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

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)*similar to 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,rt* and n,rt* 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 67dH9MC- 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:NH3) 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. (C) 2019 Elsevier B.V. All rights reserved.