Viologen–cucurbituril host/guest chemistry – redox control of dimerization versus inclusion

Abstract

Two calix[4]arene systems, C23⁴⁺ and C24⁴⁺ – where 2 corresponds to the number of viologen units and 3–4 corresponds to the number of carbon atoms connecting the viologen units to the macrocyclic core – have been synthesized and led to the formation of [3]pseudorotaxanes when combined with either CB[7] or CB[8]. The [3]pseudorotaxanes spontaneously dissociate upon reduction of the bipyridinium units as the result of intramolecular dimerization of the two face-to-face viologen radical cations. CB[7] and CB[8]-based [2]pseudorotaxanes containing monomeric viologen guest model compounds, MC3²⁺ and MC⁴⁺, do not undergo decomplexation and dimerization following electrochemical reduction of their bipyridinium units.

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Introduction

The stimuli–responsive properties of a molecule with redox-active units can be advantageously used in a broad range of applications including drug delivery and development, electronic displays, electronic memory, batteries, and multi-state switchable materials.^1–11 Among these redox-active molecules, for example metallocenes, tetrathiafulvalene derivatives, naphthalene diimides, and benzoquinones, bipyridinium derivatives¹² (also known as viologens) are of particular interest because of their vibrant electrochromic properties. Bipyridinium dications (BIPY²⁺), for example, are typically colorless whereas BIPY˙⁺ radical monocations¹³ are deep blue in color.¹⁴ Furthermore, these radical cations spontaneously undergo π-dimerization, a process known as pimerization¹⁵ that is driven by radical–radical interactions that favor the formation of the singlet state (i.e., panchromatic absorption)^14–16 to afford the (BIPY˙⁺)₂ dimer in aqueous solution. Pimerization of BIPY˙⁺ radical cations has been used in supramolecular and mechanostereochemical systems to generate translational and rotational molecular movement in many recently reported mechanically interlocked molecules.^17–19 Furthermore, the rich redox chemistry of viologens also makes them attractive for use in electrochromic, catalytic, and bioanalytical applications.^20–22

However, one major complication that limits the use of viologen-based pimerization to develop stimuli–responsive materials is that the system suffers from low dimerization constants (e.g., log K_Dim = 2.70).²³ This results in loosely associated π-dimers in solution, and thus requires the use of high concentrations of the constituent molecular components to drive the binding. The use of low temperatures, micelle environments,²⁴ or macrocyclic hosts with large cavity sizes such as cucurbiturils (e.g., CB[8]), cyclodextrins (e.g., β-CD), or Stoddart's Blue Box^17,18,25 can help favor the formation of intermolecular π-dimer species. A. E. Kaifer, K. Kim and their coworkers described for the first time the interaction of MV²⁺ with cucurbit[n]uril hosts such as CB[7] and CB[8].^26–28 With CB[8], it was found that MV²⁺ strongly binds in a 1 : 1 stoichiometry inside the macrocycle cavity, the major driving force being ion–dipole interactions between the positively charged viologen and the oxygen rich host portals atoms. One electron reduction of methyl viologen MV²⁺ allowed the formation of a bisradical species ((MV˙⁺)₂⊂CB[8]). The dimerization constant of the MV˙⁺ in the presence of the CB[8] was estimated to be ∼2 × 10⁷ M⁻¹ which is about 10⁵ times larger than MV˙⁺ alone.^28,29 Supramolecular assemblies based on CB[8] have been reviewed recently by A. E. Kaifer, M. D. Garcia et al.³⁰

Alternatively, it has been shown that pre-organizing two or more organic radicals with well-tailored anchoring platforms and appropriate chain lengths can strengthen the intramolecular binding of the π-dimers in solution.³¹ We have previously demonstrated that a hexavalent phosphazene³⁰ fulfilled most of the prerequisites including ease of functionalization, and exhibited efficient and fast pimerization. This compound was applicable towards electrochromic material research and possessed specific recognition properties as a function of the viologen redox state. In addition, alkyl,^32–37 porphyrinic,^38,39 ferrocenyl,⁴⁰ aryl,^32–44 C₆₀,⁴⁵ and calixarene^46,47 linkers, as well as dendritic^48,49 and covalent organic polymeric^50,51 viologen-based materials (Scheme S1 in the ESI†) have also been demonstrated to promote intramolecular dimerization of their corresponding bipyridinium radical cations.

In line with our ongoing research on viologen-based systems for generating redox-active devices, we turned our attention to calix[4]arene as an inert anchoring platform. Calix[4]arenes can be easily chemically modified and their structural and conformational properties make for an excellent foundation upon which to build increasingly complex architectures. Calix[4]arene can exist in four conformations in solution, the cone, the partial cone, the 1,2-alternate, and the 1,3-alternate. Furthermore, the alkylation selectivity, in regard to the number of substituted phenols, can be controlled by the reaction conditions, thereby giving rise to a broad range of conformers and cavity sizes.⁵²

Herein, we report the synthesis and physicochemical characterization of a series of host–guest monomeric and calix[4]arene-functionalized dimeric viologen⊂CB-based pseudorotaxane complexes and the structure–property relationships that enable their electrochemical-triggered decomplexation. Calix[4]arene was functionalized with two terminal viologens using spacers of varying length (Fig. 1).⁴⁵ The use of synthetic receptors that have both a high affinity and a high selectivity for the binding of guests in water is indeed a very interesting prospect.^53–56 The recognition properties of the viologen-based systems (oxidized and reduced) with CB[7] and CB[8] were analyzed using a set of complementary analytical methods which include ESI-MS, square wave and cyclic voltammetry, UV-visible-near infrared (NIR) spectrophotometry, ¹H-NMR, and electron paramagnetic resonance (EPR).

Fig. 1 Chemical structures of the calix[4]arene viologen-based systems C23⁴⁺ and C24⁴⁺ and the reference compounds MC3²⁺ and MC4⁴⁺. The number 2 stands for the number of viologens while 3–4 correspond to the number of carbons of the chain connecting the terminal viologens to the calix[4]arene backbone. Methyl-viologen (MV²⁺) has been used as a model compound in this work. C22⁴⁺ (PF₆⁻ salt) was not synthesized in this work (see ref. 45) and only considered for DFT calculations.

