First Principles Study of Nickel Complex with 1,3-dithiole-2-thione-4,5-dithiolate Ligands as Model Photosensitizers

First Principles Study of Nickel Complex with 1,3-dithiole-2-thione-4,5-dithiolate Ligands as Model Photosensitizers L. W. C. Paes,1 J. Amaya Suárez,2 A. M. Márquez and Javier. Fdez. Sanz2* 1 Departamento de Ciências Exatas, Escola de Engenharia Industrial e Metalurgia de Volta Redonda, 27255-125, Volta Redonda, RJ, Brazil 2 Departamento de Química Física, Facultad de Química, Universidad de Sevilla, E41012 Sevilla, Spain Abstract Dye-sensitized solar cells (DSSCs) have become in one important and promising technology in the photovoltaic field. The ability for a sensitizer to harvest light photons and inject the excited electrons into a photoanode, typically a metal oxide, determines the performance and operation range of the solar cell. Metal complexes with 1,3dithiole-2-thione-4,5-dithiolate (dmit) ligands, which are an important class of functional materials, have received extensive attention due to their intriguing chemical and physical properties. The electronic and molecular properties of isolated and adsorbed nickel complexes with dmit ligands have been investigated using first principles calculations based on the density functional theory (DFT). Adsorption energies of metal complexes supported on the anatase TiO2(101) surface were calculated for three different configurations, linked by sulphur atom of Sthione, SthioleSthiolate, and planar. The most stable adsorption configurations found in this study are the Sthiole-Sthiolate and the planar forms for the nickel complex. TD-DFT molecular calculations reveal that the lowest energy transition in ultraviolet visible near-infrared (UV-Vis-NIR) mainly corresponds to the HOMO-LUMO π–π* excitation for the nickel complex. The effect of the TiO2(101) surface on the absorption spectra of the nickel complex is practically limited to a red shift of about 0.1-0.3 eV. The analysis of the density of states for the dmit/TiO2(101) system shows that the LUMO of the metal complex lies at the edge of the TiO2 conduction band indicating, therefore, that electron injection from the complex excited state into the semiconductor surface is unlikely. Keywords: Metal complexes, dmit ligand, DFT, TD-DFT, Adsorption energy, Sensitizer 1 Introduction The sensitization of wide band-gap semiconductors lays at the heart of dye sensitized solar cells (DSSCs), a rising technology for solar energy harvesting that offers some advantages over classical Si-based devices[1, 2]. A key strength of DSSCs is the separation of electron generation and transport processes in two distinct materials. This allows the disconnected optimization of the dye for photon absorption and of a wide band-gap semiconductor for electron-hole separation and collection [3]. Typically metal oxides like zinc oxide (ZnO)[4, 5], stannic oxide (SnO2) [6, 7, 8] and titanium dioxide (TiO2) [2] have been used as the semiconductor material. However, different experimental results have shown that TiO2 is preferable over either ZnO or SnO2. Titanium dioxide is non-toxic, highly abundant, and provides a mesoporous structure for both organic and inorganic dye adsorption. The role of the dye is to absorb the incoming photons and to transfer the excited electron to the conduction band of the semiconductor. Thus, an efficient dye should (a) be strongly adsorbed at the semiconductor surface; (b) show intense absorption in the visible and near infrared regions of the electromagnetic spectrum; (c) be stable enough as to be capable of multiple oxidation-reduction cycles and (d) be stable enough in its oxidized form as to be reduced by the electrolyte and his lowest excited state should be higher than the semiconductor conduction band edge. Basically, photosensitizer dyes are either pure organic compounds or metal-based organometallic complexes. Metal-free organic sensitizers are cheaper, easy to modify structurally to tune the dye properties, in some cases they are environmentally benign and non-toxic and have high molar absorption coefficients. However, they also show important stability and efficiency problems. Many metal complexes-based dyes have been proposed. Of these, ruthenium (II) polypyridyl complexes have been shown to be the best so far [9]. However, the low abundance of the metal, its high cost and toxicity impose severe limitations on its practical and widespread