M6L12 Nanospheres with Multiple C70 Binding Sites for 1O2 Formation in Organic and Aqueous Media

Abstract

Singlet oxygen is a potent oxidant with major applications in organic synthesis and medicinal treatment. An efficient way to produce singlet oxygen is the photochemical generation by fullerenes which exhibit ideal thermal and photochemical stability. In this contribution we describe readily accessible M₆L₁₂ nanospheres with unique binding sites for fullerenes located at the windows of the nanospheres. Up to four C₇₀ can be associated with a single nanosphere, presenting an efficient method for fullerene extraction and application. Depending on the functionality located on the outside of the sphere, they act as vehicles for ¹O₂ generation in organic or in aqueous media using white LED light. Excellent productivity in ¹O₂ generation and consecutive oxidation of ¹O₂ acceptors using C₇₀⊂[Pd₆L₁₂], C₆₀⊂[Pd₆L₁₂] or fullerene soot extract was observed. The methodological design principles allow preparation and application of highly effective multifullerene binding spheres.

Figure 1

Schematic picture of the mechanism of photochemical generation of singlet oxygen by fullerene.

Figure 2

Illustration of fullerene binding hosts based on coordination driven self-assembly (top). Design strategy for a multiple-fullerene binding assembly (bottom).

Figure 3

Structure of the herein investigated ditopic ligand building blocks used for the preparation of Pd₆L₁₂ nanospheres.

Figure 4

Characterization of Pd₆L^N₁₂. (A) Reaction conditions for formation of nanospheres. Molecular structure of the displayed sphere was minimized at the PM3 level. Carbon is displayed in yellow, nitrogen in blue, palladium as orange spheres. (B) Overlayed DOSY NMR of the [Pd₆L^N₁₂] sphere (blue) and the building block (red). (C) ¹H NMR spectra of [Pd₆L^N₁₂] sphere and the corresponding building block. (D) ESI-MS spectrum of [Pd₆L^N₁₂].

Figure 5

Fullerene binding assay of Pd₆L₁₂ nanospheres. (A) Reaction conditions for formation of host–guest complexes. Molecular structure of the displayed assembly was minimized at the PM3 level. Carbon is displayed in yellow, nitrogen in blue, palladium as orange spheres, and fullerene C₇₀ as white spheres. (B) ¹³C NMR spectra of [Pd₆L^N₁₂] nanosphere and the corresponding fullerene adducts. (C) Distribution of fullerenes bound to different types of nanospheres based on ESI-MS analysis. (D) Example of an UV–vis titration of C₇₀ to a solution of [Pd₆L^N₁₂]. Inset: 1:2, H/G binding fit on changes of two different wavelengths. (E) Binding constant of fullerene to different types of spheres obtained by UV–vis titrations.

Figure 6

Computational investigation on C₇₀ binding of [Pd₆L^N₁₂] using molecular dynamics (MD). (A) Display of averaged total association enthalpies for different amount of C₇₀ bound to a single sphere and the obtained distribution of C₇₀ associated with [Pd₆L^N₁₂] using the MS analysis. (B) Optimized structure of four C₇₀ associated with a single sphere, displaying the window binding motif. (C) Optimized structure of 5 C₇₀ associated with a single sphere, displaying the creation of a hydrophobic interior binding site for the fifth C₇₀.

Scheme 1. Oxidation of Biomolecules by Light Induced Singlet Oxygen Formation in Aqueous Media

Top: [Pt₆L^PEGPy₁₂] sphere formation procedure and C₇₀ incorporation. Bottom: Standard reaction condition for oxidation of biomolecules using C₇₀⊂[Pt₆L^PEGPy₁₂]: (A) C₇₀⊂[Pt₆L^PEGPy₁₂] 4.16 nmol, substrate 10 μmol in 1 mL of D₂O and 5 μL of DMSO (0.5%), 4 h, room temperature, white 11W LED. Deviation from standard reaction conditions: (B) reaction performed in 0.5 mL of D₂O and 0.5 mL of (1 N) PBS_aq; (C) C₇₀⊂[Pt₆L^PEGPy₁₂] 41.6 nmol. Turnover number (TON) and conversion were determined by ¹H NMR using maleic acid as internal standard. (D) Empty [Pt₆L^PEGPy₁₂] 4.16 nmol used as a catalyst in 0.5 mL of D₂O and 0.5 mL of (1 N) PBS_aq.