Ultrafast Photoinduced Dynamics of Styryl-Substituted BODIPY Dyes and Their Aggregates

• Published in: Journal of physical chemistry. C 2024-08, Vol.128 (30), p.12762-12774
• Main Authors: Ghosh, Srijon, Mula, Soumyaditya, Biswas, Protap, Patra, Amitava
• Format: Article
• Language: English
• Subjects: C: Physical Properties of Materials and Interfaces
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.4c04236

We synthesized a series of boron dipyrromethene (BODIPY) dyes with varying p-methoxystyryl substituents to study their excited-state dynamics in both molecular and aggregated forms. Single-crystal X-ray diffraction revealed herringbone packing through π···π stacking in the unsubstituted dye, which was disrupted upon styryl substitution. Unsubstituted BODIPY formed aqueous H-aggregates with considerable emission, whereas styryl-substituted variants exhibited unprecedented and unusual nonemissive, red-shifted H-aggregates. The HOMO–LUMO gap decreased with an increase in the number of styryl substituents, aligning with quantum chemical calculations. Femtosecond transient absorption spectroscopy and global analysis suggest slower solvation, vibrational cooling, and excited-state structural relaxation with increased substitution. The unsubstituted BODIPY H-aggregates exhibited a distinct emissive state within the lower excitonic state manifold and ultrafast nonradiative relaxation from the dark state. On the other hand, styryl-substituted aggregates demonstrated vibration-assisted relaxation to the dark, singlet, lower excitonic state, which was long-lived. Our study highlights how structural modifications significantly influence the packing structure and photophysical properties of BODIPY dyes and aggregates, which are critical for biophotonics and optoelectronics applications.