Synthesis, Evaluation and Immobilisation of Anion Sensors Based on the 4-amino-1,8-naphthalimide Fluorophore

Author: Andrew James Blok

Blok, Andrew James, 2013 Synthesis, Evaluation and Immobilisation of Anion Sensors Based on the 4-amino-1,8-naphthalimide Fluorophore, Flinders University, School of Chemical and Physical Sciences

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Molecules based on the 4-amino-1,8-naphthalimide fluorophore combined with powerful urea and thiourea recognition units have been shown to be excellent sensors for anions including dihydrogen phosphate, acetate and fluoride. The majority of the literature with regards to these particular sensors however reports solution phase sensing. This thesis details the synthesis of a series of sensors based on the combination of the 4-amino-1,8-naphthalimide fluorophore and a urea recognition unit, incorporating a terminal double bond at the imide position. This terminal double bond can then be used to immobilise the sensors onto a silica surface, broadening the potential applications of this sensing technology. The synthesis of eight different sensors each containing the 4-amino-1,8-naphthalimide fluorophore and a urea or thiourea recognition group is described. The fluorophore and the recognition group are connected covalently via a spacer molecule, with the use of three different spacer molecules investigated; 2-aminobenzylamine, 4-aminobenzylamine and 3-aminobenzylamine. Previous literature reports had indicated that small changes in the sensor molecule influenced the properties of the sensors towards different anions. Several changes to the recognition group were also investigated (urea vs. thiourea, addition of a chloro group on the phenyl ring attached to the recognition group, introduction of triethoxysilyl groups to enable a different method of immobilisation). The use of microwave irradiation as an alternative to conventional heating methods was also trialled for the synthesis of three of the sensors. Reaction time was decreased, whilst in some cases purity and yield were also improved. In one step the reaction time was reduced from fourty-eight hours to sixty minutes, whilst in another a product was able to be purified using recrystallisation, whereas column chromatography was usually required when using conventional heating techniques. After successful synthesis of the sensors, their ability to sense anions (dihydrogen phosphate, acetate, fluoride and bromide) was monitored in the solution phase using both fluorescence spectrophotometry and 1H NMR spectroscopy. Strong interactions were observed upon addition of both dihydrogen phosphate and acetate to a solution of sensor in DMSO, with quenching of the fluorescent emission signal observed and also significant downfield shifts for the resonances assigned to the urea protons of each sensor in the 1H NMR spectrum. Significant shifts were also observed for the 4-amino NH proton resonance dependant on the sensor being evaluated. Little quenching or changes in the 1H NMR spectrum were observed upon addition of bromide to a solution of sensor. The most interesting results were obtained upon the addition of fluoride, with a colour change from yellow to red as greater amounts of fluoride were added due to deprotonation of the 4-amino NH proton. Again significant changes were noted in the 1H NMR spectrum of each sensor. Finally after establishing the sensors were suitable for the detection of anions, immobilisation onto a silica surface was investigated. Initially the terminal double bond included in the sensor design was used to covalently attach the sensor to a hydride modified silica gel using hydrosilation chemistry. Definitive spectroscopic characterisation of the surface was hard to obtain, however deprotonation of the 4-amino NH proton by addition of fluoride to the surface was observed, suggesting successful attachment. Alternative immobilisation methods including building the sensor onto a 3-aminopropyl functionalised silica surface and by condensing triethoxysilyl groups (introduced in three of the sensors as part of the recognition unit) onto mesoporous silica were also investigated, proving that immobilisation of the sensors onto a silica surface is viable and may be an alternative to solution phase sensing.

Keywords: anion sensing,naphthalimide,4-amino-1,8-naphthalimide,immobilisation,silica,organic synthesis,microwave irradiation,fluorescence,fluorescent sensor
Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2013
School: School of Chemical and Physical Sciences
Supervisor: Associate Professor Claire Lenehan