Glycoluril-tetrathiafulvalene molecular clips: on the influence of electronic and spatial properties for binding neutral accepting guests

Abstract

Glycoluril-based molecular clips incorporating tetrathiafulvalene (TTF) sidewalls have been synthesized through different strategies with the aim of investigating the effect of electrochemical and spatial properties for binding neutral accepting guests. We have in particular focused our study on the spacer extension in order to tune the intramolecular TTF···TTF distance within the clip and, consequently, the redox behavior of the receptor. Carried out at different concentrations in solution, electrochemical and spectroelectrochemical experiments provide evidence of mixed-valence and/or π-dimer intermolecular interactions between TTF units from two closed clips. The stepwise oxidation of each molecular clip involves an electrochemical mechanism with three one-electron processes and two charge-coupled chemical reactions, a scheme which is supported by electrochemical simulations. The fine-tunable π-donating ability of the TTF units and the cavity size allow to control binding interaction towards a strong electron acceptor such as tetrafluorotetracyanoquinodimethane (F4-TCNQ) or a weaker electron acceptor such as 1,3-dinitrobenzene (m-DNB).

Keywords: donor–acceptor interactions; glycoluril; molecular clips; supramolecular chemistry; tetrathiafulvalene.

Figure 1

Structures of molecular clips 1–4.

Scheme 1

Different routes developed for the synthesis of molecular clips 1–4.

Scheme 2

Reaction between diphenylglycoluril with 4,5-bis(bromomethyl)-2-thioxo-1,3-dithiole.

Figure 2

Intramolecular distances between TTF moieties from X-ray analysis for clips 2 and 3 and theoretical calculations for clip 4.

Figure 3

Cyclic voltammograms of molecular clips 1, 2, 3, 4 and F₄-TCNQ at 10⁻³ M in 0.1 M TBAPF₆/CH₂Cl₂/CH₃CN 3:1 on a glassy carbon electrode at 100 mV·s⁻¹.

Figure 4

Cyclic voltammograms of molecular clip 2 at different concentrations (left: 10⁻⁵ M; middle: 10⁻⁴ M; right: 10⁻³ M) in 0.1 M TBAPF₆/CH₂Cl₂/CH₃CN 3:1 on a glassy carbon electrode at 100 mV·s⁻¹. Red dashed line: electrochemical simulation performed from experiments (E₁ = 0.090 V, E₂ = 0.165 V, E₃ = 0.435 mV, K_MV = 11400 M⁻¹, K_DIM = 680 M⁻¹, k_f = 1 × 10⁹ M⁻¹·s⁻¹ for all the forward reactions, α = 0.5 and k_s = 0.01 cm·s⁻¹ for all the charge transfers).

Scheme 3

Graphical representation of the stepwise oxidation of molecular clips 1, 2 and 3.

Scheme 4

Electrochemical mechanism used to simulate the CVs of molecular clips 1, 2 and 3.

Figure 5

Chemical oxidation of molecular clip 1 (10⁻⁴ M, CH₂Cl₂) using aliquots of NOSbF₆ oxidizing reagent (5 × 10⁻³ M, CH₃CN).

Figure 6

Spectroelectrochemical experiment of molecular clip 1 during the first oxidation step at different concentrations (left: 5 × 10⁻⁵ M; right : 5 × 10⁻⁴ M) in 0.1 M TBAPF₆/CH₂Cl₂/CH₃CN (3:1) on glassy carbon on Pt electrode in thin layer conditions (close to 50 µm) electrode at 5 mV·s⁻¹ and 293 K. (top) CV in current vs time representation. (middle) 3D representation: x-axis = wavelength, y-axis = time and z-axis = absorbance. (bottom) concentration-time profiles of each simulated species in the thin layer (50 µm) calculated from electrochemical simulations of Figure 4 (E₁ = 0.090 V, E₂ = 0.165 V, E₃ = 0.435 mV, K_MV = 11400 M⁻¹, K_DIM = 680 M⁻¹, k_f = 1 × 10⁹ M⁻¹·s⁻¹ for all the forward reactions, α = 0.5 and k_s = 0.01 cm·s⁻¹ for all the charge transfers).

Figure 7

Molecular structure of molecular clip 15 and representation of its stepwise oxidation processes proposed by Chiu et al. [58].

Figure 8

Molecular packing diagram of clips 2 (left) and 3 (right) obtained from X-ray analysis. A molecule of CH₂Cl₂ is included inside the cavity of each clip. Hydrogen atoms are omitted for clarity.

Figure 9

Left: Job plot analysis for DNB vs molecular clip 3 ([3 + DNB] = 10⁻³ M in o-C₆H₄Cl₂ at 800 nm) at room temperature. Middle: Job plot for F₄-TCNQ vs molecular clip 3 at room temperature ([3 + F₄-TCNQ] = 10⁻⁵ M in CH₂Cl₂ at 860 nm) consistent with a 2:1 binding stoichiometry. Right: Job plot for F₄-TCNQ vs molecular clip 4 at room temperature ([4 + F₄-TCNQ] = 10⁻⁵ M in CH₂Cl₂ at 860 nm) in agreement with a 1:1 binding stoichiometry.

Figure 10

UV–visible absorption spectra of F₄-TCNQ (CH₂Cl₂, 10⁻⁵ M) upon titration with molecular clip 3 (CH₂Cl₂, 5 × 10⁻⁴ M).

Figure 11

Redox interaction (left) and complexation (right) of F₄-TCNQ with molecular clips 3 and 4.