A Novel Pyrene‐Based Fluorescing Amphiphile with Unusual Bulk and Interfacial Properties

Abstract

We have synthesised a new, pyrene-based, low-molecular-mass, amphiphilic molecule that displays a wealth of properties of potential interest for aggregation and interfacial applications. In order to elucidate some of the key properties of this molecule, which consists of a pyrene-containing hydrophobic head and a short PEG-based hydrophilic tail, we investigate herein some aspects of its concentration-dependent behaviour in aqueous solutions. We show that the inclusion of the hydrophobic pyrene group not only provides the molecule with intriguing bulk and interfacial properties down to low concentrations, but also with various means of assessing its aggregation behaviour by means of its well-characterised fluorescence properties. Combining a range of fluorescence techniques with microscopic imaging (optical and Cryo-TEM), interfacial tension measurements and foaming studies, we have been able to identify and characterise three concentration-dependant regimes. At low concentrations, the molecule is dissolved in monomeric form. At intermediate concentrations, labile aggregates are formed, which, at higher concentrations, give way to aggregates containing pre-associated pyrenes. Our measurements strongly imply that the latter aggregates are hexagonally close-packed tubular micelles. In this latter regime we also find a range of micron-sized precipitates. Additionally, the molecule displays strong interfacial activity, yet a surprisingly slow dynamics of interfacial adsorption. Finally, we demonstrate the possibility of using it to visualize interfaces and also create reasonably stable (1 hour) and fluorescing foams.

Solvent‐Tuned Supramolecular Assembly of Fluorescent Catechol/Pyrene Amphiphilic Molecules

“…Because the absorption measurements do not allow precise conclusions to be drawn on the degree of association, fluorescence measurements were performed at different concentrations. The excitation spectra of 1 obtained at λ em =395 nm correspond to the monomeric species ( λ max =310, 323, and 339 nm) until reaching the concentration of 7.9×10 −2 m m (see Figure a and Figure S7 in the Supporting Information). Above this concentration, the excitation spectra maintain a similar vibronic structure but shift to the red and reduce in intensity.…”

“…To obtain further information on the system from the obtained spectra, we determined the R I factor, which indicates the polarity of the environment of the pyrene molecules, which can be caused by the proximity of other molecules of the same nature. This factor is calculated as the ratio ( I I / I II ) between the band intensity of the emission spectra at λ max =376 nm ( I I ), independent of the medium polarity, and the band intensity at λ max =395 nm ( I II ), which is dependent on the environment . Therefore, the R I value is larger in polar solvents (e.g., R I =1.6 in water) and decreases in hydrophobic media (e.g., R I =0.6 in alkanes) .…”

“…On the other hand, it is well known that pyrene moieties are able to self‐aggregate, thus establishing weak intermolecular interactions (i.e., π–π interactions) to form ordered mesophases or even induce supramolecular chirality . Moreover, pyrene optical properties (absorption and luminescence) are highly sensitive to the medium polarity and to the molecular packing processes and supramolecular associations in the microenvironment that surround the probe, which makes it a good sensor for self‐assembly processes and micelle/vesicle formation …”

“…This factor is calculated as the ratio (I I /I II )b etween the band intensity of the emissions pectra at l max = 376 nm (I I ), independent of the medium polarity,a nd the band intensity at l max = 395 nm (I II ), which is dependent on the environment. [18] Therefore, the R I value is larger in polar solvents (e.g., R I = 1.6 in water) and decreases in hydrophobic media (e.g., R I = 0.6 in alkanes). [18,22] In the present system, the R I value stays above 1.4 ( Figure 1d)u ntil reaching the concentration at which the highest emission intensity is observed (7.9 10 À2 mm)a nd decreases to 1.3 when the concentration becomes one order of magnitude higher (7.9 10 À1 mm), thereby confirmingt hat the fluorescence quenching is related to the formation of labile aggregates that reduce the environment polaritya round the pyrene moiety.According to previous studies, the pyrene moleculeso ft hese labile aggregates should be presenta tavery short distance (< 40 ), but far enough to avoid the characteristic aggregatee mission.…”

“…[16] Moreover, pyrene optical properties (absorption andl uminescence) are highly sensitive to the mediump olarity andt ot he molecular packing processes and supramolecular associations in the microenvironment that surround the probe, which makes it a good sensorf or self-assembly processes and micelle/vesicle formation. [17,18] This study is primarily aimed at the control and tuning of the supramolecular self-assembly process between catecholcontaining amphiphilic molecules with two differentm oieties able to establish intermolecular interactions. We report herein the preparation of hollow nanocapsules/vesicles formed from the self-direct assembly of the novel amphiphile molecule N-(3,4-dihydroxybenzylidene)-4-(pyren-1-yl)butanehydrazide (1), composed of ah ydrophilic catecholt aila nd ap yrene hydrophobic head (Scheme 1).…”