Pd separation is 7.0330 Å. Without the basic environments created by NaOMe, the ligand is protonated on its nitrogen atoms. However, the 8-Hhq ligands do not coordinate directly with Pd(II) atoms. The PdCl42- anion is linked to the crystalline water molecules by Cl1 · · · H-O1 (3.271 Å) hydrogen bonds, and the water molecules are further linked with quinaldine hydroxyl and quinaldine nitrogen by O1 · · · H-O2 (2.714 Å) and O1 · · · H-N1 (3.002 Å) hydrogen bonds, respectively, to form a 1D chain. The chains further extend to a 3D network as a result of the π-π interactions between the ligands, with a face-to-face distance of 3.426 Å (Figure 4). A variety of complexes reported also consist of uncoordinated ligands, which are always protonated serving as cations in the compounds, and are stacked with other anion units via hydrogen bonds or π-π interactions.15f,g (8-Hhq)(tht)PtCl (5). In the structure of complex 5, Pt(II) is coordinated to four different atoms, S1 from the tetrahydrothiophene, N1 and O1 from the 8-hq-, and a terminal Cl1, to form a planar square coordination mode. The distances of the four bonds are 2.2626(18), 2.098(5), 1.988(4), and 2.3047(18) Å, respectively, which are comparable with other related complexes with similar ligands.15f The distances of the shortest metal · · · metal distance between molecules is 6.136 Å, and there is no Pt · · · Pt interaction found in this complex. Different from the other complexes, the 8-Hhq molecules are quite distant from each other and no π-π interaction exists. An intramolecular hydrogen bond exists between Cl1 and a hydrogen atom from the methyl group on the 8-Hhq ligand, with a H · · · Cl bond length of 2.657 Å (Figure 5a). The molecules then pack in 3D directions by van der Waals interactions, as seen in Figure 5b.

Discussion The reactions of 8-Hhq and Pd(II) salts with or without the addition of NaOMe have yielded the different crystalline products 1 and 4. In 1, the hydroxyl groups on 8-Hhq ligands

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are deprotoned and coordinate to metal centers, while in 4, the nitrogen atoms of 8-Hhq molecules are protonated to exhibit positive valence, and the PdCl42- ions appear to balance them in a 1:2 proportion. In the preparation of Ag(I) complex 2 without the presence of NaOMe, the hydroxyl groups of 8-Hhq ligands keep their hydrogen atoms fixed, leaving the cores of the complex positive, and the nitrate anions exist between the [Ag(8-Hhq)2]+ units for charge balance. When the aqueous ammonia solution of AgCl instead of solid AgNO3 is used for complex 3, a different result is produced, in which a half of the hydroxyl groups of ligands lose their hydrogen atoms, and thus [Ag(8-hq)(8-Hhq)] units appear to be neutral. In view of our research results and the other related investigation and reports,15,16 the formations of 8-Hhq containing complexes 1-4 were found extremely sensitive to the basicity in the reaction system. Synthesis studies here were carried out using an appropriate amount of NaOMe or ammonia as a basic reagent to adjust the reaction mixture, which is an essential condition toward the formations of 1 and 3. In the presence of basic reagents, 8-Hhq can lose one or more hydrogen atoms, favoring the coordination with metal ions. While without basicity contribution, fewer hydrogen atoms will be lost and the ligands can even be protonated. We obtained UV/vis spectra of complexes 1, 2, 3, and 5 in THF (tetrahydrofuran) solution. As seen in Figure 6, the four have similar absorptions between 200 and 300 nm, including two peaks around 250 and 280 nm, while complexes 2 and 5 have extra ones at 308.8 and 310.6 nm, respectively. All of these may be ascribed to the π-π* charge transfer of the conjugated system on the ligands,17,18 based on similarities with the absorptions of the free 8-Hhq ligand (250 and 308 nm). Special absorptions at 433.4 nm (22553 cm-1) and 430.2 nm (23260 cm-1) are found for complexes 1 and 5, respectively, which cannot be observed in complexes 2 or 3. We assign these broad absorptions as MLCT charge transitions.6a,19 To understand the phenomenon in above UV/vis spectra, we simulated the absorption spectrum of complex 1 using the Swizard program (http://www.sg-chem.net/) with the calculation results by Gaussian03 based on the time-dependent density functional theory (TD-DFT) calculations, which are shown in Figure 7. In the absorption spectrum measured in the BaSO4 plate, two peaks at 254.5 and 442.5 nm are observed, basically coherent with the calculated results (238.1 and 476.2 nm), while the 8-Hhq ligand in the BaSO4 plate is observed to absorb at 244.1 and 306.4 nm. The 254.5 nm absorption may be assigned as the π-π* charge transfer, similar to the 244.1 nm peak of free 8-Hhq, 6a and the relatively lower peak (442.5 nm) is a result of MLCT transition. TD-DFT calculations using the B3LYP functional were performed on complex 1 with its ground-state geometries adapted from the truncated X-ray data. The results indicate that HOMO is composed of d orbitals of Pd(II) and lone pair orbitals of the oxygen atoms, together with the π orbitals of the benzene rings, and LUMO mainly consists of π* orbitals of pyridine rings (Figure 8). As a result, the origin of absorption of 1 can be ascribed to MLCT and/or MMLCT (metal-metal-to-ligand charge transfer) transitions together with π-π* transitions,20 coherent with the presumed mechanism. Complex 1 exhibits orange luminescence at solid state and room temperature, giving broad and strong emission at 611 nm (λex ) 397 nm), with lifetime (τ) 9.18 µs, revealing that the emission is most likely associated with a spin-forbidden triplet parentage.21 The luminescences of complex 1 at the solution state are also studied with its crystalline samples dissolved in various solvents such as acetonitrile (MeCN), methanol (MeOH),

