Excited state proton transfer in synthetic flavylium salts: 4-methyl-7-hydroxyflavylium and 4′,7-dihydroxyflavylium Example of a four-level molecular device to invert the population of the excited state

The excited state proton transfer (ESPT) observed in the compounds 4-methyl-7-hydroxyflavylium and 4′,7-dihydroxyflavylium is described, and the results are quantitatively interpreted on the basis of the Weller approach. ESPT is especially efficient in the case of 4-methyl-7-hydroxyflavylium. For pH > 2, proton recombination in the excited state does not occur and the yield of the adiabatic formation of excited base (A*) through the excited acid (AH^+*) is 0.93. This result compares with 0.45 for 4′,7-dihydroxyflavylium under identical conditions. The use of adiabatic proton transfer to obtain the inversion of the population of the excited state is discussed.