Alkylpiperazino-1,8-Naphthalimide Fluorescent Probes for Exploring Micellar Membrane Nanospaces

Two 4-piperazine-1,8-naphthalimides were designed as fluorescent probes for exploring the local polarity and proton concentration at the interface of micelles. Designed with an anchor1-fluorophore-spacer-receptor-anchor2 layout, the hydrophobicity of the pH probes was tuned by substitution of the anchor modules with different alkyl chains (methyl, butyl and octyl) to facilitate the micellar penetration depth. Fluorescence switching ‘on’ in methanol/water media upon protonation of the piperazine receptor is driven by a competition between solvent polarity and photoinduced charge transfer. The solvatochromic properties were investigated to enhance visual naked-eye communication. The fluorescent probes were tasked with reporting on the local polarity and proton content within sodium dodecyl sulphate (SDS), cetyltrimethylammonium chloride (CTAC) and Triton X-100 micelles. The dimethyl-substituted probe reported on the environment about the micelle/bulk water interface. The more hydrophobic octyl-butyl-substituted probe explored deeper into the micelles. Insight was gained into the local polarity and proton gradients inside micelles, as corroborated by emission wavelength, fluorescence quantum yield and ΔpKa values. The findings are discussed in the context of the benzofurazan polarity–ΔpKa maps reported by Uchiyama and de Silva.