Synthesis and photophysical investigation of squaraine rotaxanes by "clicked capping"

Abstract

Pseudorotaxane complexes of squaraine dyes and tetralactam macrocycles are converted into permanently interlocked rotaxane structures using copper-catalyzed and copper-free cycloaddition reactions with bulky stopper groups. The photophysical properties of the encapsulated squaraine depend on the structure of the macrocycle. In one case, squaraine rotaxanes are produced in near-quantitative yields and with intense near-IR fluorescence. In another case, squaraine fluorescence is greatly diminished upon macrocyclic encapsulation but the signal can be restored by dye displacement with anions.

Figure 1

Partial ¹H NMR spectra of M2 (top), T1 (bottom), and the resulting clicked rotaxane M2⊃T1 (middle).

Figure 2

Fluorescence emissions of separate CH₂Cl₂ solutions of (a) D1, (b) M2⊃D1, and M2⊃D1 in the presence of (c) TBA⁺·Cl⁻, (d) TBA⁺·H₂PO₄⁻, (e) TBA⁺·CH₃COO⁻, and (f) TBA⁺·C₆H₅COO⁻ at 5 mM for each anion. The concentrations of D1 and M2 were 1 μM and 3 mM, respectively.

Figure 3

Partial ¹H NMR spectra of M3 (top), D1 (bottom), and the resulting clicked rotaxane M3⊃T2 (middle).

Figure 4

Fluorescence spectra in chloroform (ex: 590 nm); (a) pseudo-rotaxane M3⊃D1 (6 μM) yields free D1 (638 nm) and M3⊃D1 (694 nm); (b) M3⊃T2 (6 μM).

Scheme 1

Synthesis of squaraine rotaxanes via capping (top) and clipping (bottom).

Scheme 2

Capping of pseudorotaxane M3⊃D2 to produce M3⊃T3.