Excited-State Proton Transfer to H₂O in Mixtures of CH₃CN-H₂O of a Superphotoacid, Chlorobenzoate Phenol Cyanine Picolinium (CBCyP)

Steady-state and time-resolved fluorescence techniques were employed to study a superphotoacid with a pK_a∗ of ∼-7, the chlorobenzoate phenol cyanine picolinium salt (CBCyP) in acetonitrile-water mixtures. We found that the time-resolved fluorescence is bimodal. The amplitude of the short-time component depends on χ_water; the larger χ_water, the greater the amplitude. We found that the excited-state proton-transfer (ESPT) rate constant, k_PT, is ≥5 × 10¹² s^-1 in mixtures of χ_water ≥ 0.08, whereas in neat water, k_PT = 6 × 10¹² s^-1. The long-time component has a lifetime of 50 ps at χ_water = 0.75. We attribute this time component to the CBCyP molecules that are not hydrogen-bonded to H₂O clusters. The results suggest that the ESPT rate constant to water in acetonitrile-water mixtures depends only slightly on the water cluster size and structure surrounding the CBCyP molecule. We attribute the independence of the ESPT rate on the average water-cluster size to the large photoacidity of CBCyP. QM TD-DFT calculations found that in the excited-state the RO^-(S₁) species that is formed by the ESPT process is more stable than the ROH(S₁) species by -5 kcal/mol when four water molecules accept the proton, and when six water molecules accept the proton, the RO^-(S₁) drops to -10 kcal/mol. The calculations show that energy stabilities are kept constant in implicit CH₃CN-H₂O solvent mixtures of dielectric constant of ϵ ≥ 45.