Synthesis and Fluorescence Behavior of Cation Chemosensor Based on Azacrown Ether Fluoroionophore Carrying Styrylanthracene

Abstract

Recently, the development of chemosensor associated with fluoroionophore has attracted great attention. Fluoroionophore is a compound combining the ionophore and fluorophore, where ionophore binds selectively with a specific ion and fluorophore generates a fluorescence signal upon ion binding. Depending on the nature of fluorophore-ionophore interaction in fluoroionophore, fluorescence properties of fluorophore such as fluorescence intensity or fluorescence wavelength maximum may be influenced by ion capture of ionophore. Sensitivity of fluorophore may be associated with the ionic recognition ability of the ionophore, if the fluorophore may be perturbed by the coordination of cation. The recent increase of interest in the synthesis of crown ether derivatives incorporating different fluoroionophores is recently due to their application as alkali metal ion sensors in biochemical analysis and medical diagnostics. In particular, azacrown ether, a typical ionophore, acts as an electron-rich donor as well as a ligand of the cation. The electron pair on the nitrogen heteroatom in electron-rich azacrown ether could be transferred towards electron deficient fluorophore in a π-conjugated D-A arrangement, through the charge transfer interaction between ionophore, an electron donor, and fluorophore, an electron acceptor. The efficient intramolecular charge transfer takes part in the modification of the first electronic transition state of the fluorophore and may lead to weaken the fluorescence from fluorophore. Upon binding of a cation into ionophore cavity, the electron pair on nitrogen heteroatom of ionophore is stabilized and the intramolecular charge transfer is inhibited. Therefore, the original electronic configuration of the unsubstituted fluorophore is recovered, so that fluorescence may be strengthened. Many fluoroionophores including stilbenyl-, aryl-, or styryl-crown ether derivatives containing a stilbene, styrene or aryl moiety as a fluorophore and a crown ether/azacrown ether moiety as an ionophore have been investigated. However, the main disadvantage of these fluoroionophore is the relatively small changes in fluorescence intensity upon cation binding. Incorporation of more efficient fluorophore such as anthracene into fluoroionophore may lead to more remarkable change of fluorescence signal depending on the cation binding. Recent work has concerned the binding of various fluorophores to the N-phenylaza-15-crown-5 and their spectroscopic and complexation properties. This paper concerns the synthesis of N-phenylaza-15-crown-5 derivative containing anthrylethene chromophore and the effect of the complexation with various alkali metal cations on the absorption and fluorescence behavior. We prepared N-[4-(2-anthracen-9-yl-vinyl)-phenyl]-aza15-crown-5 (1), a azacrown ether derivative containing 1-(9anthryl)-2-phenylethene (9-APE) moiety, by Wittig coupling between N-[4-formyl-phenyl]-aza-15-crown-5 and 9bromomethylanthracene. It is known that t-9-APE, a styrene derivative containing highly fluorescent anthracene, shows fairly strong fluorescence (Φf = 0.45, λf = 476 nm in acetonitrile), coming from the locally excited state which the excitation energy is localized on anthracene ring. For 1-(9-anthryl)-2-(4-methoxyphenyl)ethene (MeO-APE) containing an electron donating substituent methoxy group, fluorescence becomes weak and is red-shifted (Φf = 0.028, λf = 494 nm in acetonitrile) due to the contribution of the intramolecular