Preparation , Structure , Photoluminescent and Semiconductive Properties , and Theoretical Calculation of a Novel Cadmium Complex with Mixed Ligands

A novel cadmium complex with mixed ligands {[Cd(2,2’-biim)(4,4’-bipy)(H2O)(ClO4)] (ClO4)}n (1) (2,2’-biim = 2,2’-biimidazole; 4,4’-bipy = 4,4’-bipyridine) has been synthesized through hydrothermal reaction and its crystal structure was determined by single-crystal X-ray diffraction technique. Single-crystal X-ray diffraction analyses revealed that complex 1 crystallizes in the space group Pna21 of the orthorhombic system and exhibits a one-dimensional zigzag chain structure consisting of [Cd(2,2’-biim)(4,4’-bipy)(H2O)(ClO4)]n n+ cationic chains and isolated ClO4 – anions. Powder photoluminescent characterization reveals that complex 1 has an emission in the green region of the spectrum. Time-dependent density functional theory (TDDFT) calculation showed that the nature of the photoluminescence of complex 1 is originated from the ligand-to-ligand charge transfer (LLCT; from the HOMO of the perchlorate anions to the LUMO of the 4,4’-bipy ligand). A wide optical band gap of 3.25 eV was found by the solid-state UV/vis diffuse reflectance spectrum.


Introduction
In recent years, preparation and characterization of coordination complexes have attracted increasing interest not only due to their amazing structural topologies but also their potential applications in the fields of catalyst, sensors, medicine, biology, solar energy conversion, magnetism, photoluminescence materials, and so forth. 1-,5rom the perspective of the crystal engineering, the most useful and facile way to construct coordination complexes is to adopt a suitable ligand to connect metal centers.To this end, the ligand is better to possess as much as donor atoms that enable it to bridge metal centers together to yield extended architectures.][8][9][10][11][12] 2,2'-biim and 4,4'-bipy have been confirmed to be a good chelating or bridging ligand to build coordination complexes.Being very strong N-ligating ligands, 2,2'-biim and 4,4'-bipy are also important to afford useful supramolecular recognition positions to form interesting supramolecular topologies.Therefore, 2,2'-biim and 4,4'-bipy are important and useful ligands to achieve extended structures or supramolecular geometries.Furthermore, the imidazole or pyridyl rings of the 2,2'-biim or 4,4'-bipy possess delocalized π-electron systems which allow them to become an ideal candidate to prepare photoluminescent materials.
4][15] For the sake of exploring the metal ions on the structures and properties of the coordination complexes, we often choose the IIB ions as the central ion source.In order to explore novel IIB coordination complexes with attractive structural topologies and interesting properties, we recently focus on the design and preparation of novel IIB coordination complexes with various organic ligands.We report in this work the preparation, crystal structure, photoluminescent and semiconductive properties, and theoretical calculation of a novel cadmium complex with mixed ligands, i.e. {[Cd(2,2'-biim)(4,4'-bipy)(H 2 O)(ClO 4 )](ClO 4 )} n (1) with a 1-D zigzag chain structure.It should be pointed out that this is the first IIB complex with both 4,4'-bipy and 2,2'biim ligands.

1. Materials and Instrumentation.
All chemicals and reagents were of reagent grade, commercially available and directly applied for the reaction.Photoluminescent characterization was measured using solid-state powders of the title complex at room temperature on a JY FluoroMax-3 spectrometer.Time-dependent density functional theory (TDDFT) calculation was performed by virtue of the Gaussian03 suite of program packages.Solid-state UV/vis diffuse reflectance measurement was conducted on a computer-controlled TU1901 UV/vis spectrometer equipped with an integrating sphere attachment.Finely-ground powder sample was coated on barium sulfate which acts as a reference for 100% reflectance.