Experimental section

Synthesis

Starting materials and solvents. All commercial reagents were purchased from Sigma-Aldrich and used without further purification. 1-Methyl-[4,4′-bipyridin]-1-ium iodide (I1) was prepared according to literature procedures.⁵⁷ 1-(3-Bromopropoxy)-4-(tert-butyl)benzene (I2), 1-(4-bromo-butoxy)-4-(tert-butyl)benzene (I3), 1,3-bis(3-bromopropoxy)-p-tert-butyl-calix[4]arene (I5) and 1,3-bis(4-bromobutoxy)-p-tert-butyl-calix[4]arene (I6) were also prepared according reported procedures with slight modifications.^58,59 The dicationic dimethyl viologen MV·2I (Fig. 1)¹² and tetrakis-p-tert-butyl-calix[4]arene I4 (ref. 60 and 61) were prepared according to literature procedures. Thin layer chromatography (TLC) was used to follow the reactions and was performed on aluminium sheets bearing silica gel 60 F254 (E. Merck). Column chromatography was performed using silica gel (Merck; 40–63 μm). Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AC 400 with working frequencies of 400 and 100 MHz for ¹H and ¹³C, respectively. Chemical shifts are reported in ppm relative to the signals corresponding to the residual non-deuterated solvents (CDCl₃ (δ = 7.26), CD₃OD (δ = 3.34), and d₆-DMSO (δ = 2.50). High-resolution electrospray mass (HR-ESI) spectra were measured on a micro Q-TOF (Bruker) spectrometer.

1-(3-4-(Tert-butyl)phenoxy)propyl)-1-methyl-[4,4′-bipyridin]-1,1′-diium bromide iodide (MC3²⁺). 0.8 g (2.95 mmol) of I2 was mixed with 0.88 g (2.95 mmol) of I1 in 30 mL of acetonitrile (CH₃CN) and the mixture was refluxed for two days. The reaction mixture was filtered and washed with CH₃CN to afford MC3²⁺ as a red-orange solid (yield: 31%). ¹H NMR (400 MHz, CD₃OD): δ [ppm]: 9.36 [d, 2H, J = 6.8 Hz, Ar-H], 9.23 [d, 2H, J = 6.8 Hz, Ar-H], 8.70 [t, 4H, J = 7.2 Hz, Ar-H], 7.30 [d, 2H, J = 8.8 Ar-H], 6.77 [d, 2H, J = 9.2, Ar-H], 5.04 [t, 2H, J = 6.8 Hz, N–CH₂–CH₂], 4.57 [s, 3H, N–CH₃], 4.19 [t, 2H, J = 5.6 Hz, O–CH₂–CH₂], 2.66–2.60 [m, 2H, N–CH₂–CH₂–CH₂–O], 1.29 [s, 9H, C(CH₃)₃]. ¹³C NMR (100 MHz, CD₃OD): δ [ppm]: 158.08, 152.66, 152.21, 151.75, 148.88, 148.34, 145.96, 129.04, 128.84, 128.20, 124.39, 115.78, 66.58, 62.13, 35.75, 32.78. Melting point: 258–9 °C. HR-MS for C₂₄H₃₀ON₂ [MC3²⁺]: calcd m/z = 181.1174 and exp. m/z = 181.1172.

1-(4-4-(Tert-butyl) phenoxy)butyl)-1′-methyl-[4,4′-bipyridin]-1,1′-diium bromide iodide (MC4²⁺). 1.58 g (5.53 mmol) of I3 was mixed with 1.5 g (5.02 mmol) of I1 in 30 mL of CH₃CN and the mixture was refluxed for three days. The reaction mixture was filtered and washed with CH₃CN to afford MC4²⁺ as a yellow-orange solid (yield: 71%). ¹H NMR (400 MHz, CD₃OD): δ [ppm]: 9.36 [d, 2H, J = 7.2 Hz, Ar-H], 9.23 [d, 2H, J = 6.8 Hz, Ar-H], 8.73 [d, 2H, J = 6.8 Hz, Ar-H], 8.70 [d, 2H, J = 6.8 Hz, Ar-H], 7.32 [d, 2H, J = 8.8 Ar-H], 6.88 [d, 2H, J = 8.8, Ar-H], 4.90 [t, 2H, J = 7.6 Hz, N–CH₂–CH₂], 4.57 [s, 3H, N–CH₃], 4.10 [t, 2H, J = 6, CH₂–O–Ar], 2.39–2.32 [m, 2H, N–CH₂–CH₂–CH₂], 2.00–1.93 [m, 2H, O–CH₂–CH₂–CH₂], 1.31 [s, 9H, C(CH₃)₃]. ¹³C NMR (100 MHz, CD₃OD): δ [ppm]: 158.73, 152.13, 148.64, 148.01, 145.52, 129.21, 128.84, 128.09, 115.94, 69.03, 63.95, 35.70, 32.80, 30.55, 27.92. Melting point: 254–5 °C. HR-MS for C₂₅H₃₂ON₂ [MC4²⁺]: calcd m/z = 188.1252 and exp. m/z = 188.1245.

1,3-Bis(3-(1′-methyl-[4,4′-bipyridin]-1,1′-diium)propoxy)-p-tert-butyl-calix[4]-arene dibromide diiodide (C23⁴⁺). 1.192 g (4 mmol) of I1 was added to the flask containing 0.365 g (0.4 mmol) of I5 and 30 mL of CH₃CN. The mixture was left under reflux for 3 days. The solvent was fully removed under vacuum. The product (C23⁴⁺, red-orange solid, yield: 85%) was washed with water, filtrated, and dried under vacuum. ¹H NMR (400 MHz, CD₃OD): δ [ppm]: 9.60 [d, 4H, J = 6.8 Hz, Ar-H], 9.22 [d, 4H, J = 6.8 Hz, Ar-H], 8.82 [d, 4H, J = 5.6 Hz, Ar-H], 8.74 [d, 4H, J = 6.8 Hz, Ar-H], 7.22 [s, 4H, Ar(calix)-H], 7.07 [s, 4H, Ar(calix)-H], 5.30 [t, 4H, J = 7.2 Hz, N–CH₂–CH₂], 4.56 [s, 6H, N–CH₃], 4.32–4.23 [d, 8H, Ar-CH₂-Ar and CH₂–O-Ar], 3.50 [d, 4H, J = 13.6, Ar-CH₂-Ar], 3.02–2.95 [m, 4H, CH₂–CH₂–CH₂], 1.30 [s, 18H, C(CH₃)₃], 1.02 [s, 18H, C(CH₃)₃]. ¹³C NMR (100 MHz, d₆-DMSO): δ [ppm]: 150.65, 150.36, 149.54, 148.80, 148.26, 147.52, 146.77, 142.61, 133.91, 128.37, 127.70, 127.00, 126.662, 126.31, 73.26, 58.88, 49.04, 34.92, 37.55, 32.39, 32.29, 31.76, 31.61. Melting point: 295–6 °C. HR-MS for C₇₂H₈₈O₄N₄ [C23⁴⁺ + 2e⁻]:²⁺ calcd m/z = 536.3397 and exp. m/z = 536.3382.

