The role of terminal groups in electronic structures and related properties: The case of push–pull porphyrin dye sensitizers for solar cells

In the design of novel dye sensitizers with good performance in dye sensitized solar cells, it is significantly important to understand how to tune the electronic structures and related properties of dye sensitizers by their terminal groups. Based upon the DFT and TDDFT calculated results of push–pull porphyrin sensitizers, named as YD20, YD21, YD22, 2Flu–ZnP–COOH, and 2Flu–ZnP–CN–COOH, the role of terminal groups in the modification of their electronic structures and related properties are investigated. It is found that the molecular orientation of adsorption and the driving force for electron injection can be changed by the substitution of electron-pull moiety. The data of electronic structures support that the pushing-electron capability of triphenylamino with two methoxyl substitutes is similar to that of the phenylamino group, but it is stronger than that of bis(3,3-dimethylfluorenyl) amine group. The pulling-electron capability of cyanoacrylic acid is stronger than that of carboxyl acid group. The analysis of molecular orbitals and transition configurations indicates that the triphenylamino with two methoxyl substitutes, phenylamino moieties, and bis(3,3-dimethylfluorenyl) amine groups are major chromophores for effective charge transfer excitation.