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Figure 5. The coordination environment (a) and stacking structure (b) of complex 5.

Figure 6. Observed electronic absorption spectra of complexes 1-3 and 5 in THF solutions, peaks: 258.02, 276.17, 433.39 nm for 1; 260.27, 285.93, 308.82 nm for 2; 254.09, 275.90 nm for 3; 239.67, 280.09, 310.63, 429.92 nm for 5.

acetone, dichloromethane (CH2Cl2), N,N′-dimethylformamide (DMF), and tetrahydrofuran (THF). Interestingly, the colors of emission for 1 differ in different solvents and range from 331 to 616 nm (Figure 10). At room temperature, 1 in both THF and DMF solutions produce orange luminescences with λmax ) 611 and 616 nm, respectively, close to its crystal emission. The CH2Cl2 solution of 1 gives yellow luminescence (λmax ) 574 nm), while the acetone solution exhibits green luminescence (λmax ) 506 nm), and the MeCN and MeOH solutions have emissions at 393 nm (361 nm, meanwhile) and 331 nm, respectively, all showing an obvious blue-shift compared with its solid state luminescence. For further research, the MeCN and CH2Cl2 solutions are frozen to 77 K, and similar spectrum result; both give blue emission at 436 nm and have weaker emission at 418 nm (Figure S1, Supporting Information). The above variety of solution luminescences is due to the solvent effect, in which MeCN and CH2Cl2 are regarded as uncoordinating solvents, while THF and DMF are regarded as good coordinating solvents which may have the coordinating effects

Figure 7. Observed in the solid state (in BaSO4 plate) and calculated electronic absorption spectra of complex 1.

Figure 8. Electron-density distribution of the lowest unoccupied and highest occupied frontier orbitals calculated for 1.

on the Pd(8-hq)2 framework. These solvents play different roles with photoactive metal centers in the solution state, and the primary character is the blue-shift of the emission band compared with their solid state, implying the increase of the excited-state energy influenced by different solvent molecules. Because of the similar coordination modes of Ag(I) with the 8-Hhq ligand, complexes 2 and 3 behave very much alike in their luminescent behavior at the solid state. Both give cyan emission (495 and 493 nm) at the excitation waves of 400 and

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Figure 9. Solid emission spectra of compounds 1, 2, 3, and 5 at room temperature.

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Figure 11. The emission spectra of complex 2 in different solvents.

Figure 12. The emission spectra of complex 3 in different solvents. Figure 10. The emission spectra of complex 1 in different solvents.

