Ultrafast Studies of Different Oxidation and Protonation States of Rhodamine 6G and Implications for Photocatalysis

The long-lived radical R6G•, derived from the cationic dye rhodamine 6G (R6G+) by reduction, is of growing interest in photoredox catalysis. This manuscript discusses three methods of its preparation in dimethylsulfoxide, highlighting spectral differences due to solvatochromism, co-solutes, and basicity of the solution. Upon excitation, R6G•* can release an electron to a substrate molecule or as a solvated electron, leading back to R6G+. However, a second reduction of R6G• is irreversible, decreasing the overall concentration of R6G• and R6G+ with time. R6G+ can also be deprotonated to R6G1 under basic conditions, and even double deprotonation to R6G2– is possible, though this may undergo irreversible reaction over time. Excitation of R6G1 leads to the formation of a photoproduct stable for seconds which then reforms R6G1. If R6G• is exposed to basic conditions in the presence of oxygen, it is oxidized to R6G+, which is then quickly deprotonated to yield R6G1 again. Hence, in basic solution R6G1 is the predominant species, so that other light-induced reaction pathways than with R6G+ are accessible. It remains to be determined whether the photoproduct of R6G1 could be beneficial for a photocatalytic application under strongly basic conditions.