Tuning the mechanism of proton-transfer in a hydroxyflavone derivative

Dramatic Pressure-Dependent Quenching Effects in Supercritical CO2 Assessed by the Fluorescence of 4‘-Dimethylamino-3-hydroxyflavone. Thermodynamic versus Kinetics Control of Excited-State Intramolecular Proton Transfer

“…It was recently shown that their excited states have nanosecond lifetimes and undergo reversible ESIPT, 18,19 as the fluorescence decay rates are much slower than the ESIPT rates. Under reversible conditions, the I N* /I T* ratio is governed by the ESIPT equilibrium.…”

“…Figure 3 shows the plot of log(I N* /I T* ) as a function of E T (30) using the data available in the literature. 14,18,19 Introducing the extreme log(I N* /I T* ) values measured in scCO 2 in this plot gives the range of E T (30) values that characterize scCO 2 at different pressures and temperatures. As discussed above, the value of I N* /I T* at P min is the lowest value that this ratio can attain.…”

“…The unexpected increase of I N* /I T* at lower pressures, where the polarity of scCO 2 should be lower, cannot be assigned to H-bonding of CO 2 molecules with the dye, because at lower densities this interaction should be even lower (thus contributing to the opposite, decrease in I N* /I T* ). Additionally, it cannot be H-bonding between DMA3HF molecules, because the I N* /I T* ratio is independent of the 18 and triangles 19 ) and DEA3HF (squares) 14 as a function of solvent polarity function ET (30) 31 concentration of DMA3HF. These data show that changes in the PT barrier, namely due to intermolecular hydrogen bonding, are not responsible for the decrease of the tautomer fluorescence at low pressures.…”

“…The large dipole moment of the N* state in these species and, therefore, its strong solvent-dependent stabilization leads to reversible ESIPT reactions, at least in solvents without strong quenching effects, where fluorescence lifetimes are of the order of nanoseconds. 18,19 This makes the fluorescence of the normal and tautomer bands strongly dependent on the polarity and hydrogen-bonding properties of the environment, and explains the dramatic response of DMA3HF to changes in its solvation shell. We have also attempted to resolve the controversy on the polarity of the scCO 2 microenvironment in terms of the E T (30) parameter, using literature values of I N* /I T* for DMA3HF 18,19 and DEA3HF 14,20 in various homogeneous solvents of different polarities.…”

“…18,19 This makes the fluorescence of the normal and tautomer bands strongly dependent on the polarity and hydrogen-bonding properties of the environment, and explains the dramatic response of DMA3HF to changes in its solvation shell. We have also attempted to resolve the controversy on the polarity of the scCO 2 microenvironment in terms of the E T (30) parameter, using literature values of I N* /I T* for DMA3HF 18,19 and DEA3HF 14,20 in various homogeneous solvents of different polarities. The I N* /I T ratio also provides insight into the thermodynamics and kinetics of ESIPT in scCO 2 in different pressure regimes.…”