Dansyl - naphthalimide dyads as molecular probes: effect of spacer group on metal ion binding properties

Abstract

Interaction of a few dansyl-naphthalimide conjugates 1a-e linked through polymethylene spacer groups with various metal ions was investigated through absorption, fluorescence, NMR, isothermal calorimetric (ITC), and laser flash photolysis techniques. The characteristic feature of these dyads is that they exhibit competing singlet-singlet energy transfer (SSET) and photoinduced electron transfer (PET) processes, both of which decrease with the increase in spacer length. Depending on the spacer group, these dyads interact selectively with divalent Cu²⁺ and Zn²⁺ ions, as compared to other mono- and divalent metal ions. Jobs plot analysis showed that these dyads form 2:3 complexes with Cu²⁺ ions, while 1:1 complexes were observed with Zn²⁺ ions. The association constants for the Zn²⁺ and Cu²⁺ complexes were determined and are found to be in the order 10³-10⁵ M^-1. Irrespective of the length of the spacer group, these dyads interestingly act as fluorescence ratiometric molecular probes for Cu²⁺ ions by altering the emission intensity of both dansyl and naphthalimide chromophores. In contrast, only the fluorescence intensity of the naphthalimide chromophore of the lower homologues (n = 1-3) was altered by Zn²⁺ ions. ¹H NMR and ITC measurements confirmed the involvement of both sulfonamide and dimethylamine groups in the complexation with Cu²⁺ ions, while only the latter group was involved with Zn²⁺ ions. Laser excitation of the dyads 1a-e showed formation of a transient absorption which can be attributed to the radical cation of the naphthalimide chromophore, whereas only the triplet excited state of the dyads 1a-e was observed in the presence of Cu²⁺ ions. Uniquely, the complexation of 1a-e with Cu²⁺ ions affects both PET and SSET processes, while only the PET process was partially inhibited by Zn²⁺ ions in the lower homologues (n = 1-3) and the higher homologues exhibited negligible changes in their emission properties. Our results demonstrate that the spacer length dependent variations of the photophysical properties of these novel conjugates not only enable the selective detection of Cu²⁺ and Zn²⁺ ions but also aid in discriminating these two biologically important metal ions.