Dormant acceptor activation of 10-hydroxybenzoquinline derivatives by excited-state intramolecular proton transfer

We studied the excited-state intramolecular proton transfer (ESIPT) of two derivatives of hydroxybenzo[h]quinoline (10-HBQ). We used time-resolved and steady-state techniques for this purpose. These two compounds are water soluble and can be excited by visible light, thus they have a potential use in in vitro and in vivo imaging applications. We found that the ESIPT rate of ortho-indolium-10-hydroxybenzo[h]quinoline is greater than 10¹³ s^-1, whereas for ortho-picolinium-10-hydroxybenzo[h]quinoline the rate constant is rather low (k_PT = 7 × 10¹² s^-1, τ_PT = 140 ± 20 fs). We also found a kinetic isotope effect of 1.5 ± 0.2 for ortho-picolinium-10-hydroxybenzo[h]quinoline. We observe in both compounds, a slower time component of 300 ± 50 fs with low amplitude of 0.05 ± 0.02 for the enol form decay. This slower component is also observed in the fluorescence-signal rise of the keto form, but with a higher amplitude of 0.2 ± 0.04. The fluorescence-signal rise of the keto forms of both compounds shows a third long-time component of several picoseconds. This time component in ortho-indolium-10-hydroxybenzo[h]quinoline is solvent-dependent and is assigned to solvation dynamics in protic solvents. We explain the relatively slow ESIPT rate of ortho-picolinium-10-hydroxybenzo[h]quinoline by the smaller enol-keto energy gap of this compound.