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 pKa*<−2 and with an excited-state proton transfer to water with a rate constant of kPT=1.4×1011 s−1kPT=1.4×1011 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.

► We show a process of ESPT from HCD to different solvents upon photo-excitation.
► In water, the ESPT rate is high and the rate constant is ∼1.4 × 1011 s−1.
► We show ESPT processes in linear alcohols at a high rate, exceeding 3 × 1010 s−1.
► The kinetic isotope effect (KIE) is significant in water, methanol and ethanol.