Elucidating the excited-state dynamics behavior in near-infrared Bodipy dye and aggregates toward biophotonics

Photo-induced excited-state dynamics within organic materials fundamentally determine their photophysical properties for various applications, and thus understanding the primary excited-state dynamics behavior is of fundamental and practical significance. Until recently, the excited-state dynamics of organic materials towards biophotonics have been rarely studied, although numerous endeavors have been devoted to the design of organic materials for biophotonics. Herein, various spectroscopy technologies including femtosecond transient absorption (fs-TA) spectroscopy clearly reveal a totally different excited state dynamics behavior within Bodipy monomer (2B-BDP dye) and aggregates (2B-BDP NPs), indicating strongly morphology dependent character. 2B-BDP dye undergoes an ultrafast conversion from S₁ to intramolecular charge transfer (ICT) state for subsequent photoluminescence (PL) and nonradiative (NR) decay, while 2B-BDP NPs show an accelerated excited-state deactivation mainly through S₁→S₀ NR decay. The potential bioapplications based on the corresponding excited state dynamics behavior are discussed together with experimental results. Interestingly, the accelerated NR decay in 2B-BDP NPs does not yield a stronger photoacoustic (PA) signal than that in 2B-BDP dye, which violates the conventional wisdom that faster NR decay would benefit the photothermal effects for better photoacoustic imaging (PAI). These insightful and fundamental observations of the excited-state dynamics may contribute an alternative approach at the molecular level towards the future design of functional Bodipy-based organic molecules with desirable performances.