Dramatic Pressure-Dependent Quenching Effects in Supercritical CO 2 Assessed by the Fluorescence of 4′-Dimethylamino-3-hydroxyflavone. Thermodynamic versus Kinetics Control of Excited-State Intramolecular Proton Transfer Monica Barroso, † Nitin Chattopadhyay, ‡ Andrey S. Klymchenko, § Alexander P. Demchenko, | Luis G. Arnaut,* ,† and Sebastia ˜ o J. Formosinho † Department of Chemistry, Coimbra UniVersity, P-3004-535 Portugal, Department of Chemistry, JadaVpur UniVersity, Calcutta 700 032, India, Faculte ´ de Pharmacie, Laboratoire de Pharmacologie et Physicochimie, UniVersite ´ Louis Paster, BP 24, 67401 Illkirch, France, and The Palladin Institute of Biochemistry, KieV 01030 Ukraine ReceiVed: July 11, 2006; In Final Form: October 3, 2006 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 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. Introduction Supercritical carbon dioxide (scCO 2 ) is known for its ap- plications as an extracting solvent as well as a reaction environment. 1-3 Probably, the most interesting property of scCO 2 as a reaction solvent is the tunability of the medium properties, such as local density, by changing the pressure under isothermal conditions. 4-6 Fluorometric techniques find them- selves the most effective methods for characterizing the proper- ties of supercritical fluids, because of the high sensitivity of these techniques and the strong response of some fluorophores to solvent perturbations. 4′-Dialkylamino-3-hydroxyflavones undergo excited-state in- tramolecular proton transfer (ESIPT) in different environments, similar to the parent 3-hydroxyflavone (3HF), 7-12 giving rise to dual fluorescence. The higher energy band corresponds to the normal (N*) excited state, whereas the lower energy one corresponds to the tautomeric form (T*) that originates from the ESIPT reaction. The extension of the π-electronic system and the introduction of electron-donor substituents lead to an enhanced sensitivity of the positions of these bands (λ N* , λ T* ) and, especially, their intensity ratios (I N* /I T* ), to the properties of molecular environment. The strong influence that the microenvironment exerts on the spectroscopic parameters of this family of compounds motivated their widespread use as fluorescent probes for various purposes. 11,13-17 The present work focuses on the fluorescence properties of 4′-dimethylamino-3-hydroxyflavone (DMA3HF), a remarkably sensitive fluorophore, as a function of the pressure and tem- perature of scCO 2 . The evaluation of polarity and H-bonding in scCO 2 is facilitated by the large amount of experimental data available for DMA3HF and for the closely related 4′-diethy- lamino-3-hydroxyflavone (DEA3HF) fluorescent probe in dif- ferent liquid solvents. 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 de- pendent 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 microen- vironment 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. Experimental Section DMA3HF was synthesized as described elsewhere. 21,22 The purity of the compound was checked spectroscopically before use. Absorption and emission spectra were recorded using a Shimadzu UV-2001 spectrophotometer and a SPEX Fluorolog-3 spectrofluorimeter, respectively. The supercritical fluids setup has been described elsewhere. 23,24 For the absorption and emission studies, the samples were prepared by depositing a small amount of solid DMA3HF onto a sapphire window of the cell. The cell was then evacuated and filled with CO 2 after thermostating at the desired temperature. The concentrations of * To whom correspondence should be addressed. E-mail: lgarnaut@ci.uc.pt. † Coimbra University. ‡ Jadavpur University. § Universite ´ Louis Paster. | The Palladin Institute of Biochemistry. 13419 J. Phys. Chem. A 2006, 110, 13419-13424 10.1021/jp0643606 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/22/2006 Downloaded by PORTUGAL CONSORTIA MASTER on July 13, 2009 Published on November 22, 2006 on http://pubs.acs.org | doi: 10.1021/jp0643606