Influence of conformation on the absorption spectra of flexible organic dyes used in dye-sensitized solar cells

We performed density functional theory (DFT) and time dependent-DFT (TD-DFT) calculations on indoline dye D149 to elucidate the influence and/or effect of conformation of flexible organic dyes on the light absorption spectra. Calculating the conformational energy for D149 showed that 16 kinds of conformations are possible in acetonitrile solvent. The TD-DFT calculations revealed that the 16 kinds of conformations result in two types of absorption spectra. The first type shows about 1.3 times higher oscillator strength than the second type in a long-wavelength region around 530 nm, whereas the second type shows about 1.7 times higher oscillator strength than the first type in a short-wavelength region around 390 nm. Which of the two types of absorption spectra occurs depends on whether the torsion angle between the indoline and rhodanine parts is 180° or 0°. Trying to control the torsion angle by computationally designing derivatives of D149, we demonstrated that introducing methyl, chloride, and hydroxyl groups in the indoline part enables us to fix the torsion angle on either 0° side or 180° side and thereby selectively obtain a dye with one of the two types of absorption spectra.