Ultrafast excited-state proton transfer from hydroxycoumarin-dipicolinium cyanine dyes

The photoprotolytic properties of a recently synthesized photoacid, hydroxycoumarin-dipicolinium salt (HCD) were studied by means of time-resolved emission techniques. We found that HCD is a super-photoacid with p K a * < − 2 and with an excited-state proton transfer to water with a rate constant of k PT = 1.4 × 10 11  s − 1 . We also found that HCD undergoes ESPT in a large number of protic solvents. The kinetic isotope effect is about two for water, methanol and ethanol.