TD-DFT study of charge transfer excitation in D-π-A metal complex dyes as sensitizers in DSSCs

The intra-molecular charge transfer electronic transition in a series of D-π-A metal complexes dyes which could be used as sensitizers in DSSCs have been studied utilizing the TD-DFT method. The structure of these dyes are shown in figure 1, where different π-spacers (S) namely; benzene, furan, pyrrole and thiophene have been assigned. The transition metals studied were Co(III), Ni(II), Cu(I) and Zn(II) and the acceptor part in the dye was taken as 2cyanoacrylic acid. The geometrical and electronic spectral properties of these dyes were calculated at the PBE/631G level of theory in order to understand the mechanism of the intra-molecular charge transfer electronic transition taking place in these systems. It is found that the amount of charge transferred on going from the ground state to the excited state (q) depend on the type of the metal as well as the π-linker used. For all the π-spacers (S) the amount of q was found to increase in the order Co(III) < Ni(II) < Cu(I) < Zn(II) metal complex dyes. For Co(III) and Cu(I) complexes the thiophene π-spacer was found to be the best in enhancing the CT process while the largest calculated value of q for Ni(II) and Zn(II) complexes was found with the pyrrole moiety as the π-spacer. Calculated descriptors of the charge transfer transition for all the studied dyes have been reported and discussed.