1,3-Bis(3-(1′-methyl-[4,4′-bipyridin]-1,1′-diium)butoxy)-p-tert-butyl-calix[4]arene dibromide diiodide (C24⁴⁺). 0.368 g (0.4 mmol) of I6 was dissolved in 30 mL of CH₃CN. 1.192 g (4 mmol) of I1 was added. The mixture was left under reflux for 3 days. The solvent was fully removed under vacuum. The product obtained as an orange solid (C24⁴⁺, yield: 67%) was washed with water (due to its poor solubility in water), filtrated and dried under vacuum. ¹H NMR (400 MHz, CD₃OD): δ [ppm]: 9.54 [d, 4H, J = 6.4 Hz, Ar-H], 9.21 [d, 4H, J = 6.4 Hz, Ar-H], 8.78 [d, 4H, J = 6.4 Hz, Ar-H], 8.71 [d, 4H, J = 6.8 Hz, Ar-H], 7.19 [s, 4H, Ar(calix)-H], 7.05 [s, 4H, Ar(calix)-H], 5.15 [t, 4H, J = 7.6 Hz, N–CH₂–CH₂], 4.56 [s, 6H, N–CH₃], 4.32 [d, 4H, J = 12.8, Ar-CH₂-Ar], 4.16 [t, 4H, J = 6.8 Hz, CH₂–O-Ar], 3.48 [d, 4H, J = 12.8, Ar-CH₂-Ar], 2.66–2.58 [m, 4H, N–CH₂–CH₂–], 2.25–2.17[m, 4H, –CH₂–CH₂–O], 1.29 [s, 18H, C(CH₃)₃], 1.02 [s, 18H, C(CH₃)₃]. ¹³C NMR (100 MHz, DMSO): δ [ppm]: 150.79, 150.46, 149.51, 148.89, 148.10, 147.48, 146.74, 142.42, 134.02, 128.41, 127.57, 127.01, 126.56, 126.21, 76.01, 61.22, 49.02, 34.92, 34.52, 32.29, 31.79. Melting point: 224–5 °C. HR-MS for C₇₄H₉₂O₄N₄ [C24⁴⁺ + 2e⁻]²⁺: calcd m/z = 550.3554 and exp. m/z = 550.3561.

Physico-chemistry. The experimental details and related figures are given in the ESI.†

DFT calculations. The singlet states of the C22²⁽˙⁺⁾, C23²⁽˙⁺⁾ and C24²⁽˙⁺⁾ systems were investigated using DFT calculations within the hybrid meta-GGA approximation with the M06-2X functional⁶² and the Gaussian 16 package.⁶³ The standard 6-311G(d,p) basis set was used throughout. Solvent effects (water) were incorporated with the polarized continuum model (PCM) using default settings and the integral equation formalism.⁶⁴ Frequency calculations were used to confirm that the optimized geometries corresponded to local energy minima on the potential energy surface. The M06-2X/6-311G(d,p) approach in combination with the PCM was found to provide good results for the description of π-stacking interactions of viologen derivatives.⁶⁵

EPR spectroscopy. EPR spectra were recorded on a CMS 800 EPR spectrometer 8400 and measurements were carried out with solutions of the calixarene-bis-viologens compounds C23⁴⁺ and C24⁴⁺, along with their model ligands, namely MC3²⁺ and MC4²⁺, in the absence and in the presence of CB[7]. This study was performed to evaluate whether or not the radical cations were dimerized or segregated⁶⁶ depending on the composition of the mixture. The radical cations species were produced chemically by addition of an excess of a reducing agent (e.g., sodium dithionite Na₂S₂O₄). Each solution was prepared at a concentration of ∼10⁻⁴ M. The EPR data are available in Fig. S17–S20 in the ESI.†

Electrochemistry. All the solutions used for the electrochemical experiments were prepared from 0.1 M TBACl aqueous solution with following concentrations of each species: MC4²⁺ (C = 6.3 × 10⁻⁵ M), MC3²⁺ (C = 6 × 10⁻⁵ M), C24⁴⁺ (C = 5 × 10⁻⁵ M), and C23⁴⁺ (C = 5 × 10⁻⁵ M) with CB[7] (C = 2 × 10⁻⁴ M). The setup comprises a Gamry Multipurpose instrument (Reference 600) interfaced to a PC. The experiments were performed using a glassy carbon working electrode (0.071 cm², BASi). The electrode surface was polished routinely with 0.05 μm alumina–water slurry on a felt surface immediately before use. The counter electrode was a Pt coil and the reference electrode was a Ag/AgCl electrode. Cyclic voltammetry (CV) and square wave (SW) differential pulse voltammetry were carried out at room temperature in an argon-purged H₂O solution between 0 V (initial potential) and −1.2 V (final potential). Chronocoulometry experiments were performed upon stepping from 0 to −0.7 V for 10 s before measuring the response signal corresponding to a radical-cation. Then the potential is stepped from −0.7 V to 0 V allowing measurement of the response signal corresponding to the fully oxidized species. The experimental errors on the potential values are ±10 mV.

Results and discussion

Synthesis

Two calix[4]arene systems, namely C23⁴⁺ and C24⁴⁺, whereby 2 stands for the number of viologens and 3–4 correspond to the number of carbons of the chain connecting the viologens to the calix[4]arene core (Scheme 1), were synthesized. For a deeper understanding of the host–guest complexation properties of the viologen derivatives with either CB[7] or CB[8], two model compounds, namely MC3²⁺ and MC4²⁺, were also prepared. 4,4′-bipyridine was first monomethylated using 1 equivalent of methyl iodide in acetone leading to I1 with 63% yield. Tert-butyl-phenol was reacted either with 1,3-dibromo-propane or 1,4-dibromo-butane in acetone in the presence of 1 equivalent of potassium carbonate to provide intermediates I2 or I3, respectively. The synthesis of MC3²⁺ and MC4²⁺ was then achieved following the reaction of I2 or I3 with 1-methyl-4,4′-bipyridinium (I1) in CH₃CN. The synthesis of the tetrakis-p-tert-butyl-calix[4]arene I4 (63% yield) was achieved by reaction of p-tert-butyl-phenol with excess of formaldehyde in the presence of NaOH. As described above, I4 was then reacted with 1,3-dibromo-propane and 1,4-dibromo-butane to lead to intermediates I5 (92% yield) or I6 (65% yield), respectively. Reacting these two calix[4]arene intermediates with I1 then leads to the targeted bis-viologen systems C23⁴⁺ (85% yield) and C24⁴⁺ (67% yield). All these viologen derivatives were isolated in the form of mixed iodide/bromide salts.

