Structural Origin of Intermolecular Photoexcited-State Dynamics in Flavonoids

Abstract

Flavonoids are a widespread class of natural polyphenols with diverse photophysical and photochemical properties. Understanding how molecular structure governs their photoexcited-state dynamics is essential for elucidating photoinduced energy dissipation. Here, we employ femtosecond transient absorption (TA) spectroscopy to investigate the concentration-dependent photoexcited-state dynamics of flavonoids spanning flavanone glycosides (FGs), flavanones, flavones, and polymethoxyflavones (PMFs). Although the steady-state UV–vis absorption spectra changed little with concentration, TA measurements revealed pronounced differences in the concentration-dependent photoexcited-state dynamics among the flavonoid families. With increasing concentration, FGs progressively developed a new long-lived photoinduced absorption (PIA) band centered at approximately 433 nm. Concurrently, the contribution of the long-lived photoexcited population increased from approximately 17% to 86%, whereas the fast and long-lived decay lifetimes remained nearly unchanged at approximately 0.3 and 60 ps, respectively. In contrast, PMFs displayed a characteristic delayed buildup followed by decay that remained essentially unchanged over the investigated concentration range. Systematic comparison reveals that the degree of π-conjugation associated with the C2–C3 linkage, rather than glycosylation, governs the concentration-dependent photoexcited-state dynamics. These findings establish a clear structure-dynamics relationship in flavonoids and provide a molecular framework for understanding and engineering their intermolecular photoexcited-state behavior.