An Example of How to Use AOT Reverse Micelle Interfaces to Control a Photoinduced Intramolecular Charge-Transfer Process

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 (ICT) states which can be particularly useful as sensors. In this work, we performed absorption, steady-state, time-resolved fluorescence (TRES), and time-resolved area normalized emission (TRANES) spectroscopies on PRODAN dissolved in nonaqueous reverse micelles. The reverse micelles are composed of polar solvents/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/n-heptane. Sequestered polar solvents included ethylene glycol (EG), propylene glycol (PG), glycerol (GY), formamide (FA), dimethylformamide (DMF), and dimethylacetamide (DMA). The experiments were performed with varying surfactant concentrations at a fixed molar ratio W(S) = [polar solvent]/[AOT]. In every reverse micelle studied, the results show that PRODAN undergoes a partition process between the external solvent and the reverse micelle interface. The partition constants, K(p), are quantified from the changes in the PRODAN emission and/or absorption spectra with the surfactant concentration. The K(p) values depend strongly on the encapsulated polar solvent and correlate quite well with the AOT reverse micelle interface's zones where PRODAN can exist and emits. Thus, the partition toward the reverse micelle interface is strongly favored in DMF and DMA containing micelles where the PRODAN emission comes only from an ICT state. For GY/AOT reverse micelles, the K(p) value is the lowest and only emission from the local excited (LE) state is observed. On the other hand, for EG/AOT, PG/AOT, and water/AOT reverse micelles, the K(p) values are practically the same and emission from both states (LE and ICT) is simultaneously detected. We show here that it is possible to control the PRODAN state emission by simply changing the properties of the AOT reverse micelle interfaces by choosing the appropriate polar solvent to make the reverse micelle media. Indeed, we present experimental evidence with the answer to the long time question about from which state does PRODAN emit, a process that can be controlled using the unique reverse micelle interfaces properties.

Comparison between Two Anionic Reverse Micelle Interfaces: The Role of Water–Surfactant Interactions in Interfacial Properties

“…The molecular probe located in the nonpolar organic solvent always emits from a locally excited (LE) state whereas PRODAN located at the RM interface can emit from LE or ICT or both states (dual fluorescence) at room temperature depending on the AOT RM interface properties (see Scheme 2). [24,25] Furthermore, for PRODAN molecules located at the RM interfaces the results are consistent with the emission of the probe located in a single place within the AOT RM interface from two different excited states rather than from the emission of one excited state of the probe located in a different microenvironment: the interface and the water pool. [24,25] Moreover, we have shown the possibility of switching the state from where PRODAN emits by changing the properties of the AOT RM interfaces.…”

“…It is an aromatic compound with intramolecular chargetransfer (ICT) states, and can be particularly useful as a sensor for different kinds of media such as RMs and other membrane mimickers. [24][25][26][27][28][29][30][31][32][33][34][35][36] It is a fluorescent probe that exhibits strong shifts in the absorption and emission spectra on varying the environment. The probe emits an intense, single, broad fluorescence band, strongly red-shifted upon increasing the polarity-polarizability (p*) and the hydrogen-donor ability (a) of the media.…”

“…[21][22][23][24][25] QB is a molecular probe that has a UV/Vis absorption spectrum with two major features: a band in the visible region, B 1 , that is primarily sensitive to polarity, and a band peaking at a shorter wavelength in the UV, B 2 , which reflects the hydrogen-bond donor capability of the solvent. [21,22] The absorbance of the B 2 band is highly sensitive to the molecule's environment; hence the ratio of the absorbances of B 2 to B 1 (AbsB 2 /AbsB 1 ), in combination with the absorption band shifts, provides an effective method to determine the properties of the microenvironment surrounding the probe.…”

Effect of the Constrained Environment on the Interactions between the Surfactant and Different Polar Solvents Encapsulated within AOT Reverse Micelles

“…Very recently, [28] we demonstrated that the emission state of the molecular probe 6-propionyl-2-dimethyl amino naphthalene (PRODAN) can be switched from the local exited to the intramolecular charge-transfer state or can take place from both states (dual fluorescence) by choosing the right AOT nonaqueous reverse-micelle interface. This is very interesting because it is not trivial that the emission states of molecular probes can be controlled at room temperature, and it seems that AOT reverse micelles can do it.…”

“…To do this, we used different approaches: i) invasive techniques, such as absorption and emission spectroscopy of different molecular probes dissolved in different nonaqueous reverse micelles media, [16,17,19,28,29] and ii) noninvasive techniques, such as FTIR and 1 H NMR spectroscopy, [18,20,25] where the main effort was focused on the interpretation of the spectra of different polar solvents encapsulated inside reverse micelles to gain insights into their structure and the existence or not of a polar solvent pool. Thus, we have followed how the n OD or the n ND bands for EG, GY, PG, and FA change as the W S ratio increases.…”