Dansyl-anthracene dyads for ratiometric fluorescence recognition of Cu2+

Abstract

Dansyl-anthracene dyads 1 and 2 in CH₃CN–H₂O (7 : 3) selectively recognize Cu²⁺ ions amongst alkali, alkaline earth and other heavy metal ions using both absorbance and fluorescence spectroscopy. In absorbance, the addition of Cu²⁺ to the solution of dyads 1 or 2 results in appearance of broad absorption band from 200 nm to 725 nm for dyad 1 and from 200 nm to 520 nm for dyad 2. This is associated with color change from colorless to blue (for 1) and fluorescent green (for 2). This bathochromic shift of the spectrum could be assigned to internal charge transfer from sulfonamide nitrogen to anthracene moiety. In fluorescence, under similar conditions dyads 1 and 2 on addition of Cu²⁺ selectively quench fluorescence due to dansyl moiety between 520–570 nm (for 1)/555–650 nm (for 2) with simultaneous fluorescence enhancement at 470 nm and 505 nm for dyads 1 and 2, respectively. Hence these dyads provide opportunity for ratiometric analysis of 1–50 μM Cu²⁺. The other metal ions viz. Fe³⁺, Co²⁺, Ni²⁺, Cd²⁺, Zn²⁺, Hg²⁺, Ag⁺, Pb²⁺, Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺ do not interfere in the estimation of Cu²⁺ except Cr³⁺ in case of dyad 1. The coordination of dimethylamino group of dansyl unit with Cu²⁺ causes quenching of fluorescence due to dansyl moiety between 520–600 nm and also restricts the photoinduced electron transfer from dimethylamino to anthracene moiety to release fluorescence between 450–510 nm. This simultaneous quenching and release of fluorescence respectively due to dansyl and anthracene moieties emulates into Cu²⁺ induced ratiometric change.