Quantum chemical study of the donor-bridge-acceptor triphenylamine based sensitizers

• HOMO is delocalized on TPA moieties while LUMO is localized on anchoring group.
• Electron transfer from the dye to the TiO2 can occur by the HOMO–LUMO transition.
• Light harvesting efficiency would be improved by elongating the bridge.
• Enhanced bridge is encouraging to promote the electron injection and electronic coupling constant.
• The distortion between LUMO and TPA unit would hinder the recombination process.

Quantum chemical calculations were carried to investigate the electron coupling, electron injection, electronic and photophysical properties of 2-cyano-5-(4-(phenyl(4-vinylphenyl)amino)phenyl) penta-2,4-dienoic acid (TC4) and its derivatives. Geometries have been optimized by using density functional theory at B3LYP/6-31G** level of theory. The highest occupied molecular orbitals (HOMOs) are delocalized on triphenylamine (TPA) units while lowest unoccupied molecular orbitals LUMOs are localized on anchoring groups. The mono-methyl is more significant to lowering the energy gap than di and tri-methyl substituted ones. The HOMOs of the dyes are below the redox couple and LUMOs are above the conduction band of TiO2. We have explained the recombination barrier on the basis of distortion and coplanarity. The excitation energies have been computed by time dependent density functional theory at PCM-CAM-B3LYP/6-31G** level of theory. Enhanced bridge is encouraging to promote the electron injection, electronic coupling constant and light harvesting efficiency. Generally, electron injection, electronic coupling constant and light harvesting efficiency of new designed sensitizers are higher than TC4. This revealed that new materials would be efficient photosensitizers.

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