use. Thus, transition metal complexes based on iron (Fe), nickel (Ni), cobalt (Co), palladium (Pd), platinum (Pt), and zinc (Zn), among others, have been proposed as alternatives in the design of photovoltaic sensitizers [2,3,10,11,12,13,14]. In this regard, square-planar complexes with sulfur-containing ligands that absorb in the near infrared region (NIR) of the spectrum have attracted special interest and have been examined both experimentally and theoretically [12,15,16,17,18,19]. Islam et al. [15] were the first to explore the application of a series of square-planar 2 diimine-dithiolate complexes as sensitizers. They synthesized and characterized a series of platinum-based polypyridyl complexes with dithiolate ligands that were also anchored to nanocrystalline TiO2 in photoelectrochemical cells. The intense charge transfer band in these complexes was shown to be tunable by changing the dithiolate ligands. Geary et al. [16] prepared and examined a family of Pt(II)(diimine)(dithiolate) complexes, analyzing the influence of 3,3’-, 4,4’-, and 5,5’- bipyridyl substituents on their electronic properties. All synthesized complexes where attached to a TiO2 substrate and tested as solar cells sensitizers with the 3,3’-disustituted bipyridyl complex showing the highest photovoltaic performance. In a later study [17] the superior performance of the 3,3’- bipyridyl complex was rationalized by using density functional theory calculations based on a hybrid functional that suggested that the longer-lived charge-separated state for this complex on TiO2 was related to the non-planar geometry of the complex, reducing the electronic coupling between ligands. Lazarides et al. [18] have attempted to increase the light absorption properties of Pt(II)(diimine)(dithiolate) chromophores by combining them with boron-dipyrromethene, a strongly absorbing dye, in a dual chromophore system. By using time-dependent DFT calculations, the authors show that the many paths for electron transfer that exist in these systems result in unexpected routes for excited-state relaxation and loss of the desired properties of the excited charge transfer state. Despite the intense work developed on examining the potential of Pt(II)(diimine)(dithiolate) complexes as sensitizers for DSSC cells, only the paper by Linfoot et al. [12] has studied some Ni(II)(diimine)(dithiolate) dyes in relation with their use as dyes in a DSSC cell. The authors characterized the complexes using electrochemical, spectroscopic and computational techniques and assigned intense visible absorptions to ligand-to-ligand charge transfer transitions that would suggest appropriate charge separation for using on a photoelectrochemical device. However, low photocurrents were found when the complex was adsorbed on a TiO2 film, a problem that was linked to a short-lived excited state of the Ni(II) complex. Because of their unique properties related to applications in fields as diverse as conducting and superconducting materials, non-linear optics, catalysis, and dyes, metal dithiolene complexes, R2M(dmit)2, R=PyMe, NEt4, NMe4, NPr4, NBu4, and dmit=1,3dithiole-2-thione-4,5-dithiolate) have been extensively studied for more than forty years [20]. These applications result from an interplay of different properties, including highly delocalized frontier orbitals that allow direct electron transfer through the ligand 3 π orbitals. For this reason, these complexes are considered promising candidates for photochemical devices [21]. Here, we present a study of the structural and spectroscopic properties of the model dithiolene complex [(CH3)2][Ni(dmit)2] by combining DFT and TD-DFT calculations. First we analyze the properties of the isolated complex using the B3LYP functional and an atom-centered basis set. Second, the geometric and electronic properties of the complex adsorbed on a model TiO2 (101) anatase surface have been examined by using plane-wave calculations that include both the use of a Hubbard correction to properly localize the metal d-electrons and an approximate functional to improve the description of the dispersion forces on the DFT calculations. Finally, we theoretically examine the performance of the model [(CH3)2][Ni(dmit)2] complex as a sensitizer. Computational details For the isolated [CH3]2[Ni(dmit)2] complex, DFT calculations have been performed using the Gaussian 09 quantum chemical package [22]. Equilibrium geometry