3. X-ray Structure Determination
The single-crystal data of the title complex were collected on a SuperNova X-ray diffractometer equipped with a graphite monochromated Mo-Kα radiation source (λ = 0.71073 Å) at 293(2) K.The diffraction was performed by means of a ω scan mode.Using the CrystalClear software, we reduced the data set and corrected the empirical absorption. 16The crystal structure was successfully solved by using the direct methods and Siemens SHELXTLTM Version 5 software package. 17The non-hydrogen atoms were generated based on the subsequent Fourier difference maps and refined anisotropically.The hydrogen atoms were located theoretically and ride on their parent atoms;  (12)  N4-Cd1-O7 92.53 (14)   Symmetry code: (i) -x -½, y + ½, z -½.
Yi et al.: Preparation, Structure, Photoluminescent ... however, the hydrogen atoms of the coordinated water molecule were not located and are not included into the model.The single-crystal structure was finally refined by using the full-matrix least-squares procedure on F 2 .Crystallographic data and structural refinements for the title complex are summarized in Table 1.Selected bond lengths and bond angles for the crystal structure are displayed in Table 2.The hydrogen bonding interactions are presented in Table 3.

Results and Discussion
Single-crystal X-ray diffraction measurement revealed that complex 1 crystallizes in the space group Pna2 1 of the orthorhombic system with four formula units in one unit cell and the crystallographically asymmetric unit is comprised of one cadmium ion, one 4,4'-bipy ligand, one 2,2'-biim ligand, one isolated perchlorate anion, one coordinating perchlorate anion and one coordinating water molecule, as presented in Fig. 1.Complex 1 is characterized by a 1-D zigzag chain structure, consisting of [Cd(2,2'biim)(4,4'-bipy)(H 2 O)(ClO 4 )] n n+ cationic chains and isolated ClO 4 -anions.The cadmium ion displays a slightly distorted octahedral geometry with the equatorial positions inhabited by three nitrogen atoms from one 2,2'-biim and one 4,4'-bipy ligand as well as one oxygen atom from one coordinating water molecule, and the apical sites are occupied by one oxygen atom from one coordinating perchlorate anion as well as one nitrogen atom from one 4,4'bipy ligand (Fig. 1).
nally coordinating and isolated.The Cd•••Cd distance is 11.5886(2) Å because of the distraction of the long rodlike bridging 4,4'-bipy ligand.The bond lengths of Cd-N are in the range of 2.290(3)-2.369(4)Åwith a mean value of 2.329(4) Å.9][20] The bond lengths of Cd-O for water and for perchlorate ligand are 2.296(3) Å and 2.499(4) Å, respectively.2][23][24] The bond angle of N5'-Cd1-N2 is 170.58( 14 The imidazole rings of the 2,2'-biim ligand is nearly coplanar with a small dihedral angle of 4.64(2)°, which is close to that in another cadmium 2,2'-biim complex (3.23°). 25  The neighboring cadmium ions are interconnected by the 4,4'-bipy ligands through the nitrogen atoms to construct a 1D zigzag chain structure running along the b-axis, as depicted in Fig. 2. Different from the bridging 4,4'-bipy ligand, the 2,2'-biim molecule acts as a terminal ligand and chelates to one cadmium ion with the chelating angle N2-Cd1-N4 being of 72.89(11)°.[28][29] cited by the wavelength of 424 nm, the photoluminescent emission spectra yield a strong emission peak at 493 nm in the green region of the spectrum.As a result, complex 1 could be a candidate material for green photoluminescence.
Trying to unveil the nature of the photoluminescence spectra of complex 1, we truncated ground state geometry from its single-crystal X-ray diffraction data set and carried out its theoretical calculation in light of the time-dependent density functional theory (TDDFT) based on this ground state geometry.The TDDFT investigation was performed with the B3LYP function and by virtue of the Gaussian03 software package.After successfully calculating, the theoretical electron-distribution diagrams were obtained using the ChemOffice Ultra 7.0 graphics program and the results are given in Fig. 5.It is easy to find out that the electron-density distribution of the singlet state of HOMO is totally resided at the coordinating perchlorate anion with an energy being of -0.213039 Hartrees; however, the electron-density population of the singlet state of the LUMO locates at the 4,4'-bipy ligand and the energy of  In general, cadmium compounds can exhibit attractive photoluminescent properties and, therefore, they have potential application in the areas of light-emitting diodes, electrochemical displays, photoluminescent materials, sensors. 