Ultrafast Spectroscopic Investigation of Intermolecular Excited-State Proton Transfer of Fisetin at an Abasic Site

Fluorescent probes based on excited-state proton transfer (ESPT) have attracted considerable attention because of their large Stokes shifts and high sensitivity to the environment. However, their excited-state behavior in confined nucleic acid microenvironments remains unclear. Here, we introduced fisetin, an ESPT-active flavonoid fluorophore, into a DNA duplex containing an abasic site opposite thymine (AP-T DNA) and investigated its excited-state dynamics by ultrafast time-resolved spectroscopy. Femtosecond time-resolved transient absorption measurements reveal that the proton-transfer tautomer lifetime of fisetin increases from 0.69 ns in a homogeneous solution to 2.0 ns in AP-T DNA. Meanwhile, there is an additional emissive component with a lifetime of 7.2 ns. Time-resolved mid-infrared spectroscopy combined with molecular docking calculations further reveals that the long-lived component should arise from a distinct intermolecular excited-state proton transfer (inter-ESPT) species between fisetin and thymine base opposite to the abasic site. Our discoveries not only clarify the excited-state behavior of fisetin in the AP-T DNA but also reveal a distinct inter-ESPT pathway. These findings provide valuable insights into the ESPT mechanism in nucleic acid microenvironments and for further development of ESPT-active fluorescent probes.