The photoacidity of phenol chloro benzoate cyanine picolinium salt photoacid in alkanols

Steady-state and time-resolved fluorescence techniques were employed to study the excited-state proton transfer (ESPT) rate to methanol, ethanol and propanol of a new photoacid, the chloro benzoate phenol cyanine picolinium salt (CBCyP). We found that the ESPT rate constants for methanol, ethanol and propanol are about 3 × 10¹² s⁻¹, 2 × 10¹² s⁻¹ and 1.2 × 10¹² s⁻¹ respectively, whereas for water it is 6 × 10¹² s⁻¹. The photoacid pK_a^* in water is about pK_a ∼ −7. The kinetic isotope effect as measured from the fluorescence decay rate of the protonated form is 1.5 and 1.25 for methanol and ethanol respectively, whereas in water it is 1.7. We suggest that a nonradiative process takes place and reduces the measured kinetic isotope effect in both methanol and ethanol.