New Insights on the Photophysical Behavior of PRODAN in Anionic and Cationic Reverse Micelles. From Which State or States does it Emit?

NOVAIRA M.; BIASUTTI M.A.; SILBER J.J.; CORREA N.M*

JOURNAL OF PHYSICAL CHEMISTRY B

American Chemical Society

Lugar: Washington DC, USA; Año: 2007 vol. 111 p. 748 - 759

1089-5647

Abstract 6 - propionyl ? 2 - (N,N-dimethyl) aminonaphtahalene, PRODAN, is widely used as a fluorescent molecular probe due to its significant Stokes shift in polar solvents. It is an aromatic compound with intramolecular charge-transfer states (ICT) which can be particularly useful as sensor. The nature of the emissive states has not yet been established despite of the detailed experimental and theoretical investigations made on this fluorophore. In this work, we performed absorption, steady-state, time-resolved fluorescence (TRES) and time?resolved area normalized emission (TRANES) spectroscopies, on the molecular probe PRODAN in the anionic: water/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/n-heptane and the cationic water/benzyl?n-hexadecyl dimethylammonium chloride (BHDC)/benzene reverse micelles (RMs). The experiments were done varying the surfactant concentrations at a fixed molar ratio W = [H2O]/[Surfactant] and changing the water content at constant surfactant concentration. The results obtained varying the surfactant concentration at W = 0, show a bathochromic shift and an increase in the intensity of the PRODAN emission band due to the PRODAN partition process between the external solvent and the RMs interface. The partition constants, Kp, are quantified from the changes in the PRODAN emission spectra and the steady state anisotropy (<r>) with the surfactant concentration in both RMs. Kp value is larger in the BHDC than the AOT RMS, probably due to the interaction between the cationic polar head of the surfactant and the aromatic ring of PRODAN. The partition process is confirmed with the TRES experiments, where the data fits to a continuous model and, with the time?resolved area normalized emission spectroscopy (TRANES) spectra where only one isoemissive point is detected. On the other hand, the emission spectra at W = 10 and 20 show a dual fluorescence with a new band that emerge in the low energy region of the spectra, band that was previously assigned to the PRODAN emission from the water pool of RMs. Our studies demonstrate that this band is due to the emission from and ICT state of the molecular probe PRODAN located at the interface of the RMs. These results are also confirmed by the lifetime measurements, the TRES experiments where the results fit to a two-state model and the time?resolved area normalized emission spectroscopy (TRANES) spectra where three or two isoemissive points are detected in the AOT and BHDC RMs respectively. In the AOT RMS, Kp values obtained at W = 10 and 20 are almost independent of the water content while, the values are higher for the BHDC RMs due to the higher micropolarity of this interface.