Kinetic detection of the geminate ion pair in the super Photoacid 3-Cyano-7-Hydroxycoumarin: excited-state characterization in aqueous and mixed solvents

Hydroxycoumarins are aromatic hydroxyl compounds of natural or synthetic origin characterized by multi-state excited state equilibria. 3-cyano-7-hydroxycoumarin (3CN7HC) was here the focus of a comprehensive study of its spectral and photophysical properties in both neutral and anionic forms, together with its model compound, 3-cyano-7-methoxycoumarin (3CN7OMeC). The study was carried out in different solvents at room temperature (293 K) and at low temperature (77 K) in ethanol (neutral, acidic and alkaline), from where all the deactivation rate constants of the singlet (k_F, k_IC and k_ISC) and triplet (k_Ph, and k_ISC) states could be obtained. In contrast to other coumarins, the two 3-cyanocoumarins exhibited high fluorescence quantum yields solvents (φ_F = 0.85), with the S₁ ∼ ∼∼ > T₁ intersystem crossing found to be a negligible deactivation route. The prototropic reactions of 3CN7HC were further investigated in dioxane:water (Dx:H₂O) mixtures with molar water fractions (x_w) ranging from 0 to 1. In aqueous solution (x_w = 1), 3CN7HC undergoes a reversible intermolecular proton transfer to water with pK_a = 6.2 and pK_a* ≈ 0.5 in the ground and excited states, respectively, making it a super photoacid. The emission spectra showed two bands assigned to the neutral (N*) and anionic (A*) forms of 3CN7HC. While the decays in non-protic solvents (or at alkaline pH values) were mono-exponential, those in Dx:H₂O mixtures with x_w > 0.5 were triple-exponential, indicating the presence of an additional species. The presence of the tautomeric form (T*) is excluded and the additional species is clearly assigned to the geminate ion pair (A*^…H⁺).