and electronic properties were determined by employing the hybrid Becke three-parameter functional with the Lee, Yang, and Parr (B3LYP) exchange correlation functional [23,24], with CEP-121G [25,26,27] effective core potentials and basis sets for Ni and S atoms and 6-31++G basis set for C and H atoms. Given that some chargetransfer character was found in the first excited state, the CAM-B3LYP hybrid functional, that includes long-range corrections was also employed [28]. TD-DFT single-point energy calculations were performed on optimized geometries. To determine the geometric and electronic properties of the TiO2 (101) surface and TiO2 (101) surface with the adsorbed nickel complex, we performed periodic DFT calculations using the Vienna ab initio Simulation Package (VASP) [29,30,31]. The projector augmented wave (PAW) method was used, and the cutoff energy was set to 400 eV for slab and adsorption calculations. The generalized gradient approximation (GGA) functional was used (Perdew-Burke-Ernzerhof, PBE) [32]. In order to better render the anatase band gap, usually underestimated in plain GGA DFT calculations, a Hubbard type on-site Coulomb correction term was used as implemented by Dudarev et al. [33]. The GGA+U procedure was applied on the transition metal d electrons, being the 𝑈eff values employed in this work 4.5 eV and 5.5 eV for 3d levels of Ti and Ni, 4 respectively [34,35]. Optical spectra were obtained from the frequency dependent dielectrical functions as proposed by Gajdoš et al. [36]. Because GGA functionals neglect attractive long-range contributions, computed adsorption energies are generally underestimated [37]. To include the van der Waals corrections into the density functional approach (vdW-DF) and obtain a more accurate description, the method proposed by Tkatchenko and Scheffler was employed in this work [38]. The slab model of anatase surface was obtained by appropriately cutting the most stable TiO2 (101) surface, and is represented by 96 [TiO2] units arranged according to anatase crystalline structure. The model 5x3 supercell consisted of two O-Ti-O trilayers, 144 atoms each, where the bottom layer was fixed. The orthorhombic supercell has, thus, dimensions: a = 31.254 Å, b = 15.288 Å and c = 35.916 Å, including a vacuum space of 20 Å in the c direction. All calculations were performed at the Γ point. Adsorption of [CH3]2[Ni(dmit)2] complex on the TiO2 (101) surface was done in three different adsorption configurations: linked by Sthione, linked by Sthiole-Sthiolate (bridge) and plane (Fig. 1). Fig. 1: Schematic structure of adsorption form Sthione, Sthiole-Sthiolate. Adsorption energies (𝐸𝐴𝐷𝑆 ) for the optimized metal complexes on the TiO2(101) surface were calculated using 𝐸𝐴𝐷𝑆 =𝐸(TiO2)+(Nidmit)− (𝐸(TiO2)+ 𝐸Nidmit ) where 𝐸(TiO2)+(Nidmit) is the energy of (Ni-dmit) complex adsorbed on the TiO2 (101) surface, 𝐸Nidmit R and 𝐸(TiO2)R are the energies of the isolated Ni-dmit complex and clean TiO2 (101) surface respectively. With this definition, negative adsorption energies represent bound states stable with respect to desorption. 5 Results and discussion Structure and Electronic Properties of [CH3]2[Ni(dmit)2] complex Fig. 2 shows the optimized structure of the square planar complex [CH3]2[Ni(dmit)2]. Table 1 presents the geometric parameters obtained at the B3LYP and PBE+U levels, in comparison to the experimental structure. We report only the relevant bond lengths and bond angles. Fig. 2: Optimized structure of [CH 3 ] 2 [Ni(dmit) 2 ] complex. Atoms colors code: Ni, gray; C, black; S, yellow; H, white. Table 1: Main geometrical parameters calculated for [CH3]2[Ni(dmit)2] complexes B3LYP CAM-B3LYP PBE+U Exp [39] Bond distances Ni-S 2.244 2.230 2.186 2.16-2.17 S=C 1.747 1.749 1.721 1.66 C=C 1.401 1.400 1.416 1.39 Bond angles S-Ni-S 92.6 92.8 93.1 92.2 S-Ni-S 87.4 87.2 93.2 86.6 Ni-S-C 101.5 101.4 102.4 102.8 *Bond lengths in Å and bond angles in degrees From the data shown in Table 1, a general agreement between calculated and experimental values is observed. Optimized bond distances are systematically overestimated, and the values obtained from PBE+U calculations in general are in better 6 agreement than those estimated with either the B3LYP or the CAM-B3LYP functionals, except for the C=C double bond. Ni–S bond lengths are very similar to each other and in agreement with the experimental results. In contrast, the S=C bond is significantly