Chelating Rotaxane Ligands as Fluorescent Sensors for Metal Ions

Abstract

Although metal-ion-binding interlocked molecules have been under intense investigation for over three decades, their application as scaffolds for the development of sensors for metal ions remains underexplored. In this work, we demonstrate the potential of simple rotaxanes as metal-ion-responsive ligand scaffolds through the development of a proof-of-concept selective sensor for Zn²⁺ .

Keywords: fluorescent probes; rotaxanes; sensors; supramolecular chemistry; zinc.

Figure 1

Emission profile of rotaxane 4 (MeCN, 100 μm, λ _ex=343 nm) in the presence of varying amounts of Zn(ClO₄)₂⋅6 H₂O (a), and in the presence of 5 equiv M(ClO₄)₂ (b). No emission was observed with Fe(ClO₄)₂.

Figure 2

Rotaxanes a) 5 (λ _ex=435 nm), b) 6 (λ _ex=380 nm), and c) 7 (λ _ex=379 nm) and their fluorescence response to 5 equiv M(ClO₄)₂ (MeCN, 100 μm; blue bars). Red bars in (c) refer to the fluorescence response upon sequential addition of M²⁺ followed by Zn²⁺ (5 equiv each). R=CH₂C(H)Ph₂, R′=3,5‐di‐^tBu‐C₆H₃.

Figure 3

Partial ¹H NMR spectra (CD₃CN, 400 MHz, 298 K) of a) the non‐interlocked axle of rotaxane 7, b) rotaxane 7, and c) rotaxane 7+Zn(ClO₄)₂⋅6 H₂O. For labelling scheme, see Scheme 1 (macrocycle) and Figure 2 (axle).

Figure 4

a) Solid‐state structure of rotaxane 7 (selected distances in Å: C−H_m⋅⋅⋅N=2.46, C‐H_k⋅⋅⋅N=2.71, C−H_k⋅⋅⋅π=2.66, C‐H_l⋅⋅⋅π=2.79; dihedral angle C_k‐S‐C‐C_ipso=8.2°). b) Solid‐state structure of [Zn(7)]²⁺ (selected distances: C−O⋅⋅⋅Zn=2.28, C−H_m⋅⋅⋅π=2.74, C−H_F⋅⋅⋅π=2.86; dihedral angle C_k‐S‐C‐C_ipso=29.9°).

Scheme 1

Synthesis of fluorescent rotaxane 4 using the AT‐CuAAC reaction. R=CH₂C(H)Ph₂.

Figure 5

UV/Vis (λ=322 nm) titrations of 7 (100 μm) with M(ClO₄)₂ as a function of a) Zn²⁺, b) Zn²⁺ (2 % H₂O/MeCN), c) Cd²⁺ (MeCN), and d) Cd²⁺ (2 % H₂O‐MeCN). e) Emission spectra (λ _ex=380 nm) of 7 (black), 7+5 equiv Cd²⁺ (blue), 7+5 equiv Zn²⁺ (red), 7+5 equiv each Zn²⁺ and Cd²⁺ (black dashed) in 2 % H₂O/MeCN.