Chemical Photocatalysis with Rhodamine 6G: Investigation of Photoreduction by Simultaneous Fluorescence Correlation Spectroscopy and Fluorescence Lifetime Measurements.

Recent research has demonstrated that consecutive excitation of the radical anion state of commercially available dye molecules-generated by a photoinduced electron-transfer process-yields sufficient energy to stimulate challenging chemical reactions in photocatalysis. For this reason, an efficient transfer of dye molecules into their radical anion states upon photoexcitation is highly desirable, as is a long radical lifetime. However, the formation of these reactive states is strongly dependent on the redox agent, the local environment, for example, the solvents and additives, as well as on the properties of the excited states of the dye molecule. Finding the best conditions for radical formation is crucial, but, owing to the complexity of the underlying photochemical process, this is usually only achieved by an iterative exploratory approach. Here, we demonstrate that the formation and lifetime of the rhodamine 6G (Rh6G) radical anion can be followed in detail by single-molecule fluorescence correlation spectroscopy combined with simultaneous fluorescence lifetime measurements. We elucidate the role of the first excited singlet and triplet states of the dye molecule in the formation of the radical at different concentrations of the reducing agent, ascorbic acid (AscA); for different solvents, water and dimethyl sulfoxide; and for different reducing agents, AscA and N, N-diisopropylethylamine; as well as for varying pH values. The results provide a guideline toward generating an increased yield of radical anions of the dye under photoexcitation. As an example, we find that the lifetime of the radical anion state of Rh6G can be increased by over an order of magnitude from 7 to 110 μs in an aerated solution.