Propping the optical and electronic properties of potential photo-sensitizers with different π-spacers: TD-DFT insights Basant A. Ali, Nageh K. Allam ⁎ Energy Materials Laboratory, School of Sciences and Engineering, The American University in Cairo, New Cairo 11835, Egypt abstract article info Article history: Received 26 May 2017 Received in revised form 4 July 2017 Accepted 11 July 2017 Available online 12 July 2017 We report density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations on the widely used N3 dye (cis-[Ru(2,2′-bipyridine-4,4′-dicarboxylic acid) 2 (NCS) 2 ] and its trans isomer with dif- ferent π-spacers. The study compared the sensitization properties of the two isomers in terms of their electronic properties such as light harvesting efficiency (LHE), absorbed wavelength (λ Max ) and molecular orbital distribu- tion. Also, charge transfer descriptors, such as the charge transfer distance (D CT ), dipole moment (μ CT ), and the amount of charge transferred (q CT ) were investigated. Upon replacing the two “2,2′-bipyridine-4,4′-dicarboxylic acid” ligands of the N3 dye with extended π-spacers of “1,4-benzene and 2,5-thiophene” for both the cis and trans isomers, the LHE of the trans isomer was increased by 70% compared to the cis counterpart. The complexes with thiophene spacers showed the highest LHE. The trans isomers showed wider absorbance range of wavelengths and equal wide distribution of charge density in the excited state along the organic ligands. These findings high- light the importance of using π-spacers between the organic ligands and the carboxylate groups to boost the LHE of DSSCs. Also, our study showed that the trans isomer is superior in its optical and electronic properties than the cis counterpart. However, the trans isomer is yet to be tested experimentally in DSSCs. © 2017 Elsevier B.V. All rights reserved. Keywords: DFT TD-DFT Charge transfer N3 dye DSSCs 1. Introduction Energy is the backbone of our daily life [1]. However, the currently used energy sources are based mainly on fossil fuel (coal, oil, and natural gas), which faces two main problems; limited reserves and the environ- mental damaging effects [1]. On the other hand, the Sun gives the Earth about 100,000 TW per m 2 per year, which is 10,000 times more than what we currently need [2]. Therefore, if we succeed to construct solar energy conversion devices with only 15% efficiency, only 250 km of the whole earth will need to be covered with those devices to satisfy the world energy needs [3]. In this regard, there is an ongoing interest to convert solar energy into electricity [3] using a variety of devices such as silicon solar cells [4], organic, and hybrid solar cells [5], among others [6]. However, the capital cost of such devices is still inconvenient for large-scale implementation. To this end, dye-sensitized solar cells (DSSCs) have attracted consid- erable attention due to their high quantum efficiency giving the oppor- tunity of low-cost conversion of solar energy into electrical power [7– 10]. In DSSCs, light is harvested by molecules bonded to a large surface, wide bandgap semiconductor (such as TiO 2 or ZnO) in contact with an electrolyte containing a redox couple as a charge mediator while the circuit is closed by a counter electrode, usually made of platinum [3, 11]. The ongoing research in DSSCs is mainly focusing on the optimiza- tion of all cell components; namely, the photoactive material, the dye and the redox electrolyte, with particular focus on the dye [12–17. Spe- cifically, dyes with porphyrin ring gain much attention due to their good sensitization properties [18–21]. The π-spacer between the ligand and the linkage group has also been investigated to improve the efficiency of the sensitization process [22,23], especially in Ruthenium-based complexes [24,25]. Those complexes have shown a broad range of ab- sorptivity [26], which can be tuned into the visible region by increasing the conjugation of the ligands used in the metal complex [27,28]. N3 and N719 Ruthenium complexes are the commonly used sensitizers with reported power conversion efficiencies exceeding 11% [29–35]. For example, the N3 dye has shown a broad absorption band (450– 750 nm) that is centered at ~538 nm [35–37]. However, the optimization of such sensitizers is mainly based on “guess-and-check” procedures, necessitate the search for a more sys- tematic approach to discover optimum sensitizers with the required specifications. In this regard, time-dependent density function theory (TDDFT) calculations are considered ideal to overcome the “guess- and-check” problem involved in the design of DSSCs systems [38–40]. Specifically, it can be used to study the changes in the optical and elec- tronic structures of the sensitizers, such as the metal-to-ligand charge transfer (MLCT) [41] and the injection of electrons to the oxide semi- conductor (CT excitation) [42]. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 188 (2018) 237–243 ⁎ Corresponding author. E-mail address: nageh.allam@aucegypt.edu (N.K. Allam). http://dx.doi.org/10.1016/j.saa.2017.07.009 1386-1425/© 2017 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa