научная статья по теме DFT STUDY OF THE EFFECT OF DIFFERENT METALS ON STRUCTURES AND ELECTRONIC SPECTRA OF SOME ORGANIC-METAL COMPOUNDS AS SENSITIZING DYES Физика

Текст научной статьи на тему «DFT STUDY OF THE EFFECT OF DIFFERENT METALS ON STRUCTURES AND ELECTRONIC SPECTRA OF SOME ORGANIC-METAL COMPOUNDS AS SENSITIZING DYES»

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DFT STUDY OF THE EFFECT OF DIFFERENT METALS ON STRUCTURES AND ELECTRONIC SPECTRA OF SOME ORGANIC-METAL COMPOUNDS

AS SENSITIZING DYES

© 2014 г. Guodong Tang*, **, Rongqing Li*, Shanshan Kou*, **, Tingling Tang*, **,

Yu Zhang*, Yiwei W&ng*

*Jiangsu Key Laboratory for Chemistry of Low-Dimensional Materials, School of Chemistry and Chemical Engineering, Huaiyin Normal University, Huai'an 223300, Jiangsu Province, People's Republic of China **School of Chemistry & Chemical Engineering, Ningxia University, Yinchuan 750021, People's Republic of China

E-mail: hysytanggd@hotmail.com, yuzhang@hytc.edu.cn Received May 6, 2013

Ruthenium polypyridined-derivative complexes are used in dye-sensitized solar cell (DSSC) as a light to current conversion sensitizer. In order to lower the cost of the DSSC, the normal transition metals were used to replace the noble metal ruthenium, and some compounds [ML2L'] (M = Pt, Fe, Ni, Zn; L = isonicotinic acid, L' = maleonitriledithiolate, I = PtL2L', II = FeL2L', III = NiL2L', IV = ZnL2L') were selected as the replacement. The geometries, electronic structures and optical absorption spectra of these compounds have been studied by using density functional theory (DFT) calculation at the B3LYP/LANL2DZ, B3P86/LANL2DZ, B3LYR/GEN level of theory. All the geometric parameters are close to the experimental values. The HOMOs are mainly on the maleonitriledithiolate groups mixed with fewer characters of the metal atom, the LUMOs are mainly on the two pyridine ligands. This means that the electron transition is attributed to the LLCT. The maximum absorptions of complexes are found to be at 351 nm, 806 nm for compound I and 542 nm for compound II. The maximum absorptions of complexes are found to be at 884 nm for compound III, and 560 nm for compound IV. This means that those compounds may be as a suitable sensitizer for solar energy conversion applications.

DOI: 10.7868/S0030403414010085

1. INTRODUCTION

With the global challenge for sustainable development of renewable energy sources, photovoltaic technologies play a major role. In the early 1990s, the new types of solar cell technology were discovered with the advent of dye sensitized solar cells (DSSC) by Grätzel and coworkers [1—5]. DSSC have attracted much attention due to their potentially low fabrication costs, easy production and flexibility compared to conventional photovoltaic cells.

To increase the photoconversion efficiency, different approaches are still being studied to improve the photoelectricity conversion efficiency of the DSSC, such as the nature of semiconductors (TiO2, SnO2, ZnO, NiO, etc) [5—8], the morphology of semiconductors (TiO2, ZnO, etc.) [9—11], the forms of electrolytes [12, 13], and the singlet excitation fission materials were used in the DSSC [14-18].

In the DSSC, the sensitizer is one of the key components, harvesting the solar radiation and converting it into electric current. It has been found that many of the organic-metal compounds possess promising high conversion efficiency since these kinds of organic-metal compounds were found to combine the advantages of molecular tunability with the material properties ofwide band-gap semiconductors, such as stability toward corrosion, charge transport and mechanical

resilience. Until now, many efforts have been done on the syntheses of new ruthenium complexes and determinations of their electronic spectra [19—25], and state-of-art DSSCs based on ruthenium (Il)-polypy-ridyl complexes as the active material have an overall power conversion efficiency (n) approaching 11% under standard (Global Air Mass 1.5) illumination [26— 29]. The ruthenium polypyridine and polypyridine-derivative complexes have been widely investigated for many years both experimentally and theoretically. Ko-matsuzaki and co-workers [30] synthesized a series of ruthenium (H)-polypyridyl complexes as a sensitizer for DSSC and characterized their photophysical and photochemical properties; the complexes showed broad electronic absorption bands in the near-IR region. Xu and co-workers [31] reported the behaviors of RuL2 (L = 5'-methyl-2,2'-bipyridine-6-carboxyl) in the gas phase and DMF solution by the TD—DFT method. Their calculation results indicate that the two maximum absorption peaks are blue-shifted in DMF solution in comparison with those in the gas phase. Nazeeruddin [32] and co-workers reported a ruthenium complex trans-[Ru(L')(NCS)2], where L' = 4, 4"'-di-tert-butyl-4', 4''-bis(carboxylic acid)-2, 2': 6', 2'': 6'', 2''' quaterpyridine (N886), and the complex was characterized by spectroscopic and electrochemical methods. The electronic spectrum of the N886 com-

Fig. 1. The optimized geometries of M^L' complexes (M = Pt, Fe, Ni, Zn; L = isonicotinic acid, L' = 2,3-dimercapto-but-2-enedinitrile I = PtL2L', II = FeL2L', III = NiL2L', IV = ZnL2L').

plex was also calculated by TD-DFT method. The result showed its absorption bands as mixed Ru/SCN-to-quarterpyridine charge transfer transitions, which extend from the near-IR to the UV regions. This suggests that it can act as a suitable sensitizer for solar energy conversion applications. These studies mainly focus on the effect of the different ligands on the DSSC properties. However, ruthenium is a noble metal, and it may be a bottleneck for the development of the low-cost DSSC. At present, little attention has been paid to the normal transition metals and their effect on the DSSC.