Scheme 1 Synthetic route to the MC3²⁺ and MC4²⁺ monomers as well as the calix[4]arene-bis-viologen systems C23⁴⁺ and C24⁴⁺. Conditions: (i) CH₃I, acetone, reflux, 24 h, 63%; (ii) I2: 1,3-dibromopropane, K₂CO₃, acetone, reflux, 48 h, yield 70%; I3: 1,4-dibromobutane, K₂CO₃, acetone, reflux, 72 h, yield 27%; (iii) MC3²⁺: I1, CH₃CN, reflux, 72 h, 31%; MC4²⁺: I1, CH₃CN, reflux, 72 h, 71%; (iv) NaOH, HCHO, 100 °C, then heating at 195–250 °C in diphenyl ether, toluene, xylene, yield 63%; (v) I5: 1,3-dibromopropane, K₂CO₃, acetone, reflux, 96 h, yield 92%; I6: 1,4-dibromobutane, K₂CO₃, acetone, reflux, 96 h, yield 65%; (vi) (iii) C23⁴⁺: I1, CH₃CN, reflux, 72 h, 85%; C24⁴⁺: I1, CH₃CN, reflux, 72 h, 67%.

Characterization of the thread/CB[7] [n]pseudorotaxanes

Recognition of MC3²⁺ and MC4²⁺ by CB[7]. The UV-vis absorption spectrophotometric binding titration of MC3²⁺ (Fig. S1 and S2 in the ESI†) and MC4²⁺ (Fig. S3 and S4 in the ESI†) with CB[7] and their corresponding Job plots provided evidence that MC3²⁺⊂CB[7] and MC4²⁺⊂CB[7] exist as 1 : 1 stoichiometric host–guest complexes. Both MC3²⁺ and MC4²⁺ are characterized by two main absorptions at ∼225 nm and 260 nm (Table 1) that are attributed to the π–π* transitions of the tert-butyl-phenyl and BIPY²⁺ chromophoric units, respectively. It was observed that complexation with CB[7] induces a hypochromic shift of the absorption band attributed to the BIPY²⁺ transitions, while those of the tert-butyl-phenyl units remain almost unaltered. This suggested that CB[7] is bound to the BIPY²⁺ subunit rather than the apparently more hydrophobic alkoxy-tert-butyl-phenyl group. This binding behavior is most likely explained by the weaker affinity⁶⁷ of the alkoxy-tert-butyl-phenyl moiety for CB[7] and steric hindrance of its bulky tert-butyl group. The calculated binding constants (log K_{MC3²⁺⊂CB[7]} = 4.51(6) and log K_{MC4²⁺⊂CB[7]} = 4.68(5) determined in 0.1 M phosphate buffer, pH 7.0) were found to be lower than that determined for MV²⁺, most likely as a consequence of statistical effects. In this case, the two pyridinium aromatic faces of MV²⁺ are not sterically blocked to CB[7], while one side of the bipyridinium units of MC³⁺ and MC⁴⁺ are capped with a tert-butyl-phenyl bulky stopper. Consequently, the K_MC3²⁺/K_MV²⁺ (0.16) and K_MC4²⁺/K_MV²⁺ (0.24) ratios are close to the statistical value of 0.25.⁶⁸ At the higher concentrations that were used for ¹H NMR (Fig. S12 and S13 in the ESI†) and the higher CB[7]/MC3²⁺ or CB[7]/MC4²⁺ host–guest ratios employed for ESI-MS measurements (Fig. S8 and S9 in the ESI†), other host–guest complexes were observed, namely the [3]pseudorotaxanes, MC3²⁺⊂(CB[7])₂ and MC4²⁺⊂(CB[7])₂. For these complexes, the ¹H NMR titrations suggested that the second CB[7] macrocycle is sitting in close proximity to the tert-butyl-phenyl unit (Fig. 2a). Binding of the first CB[7] indeed mainly affects the β and β′ protons of the BIPY²⁺ unit, while recognition of the second CB[7] macrocycle influences the protons of the tert-butyl-phenyl residue (Fig. S12 and S13 in the ESI†). The inability to quantify the second recognition event leading to the formation of MC3²⁺⊂(CB[7])₂ or MC4²⁺⊂(CB[7])₂ by absorption spectrophotometry likely resulted from weak spectral variations and their much lower stability constants.

Table 1 Thermodynamic and spectroscopic parameters of [n]pseudorotaxanes formed with CB[7]^a

Viologen V	log KV⊂CB[7]	V	V⊂CB[7]
		λ (ε) nm (10^4 M^−1 cm^−1)	λ (ε) nm (10^4 M^−1 cm^−1)
a Solvent: water buffered at pH 7.0 with 0.1 M Na2H2PO4/NaH2PO4); l = 1 cm; T = 25.0(1) °C. The error (indicated in brackets) on the stability constants correspond to 3σ with σ = standard deviation. sh = shoulder.
MV^2+	5.30 (2)	227 (2.96)/257 (2.06)	226 (3.13)/281 (sh)
MC3^2+	4.51 (6)	224 (2.29)/259 (2.17)	223 (2.49)/255 (1.51)
MC4^2+	4.68 (5)	224 (2.17)/260 (2.29)	224 (2.48)/252 (1.40)
C23^4+	4.5 (1)	224 (7.42)/262 (3.20)	225 (7.50)/275 (2.10)
C24^4+	4.44 (8)	220 (7.16)/263 (2.99)	220 (7.29)/280 (1.83)

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Fig. 2 Schematic representation of (a) the [2]pseudorotaxanes MC3²⁺⊂CB[7] and MC4²⁺⊂CB[7] (CB[7] mainly resides on the bipyridinium group) and of MC3²⁺⊂(CB[7])₂ and MC4²⁺⊂(CB[7])₂ (i.e., the hydrophobic cavity of the second CB[7] is sitting close to the terminal phenyl group); and (b) the [3]pseudorotaxanes C23⁴⁺⊂(CB[7])₂ and C24⁴⁺⊂(CB[7])₂ (i.e., the CB[7] mainly resides on the bipyridinium groups). n = 2, or 3. CB[7] is represented with the grey cylinder.

To gain further insight into the molecular recognition properties, diffusion coefficients were determined by ¹H NMR-DOSY for MC3²⁺ and MC4²⁺ both in the absence and in the presence of increasing amounts of CB[7] (Table 2). Firstly, a comparison of the diffusion coefficients of the free MC3²⁺ and MC4²⁺ revealed similar values, which indicated that both model systems share common structural properties (i.e., comparable volume of 303 cm³ mol⁻¹ for MC3²⁺ and 318 cm³ mol⁻¹ for MC4²⁺ estimated with DFT) notwithstanding the slightly elongated carbon spacer for MC4²⁺. Secondly, the values of the diffusion coefficients decreased as a function of the number of equivalents of CB[7], demonstrating the successive formation of host–guest species of increasing volume and weight (e.g., volume of 928 cm³ mol⁻¹ obtained with DFT for MC3²⁺⊂CB[7]). Lastly, the diffusion coefficients measured for MC3²⁺ and MC4²⁺ using the same amount of CB[7] are comparable to one another, which suggested that the [2]pseudorotaxanes, MC3²⁺⊂CB[7] and MC4²⁺⊂CB[7], and the [3]pseudorotaxanes, MC3²⁺⊂(CB[7])₂ and MC4²⁺⊂(CB[7])₂, are roughly structurally equivalent. The inability to determine the stability constant of [3]pseudorotaxane prevents the evaluation of the diffusion coefficients specific to each inclusion complex (i.e., [2]- and [3]pseudorotaxanes).

