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.