Cyclic imide probes in cation sensing

Chapter 6: Chapter 6: Cyclic imides containing hydroxy carboxylic acids in the syntheses of manganese(II), zinc(II) and cadmium(II) complexes

1.5 Cyclic imides as sensors for anions and cations

1.5.2 Cyclic imide probes in cation sensing

The 4-amino-1,8-naphthalimide derivatives are usually found to be highly emissive in organic solvents such as dichloromethane and chloroform, with quantum yield often being reported to be close to unity; while in water, significant quenching is observed. Nevertheless, the use of 4- aminonaphthalimide for sensing of cations in water is well established. Sensors 1.45-1.50 are just a few recent examples where the receptors are connected to the fluorophore via the amino functionality of the aryl ring. In these, depending on the design principles employed, the naphthalimide absorption and emission spectra were highly modulated upon binding of cations at the respective binding sites. Of these, 1.45 was developed for the sensing of H⁺, where through an electron-transfer mechanism, the excited state of the naphthalimide is quenched

upon protonation of the tertiary aliphatic amine.⁸¹ Many other examples of such pH dependent PET naphthalimide sensors have been developed to date, for use in solution or on solid supports.⁸² In contrast, compound 1.46, possessing a crown ether receptor was developed for

analysis of Na⁺ in blood samples,⁸³ while 1.47⁸⁴ and 1.48⁸⁵ have been developed for detecting Zn(II). Compound 1.47 has also recently been used for imaging of bone structures using epifluorescence microscopy.⁸⁶ The Zn(II) sensors 1.47a and 1.47b are based on the fluorophore–spacer–receptor principle, and were shown to bind the Zn(II) ion in a highly selective manner at the iminodiacetate moiety in competitive media at pH 7.4; this increases the oxidation potential of the receptor, preventing photoinduced electron transfer quenching taking place from the receptor to the excited state of the fluorophore, and hence, this caused the naphthalimide fluorescence to be ‘switched on’ . Here, the 4-amino moiety does not participate directly in ion binding and hence, the absorption spectrum was not significantly affected.

Similarly, large enhancements were seen in the emission spectra of 1.48, developed by Watkinson et al,⁸⁵ upon binding to Zn(II). Recently, Watkinson et al, have extended their design to form naphthalimide dimers, using Cu(II) catalysed click chemistry.⁸⁷ In contrast, compound 1.49, developed as sensors for Cu(II), where binding of Cu(II) engages the two aryl amines in the binding.⁸⁸ Hence, the absorption spectrum is significantly affected, and indeed so much so that the binding is visible to the naked eye; hence 1.49 is a colorimetric as well as a fluorescent sensor for Cu(II). Similarly, 1.50 showed large changes in the absorption and the emission spectra of the naphthalimide moiety upon sensing of Cu(II).⁸⁹

Recently, Xu et al reported an amide-containing receptor for Zn²⁺, combined with a naphthalimide fluorophore (1.51). The fluorescence, absorption detection, NMR, and IR studies indicated that 1.51 bound Zn²⁺ in an imidic acid tautomeric form of the amide/di-2- picolylamine receptor in aqueous solution, while most other transition metal ions were bound to the sensor in an amide tautomeric form (Scheme 1.31). Due to this differential binding mode, 1.51 showed excellent selectivity for Zn²⁺ over most competitive transition metal ions with an enhanced fluorescence (22-fold) as well as a red-shift in emission from 483 to 514 nm.⁹⁰

Scheme 1.31^.

In chemosensor 1.52, reported by the Qian group,⁹¹ the receptor N,N-di-(2- picolyl)ethylenediamine (DPEN) was attached to the fluorophore through a benzene ring on the naphthalimide moiety using the virtually decoupled fluorophore–receptor linking strategy. The lone pair electron of the aniline nitrogen quenches the fluorescence of the excited fluorophore 4-aminonaphthalimide by the PET mechanism. When the electron-donating aniline nitrogen is complexed with Zn²⁺, the PET process is blocked to get a 6-fold increase in emission. Xu et al.

incorporated N,N,N -tris(pyridin-2-ylmethyl)ethylenediamine (TRPEN) with N-substituted-4- bromo- 5-nitro-1,8-naphthalimide to develop a ratiometric chemosensor 1.53 for Zn²⁺.⁹² The capture of Zn²⁺ by the receptor resulted in the deprotonation of the secondary amine conjugated to 1,8-naphthalimide so that the electron-donating ability of the N atom would be greatly enhanced; thus 1.53 showed a 56 nm red-shift in absorption (507 nm) and fluorescence spectra (593 nm), respectively, from which one could sense Zn²⁺ ratiometrically and colorimetrically.

Xu et al then extended the Zn²⁺ deprotonation mechanism to construct chemosensor 1.54 which can discriminate Zn²⁺ and Cd²⁺ by undergoing different intermolecular charge transfer (ICT) processes.⁹³ Cd²⁺ binding induces a blue shift in emission from 531 nm to 487 nm based on a general ICT mechanism, while Zn²⁺ binding produces a red shift in emission from 531 nm to 558 nm through the deprotonation-ICT mechanism.

The development of naphthalimide based optical probes as fluorescent and colorimetric chemosensors for the detection of precious metal ions such as silver, gold and platinum ions has also been reported recently. Zhu and Qian et al described a fluorescent sensor 1.55, based on the naphthalimide chromophore, exhibiting dual signalling behaviors for Hg²⁺ and Ag⁺ in ethanol–water solution at pH 7.14.⁹⁴ Upon the addition of Hg²⁺, a 5-fold fluorescence enhancement and a slight blue shift in the emission maximum from 547 to 532 nm were observed while upon the addition of Ag⁺ ions, 1.55 exhibited a quenched fluorescence due to the intramolecular d– interaction between the fluorophore and Ag⁺. Two naphthalimide derivatives 1.56 and 1.57 were reported by Yoon and Spring et al. in 2010.⁹⁵ Compound 1.56 can detect Ag⁺ with a selective fluorescence enhancement (14 fold) in CH3CN–H2O solution at pH 7.4. Furthermore, Ag⁺ could be detected at least down to 1.0 ×10^-8 M, which suggested that 1.56 is a highly selective chemosensor for Ag⁺ with turn-on fluorescence in aqueous solution.

On the other hand, the reference compound 1.57 without the carbonyl group does not show a strong binding with Ag⁺, therefore proving that the carbonyl group positioned between the 1,8- naphthalimide and [15]aneNO₂S₂ plays a key role in displaying the selective fluorescence enhancement.

These examples are all based on the use of 4-amino-1,8- naphthalimide structures, where the focus has been on the detection of cations, but the 3-amino-1,8-naphthalimide structures have also been employed in such sensing, as demonstrated elegantly by de Silva et al.⁹⁶