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

Abstract

Steady-state fluorescence of 4′-dimethylamino-3-hydroxyflavone (DMA3HF) was observed in supercritical carbon dioxide (scCO 2 ). Excited-state intramolecular proton transfer (ESIPT) occurs resulting in two wellseparated emission bands corresponding to the normal and tautomer forms. As the scCO 2 density exceeds 0.7 g/mL, the relative intensity of the two bands tends to a constant value, comparable to that observed for organic solvents with E T (30) ) 33.0 ( 0.5 kcal/mol, such as toluene and di-n-butyl ether. At lower densities, the substantial decrease of the total fluorescence intensity (a 600-fold decrease as the pressure decreases from 100 to 80 bar) is accompanied by an even more accentuated decrease of the tautomer fluorescence. This can be explained by a shift in the equilibrium between normal and tautomer forms, concomitant with a more efficient quenching of the less solvated fluorophore, that may change the thermodynamic control of the relative population of the two emissive species to a kinetic control.

Excited-state proton coupled charge transfer modulated by molecular structure and media polarization

“…This transformation, reducing the dipole moment of 4 0 -diethylamino-3HF, shifts the reversible ESIPT reaction towards the ESIPT product with the two-band sensitivity in a new, shifted, polarity range. 162 Presently, for any application in any condensed medium (including even supercritical fluids), 163 a 3HC or 3HF dye can be suggested for detection of fine changes in molecular properties with a strong variation of fluorescence emission color.…”

“…The host-guest interactions in cyclodextrins have been studied 181 based on dramatic differences in response between unbound dye and that located in a low-polar molecular cavity. The properties of thermosensitive polymers dependent on their composition 182 and dynamic solvation in supercritical fluids 163 can also be investigated by this method. The intriguing properties of ionic liquids were characterized based on the dye two-color response in steady-state and time domains.…”

External Electric Field Effects on Excited-State Intramolecular Proton Transfer in 4′-N,N-Dimethylamino-3-hydroxyflavone in Poly(methyl methacrylate) Films

“…Therefore, in polar environments, stabilization of the energy of the N* form of DMHF and DEHF is much larger than that of the T* form, leading to significant changes in the shape of the excited-state potential energy surface (PES) on which CT and ESIPT reactions occur. Thus, DMHF and DEHF are suitable for the investigation of intermolecular interaction effects on excited-state dynamics, as well as applications that probe various environments. − …”

Proton Transfer from 2-Naphthol to Aliphatic Amines in Supercritical CO2

“…Excited state intramolecular proton transfer (ESIPT) in scCO 2 was observed in 4′-dimethylamino-3-hydroxyflavone . Steady state emission spectra show the two emission bands due to the tautomers.…”

“…Excited state intramolecular proton transfer (ESIPT) in scCO 2 was observed in 4 0 -dimethylamino-3-hydroxyflavone. 8 Steady state emission spectra show the two emission bands due to the tautomers. The total emission intensity, and more notably the lower energy tautomer emission, rapidly decreases at pressures lower than 97 bar in CO 2 at 34 °C.…”