IR, Raman and UV-Visible Spectra of the Ru(II) Cyano Complexes Studied by DFT B. Minaev, V. Minaeva, G. Baryshnikov * , M. Girtu ** and H. Agren *** * Bogdan Khmelnitskii National University, Cherkassy, Ukraine, bfmin@rambler.ru ** Ovidius University, Constanca, Romania, girtu@univ-ovidius.ro *** The Royal Institute of Tehnology, Stockholm, Sweden, agren@theochem.kth.se ABSTRACT Harvesting energy directly from sunlight using photovoltaic technology is an essential component of the future global energy production programs [1]. Photovoltaic devices, or solar cells, are based on the photoelectric effect, in which the incoming solar photons are absorbed in a sensitized semiconductor material freeing electric charges that are used to energize an external circuit [2]. The solar cells on the bases of nanocrystalline TiO 2 are usually sensitized by metal-organic dyes such as Ru- complexes with phenylpyridine and bipyridine ligands. The incident photon to current conversion efficiency (IPCCE) in the wavelength region 400-600 nm can be rather high [1, 2]. In order to improve the IPCCE value the heavier metal ions can be tested. Sensitizers like coumarin, porphyrins, chlorophyll derivatives, antenna-sentsitizer polynuclear complexes and eosin have been studied and reported to be less efficient than the most effective Ru- based sensitizing dyes, called the black dye (BD) and N3 dye [2–4], used for nanocrystalline TiO 2 solar cells. In this paper we aim to understand the IPCE of a new dye- sensitizer in order to predict a higher absorption efficiency the solar spectrum and higher electron transfer rate from the redox systems to the oxidized dye. A simple Ru(II)(bpy) 2 (CN) 2 complex is studied by DFT method with optimized structure and vibrational analysis in order to predict the role of vibronic perturbations in the spectra and in the interface electron transfer rate. Intermolecular interactions with solvent and nanocrystalline surface are also discussed in terms of vibronic mixing of states. Keywords: solar cell, density functional, Ru-complexes, IR and Raman spectra. 1. METHOD OF CALCULATION The density functional method (DFT) with the hybrid B3LYP functional and the Lanl2DZ basis set [5–9] is used for geometry optimization of the Ru(II)(bpy) 2 (CN) 2 complex (Fig. 1) and its vibrational spectra (Table 1) calculations. The Hessian matrix in the ground singlet state (S 0 ) and in the first excited triplet (T) state are used also for Frank-Condon factor estimations in the absorption and emission spectra [10–13]. Infrared (IR) and Raman band intensities are calculated by the derivatives of the dipole moment and of the polarizability, respectively. Mulliken atomic charges in the ground S 0 state indicate that the Ru(II) ion is more close to the Ru(I) ion in the complex (0.726). Thus the polarization in the ligands can be analyzed from the DFT calculations. We use a scaling factor of 0.9756 for fitting the calculated vibrational frequencies to the available experimental data on the 2,2′- bipyridine and bpy complexes with metals. 2. RESULTS AND DISCUSSION Numeration of atoms of the optimized Ru(II)(bpy) 2 (CN) 2 complex structure is presented in Fig. 1. The Ru–N bonds, which are opposite to the CN – ligands, have longer length (2.124 Å) than the Ru–N 3 (Ru–N 16 ) bonds (2.076 Å) in agreement with x-ray data (2.05 and 2.11 Å, respectively) [14]. The calculated C–C links in the ground S 0 state (1.474 Å) agree well with the x-ray data (1.481 Å); they are shortened to 1.425 Å in the T state. The bpy ligand bears the whole positive charge (0.118), while the CN – ligand bears the whole negative charge (-0.481), the complex being electroneutral. Such polarization is quite convenient for interaction with TiO 2 surface. Figure 1. Optimized structure of [Ru(bpy) 2 (CN) 2 ] complex. Mode Type of vibration ω І IR І Ram 113 ν(CN), in phase 2066 61.7 257.7 112 ν(CN), out-of-phase 2059 46.6 221.0 111 ν(C=С), ν(C–C) 1603 3.7 145.9 110 ν(C=С), ν(C–C) 1603 2.7 75.0 109 ν(C=C), ν(C–C) 1598 0.01 462.9 108 ν(C=C), ν(C–C) 1598 1.2 224.1