Cationic molecular rulers: Synthesis, characterization, and intramolecular complications

A new family of cationic, solvent-sensitive surfactants is synthesized and characterized. The surfactants consist of a solvatochromic, hydrophobic chromophore connected to a quaternized ammonium headgroup by short alkyl spacers of varying lengths. The chromophore is based on p-nitroanisole (pNAs), a simple aromatic probe whose first electronic excitation wavelength shifts more than 20 nm as the local dielectric environment varies between nonpolar and polar limits. Given that pNAs is 20 times more soluble in cyclohexane than in water, lengthening the separation between the chromophore and the charged headgroup enables the chromophore to extend further into the organic phase when surfactants adsorb to an aqueous/organic interface. Thus, in principle, these surfactants can function as molecular rulers by measuring the distance required for solvent polarity to converge from an aqueous to an organic limit across different liquid/liquid boundaries. Preliminary resonance-enhanced second harmonic generation spectra of surfactants adsorbed to an aqueous/cyclohexane interface support this idea. In bulk solution, short-chain surfactants appear to self-associate, forming ringlike structures. This behavior manifests itself as diminished solvatochromic activity as well as clear, through-space coupling in nuclear Overhauser enhancement spectra. The strength of intramolecular association diminishes with increasing alkyl spacer length and disappears completely with alkyl chains having six methylene groups.