30,31Moreover, complexes containing 4,4'-bipy or 2,2'biim ligands can generally also show good photoluminescence due to the existence of their delocalized π electrons.As a result, the title complex is expected to possess photoluminescent behavior.Based on these considerations, in the present work, we measured the photoluminescent properties of complex 1 using powder-like samples under room temperature.Fig. 4 gives the photoluminescent excitation and emission spectra of complex 1.As displayed in this figure, the photoluminescent emission spectra of complex 1 show a wide and intensive emission peak, while the photoluminescent excitation spectra show that the effective energy absorption mainly resides in the wavelength span of 250-430 nm.The photoluminescent excitation spectra display a main peak at 424 nm and a shoulder at 355 nm.When it was ex-  the LUMO is calculated to be -0.122838Hartrees.The energy difference between LUMO and HOMO is 0.090201 Hartrees that is small enough to allow the charge transfer from HOMO to LUMO.Based on these observation, it is proposed that the essence of the photoluminescence of complex 1 could be assigned to the ligand-to-ligand charge transfer (LLCT; from the HOMO of the perchlorate anion to the LUMO of the 4,4'-bipy ligand).
Cadmium compounds are well-known not only for their photoluminescent behaviors but also for their semiconductive properties and the latter enable them to be widely applied in military or civil areas.For example, Hg-CdTe, known as MCT, is one of the most famous military infrared detectors based on the semiconductive properties.Therefore, it could be worthy to measure the semiconductive properties of the title complex.Powder-like barium sulfate acts as a reference for 100% reflectance and finely-ground powder sample was coated on the surface of the barium sulfate.After measuring the solid-state UV/vis diffuse reflectance spectra, the data was treated carefully with the Kubelka-Munk function which is known as α/S = (1 -R) 2 /2R.With regard to this function, α means the absorption coefficient, S refers to the scattering coefficient which is actually wavelength independent when the size of the particle is larger than 5 µm, and the R is related to the reflectance.From the α/S vs. energy gap diagram, the value of the optical band gap could be determined via extrapolating the linear portion of the absorption edges.In this way, the solid-state UV/vis diffuse reflectance spectra showed that complex 1 has a wide energy band gap of 3.25 eV, as depicted in Fig. 6.As a result, complex 1 could be a possible candidate for the wide optical band gap semiconductors.The slow slope of the optical absorption edge of complex 1 suggests that it must be an indirect transition. 32he energy band gap of 3.25 eV of complex 1 is obviously larger than those of CuInS 2 (1.55 eV), CdTe (1.5 eV) and GaAs (1.4 eV), all of them are broadly applied as efficient photovoltaic materials. 33,34

Conclusions
In conclusion, a novel cadmium complex with mixed ligands has been synthesized and characterized by single-crystal X-ray diffraction.It exhibits a 1-D zigzag chain structure.It is the first cadmium complex with both 4,4'bipy and 2,2'-biim ligands.Powder photoluminescent characterization reveals that it displays an emission in the green region.TDDFT calculation revealed that the nature of the photoluminescence is originated from the ligand-to-ligand charge transfer (LLCT; from the HOMO of the perchlorate anions to the LUMO of the 4,4'-bipy ligand).A wide optical band gap of 3.25 eV was determined by the solid-state UV/vis diffuse reflectance spectrum.

Figure 3 .
Figure 3. Packing diagram of complex 1 with the dashed lines representing the hydrogen bonding interactions.

Figure 4 .
Figure 4. Photoluminescence spectra of 1 with the red and green lines representing excitation and emission spectra, respectively.

Table 1 .
Crystallographic data and structural analysis for complex 1