Tracking the Intramolecular Charge Transfer Process of 2,6-Substituted D-A BODIPY Derivatives

Three 2,6-electron donor-substituted boron dipyrromethene (BODI-PY) exhibiting an intramolecular charge transfer (ICT) character with large Stokes shift and moderate fluorescence quantum yields were designed and synthesized. Broadband femtosecond transient absorption (fs-TA) spectroscopy measurements were performed to directly detect the CT state in nonpolar or less polar solvents and the charge separation (CS) state in more polar solvents. A solid foundation for the fs-TA assignment can be found in electrolysis experiments. In addition, the ICT character of the newly designed compounds was investigated by density functional theory (DFT) calculations. Meanwhile, the reference compounds without the donor groups were synthesized, and their photophysical behaviors and ultrafast time-resolved spectra confirmed that no ICT process occurred regardless of the nature of the solvent. This work emphasizes the importance of decorating the BODIPY core with electron-donating substituents at 2,6-positions to efficiently adjust its photofunctional behaviors demonstrating the ICT character. Importantly, the photophysical processes could be easily regulated by changing the solvent with different polarities.

Automated summary

Three new boron dipyrromethene derivatives with electron donor substituents at 2,6-positions were synthesized and exhibited intramolecular charge transfer (ICT) character. Broadband femtosecond transient absorption spectroscopy was used to detect the CT and CS states in different solvents. Electrolysis experiments provided a solid foundation for the assignment of the fs-TA results. In addition, density functional theory calculations confirmed the ICT character of the compounds. Decorating the BODIPY core with electron-donating substituents is important for adjusting its photofunctional behaviors, and the solvent polarity can easily regulate the photophysical processes.