Excited-State Proton-Transfer Processes of DHICA Resolved: From Sub-Picoseconds to Nanoseconds.
Excited-state proton transfer has been hypothesized as a mechanism for UV energy dissipation in eumelanin skin pigments. By using time-resolved fluorescence spectroscopy, we show that the previously proposed, but unresolved, excited-state intramolecular proton transfer (ESIPT) of the eumelanin building block 5,6-dihydroxyindole-2-carboxylic acid (DHICA) occurs with a time constant of 300 fs in aqueous solution but completely stops in methanol. The previously disputed excited-state proton transfer involving the 5- or 6-OH groups of the DHICA anion is now found to occur from the 6-OH group to aqueous solvent with a rate constant of 4.0 × 10(8) s(-1).
†Department of Chemical Physics, Lund University, Box 124, 22100 Lund, Sweden.
Alice
A
Corani
‡Department of Chemical Sciences, University of Naples, Federico II Via Cintia, 80126 Naples, Italy.
Alessandro
A
Pezzella
†Department of Chemical Physics, Lund University, Box 124, 22100 Lund, Sweden.
Torbjörn
T
Pascher
§Laboratoire Francis Perrin, CEA/DSM/IRAMIS/SPAM-CNRS, URA 2453, CEA/Saclay, F-91191 Gif-sur-Yvette, France.
Thomas
T
Gustavsson
§Laboratoire Francis Perrin, CEA/DSM/IRAMIS/SPAM-CNRS, URA 2453, CEA/Saclay, F-91191 Gif-sur-Yvette, France.
Dimitra
D
Markovitsi
†Department of Chemical Physics, Lund University, Box 124, 22100 Lund, Sweden.
Annemarie
A
Huijser
‡Department of Chemical Sciences, University of Naples, Federico II Via Cintia, 80126 Naples, Italy.
Marco
M
d'Ischia
†Department of Chemical Physics, Lund University, Box 124, 22100 Lund, Sweden.
Villy
V
Sundström
DHICA; ESPT; eumelanin; solvent; time-resolved fluorescence