Molecular design of new hydrazone dyes for dye-sensitized solar cells: Synthesis, characterization and DFT study

Three new sensitizers 2-{4-[2-(4-Nitrobenzylidene)hydrazino)]phenyl}ethylene-1,1,2-tricarbonitrile (NBHPET), 2-{4-[2-p-Chlorobenzylidenehydrazino]phenyl}- ethylene-1,1,2-tri carbonitrile (CBHPET) and 2-{4-[2-p-Bromobenzylidenehydrazino] phenyl}ethylene-1,1,2-tricarbonitrile (BBHPET) have been synthesized. The dyes showed pronounced solvatochromic effects as the polarity of the solvents increased. The structures have been optimized at B3LYP/6-31G(d) level of theory. The torsion in E-isomer is smaller than Z-isomer and azo isomers. The highest occupied molecular orbitals are delocalized on whole molecule while lowest unoccupied molecular orbitals are distributed on the tricarbonitrile. The lowest unoccupied molecular orbital energies are above the conduction band of titanium dioxide, highest occupied molecular orbitals of the dyes are below the redox couple of new synthesized dyes and small energy gap revealed these dyes would be better sensitizers for dye-sensitized solar cells.

► By increasing the solvent polarity, bathochromic shift in absorption spectra has been observed. ► The HOMO is localized on whole molecule while LUMO is distributed on the tricarbonitrile. ► The LUMO energies of NBHPET, CBHPET, and BBHPET are above the conduction band of TiO2. ► The HOMOs of the dyes are below the redox couple. ► Smaller HOMO–LUMO energy gaps revealed that these dyes would be efficient for DSSC.