Table 2 Diffusion coefficients of MC3²⁺, MC4²⁺, C23⁴⁺ and C24⁴⁺ in the absence and the presence of CB[7] ^a

D (×10^−6 cm^2 s^−1)
No CB[7]		+1 eq. CB[7]		+2 eq. CB[7]		No CB[7]		+2 eq. CB[7]
MC3^2+	MC4^2+	MC3^2+	MC4^2+	MC3^2+	MC4^2+	C23^4+	C24^4+	C23^4+	C24^4+
a Measured by DOSY experiments on a 600 MHz spectrometer, D2O, 298 K. The errors on these values are estimated to less than 5%.
6.51	6.57	4.89	4.17	3.56	3.29	2.58	4.30	3.85	3.11

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Recognition of C23⁴⁺ and C24⁴⁺ by CB[7]. Altogether, absorption binding titrations (Fig. S6 and S7 in the ESI†), ¹H-NMR data (Fig. S14 and S15 in the ESI†), and ESI-MS experiments (Fig. S10 and S11 in the ESI†) demonstrated that the [3]pseudorotaxanes, C23⁴⁺⊂(CB[7])₂ and C24⁴⁺⊂(CB[7])₂, formed predominantly. Hypochromic and bathochromic shifts of the absorption bands which correspond to the π–π* transitions of the BIPY²⁺ cores were observed in both cases (Table 1) as a consequence of the encapsulation of the terminal BIPY²⁺ chromophores within the hydrophobic macrocyclic cavity of CB[7]. The absorption bands which correspond to the π–π* transitions for the calix[4]arene core remained unaffected by CB[7] complexation. As previously shown with the model systems, MC3²⁺ and MC4²⁺, the presence of a hydrophobic alkoxy substituent did not alter the molecular recognition properties. As a result, the two CB[7] macrocycles remained bound to the middle of each of the two BIPY²⁺ terminal electrophores as evidenced by ¹H NMR spectroscopy (Fig. S14 and S15 in the ESI†). The apparent stability constants (log K*_{CB23⁴⁺⊂(CB[7])2} = 4.5(1) and log K*_{CB24⁴⁺⊂(CB[7])2} = 4.44(8)) were found to be close to those evaluated for the corresponding models, MC3²⁺ and MC4²⁺ (Table 1). Again, these stability constant values were found to be lower than those measured for MV²⁺ (log K_MV²⁺⊂_CB[7] = 5.30(2)) as a consequence of the presence of the bulky calix[4]arene core (Fig. 2b).

Diffusion coefficients were also determined for C23⁴⁺ and C24⁴⁺ both in the absence and the presence of CB[7]. In contrast to the values measured for the model ligands, the D value measured for C23⁴⁺ was found to be lower than that of C24⁴⁺. One would expect that the extended spacer of C24⁴⁺ should increase the global volume of the molecule (i.e., DFT calculated volumes of 824 cm³ mol⁻¹ for C23⁴⁺ and 873 cm³ mol⁻¹ for C24⁴⁺), thereby causing it to diffuse slower in solution. This unexpected result thus might be explained by the slightly more flexible arms of C24⁴⁺ which should minimize the steric interactions of the viologen units with the bulky calix[4]arene core, thereby resulting in markedly different solvation properties with respect to C23⁴⁺. As a consequence, the diffusion coefficient decreases in the presence of CB[7] for C24⁴⁺, while it increases for C23⁴⁺, so that both converge to a comparable value, the volumes of host–guest species with C23⁴⁺ and C24⁴⁺ being then globally equivalent in the presence of CB[7].

Characterization of the radical cations

Intermolecular versus intramolecular dimerization of the radical cations. We have demonstrated that MC3˙⁺ and MC4˙⁺ rapidly pimerize (Fig. S21 and S22 in the ESI†) in aqueous solution with a log K_Dim value of 3.4. This value is very similar to the value previously determined for the benzyl methyl viologen radical cation BMV˙⁺ (log K_Dim = 3.46(5)),³⁰ but much higher than that reported for the methyl viologen radical cation MV˙⁺ (log K_Dim ∼2.5–2.9).^69,70 The values of K_Dim are also similar to those measured for viologens decorated with hydrophobic alkyl substituents.¹⁵ This suggested that increasing the hydrophobicity around the BIPY˙⁺ unit with aryl/alkyl substituents favors the intermolecular pimerization in aqueous solution. Moreover, we could again demonstrate that the extended spacer for MC4˙⁺ does not drastically alter its pimerization since similar K_Dim values were calculated for MC3˙⁺ and MC4˙⁺. These two radical cations are characterized by intense and structured absorption bands in the visible region (MC3˙⁺: λ_max ∼602 nm, ε⁶⁰² = 1.01 × 10⁴ M⁻¹ cm⁻¹; MC4˙⁺: λ_max ∼602 nm, ε⁶⁰² = 9.56 × 10³ M⁻¹ cm⁻¹, Fig. S21 and S22 in the ESI†), in agreement with the spectroscopic parameters determined for MV˙⁺ (600–606 nm)^15,23,67,71 and BMV˙⁺ (600 nm).³⁰ Formation of the radical cation dimers induced a significant hypsochromic shift of the absorption band at ∼600 nm (Δλ ∼50 nm) and gave rise to intense absorption bands corresponding to radical–radical transitions in the NIR region (λ_max > 850–900 nm).¹⁵ These UV-vis absorption studies also demonstrated that no significant pimerization occurs unless the concentration of the radical cations is high enough (>10⁻³ M, Fig. S21 and S22 in the ESI†). As previously reported,^31–45 preorganization of the BIPY²⁺ electrophores to within close proximity of one another around a robust and inert molecular platform can enhance their pimerization, leading to very stable π-dimers in aqueous solution.

