Molecular Insights into the Quenching Mechanism of the Triplet Excited State of Rose Bengal through Oxidative and Reductive Organic Compounds.

In oxygenated aquatic environments, the predominant scavenging of the triplet excited state of chromophoric dissolved organic matter (CDOM*) involves dissolved ground-state oxygen, diverting attention away from the scavenging mechanisms of CDOM* mediated through specific organic compounds. Previous studies demonstrated that model CDOM* exhibited quantum yields (i.e., 1-56%) in the formation of radical ions, resulting from the competition between physical and chemical quenching through a common exciplex intermediate. Physical quenching was rationalized through the reverse intersystem crossing of the exciplex, followed by back electron transfer, yielding ground-state reactants. Despite this, direct experimental evidence for exciplex involvement has been elusive, owing to detection challenges. Herein, employing density functional theory (DFT) and time-dependent DFT specifically for excited state surrogate CDOM and organic scavengers, we unveil, for the first time, the underlying mechanisms responsible for the quenching of Rose Bengal through oxidative and reductive scavengers. Our computational findings provide evidence for the involvement of exciplexes during the quenching process of the excited triplet state of Rose Bengal, highlighting the impact of electronic coupling between Rose Bengal and quenchers on the quantum yield for radical ion formation.