Quantum chemical study in the direction to design efficient donor-bridge-acceptor triphenylamine sensitizers with improved electron injection.

The ground state geometries have been computed by using density functional theory. The excitation energies for dye sensitizers were performed by using time dependant density functional theory. The polarizable continuum model (PCM) has been used for evaluating bulk solvent effects at all stages. The calculations have been carried out in methanol according to the experimental set up. The long-range-corrected functional (PCM-TD-LC-BLYP) underestimate the absorption spectrum of parent molecule while PCM-TDBHandHLYP is in good agreement with the experimental data. The highest occupied molecular orbital (HOMO) is delocalized on TPA moiety while lowest unoccupied molecular orbital (LUMO) is localized on anchoring group, conjugated chain and the benzene ring near to the anchoring group. The LUMO energies of all the investigated dyes are above the conduction band of TiO₂, HOMOs are below the redox couple and HOMO-LUMO energy gaps of studied dyes are smaller compared to TC4. The 1 and 3 are 7 and 12 nm blue shifted while 2 and 4 are 25 and 22 nm red shifted, respectively compared to TC4. The trend of electron injection (ΔG(inject)), relative electron injection (ΔG(r)(inject)), and electronic coupling constant (|VRP|) has been observed as 3 > 1 > 4 > 2 > TC4. The improved ΔG(inject), |VRP| and light harvesting efficiency (LHE) of new designed sensitizers revealed that these materials would be excellent sensitizers. The broken coplanarity between the benzene near anchoring group having LUMO and the last benzene attached to TPA unit in 1-4 consequently would hamper the recombination reaction.