Upon chemical generation of the radical cations, C23²⁽˙⁺⁾ and C24²⁽˙⁺⁾ (Fig. S27 and S28 in the ESI†), intense absorption bands appeared at ∼530 nm (Fig. 3). An additional intense absorption band centered at 1072 nm for C23²⁽˙⁺⁾ and at 925 nm for C24²⁽˙⁺⁾ were observed and are unambiguously assigned to the intramolecular charge resonance that occurs in the dimerized viologen radical cation species. This feature indicated that a two-electron reduction of C23⁴⁺ and C24⁴⁺ leads to two radical divalent cations, C23²⁽˙⁺⁾ and C24²⁽˙⁺⁾, that spontaneously pimerize intramolecularly, leading to a stable radical dimeric species.

Fig. 3 Electronic UV-vis-NIR absorption spectra of C23²⁽˙⁺⁾ and C24²⁽˙⁺⁾ in water. Solvent: water buffered at pH 7.0 with 0.1 M Na₂HPO₄/NaH₂PO₄. T = 25.0(1) °C.

The NIR absorption spectroscopic signatures of the π-dimerized complexes were demonstrated to be closely related to the extent of overlapping between both π-orbitals of the BIPY˙⁺ radicals.⁴⁵ When a propyloxy chain links the viologens to the calix[4]arene moiety (C23²⁽˙⁺⁾: λ_max ∼960 nm in CH₃CN⁴⁵ and λ_max ∼1070 nm in water), DFT calculations at the BLYP-D3/DZVP level suggested that only one pyridinium ring per viologen radical is involved in the intramolecular π-dimerization process. This result, which is imposed by geometric constraints that is inherent to the length and structure of the propyl chain, is not observed with the ethyloxy-derived calix[4]arene-bis-viologen (C22²⁽˙⁺⁾: λ_max ∼860 nm in CH₃CN⁴⁵), which affords a fully face-to-face stacked arrangement. DFT calculations performed at the M06-2X/6-311G(d,p) level (Fig. 4) provided a minimum energy conformation for C22²⁽˙⁺⁾ in which the two BIPY˙⁺ units display an almost perfect face-to-face interaction. Similarly, the BIPY˙⁺ units in C23²⁽˙⁺⁾ are twisted with respect to each other by an angle of ∼78°, reducing the overlap between the corresponding π-orbitals. Increasing the length from 3 to 4 carbons (butyloxy derivative, C24²⁽˙⁺⁾: λ_max ∼925 nm in water) does not completely reinstate the face-to-face dimerized π-stacking orientation, but it does increase the overlap of the π-orbitals with respect to C23²⁽˙⁺⁾ with a twist angle of ∼60°. Thus, the nature and size of the linker between the calix[4]arene platform and the terminal BIPY˙⁺ radical cations, as well as the nature of the platform, with the calix[4]arene displaying a cone conformation, appear to be crucial factors that govern the arrangement of the π-dimers. It is noteworthy that preorganization of the designed systems apparently allows for efficient intramolecular dimerization of the two viologens upon reduction in a range of solvents (e.g., water and CH₃CN⁴⁵).

Fig. 4 Geometries obtained with DFT (M062X/6-311G(d,p)) (left panel), the corresponding HOMOs (central panel) and detail of the arrangements of the redox-active BIPY˙⁺ units within the two electrons reduced calix[4]arene-bis-viologens C22²⁽˙⁺⁾, C23²⁽˙⁺⁾ and C24²⁽˙⁺⁾ that undergo intramolecular pimerization.

Reduction of the [n]pseudorotaxanes with CB[7]

Monocationic monoradicals MC3˙⁺ and MC4˙⁺. Cyclic voltammetry (Fig. S16 in the ESI†), square-wave voltammetry (Fig. 5), and EPR (Fig. S17 and S18 in the ESI†) measurements in phosphate-buffered solutions at pH 7 were performed on the model systems, MC3²⁺ and MC4²⁺, in the absence and presence of CB[7]. Both systems were characterized by two successive one-electron reversible redox waves: E_1/21 (MC3²⁺ → MC3˙⁺) = −0.56 V and E_1/22 (MC3˙⁺ → MC3⁰) = −0.83 V; E_1/2¹ (MC4²⁺ → MC4˙⁺) = −0.56 V and E_1/2² = −0.83 V. In the presence of three equivalents of CB[7], where it is assumed that MC3²⁺⊂CB[7] and MC4²⁺⊂CB[7] predominate, both redox waves shifted slightly to more negative potentials (Table 3) while retaining their reversible shapes.

Fig. 5 Square wave voltammograms of (a) MC3²⁺, [MC3²⁺] = 0.05 mM and b) MC4²⁺, [MC4²⁺] = 0.06 mM in the absence (orange) and the presence of CB[7] (blue, 3 equivalents of CB[7]). All voltammograms were recorded in argon-purged phosphate buffer solutions (pH 7) at 298 K (E versus Ag/AgCl).

Table 3 Values of E_1/2 (in V) for MC3²⁺ (0.05 mM), MC4²⁺ (0.063 mM), C23⁴⁺ (0.05 mM) and C24⁴⁺ (0.05 mM) measured by cyclic voltammetry (CV) and square wave voltammetry (SWV) in the absence and in the presence of 3 equivalents of CB[7] for MC3²⁺, MC4²⁺ and C23⁴⁺, and 4 equivalents of CB[7] forC24⁴⁺

Species	E1/2^1 (CV)	E1/2^1 (SW)	E1/2^2 (CV)	E1/2^2 (SW)
MC3^2+	−0.56 V	−0.56 V	−0.83 V	−0.83 V
MC3^2+ + 3 eq. CB[7]	−0.60 V	−0.59 V	−0.85 V	−0.86 V
MC4^2+	−0.56 V	−0.56 V	−0.83 V	−0.84 V
MC4^2+ + 3 eq. CB[7]	−0.58 V	−0.58 V	−0.83 V	−0.86 V
C23^4+	−0.42 V	−0.43 V	−0.84 V	−0.85 V
C23^4+ + 3 eq. CB[7]	−0.44 V	−0.46 V	−0.84 V	−0.85 V
C24^4+	−0.45 V	−0.47 V	−0.85	−0.85
C24^4+ + 4 eq. CB[7]	−0.50	−0.51	−0.85	−0.85

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These shifts corresponded to the signatures of the relative affinities of CB[7] for the different redox states of the model systems. Assuming a log K_{MC3²⁺⊂CB[7]} value of 4.51(6) for MC3²⁺⊂CB[7] (Fig. S1 in the ESI†), a log K_{MC3˙⁺⊂CB[7]} of ∼4–4.2 and a log K_{MC3⁽⁰⁾⊂CB[7]} of ∼3.5–3.9 values were accordingly calculated.^26,29 Similarly, using a log K_{MC4²⁺⊂CB[7]} value of 4.68(5) (Fig. S3 in the ESI†), a log K_{MC4˙⁺⊂CB[7]} of ∼4.2 and a log K_{MC4⁰⊂CB[7]} of ∼3.8 were calculated. Using an electrochemical approach, these calculated values were in reasonably good agreement, within error, with those derived from direct absorption binding titrations (log K_{MC3˙⁺⊂CB[7]} = 3.8(4) (Fig. S24 in the ESI†) and log K_{MC4˙⁺⊂CB[7]} = 4.54(5) (Fig. S26 in the ESI†). In contrast to BMV²⁺, where translational motion of the CB[7] macrocycle occurred upon electrochemical reduction of the bipyridinium unit,³⁰ the CB[7] macrocycle remained firmly bound to the BIPY²⁺ electroactive units of the MC⁴⁺ derivatives regardless of their oxidation state.