In order to reduce the cost of the DSSC, we want to use the normal transition metals to replace the noble metal. We selected a reported platinum complex with formula [Pt{4,4'(CO2H)2-2,2'-bipyridyl} (maleoni-triledithiolate)] [33] as template; but we used the isonicotinic acid ligand replacement of the 4,4'(CO2H)2-2,2'-bipyridyl ligand. In order to understand the relationship between the metals and the DSSC properties, the DFT methods were used to optimize the structures of those organic metals without any constrains in bond lengths, bond angles and dihedral angles, and then calculated the electronic spectra of those optimized structures. The results would be helpful in the simulation, screening and design ofnew organic metal complexes as new type DSSC sensitizers.

2. COMPUTATIONAL PROCEDURES

Due to the high accuracy, DFT has been proved to be more useful in calculating molecular properties of the organometallic compounds than the traditional ab initio electronic structure methods [34—36]. Some re-

ports indicated that the Ruthenium polypyridined-de-rivative complexes are efficient photosensitizers because of their broad range of visible light absorption and relatively long excited states, with energies almost matching those of TiO2 conduction band states [37—40].

The geometries of these complexes were optimized using the DFT (B3LYP and B3P86) methods and the LANL2DZ basis set, in order to check the affection with different basis set, we used the hybrid-basis set, that is LANL2DZ basis set for metal element while the 6-311G** for the nonmetal elements [41—46]. The UV-Vis spectra of these complexes were calculated with the same method. Geometry optimization is one of the most important steps in the theoretical calculations. This procedure proceeds in two steps. Firstly the geometry was constructed by MM+ molecular dynamics in Hyper-Chem. 6.0 package [47], and then optimized by the DFT methods at B3LYP or B3P86 level with LANL2DZ or mixed basis set using Gaussian 03W program package. The maximum values of the converged criterion are default. All geometries converged perfectly. Meanwhile, the time-dependent density functional theory (TD-DFT) was employed to describe the electron absorption spectra of all organic metals.

3. RESULTS AND DISCUSSION 3.1. Structures

Any comparison between experimental and calculated data should require a precise knowledge of the molecular structure of the complexes. However, only a few crystal structures of [Pt(bpy)(l,2-dithiolate)] systems have previously been reported [33], and no crystal structures of [Pt(ina)(mndt)]) (ina = isonicotinic acid and mndt = maleonitriledithiolate) was reported. We only selected the similar reported crystal structure as comparison [48].

The fully optimized geometries of [ML2L'] (M = = Pt, Fe, Ni, Zn; L = isonicotinic acid, L' = 2,3-dimercapto-but-2-enedinitrile) are shown in Fig. 1. The selected bond lengths and bond angles are given in Table 1, along with the available experimental data [48].

In order to evaluate the method exactness, we selected the similar reported structure [48] for comparison. The relative errors for different methods (B3LYP/LANL2DZ, B3P86/LANL2DZ and B3LYP/GEN) were in the range of 1.7-6.0%, 0.35.0%, and 0.5-3.7% for bond lengths, respectively. These geometric parameters are close to the experimental values, suggesting that these methods were appropriate for these types of compounds, and the result by using the B3LYP/GEN is relatively accurate.

Since the exact crystal structures of the other title compounds are not available by now, the optimized structures can only be compared with other similar systems which the crystal structures have been reported [49-51].

DFT STUDY OF THE EFFECT OF DIFFERENT METALS Table 1. The select bond length (A) and bond angles (°) for all compounds

Compounds Parameters B3LYP/LANL2DZ B3P86/LANL2DZ mix/LANL2DZ Exp. [48]

I Pt1-N2 2.090 2.060 2.130 2.055

Pt1-N3 2.090 2.060 2.130

Pt1-S4 2.379 2.356 2.314 2.245/2.302

Pt1-S5 2.379 2.356 2.314 2.244/2.300

C10-C16 1.429 1.424 1.420

C11-C17 1.429 1.424 1.420

C16-N24 1.185 1.183 1.157

C17-N25 1.185 1.183 1.157

C30-O32 1.240 1.237 1.204 1.21

C30-O33 1.384 1.376 1.349 1.33

C31-O34 1.241 1.238 1.204 1.21

C31-O35 1.382 1.374 1.347 1.33

N2-Pt1-N3 92.1 92.1 91.2 79.8

N2-Pt1-S4 178.6 178.7

N2-Pt1-S5 89.1 89.0 89.6

N3-Pt1-S4 89.2 89.1 89.6

N3-Pt1-S5 178.7 178.8

S4-Pt1-S5 89.5 89.8 89.6 89.

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