Propping the optical and electronic properties of potential photo-sensitizers with different π-spacers: TD-DFT insights.

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 different 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 (λMax) and molecular orbital distribution. Also, charge transfer descriptors, such as the charge transfer distance (DCT), dipole moment (μCT), and the amount of charge transferred (qCT) 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 highlight 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.