Fluorescent molecular logic gates for pE and pH

Abstract

Three novel molecules were designed for sensing pH and redox potential. The fluorophore for two of the molecules was based on an asymmetrical (PDI) motif. Receptors for these molecular logic gates were attached to the N-imide positions of a perylene-3,4,9,10-tetracarboxylic acid diimide, the main precursor. Through asymmetrical attachment of pE and pH receptors these perylene based moieties were coined as ‘Pourbaix’ sensors. An ‘electron-donor-spacer1-fluorophore-spacer2-receptor’ format was used in their design. A third molecule, 1 was built in ‘electron-donor-spacer1-fluorophore-spacer2- receptor1-spacer3-receptor2’ format incorporating a naphthalimide fluorophore. A ferrocene with a methylene spacer was attached on the N-position through an imide bond to the naphthalimide fluorophore. As proton receptor, a piperazine moiety was attached at the 4 position on the naphthalimide. By incorporating a 2-methoxybenzo-1-aza-15- crown-5, 1 could also detect Na+. The presence of a methoxy group on the benzene was devised as a way of increasing specificity to Na+. Photophysical and logic properties of the molecule 1 were studied in 1:1 (v/v) methanol/water and 2 and 3 were studied in THF. The study also investigated the physicochemical behaviour of molecule 19 in polyacrylamide hydrogel and both molecule 3 and 19 in polyurethane polymer. Logic characteristics were determined in aqueous solution with UV-vis absorption and fluorescence spectroscopy. Two mechanisms, photon electron transfer (PET) and internal charge transfer mechanisms (ICT) drive 1 and PET drives 2 and 3. With molecules 2 and 3 having respective fluorescence quantum yields of Ф = 0.20 and Ф = 0.22 respectively. Molecule 2 showed a 3-fold enhancement while 3 showed a 2-fold enhancement. Molecule 1 had a fluorescence quantum yield of Ф = 0.22 and a 9-fold enhancement between the ‘off’ and ‘on’ state. At high H+ concentrations and Na+ presence and high pE. Confocal microscopy was used to determine the progress of corrosion on cold rolled steel coupons coated with a polyurethane coating impregnated with Pourbaix sensor 19 from previous work and 3, one of the novel PDI molecules synthesised in this work. The impregnation of the polyurethane polymer was enough to indicate early signs of corrosion.