Computational evaluation of triphenylimidazole-coumarin-3-carboxylic acid derivatives as potential sensitizers for dye sensitized solar cells

Abstract

Triphenylimidazole derivatives linked with coumarin-carboxylic acid were evaluated as sensitizers for DSSC applications through density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. Coumarin-3-carboxylic acid as acceptor moiety was linked with the triphenylimidazole derivatives at C2 position on the imidazole ring. Phenyl groups on the 4 and 5 positions of the imidazole core were substituted with Cl, OCH 3 , & N(CH 3 ) 2 at the para positions. Further, the 1-H position of the core imidazole ring was substituted with pyrimidine ring. The resulting molecules are donor–acceptor dyads. The DFT calculations revealed that the geometry of the four dyads are very similar. The lowest unoccupied molecular orbital (LUMO) of the dyes were localized in the coumarin-carboxylic acid moiety. For dyes bearing Cl & H, the highest occupied molecular orbital (HOMO) is delocalised over the whole molecule. While the HOMO is localised on the 1,4,5-triphenyl-1H-imidazole fragments for the OCH 3 , & N(CH 3 ) 2 dyes. The LUMO energy levels of the dyads are energetically favourable for electron injection into TiO 2 . The dyes bearing OCH 3 and N(CH 3 ) 2 groups gave HOMO–LUMO gap 2.73 and 2.32 eV respectively. Regeneration of oxidised dyads by I − /I 3 − redox mediator was also energetically favourable except the dyad with N(CH 3 ) 2 group. The HOMO–LUMO gaps of the dyads decreases with electron releasing strength of the substituents. The decrease in the HOMO–LUMO gap is accompanied by decrease in the oscillator strength of the HOMO–LUMO transition. The HOMO of the dyad containing N(CH 3 ) 2 was raised to a level that is detrimental to regeneration of the oxidised dyad. Thus, triphenylimidazole is strong donor moiety and its use in DSSC dye design do not require further substitutions with strong electron releasing groups.