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 ultrafast ESPT from the CouOH moiety to hydrogen bonded MI molecules occurs within ~1 ps, tracked by unequivocal spectroscopic signatures of CouO–* photoproducts which are formed in tandem with HMI+. Some of the ESPT photoproducts subsequently pi-stack to form exciplexes on a ~35 ps timescale, 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.