Assuming that only the first one-electron reduction process dominates from 0 V to −0.7 V (Fig. S16 in the ESI† and Fig. 5), the diffusion coefficients of MC3²⁺ and MC4²⁺ in the absence and in the presence of CB[7] (Table 4) can be evaluated by chronocoulometric experiments. The differences observed between the values of the diffusion coefficients of MC3˙⁺/MC3²⁺ and MC4˙⁺/MC4²⁺ could be an indication of the dimerization of the model systems upon the one -electron reduction reactions. Addition of CB[7] suppresses the pimerization process in favor of the inclusion complexes. As a consequence, the inclusion complexes with CB[7] have lower diffusion coefficients than the corresponding free viologens.

Table 4 Diffusion coefficients D (cm² s⁻¹) of MC3²⁺, MC4²⁺, C23⁴⁺ and C24⁴⁺ measured by chronocoulometry in H₂O (0.1 M TBACl) in the absence and the presence of CB[7]. The errors on these values are 10%

No CB[7]/D	+CB[7]/D
MC3˙^+/0.7 × 10^−5	MC3˙^+ + 2 eq. CB[7]/1.9 × 10^−5
MC3^2+/5.4 × 10^−5	MC3^2 + 2 eq. CB[7]/14.2 × 10^−5
MC4˙^+/3.1 × 10^−5	MC4˙^+ + 2 eq. CB[7]/6.2 × 10^−5
MC4^2+/81.0 × 10^−5	MC4^2+ + 2 eq. CB[7]/22 × 10^−5
C23˙^+/1.41 × 10^−5	C23˙^+ + 3 eq. CB[7]/2.9 × 10^−5
C23^2+/7.58 × 10^−5	C23^2 + 3 eq. CB[7]/3.91 × 10^−5
C24˙^+/0.81 × 10^−5	C24˙^+ + 4 eq. CB[7]/0.16 × 10^−5
C24^2+/20.1 × 10^−5	C24^2+ + 4 eq.CB[7]/42.1 × 10^−5
C23˙^+/1.41 × 10^−5	C23˙^+ + 3 eq. CB[7]/2.9 × 10^−5
C23^2+/7.58 × 10^−5	C23^2 + 3 eq. CB[7]/3.91 × 10^−5
C24˙^+/0.81 × 10^−5	C24˙^+ + 4 eq. CB[7]/0.16 × 10^−5
C24^2+/20.1 × 10^−5	C24^2+ + 4 eq. CB[7]/42.1 × 10^−5

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Investigations of the UV-vis-NIR absorption spectra of MC3²⁺ and MC4²⁺ in water at pH 7.0 in the absence or in the presence of a reducing agent were carried out to evaluate the effect of CB[7] addition (Fig. S23 and S25 in the ESI†). MC3˙⁺⊂CB[7] and MC4˙⁺⊂CB[7] both clearly exhibited a significant hypochromic shift of the visible absorption band associated with the radical cation with increasing concentration of CB[7], strong evidence for the inclusion of the BIPY˙⁺ radical cation within the hydrophobic cavity of CB[7]. This binding event effectively hampers radical cation pimerization.

Bisradicals C23²⁽˙⁺⁾ and C24²⁽˙⁺⁾. Cyclic (Fig. S16 in the ESI†) and square wave (Fig. 6) voltammetry studies were undertaken to evaluate the impact of CB[7] addition on the intramolecular pimerization of the BIPY˙⁺ radical cations of the calix[4]arene-bis-viologens, C23⁴⁺ and C24⁴⁺. The electrochemical data clearly showed two distinct and reversible redox waves. The relative amplitudes and shapes of the peaks for each wave at the anode (oxidation) are similar to those observed at the cathode (reduction). Such a pattern is an indication of the redox reversibility of the system.

Fig. 6 Square wave voltammograms of (a) C23⁴⁺, [C23⁴⁺] = 0.05 mM and (b) C24⁴⁺, [C24⁴⁺] = 0.05 mM in the absence (orange line) and the presence of CB[7] (blue line, respectively 3 and 4 equivalents of CB[7]). All voltammograms were recorded in argon-purged phosphate buffer solutions (pH 7) at 298 K (E versus Ag/AgCl).

The square wave voltammograms of C23⁴⁺ and C24⁴⁺ (Fig. 6) displayed two reversible two-electron reduction processes: E¹_1/2 (C23⁴⁺ → C23²⁽˙⁺⁾) = −0.43 V and E_1/2² (C23²⁽˙⁺⁾ → C23⁰) = −0.85 V; E_1/2¹ (C24⁴⁺ → C24²⁽˙⁺⁾) = −0.47 V and E_1/2² (C24²⁽˙⁺⁾ → C24⁰) = −0.85 V. Interestingly, for the [3]pseudorotaxanes, C23⁴⁺⊂(CB[7])₂ and C24⁴⁺⊂(CB[7])₂, no shifts were observed for the second reduction wave (Table 3), which would have corresponded to the following electrochemical processes: C23²⁽˙⁺⁾⊂(CB[7])₂ → C23⁰⊂(CB[7])₂ and C24²⁽˙⁺⁾⊂(CB[7])₂ → C24⁰⊂(CB[7])₂. Rather, our observations indicated that after the first two-electron reduction process, the threads behave as if they are unbound C23²⁽˙⁺⁾ and C24²⁽˙⁺⁾. In other words, the first reduction of C23⁴⁺⊂(CB[7])₂ and C24⁴⁺⊂(CB[7])₂ induced dethreading of the CB[7] macrocycles, which is spontaneously followed by intramolecular dimerization of the two terminal BIPY˙⁺ groups, leading to the formation of intramolecular dimeric species, C23²⁽˙⁺⁾ and C24²⁽˙⁺⁾. These results were consistent with the absorption spectrophotometric analyses (Fig. S27 and S28 in the ESI†) and EPR behavior (Fig. S19 and S20 in the ESI†) of the C23⁴⁺/CB[7] and C24⁴⁺/CB[7] [3]pseudorotaxanes.

