Moderately Strong Photoacid Dissociates in Alcohols with High Transient Concentration of the Proton-Transfer Contact Pair.
Proton transfer from strong photoacids to hydroxylic solvents is much under debate. Experimentally, the main issue stems from relaxation and diffusion processes that are concomitant with ultrafast proton transfer and blur population dynamics. To overcome this, we propose a fast photodissociation reaction that, however, proceeds slower than solvent relaxation. Fluorescence spectroscopy of the cationic photoacid 2-(1'-hydroxy-2'-naphtyl)benzimidazolium reveals a two-stage mechanism: (a) reversible elementary proton transfer inside the solvent shell and (b) irreversible contact-pair splitting. The time evolution of the fluorescence signal is complex, yet this is explained quantitatively by simultaneous, spectrally overlapping emission of the acid, the conjugate base, and the contact proton-transfer pair. The latter attains high transient concentration in linear alcohols. Microscopic rate constants of dissociation are determined.
Center for Research in Biological Chemistry and Molecular Materials (CIQUS) and Department of Physical Chemistry, University of Santiago de Compostela, C/ Jenaro de la Fuente s/n, E-15782 Santiago de Compostela, Spain.
Alfonso
A
Brenlla
Center for Research in Biological Chemistry and Molecular Materials (CIQUS) and Department of Physical Chemistry, University of Santiago de Compostela, C/ Jenaro de la Fuente s/n, E-15782 Santiago de Compostela, Spain.
Manoel
M
Veiga Gutiérrez
Center for Research in Biological Chemistry and Molecular Materials (CIQUS) and Department of Physical Chemistry, University of Santiago de Compostela, C/ Jenaro de la Fuente s/n, E-15782 Santiago de Compostela, Spain.
M Carmen
MC
Ríos Rodríguez
Center for Research in Biological Chemistry and Molecular Materials (CIQUS) and Department of Physical Chemistry, University of Santiago de Compostela, C/ Jenaro de la Fuente s/n, E-15782 Santiago de Compostela, Spain.
Flor
F
Rodríguez-Prieto
Center for Research in Biological Chemistry and Molecular Materials (CIQUS) and Department of Physical Chemistry, University of Santiago de Compostela, C/ Jenaro de la Fuente s/n, E-15782 Santiago de Compostela, Spain.
Manuel
M
Mosquera
Center for Research in Biological Chemistry and Molecular Materials (CIQUS) and Department of Physical Chemistry, University of Santiago de Compostela, C/ Jenaro de la Fuente s/n, E-15782 Santiago de Compostela, Spain.
J Luis
JL
Pérez Lustres
acids and bases; fluorescence spectroscopy; kinetics and photochemistry; proton transfer; solution chemistry; time-resolved spectroscopy