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Following Bimolecular Excited-State Proton Transfer between Hydroxycoumarin and Imidazole Derivatives
The
ultrafast dynamics of a bimolecular excited-state proton transfer
(ESPT) reaction between the photoacid 7-hydroxy-4-(trifluoromethyl)-1-coumarin
(CouOH) and 1-methylimidazole (MI) base in aprotic chloroform-d1 solution were investigated using ultrafast
transient infrared (TRIR) and transient absorption (TA) spectroscopies.
The excited-state lifetime of the photoacid in solution is relatively
short (52 ps), which at the millimolar photoacid and base concentrations
used in our study precludes any diffusion-controlled bimolecular ESPT
reactions. This allows the prompt ESPT reaction between hydrogen-bonded
CouOH and MI molecules to be studied in isolation and the “contact”
ESPT dynamics to be unambiguously determined. Our time-resolved studies
reveal that ultrafast ESPT from the CouOH moiety to hydrogen-bonded
MI molecules occurs within ∼1 ps, tracked by unequivocal spectroscopic
signatures of CouO–* photoproducts that are formed
in tandem with HMI+. Some of the ESPT photoproducts subsequently
π-stack to form exciplexes on a ∼35 ps time scale, minimizing
the attractive Coulombic forces between the oppositely charged aromatic
molecules. For the concentrations of CouOH and MI used in our study
(up to 8 mM), we saw no evidence for excited-state tautomerization
of coumarin anions