Inclusion of Two Push-Pull N-Methylpyridinium Salts in Anionic Surfactant Solutions: A Comprehensive Photophysical Investigation

Two N-methylpyridinium salts with pushpull properties have been investigated in the aqueous solution of anionic micelles of sodium dodecyl sulfate (SDS) and potassium p-(octyloxy)benzenesulfonate (pOoBSK) surfactants. These molecules are known to be extremely sensitive to the local environment, with their absorption spectrum being subjected to a net negative solvatochromism. These compounds are also characterized by an excited state deactivation strictly dependent on the physical properties of the chemical surrounding, with the formation of intramolecular charge-transfer (ICT) states accordingly stabilized. Thanks to steady-state and femtosecond resolved spectroscopic techniques, the photophysical properties of these molecules in the presence of anionic micelles have been fully characterized and an efficient permeation within the micellar aggregates can thus be inferred. The extent of the changes in the photophysical properties of these molecules (with respect to what is observed in water) is an indicator of the medium experienced in the nanoheterogeneous solutions: enhanced fluorescence emissions, reduced Stokes shifts and slowed-down excited state decays strongly confirm the confinement within a scarcely polar and restraining environment. The slightly different behavior shown in the two types of micelles can be ascribed to a peculiar interaction between the aromatic moiety of the surfactant and that of the cations. Additionally, the inclusion promotes the solubilization of these poorly water-soluble salts, which is alluring in their promising use as DNA binders for antitumor purposes. Thus, the anionic micelles allowed the solubilization of the pyridinium salts under investigation, which in turn allowed the characterization of the nonhomogeneous medium established by the micellar aggregates.