Computational design of 6-substituted phenalenone fluorophores: Impact of electron-donating groups.

Fluorophores and their properties can be modified by substituents based on the application area. Phenalenone is commonly used as a photosensitizer due to its high singlet oxygen quantum yield. Also, its derivatives are used in sensor implementations. This study focused on designing novel phenalenone-based fluorophores by introducing electron-donating substituents at the 6-position and evaluating the influence of a spacer group on their photophysical properties through quantum chemical calculations. DFT (density functional theory) and TD-DFT (time-dependent density functional theory) calculations were performed on 6-substituted phenalenone-based fluorophores to investigate their photophysical properties. Moderate rotational changes in some derivatives supported ICT (intramolecular charge transfer), while others exhibited smaller geometric changes but still showed donor-to-acceptor charge transfer. Introducing an amino (–NH₂) spacer induced pronounced geometric changes around the C–N single bond, consistent with the TICT (twisted intramolecular charge transfer) mechanism and indicative of potential fluorescence quenching pathways with large Stokes shifts. In contrast, derivatives with minimal rotational changes and no charge transfer character exhibited smaller Stokes shifts and a potential for fluorescence, demonstrating promise as phenalenone-based fluorophores suitable for sensor applications. The findings of this study have provided fundamental insights for future experimental studies into fluorophore design for optical sensing systems.