Structure and electronic absorption spectra of isotruxene dyes for dye-sensitized solar cells: Investigation by the DFT, TDDFT, and QTAIM methods

The electronic structure of two photosensitizing isotruxene dyes used in Gratzel photoelectro-chemical cells has been investigated by the quantum-chemical methods of the density functional theory (DFT) and Bader's topological analysis of the electron density. It has been shown that intramolecular interactions in the dye molecules stabilize the planar relative arrangement of isotruxene and cyano-acrylic structural fragments, which provides pi-conjugation over the whole molecular backbone. Based on the time-dependent density functional theory (TDDFT) calculations and analysis of the Kohn-Sham orbitals, it has been found that the first vertical electronic transition in the spectra of the isotruxene dyes is a combination of a poorly resolved l pi*-type excitation and a well-resolved pi-pi* excitation. The pi-pi* excitation admixture is associated with the asymmetric structure of the isotruxene fragment and contributes significantly to the intensity of the first electronic transition.