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.

Photoacids are a class of organic molecules with weak Brønsted acidity in their ground electronic-state with pK_a in the range of 5–10. In their first singlet electronic state their pK_a* in water is smaller by 6–13 pK_a units. Their pK_a* values range from 3.4 to −8 in water. Photoacids with high photoacidities (pK_a*< −2) are termed superphotoacids.

The time constant of the excited-state proton transfer (ESPT) to water of weak photoacids with pK_a* ∼ 2.7 (like 2-naphthol) is τ_PT = 10⁸ s^–1, and their ESPT efficiency is low since the excited-state lifetime is less than 10^–8 s. During the last 30 years, this field of photoacidity has been studied (1−18) by time-resolved techniques and has provided pertinent information on the properties of photoacids.

For the superphotoacid, the ESPT rate constant k_PT > 10¹¹ s^–1.

For photoacids with pK_a* of about −8, we have found (19) for QCy9, shown in Scheme 1a, that the k_PT to water can reach ∼10¹³ s^–1.

Tagged 2018