Excited-State Proton Transfer to HO in Mixtures of CHCN-HO of a Superphotoacid, Chlorobenzoate Phenol Cyanine Picolinium (CBCyP).

Steady-state and time-resolved fluorescence techniques were employed to study a superphotoacid with a p K* 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; the larger χ, the greater the amplitude. We found that the excited-state proton-transfer (ESPT) rate constant, k, is ≥5 × 10 s in mixtures of χ ≥ 0.08, whereas in neat water, k = 6 × 10 s. The long-time component has a lifetime of 50 ps at χ = 0.75. We attribute this time component to the CBCyP molecules that are not hydrogen-bonded to HO 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(S) species that is formed by the ESPT process is more stable than the ROH(S) species by -5 kcal/mol when four water molecules accept the proton, and when six water molecules accept the proton, the RO(S) drops to -10 kcal/mol. The calculations show that energy stabilities are kept constant in implicit CHCN-HO solvent mixtures of dielectric constant of ε ≥ 45.