In Situ Preparation , Structure , Photoluminescence and Theoretical Study of an Unusual Bismuth Complex

A novel bismuth photoluminescent material, (N,N’-dimethyl-2,2’-bipy)2(Bi2Cl10) · 2H2O (1) (bipy = bipyridine), with the N,N’-dimethyl-2,2’-bipy moiety obtained in situ, has been synthesized under solvothermal conditions and characterized by single-crystal X-ray diffraction. Compound 1 is characterized by an isolated structure, consisting of N,N’-dimethyl-2,2’-bipy cations, Bi2Cl10 4– anions and lattice water molecules. Photoluminescence experiments with solidstate samples discover that compound 1 exhibits a strong emission in the green region. Time-dependent density functional theory (TDDFT) calculation reveals that the essence of the photoluminescence of 1 can be assigned to the combination of the metal-to-ligand charge transfer (MLCT) (from the HOMO of the bismuth ion to the LUMO of the N,N’-dimethyl-2,2’-bipy moiety) and the ligand-to-ligand charge transfer (LLCT) (from the HOMO of the chloride ions to the LUMO of the N,N’-dimethyl-2,2’-bipy moiety).


Introduction
In recent years, bismuth compounds have gained increasing interest and have been broadly investigated because they can generally exhibit attractive properties for applications in the fields of medicine, biology, chemistry and so forth. 1,2To our knowledge, as a heavy p-block element of the group 15, bismuth displays variable stereochemical activity, large ionic radius and flexible coordination geometries which allow it to be applied to build different crystal architectures.The good deformation and polarization ability of bismuth make it to conquer the steric hindrance and achieve high coordination number compounds.3][4][5] Therefore, some bismuth compounds have been applied in medicine to destroy cancer cells in high efficiency without side effect. 6,7[10] Organic ligands with various functional groups are important in the construction of metal compounds.Ncontaining heterocyclic ligands (e.g., 2,2'-bipy, 4,4'bipy) have been broadly exploited for the syntheses of metal compounds due to their plenty of coordination sites and different coordination patterns. 112,2'-bipy has been proved to be a good chelating and bridging ligand to construct metal compounds.As a very strong N-ligating ligand, 2,2'-bipy is also important to offer potential supramolecular recognition sites to give attractive supramolecular structures.As a result, 2,2'-bipy is an important and useful building block to construct extended structures or supramolecular structures.][14][15] Recently we become interested in the crystal engineering of bismuth compounds.For the sake of exploring novel bismuth compounds with new structural motifs and interesting properties, we aim at the design and synthesis of novel bismuth compounds with different organic ligands under solvothermal conditions.In this work, we report the preparation, crystal structure, photoluminescence, TDDFT study of a novel bismuth photoluminescent compound, i.e. (N,N'-dimethyl-2,2'-bipy) 2 (Bi 2 Cl 10 ) • 2H 2 O (1) (bipy = bipyridine) that is synthesized by a solvothermal reaction.The N,N'-dimethyl-2,2'-bipy 2+ moiety is obtained in situ.It should be pointed out that this is the first bismuth compound with a methylated 2,2'-bipy moiety.

1. Materials and Instrumentation.
The reagents and chemicals were commercially purchased and directly used for the reactions.FT-IR was conducted with KBr pellets on a PE Spectrum-One spectrometer.Photoluminescence measurements were performed with solid state samples at room temperature on a JY FluoroMax-3 spectrometer.Time-dependent density functional theory (TDDFT) investigations were carried out by means of Gaussian03 suite of program packages.

3. X-ray Structure Determination
The crystal data of the title compound were obtained from a Rigaku Mercury CCD X-ray diffractometer equipped with graphite monochromated Mo-Kα radiation source (λ = 0.71073 Å) at room temperature.The data reduction and the empirical absorption corrections were carried out using the CrystalClear software. 16We solved the crystal structure by virtue of the direct method.The Siemens SHELXTLTM Version 5 software package was applied to solve the structure. 17The non-hydrogen atoms were located from the subsequent difference electron density maps and refined anisotropically, while the hydrogen atoms were added theoretically.The final structure was refined by applying the full-matrix least-squares refinement on F 2 .Crystallographic data and structural parameters for the title compound are summarised in Table 1.Selected bond lengths and bond angles are depicted in Table 2.   Symmetry codes: (i) -x, -y, -z.