419 nm, respectively, as shown in Figure 9. Without any obvious Ag-Ag interactions, the strong sharp emissions may be denoted as ligand-centered π-π* transitions, and the coordination of the ligands to Ag(I) ions just leads to reduce the nonradiative decay of the intraligand π-π* excited states.22–24 At 77 K, the two complexes also give similar green emission at 505 and 504 nm, respectively (Figure S2, Supporting Information). Complexes 2 and 3 also can dissolve in various organic solvents and give light yellow solutions. We measured the fluorescent spectra of their solutions in MeCN, acetone, CH2Cl2, DMF, THF, and MeOH, and gained different results as shown in Figures 11 and 12. For complex 2, the solutions behave much differently from its solid state for the luminescent measurements. In all of the above solutions, complex 2 gives bands that range from 531 to 572 nm, between the green and yellow emissions, showing an obvious red-shift compared with its luminescence at the solid state. As the molecules of [Ag(8-Hhq)2]NO3 may exist as [Ag(8Hhq)2]+ and NO3-, the red-shift phenomenon may result from the decrease of the excited-state energy when the solvent molecules get close to the [Ag(8-Hhq)2]+ cations. For DMF and acetone solutions, the extra bands are also found at shorter wavelengths of 406 and 435 nm, respectively. Different from

Figure 13. The emission spectra of complex 5 in different solvents.

the others, the blue emission of the acetone solution (435 nm) is much stronger than its low energy band at 572 nm. The emission wavelength depends greatly on the solvent polarity.25 As mentioned above, in solvents with higher polarity like DMF,

Metal-8-Hydroxyquinaldine Complexes

MeOH, and MeCN, complex 2 emits a longer wavelength compared with that in THF, CH2Cl2, and acetone. For the emission spectra of complex 3, in spite of the obvious difference from its solid state, different solutions do not make as much variety as complex 2. In CH2Cl2, MeOH, and DMF, it emits green at 541 nm, yellow at 574 and 569 nm, respectively, with just one peak. In THF and MeCN, the emissions are not only found around such positions at 536 and 584 nm, respectively, but also at a shorter wavelength of 420 and 403 nm, respectively. In acetone, a slight peak at about 453 nm appears cyan near the major strong green emission at 543 nm. Similar to complex 2, the solution luminescence of 3 exhibits an obvious red-shift compared with its luminescence at the solid state, which can be ascribed as the decrease of the excited-state energy caused by the solvent molecules. Considering the solvent polarity, a similar trend was also found in complex 3, with its red-shift emission in DMF, MeOH, and MeCN compared with the other solvents.26 In Figure 9, the red crystalline complex 5 produces a green luminescence at 522 nm under UV irradiation, and a shoulder peak at the orange luminescence area (about 614 nm) is also found. Yellow solutions are gained when 5 is dissolved in the above-mentioned solvents. Using MeOH, DMF, and THF as the coordinating solvents, their solutions of 5 all produce green luminescences at 506-542 nm (Figure 13). While in MeCN, acetone, and CH2Cl2 solutions, they have cyan emissions at 484-476 nm, showing a blue-shift compared with its luminescence at the solid state. With higher polarity, the DMF and MeOH solutions emit at 540 and 542 nm, which are obviously longer than the other four. On the basis of the above analysis, in the solution luminescence we can note that the emission in DMF locates at a comparably longer wavelength, leading the conclusion that the higher polarity of the solvents may lower the excited-state energy and cause lower emission energy.27 The synthesized products of 1-5 were also characterized by X-ray powder diffraction (XRD). As shown in Figure 14, the XRD patterns are very consistent with the results simulated from single crystal data, illuminating the high purity of the assynthesized sample. To investigate their thermal stabilities, thermogravimetric analyses (TGA) of 1, 2, 3, and 5 were carried out at a rate of 10 °C/min in air atmosphere. As shown in Figure S3 in Supporting Information, all four are stable until 180 °C. Heating between 220 and 413 °C results in the collapse of complex 1 with a weight loss of 71.60%, implying that its black residue is PdO (as calculated, PdO takes 28.95% of the molecule mass). Complex 2 starts to collapse at 190 °C with a two-step weight loss, and the framework is destroyed thoroughly after the temperature increases to 391 °C. Complex 3 also gives a two-step weight loss between 201 and 460 °C. The residues of complex 2 and 3 can both be taken as Ag2O, with weight percent of 23.73 and 27.22%, respectively (calculated: 23.73%, 27.25%, respectively). Complex 5 seems not as stable as the other three, and it collapses under heating at 180 °C with a two-step weight loss. After a quick collapse above 180 °C, the second weight loss begins at 337 °C, and the continuous heating until 411 °C brings the thorough collapse of the complex. Conclusion In summary, by introducing optically active 8-Hhq ligands with Ag(I), Pd(II), or Pt(II) salts in the different reaction atmospheres we have successfully synthesized five new d8 or d10 metal complexes with distinct packing modes, favoring the existence of weak metal-metal interaction and interesting

