Fluorescent Naphthalimide logic gates in Micelles

Abstract

A series of novel 4-amino-1,8-naphthalimide H± sensors were designed and synthesised with an ‘anchor1-spacer1-fluorophore-spacer2-receptor1-anchor2’ format. A scaffold based on the naphthalimide fluorophore, piperazine receptor and butyl chain anchor remained constant throughout the compounds. The variable anchor1 at the imide position was either a methyl, phenyl or 3,4-dimethoxyphenyl substituent. A fourth sensor bears a crown ether with a ‘receptor1-spacer1-fluorophore-spacer2-receptor2-anchor1’ format. A green fluorescence emission is modulated by photoinduced electron transfer (PET) and internal charge transfer (ICT) mechanisms, and by solvent polarity. Experiments were performed in aqueous solutions of CTAC (cetyltrimethylammonium chloride), Triton X-100TM and SDS (sodium dodecyl sulphate). UV-Vis absorbance and steady-state fluorescence spectroscopy were used to study the response of these novel compounds upon altering the nanoenvironment of the micelles. Acid titrations were conducted in surfactant solutions to obtain ΔpKa , lambda maximas and fluorescence quantum yields. These results clearly highlight the repulsive and attractive forces of each surfactant brought about by the charged headgroups and hydrophobic effects. Another set of acid titrations were carried out using model pH probes based on a ‘receptor1-spacer1- fluorophore-spacer2-receptor2’ format without an anchor on the piperazine receptor. These models exhibits a lack of fluorescence switching indicating the butyl chain is essential as an anchor for sensing in micelles. Na± titrations with the crown ether variant uncovered the redundancy of the metal ion as it is dominated by the H± input. Hence all novel sensors function as single-input H±-driven YES gates when probing the micellar interface.