Systematic molecular engineering of π-spacer in Indoline-based dyes with D-A'-π-A framework to enhance the intramolecular charge transfer and photovoltaic properties in DSSC and NLO applications: DFT insight

This study designed 15 D-A´-π-A indoline-based dyes using four strategies: modifying the π-spacer form, introducing heteroatoms, extending π-conjugation, and altering π-spacer order relative to the WS-2 dye. Utilizing DFT and TD-DFT, we explored the geometrical, electronic, and optical properties of the dyes and their interactions with TiO₂. Results showed a reduced energy gap, decreasing from 1.911 eV to a range of 1.295 eV–1.622 eV, and a redshifted absorption from 547.30 nm to 602.24 nm–739.31 nm. The introduction of a thiophene (Th) unit improved absorption, intramolecular charge transfer (ICT), and charge mobility. The D-A'-Th-π-A configuration enhanced energy gaps and non-linear optical (NLO) properties, while D-A´-π-Th-A had limitations. Notably, IND14 and IND05 exhibited promising energy gaps and NLO properties, suggesting their potential for increasing short-circuit photocurrent density, offering insights for optimizing D-A´-π-A dyes for efficient dye-sensitized solar cells (DSSCs).