Molecular design and photophysical tuning of iminocoumarin-based D–π–A Dyes: Structure–Property relationships and DSSC performance

The development of metal-free organic dyes with tunable optoelectronic properties is crucial for advancing dye-based technologies. In this work, a series of coumarin–benzothiazole and iminocoumarin–benzothiazole derivatives were designed as D–π–A systems to investigate structure–property relationships governing light absorption and charge transfer. Photophysical and electrochemical analyses revealed that substitution at the iminocoumarin nitrogen significantly modulates the HOMO–LUMO energy levels and the intramolecular charge-transfer character. UV–Vis and diffuse reflectance measurements showed that electron-donating substituents enhance both absorption intensity and spectral broadening in the visible region. TD-DFT calculations support these results, indicating a dominant donor-to-acceptor charge-transfer mechanism and improved electron–hole separation in substituted systems. Despite the limited absorption beyond 500 nm, dye-sensitized solar cells (DSSC) devices fabricated with these dyes demonstrate that molecular modifications directly influence photocurrent generation and interfacial charge-transfer processes. Among the investigated compounds, ICO–BTe exhibited the best experimental photovoltaic performance, reaching Jsc ≈1.05 mA cm −2 , Voc ≈0.46 V, and a power conversion efficiency of 1.34%, consistent with its improved light-harvesting capability and favorable charge-transfer characteristics. These findings provide molecular-level insights into how structural modifications in iminocoumarin-based dyes affect photophysical behavior and device performance, offering useful guidelines for the rational design of efficient metal-free photosensitizers.