Crystal Growth & Design, Vol. 8, No. 8, 2008 2727

Figure 14. XRD patterns of complexes 1-5 (black: calculated, red: as-synthesized sample). Table 3. Selected Bond Lengths (Å) and Angles (°) for 2a Ag(1)-N(1) 2.197(2) Ag(1)-N(1)#1 2.197(2) Ag(1)-O(1)#1 2.508(3) Ag(1)-O(1) 2.508(3) N(1)-Ag(1)-N(1)#1 173.38(10) N(1)-Ag(1)-O(1)#1 112.78(8) N(1)#1-Ag(1)-O(1)#1 69.90(8) N(1)-Ag(1)-O(1) 69.90(8) N(1)#1-Ag(1)-O(1) 112.78(8) O(1)#1-Ag(1)-O(1) 135.62(15) a Symmetry transformations used to generate equivalent atoms: #1 -x, y, -z + 1/2.

Table 4. Selected Bond Lengths (Å) and Angles (°) for 3 Ag(1)-N(2) Ag(1)-O(1) Ag(2)-N(3) Ag(2)-O(4) N(2)-Ag(1)-N(1) N(1)-Ag(1)-O(1) N(1)-Ag(1)-O(2) N(3)-Ag(2)-N(4) N(4)-Ag(2)-O(4) N(4)-Ag(2)-O(3)

2.2218(18) 2.4119(17) 2.2253(19) 2.3957(17) 165.58(7) 72.13(6) 110.32(7) 153.66(7) 71.40(7) 119.03(6)

Ag(1)-N(1) Ag(1)-O(2) Ag(2)-N(4) Ag(2)-O(3) N(2)-Ag(1)-O(1) N(2)-Ag(1)-O(2) O(1)-Ag(1)-O(2) N(3)-Ag(2)-O(4) N(3)-Ag(2)-O(3) O(4)-Ag(2)-O(3)

2.2257(18) 2.4584(18) 2.242(2) 2.4573(18) 119.73(6) 71.65(7) 121.48(7) 132.57(7) 71.24(6) 102.90(7)

chains. Their UV/vis and photoluminescent properties are characterized in both solution and solid states. The crystalline complexes all give strong emissions at photoexcitation and, due to their excellent solubilities, in various organic solvents they can give emissions of different colors within a wide range of wavelengths implying the effect of solvent polarity on their absorption and fluorescence properties. The TD-DFT calculation is performed on complex 1, agreeing well with the experimental

2728 Crystal Growth & Design, Vol. 8, No. 8, 2008 Table 5. Selected Bond Lengths (Å) and Angles (°) for 4 Pt(1)-O(1) Pt(1)-N(1) O(1)-Pt(1)-N(1) O(1)-Pt(1)-S(1) N(1)-Pt(1)-S(1)

1.988(4) 2.098(5) 82.13(18) 89.45(12) 171.51(14)

Pt(1)-S(1) Pt(1)-Cl(1) O(1)-Pt(1)-Cl(1) N(1)-Pt(1)-Cl(1) S(1)-Pt(1)-Cl(1)

2.2626(18) 2.3047(18) 173.02(14) 103.81(14) 84.53(7)

Table 6. Selected Bond Lengths (Å) and Angles (°) for 5 Pt(1)-O(1) Pt(1)-N(1) O(1)-Pt(1)-N(1) O(1)-Pt(1)-S(1) N(1)-Pt(1)-S(1)

1.988(4) 2.098(5) 82.13(18) 89.45(12) 171.51(14)

Pt(1)-S(1) Pt(1)-Cl(1) O(1)-Pt(1)-Cl(1) N(1)-Pt(1)-Cl(1) S(1)-Pt(1)-Cl(1)

2.2626(18) 2.3047(18) 173.02(14) 103.81(14) 84.53(7)

results, and confirms the luminescence origin as MLCT and/or MMLCT and intraligand π-π* charge transition. TGA results of 1-3 and 5 imply that they are stable until 180 °C. Acknowledgment. We are thankful for financial support from 973 Program (2006CB932900), the National Nature Science Foundation of China (20631050, 20521101) and the Nature Science Foundation of Fujian Province. Supporting Information Available: Crystallographic information files; emission spectra for complex 1-3 and thermogravimetric analysis of complexes 1-3 and 5. This material is available free of charge via the Internet at http://pubs.acs.org.

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