Chronocoulometry was also used to evaluate the diffusion coefficients of the fully oxidized and radical cationic states of the different species in aqueous solution. During the two electrons reduction process (0 V → −0.7 V) the fully oxidized species predominate and diffuse to the electrode surface where they are reduced. By measuring the reduction rates, the diffusion constants for C23⁴⁺ and C24⁴⁺ species can be calculated in the absence and in the presence of CB[7] (Table 4). Alternatively, by setting the voltage to −0.7 V and performing the oxidation process, the rates of oxidation and the diffusion coefficients of the corresponding radical cationic species can be measured as well. In the absence of CB[7], larger diffusion coefficients were observed for C23⁴⁺ and C24⁴⁺ than for their corresponding fully reduced forms. This could be explained by the ability of higher charged species to diffuse faster towards or away from an electrode. In the presence of CB[7], larger diffusion coefficients were measured for the [3]pseudorotaxanes C23⁴⁺⊂(CB[7])₂ and C23⁴⁺⊂(CB[7])₂ than the corresponding reduced species (i.e., upon reduction, a dethreading of the CB[7] occurs as a consequence of favored intramolecular pimerization with respect to the recognition process). Here also, one would expect that the [3]pseudorotaxanes C23⁴⁺⊂(CB[7])₂ and C23⁴⁺⊂(CB[7])₂ display larger hydrodynamic size by comparison with C23²⁽˙⁺⁾ or C24²⁽˙⁺⁾. Markedly different solvation of the latter species is suggested to explain these peculiar properties. The close diffusion coefficients of the radical cation species in the absence and the presence of CB[7] indicate that the dimerization of the viologen radical cations induce a dethreading of the CB[7] macrocycles.

Recognition of the viologen derivatives by CB[8]. The characterization of the host–guest complexes formed between MC3²⁺, MC4²⁺, C23⁴⁺, and C24⁴⁺ with CB[8] was investigated by absorption spectrophotometry (Fig. S29 to S32 in the ESI†) and ESI-MS measurements (Fig. 7). Due to the limited solubility of CB[8], we were not able to perform ¹H NMR titrations of the corresponding viologens. Similarly to the [n]pseudorotaxanes formed with CB[7], we were able to observe evidence for the formation of [2]pseudorotaxanes with MC3²⁺ and MC4²⁺ (namely MC3²⁺⊂CB[8] and MC4²⁺⊂CB[8]) and [3]pseudorotaxanes with C23⁴⁺ and C24⁴⁺ (namely a C23⁴⁺⊂(CB[8])₂ and C24⁴⁺⊂(CB[8])₂).

Fig. 7 ESI mass spectra of inclusion complexes of a) C23⁴⁺ and b) C24⁴⁺ (bottom) with CB[8]. (a) [C23⁴⁺]₀ = 5 × 10⁻⁵ M; [CB[7]]₀ = 10⁻⁴ M. Solvent: H₂O; positive mode. V_c = 200 V. (b) [C24⁴⁺]₀ = 5 × 10⁻⁵ M; [CB[7]]₀ = 10⁻⁴ M. Solvent: H₂O; positive mode. V_c = 100 V.

For the latter [3]pseudorotaxanes, chemical reduction of the BIPY²⁺ electroactive units led to dethreading of the CB[8] macrocyclic host despite its larger cavity size (Fig. S31 and S32 in the ESI†). This suggests significant steric constraints with the calix[4]arene platform and/or poor flexibility of the designed systems even though longer and apparently more flexible spacers were introduced. However, the MC3²⁺ and MC4²⁺ models clearly led (Fig. S29 and S30 in the ESI†) to the expected [3]pseudorotaxanes (MC3˙⁺)₂⊂CB[8] and (MC4˙⁺)₂⊂CB[8] thus emphasizing the detrimental role of the anchoring platform rather than the length of the spacers. The occurrence of [3]pseudorotaxanes (MC3˙⁺)₂⊂CB[8] and (MC4˙⁺)₂⊂CB[8] upon reduction is in agreement with the published reports^26–28 that showed that the stability of methyl viologen dimer (MV˙⁺)₂⊂CB[8]) was significantly increased in the presence of CB[8] following the formation of a 2 : 1 (MV˙⁺)₂⊂CB[8]) supramolecular complex in water.

Conclusions

Two calix[4]arene-bis-viologen systems, namely C23⁴⁺ and C24⁴⁺ were synthesized. The host–guest properties were extensively studied using a large set of analytical methods and led to the characterization of [3]pseudorotaxanes in combination with either CB[7] or CB[8]. For each of these host–guest species, CB[7] or CB[8] was demonstrated to reside in the middle of the BIPY²⁺ cations as a result of steric interactions with the anchoring platform. Upon reduction of the terminal BIPY²⁺ cations, these [3]pseudorotaxanes spontaneously dissociate as the result of a strong intramolecular dimerization of the two face-to-face viologen radical cations. The arrangement of the BIPY˙⁺ radical cations within the dimeric species seemingly relies on the length of the spacer that links the electroactive units to the anchoring moiety. Dethreading and dimerization do not occur in experiments involving CB[7] (or CB[8]) and either of the two monomeric viologen guests MC3²⁺ and MC4²⁺, which were used a models. On the other hand, the model compounds provided unexpected and valuable properties. Thanks to the presence of a alkoxy-phenyl substitution, [3]pseudorotaxanes such as MC3²⁺⊂(CB[7])₂ and MC4²⁺⊂(CB[7])₂ were characterized. For the MC3²⁺⊂CB[7] and MC4²⁺⊂CB[7] [2]pseudorotaxanes, no redox-triggered translocation of CB[7] between the two binding stations were observed indicating that the BIPY²⁺ unit remained the favored recognition site whatever its redox state. In the presence of CB[8], similar properties were emphasized with the oxidized form of MC3²⁺ and MC4²⁺ threads. However, [3]pseudorotaxanes (MC3⁽˙⁺⁾)₂⊂CB[8] and (MC4⁽˙⁺⁾)₂⊂CB[8]) were predominantly formed as a consequence of the larger cavity size of the CB[8] host. This preliminary study provided interesting information for the further development of functional electroactive systems and will be extended to tetrakis analogues displaying either a cone or 1,3-alternate arrangement of the anchoring calix[4]arene unit.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

M. E. thank the CNRS (LIMA, UMR 7042) and the University of Strasbourg for funding this research work. C. P.-I. acknowledges Centro de Supercomputación de Galicia (CESGA) for providing access to supercomputing facilities. A. T. thank NYUAD for its generous support of the research program at NYUAD.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Physicochemical characterisation, spectrophotometric titrations, Job's plots, NMR, MS and EPR spectra, and DFT structures. See DOI: 10.1039/d1ra05488k

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