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.
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 well-separated 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 ET(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.
Barroso
Monica
M
Department of Chemistry, Coimbra University, P-3004-535 Portugal.
Chattopadhyay
Nitin
N
Klymchenko
Andrey S
AS
Demchenko
Alexander P
AP
Arnaut
Luis G
LG
Formosinho
Sebastião J
SJ
D002245:Carbon Dioxide / Q000737:chemistry*; D005419:Flavonoids / Q000737:chemistry*; D007700:Kinetics; D011522:Protons; D013050:Spectrometry, Fluorescence; D013816:Thermodynamics
C090856:4'-diethylamino-3-hydroxyflavone; D005419:Flavonoids; D011522:Protons; D002245:Carbon Dioxide