Fluorescent sensors for metal ions

Abstract

The considerable current interest in the use of fluorescent molecules in molecular recognition is not new, but is one that has increased in importance in recent years due to their applications in many fields including trace analysis in chemistry and
biochemistry. Using the "dual fluorescence" properties of TICT state compounds (Twisted Intramolecular Charge Transfer which gives an extra red-shifted emission band) and the key property of crown ether compounds which is their ability to complex ions, we designed and synthesised a range of new fluoroionophores and studied their photophysical behaviour towards a selection of alkaline and alkaline-earth metal ions.
The new fluoroionophores (4-(i '-aza-4'-7'- 10 ,-i 3 '-tetraoxapentadecyl) benzonitrile, 4-( 1 '-aza-4'-7'- 10-i 3'-! 6'-pentaoxapentadecyl)benzonitrile, methyl-4- (1 '-aza-4'-7'-10'-13'-tetraoxapentadecyl)benzoate, 4'-N,N-(i "-aza-4",7",iO' ',l 3"-
tetra oxapentadecyl)-3-hydroxyflavone and 4'-N,N-(l "-aza-4' ',7' ',lO", 13"- tetraoxapenta decyl)-3-methoxyflavone) have been shown to form TICT states upon excitation. The photophysics (absorption, emission, lifetimes) of these compounds are
affected by the complexation of cations which reduce the donor character of the nitrogen belonging to the crown. The studied dications (Mg2 , Ca2 and Ba2') lead to larger spectral shifts and bigger values of the binding constant Ks than the studied monocations (Lie, Nat, K, CC) which can be interpreted in terms of a stronger electrostatic interaction between the nitrogen lone pair and the alkaline-earth metals than the alkali-metals.
It appears that the selectivity demonstrated by these crown ethers towards the alkaline earth and alkali metals is not only explained by the "hole-size relationship" between the size of the crown ether cavity and the ionic mdii of the metal ions. The type of substituent on the phenyl of the azacrown ether has been shown to further influence the spectroscopic properties (shift of the absorption ansi emission bands). Nevertheless the photophysical properties of the azacrown ether derivatives studied does not show enhancement of the selectivity towards the studied metals on expansion of the delocalisation system. More complex mechanisms are in operation in flexible systems such as the azacrown derivatives used in this study geometry, conformation and rigidity all play roles in determining the binding strength and selectivity of these molecules.