Characterization of 4-methylesculetin and of its mono- and di-methoxylated derivatives in water and organic solvents in its ground, singlet and triplet excited states

Abstract A comprehensive investigation of the multiequilibria of 6,7‑dihydroxycoumarin (4‑methylesculetin, 67dH4MC ) and of the monomethoxylated, 7‑hydroxy‑6‑methoxycoumarin or Scopoletin ( 7H6MetC ), and the dimethoxylated, 6,7‑dimethoxy‑4‑methylcoumarin ( 67dMet4MC ) in the singlet and triplet states has been undertaken in organic solvents and in aqueous solution. In the singlet state (S 1 ), the obtained ground and excited state acidity constants (pK a and pK a *) of 7H6MetC (pK a  = 7.4 and pK a *~ 0.95) are different to the 67dH4MC with pK a1,2 (and pK* a1,2 ) approximately identical in the ground and excited state: 7.1 (7.7) and 12.2 (12.1). The fluorescence quantum yields and decay times were obtained in dioxane: water mixtures and found, in the case of 67dMet4MC , to increase with the solvent polarity thus showing the presence of nearby n,π* and π,π* states. In the case of the hydroxylated 67dH4MC and 7H6MetC multiequilibria involving the excited neutral (N*), anionic (A*), dianionic (A 2− *) and tautomeric (T*) - in the case of 67dH4MC - forms were observed. The phosphorescence emission (and lifetimes) obtained in acidic, neutral and alkaline media, allowed to obtain the spectral features of the different T 1 species present. The data indicate that for 67dMet4MC only a neutral emissive triplet exists (N* T1 ) whereas in the case of 7H6MetC and 67dH4MC in an alkaline matrix (ether:ethanol:NH 3 ) the anionic triplet (A* T1 ) exists and is identical for both compounds in agreement with the similar pK a1,2 values for the S 1 state of the di-hydroxy 67dH4MC . Kinetic schemes coupling the ground, singlet and triplet states are proposed.