Results and Discussion
For the sake of getting deep insight into the crystal structure, we carried out the FT-IR spectra of the title compound.The FT-IR peaks of the title compound are mainly located in the frequency region of 780∼1690 cm -1 .The peaks at 3847 cm -1 and 3739 cm -1 should be attributed to the stretching and bending vibrations of the methyl groups of the N,N'-dimethyl-2,2'-bipy 2+ cations.The very strong intensity peak of 3473 cm -1 and 1617 cm -1 can be ascribed to the O-H stretching and bending vibration of the water molecules, respectively. 18The peaks at 3049 cm -1 and 2941 cm -1 are probably assigned to the N-H and C-H bent vibrations, respectively. 19The peaks at 2360 cm -1 and 1690 cm -1 should be assigned to the stretching and bending vibrations of the C-N bonds. 20The peaks at 1504, 1449 and 1277 cm -1 can be attributed to the stretching vibrations of the pyridyl rings of the N,N'-dimethyl-2,2'-bipy 2+ cations. 21The peaks residing in the region of 888∼1173 cm -1 can be ascribed to the bending vibration of the pyridyl rings. 18The peaks at 425, 519 and 780 cm -1 should be attributed to the stretching vibrations of the Bi-Cl bonds. 22The N,N'-dimethyl-2,2'-bipy 2+ cations in the title compound was synthesized with the existence of methanol 2,2'-bipy and concentrated hydrochloric acid.The methyl groups of the N,N'-dimethyl-2,2'-bipy 2+ cations must come from the methanol.Therefore, we deem that the in situ reaction has happened during the solvothermal process.
As revealed by the single-crystal X-ray diffraction, compound 1 crystallized in the monoclinic space group P2 1 /n with two formula units in a unit cell.The asymmetric unit is consisted of half Bi 2 Cl 10 4-anion, one N,N'-di-methyl-2,2'-bipy 2+ cation and one lattice water molecule, as presented in Fig. 1.All of the crystallographic independent atoms are located in the general positions.There are two types of chloride ions, i.e. μ 2 -bridging Cl4 and terminal Cl1, Cl2, Cl3 and Cl5 ions.The Bi 3+ cation is surrounded by two μ 2 -bridging chloride ions and four terminal chloride ions, giving a distorted octahedron.Two octahedra link together through edge-sharing to yield a Bi 2 Cl 10 4-anion.The bond lengths of Bi-Cl are in the range of 2.5594(8)-2.9350(8)4][25][26][27] The bond angle of Bi-Cl-Bi is 97.30( 2 3).][30][31] Bismuthate compounds are well-known for their interesting photoluminescent behaviors and they have been found applications in the fields of chemical sensors, electrochemical displays, light-emitting diodes (LEDs), photoluminescent materials, and so on. 32,33aking into account the good photoluminescent properties of bismuthate and 2,2'-bipy, in the present work, we investigate the photoluminescent behaviors of compound 1 with solid-state samples at room temperature.The result of the photoluminescent measurement of compound 1 is depicted in Fig. 3.As presented in the figure, the emission spectrum of compound 1 displays a broad and strong emission band.The excitation spectrum of compound 1 exhibits that the effective energy absorption dominantly locates in the ultraviolet range of 350-400 nm.The excitation spectrum shows one peak at 385 nm.When excited at 385 nm, the emission spectrum displays one strong emission band at 551 nm in the green region, accompanied by a shoulder band at 479 nm.As a result, compound 1 could be a candidate of potential green photoluminescent materials.ChemOffice Ultra 7.0 graphics program, as depicted in Fig. 4. We can easily observe from this figure that the characteristics of the highest occupied molecular orbital (HOO) and the lowest unoccupied molecular orbital (LUMO) of compound 1.The electron-population of the singlet state of the HOMO dominantly locates at the Bi 2 Cl 10 4-anions.However, the electron-population of the singlet state of the LUMO mainly resides on the N,N'-dimethyl-2,2'-bipy 2+ moiety.As a result, the essence of the photoluminescence of compound 1 can be assigned to the metal-to-ligand charge transfer (MLCT) (from the HOMO of the bismuth ion to the LUMO of the N,N'-dimethyl-2,2'-bipy 2+ moiety) and ligand-to-ligand charge transfer (LLCT) (from the HOMO of the chloride ions to the LUMO of the N,N'-dimethyl-2,2'bipy 2+ moiety).In order to reveal the nature of the photoluminescence of compound 1, we conducted the time-dependent density functional theory (TDDFT) calculations based on the ground state geometry of compound 1 obtained from the single-crystal X-ray diffraction data.TDDFT calculations with B3LYP function were carried out by means of the Gaussian03 program package.The final electron-population diagrams were drawn by the

Conclusions
In conclusion, we have prepared a novel photoluminescent bismuth compound via a solvothermal in situ reaction.The title compound is characterized by an isolated structure.It exhibits a strong photoluminescent emission in the green region.TDDFT calculation reveals that the essence of the photoluminescence can be assigned to the combination of the MLCT and LLCT.The title compound is a potential green-light photoluminescent material.

Fig. 3 .
Fig. 3. Photoluminescence spectra of 1 with the green and red lines representing excitation and emission spectra, respectively.

Table 1 .
Crystallographic data and structural analysis