Charge Transfer Initializes Photoexcited YOYO-1 Intramolecular Rotation and Fluorescent Quenching

Abstract

YOYO-1 is a commonly used cyanine dye for DNA staining that is fluorescently bright in DNA but very dim in water. The major assumption of its excited electron decay pathway is thermal relaxation via the rotation at a bridging methine that connects the two moieties of the molecule, i.e. photo-isomerization. In this report, we use femtosecond transient absorption spectroscopy to directly measure the excited electron decay, the hole refill, and the hot ground state rise and decay. The data suggest that the first step of the photo-isomerization involves a charge transfer to quench the holes and vibrational activation of the molecules to a hot ground state.YOYO-1, an oxazole yellow (YO) dimer ( . However, the initial step that causes the intramolecular rotation is unclear. We hypothesize that the first step of the methine rotation is enabled by accepting or donating an electron that breaks the big π bond holding the indole and quinolinium moieties in the plane. There has been evidence in the literature to support this hypothesis. Firstly, fluorescent quenching via intermolecular charge transfer to solvents with hydrogen bonds has been considered significant in water [10][11][12][13] . Charge transfer between water and photosynthetic systems has been commonly observed after excitation 14,15 . Experiments has shown that the N +