overestimated. The disagreement found can be related to the fact that experimental data derive from solid crystal structure diffraction experiments in which packing forces may alter the geometry of individual molecules. The calculated harmonic vibrational frequencies and band assignments for the nickel complex are presented in Table 2. Table 2: Comparison between the experimental and calculated frequencies and assignments of vibrational modes of [CH3]2[Ni(dmit)2] (cm-1) The calculated peaks associated with the C-H stretch modes of the [CH3] groups were found at 3073/3098/3001 cm−1 for B3LYP, CAM-B3LYP and PBE+U respectively. The bands at 1331/1374/1297 cm−1 and 993/1063/950 cm−1 were assigned to C=C and C=S stretching modes, respectively, and were compatible with other published results [40]. Valade et al. [41] also reported the C=C peak at 1430 cm-1, and listed two peaks at 455 cm-1 and 310 cm-1; both were assigned as Ni-S vibration. The bands at 496/509/490 cm-1 are characteristic of the fundamental vibrations of the thiocarbonate group (–S–(C=Sthione)-S–). The C=S stretching vibration is the characteristic vibration in the IR spectra of DMIT complexes. According to various B3LYP CAMB3LYP PBE+U Exp [40,41,42,43,44 ] υ s (C-H) 3073 3098 3001 3000 υ s (C=C) 1331 1374 1297 1454 υ(C-S thiole ) 943 984 933 940 υ(S-(C=S thione )-S) 496 509 490 531 υ(C=S) + υ(S thiole -C thione -S thiole ) 993 1063 950 1039 υ(Ni-S thiolate ) 418 438 414 455 υ(Ni-S thiolate ) 317 337 318 310 7 authors, several peaks appear in the 1050-995 cm−1 range, making it difficult to assign as C=S [42,43]. The results show the same tendency for different levels of calculations in describing the vibrational properties. Considering now the electronic properties of metal complexes we first start analyzing the HOMO and LUMO Kohn–Sham frontier orbitals of [CH3]2[Ni(dmit)2]. As can be seen in Fig. 3, the HOMO is of π-character and mainly corresponds to the C2S22− unit of dmit ligand and Ni(II) d orbital center. The LUMO is mainly contributed from the thiole ring of dmit ligand without metal participation. The same profile was observed by Fan et al. [45]. Fig. 3: Kohn–Sham frontier orbitals of isolated [CH 3 ] 2 [Ni(dmit) 2 ] HOMO (left); LUMO (right). Absorption electronic spectra were obtained from TD-DFT calculations performed at the optimized ground-state geometries (Fig. 4). Calculated oscillator strengths, transition energies, and wavefunction for the most relevant transitions of electronic absorption bands are listed in Table 3. The influence of the solvent environment on the absorption spectra was not considered in our calculations. Only transitions with significant oscillator strengths are presented. Fifty singlet electronic excited states were included. 8 Table 3: Excitation energy (E in eV), oscillator strength (ƒ) and main configurations of the wavefunction of [CH3]2[Ni(dmit)2] at TDDFT/B3LYP and TDDFT/CAM-B3LYP levels of theory (H=HOMO, L=LUMO). B3LYP CAM-B3LYP E ƒ Main configurations E f Main configurations 1.51 0.33 H→L 2.03 0.50 H→L/H-1→L+1 2.49 0.12 H-1→L+1 3.35 0.07 H→L/H-1→L+1 3.29 0.08 H-6→L 4.69 0.15 H-7→L 3.97 0.04 H→L+5/H→L+8 4.79 0.54 H-5→L+2 4.32 0.60 H-5→L+2/H→L+8/H→L+10 Fig 4.: Gas phase UV-vis absorption spectrum of [CH 3 ] 2 [Ni(dmit) 2 ] computed at the TDDFT/B3LYP and TDDFT/CAM-B3LYP levels of theory. As shown both in Table 3 and Fig. 4, the theoretical description of the UV-vis absorption spectrum of [CH3]2[Ni(dmit)2] complex is, quantitatively, quite different for the two DFT functionals tested. The first absorption appears at a wavelength of  800 nm when the B3LYP functional is used, However, the CAM-B3LYP functional offers a different picture, with a first, quite intense band appearing at  590 nm, in much 9 References 1. Nazeeruddin MK, Baranoff E, Grätzel M (2011) Dye-sensitized solar cells: A brief overview, Sol Energy 85:1172-1178 2. Hagfeldt A, Boschloo G, Sun L, Kloo L, Pettersson H (2010) Dye-Sensitized Solar Cells, Chem Rev 110:6595-6663 3. Robertson N (2006) Optimizing Dyes for Dye ‐ Sensitized Solar Cells, An gew Chem Int Edit 45:2338-2345 4. Lupan O, Guérin VM, Tiginyanu IM, Ursaki VV, Chow L, Heinrich H, Pauporté T (2010) Well-aligned arrays of vertically oriented ZnO nanowires electrodeposited on ITO-coated glass and their integration in dye sensitized solar cells, J Photoch Photobio A 211:65-73 5. 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