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 (S1), the obtained ground and excited state acidity constants (pKa and pKa*) of 7H6MetC (pKa = 7.4 and pKa*~ 0.95) are different to the 67dH4MC with pKa1,2 (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 (A2−*) 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 T1 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:NH3) the anionic triplet (A*T1) exists and is identical for both compounds in agreement with the similar pKa1,2 values for the S1 state of the di-hydroxy 67dH4MC. Kinetic schemes coupling the ground, singlet and triplet states are proposed.