Spin–spin interactions between two identical aromatic radicals have been studied extensively and utilized to establish supramolecular recognition. Here we report that spin-pairing interactions could also take place between two different π-electron radicals, namely a bipyridinium radical cation (BPY+•) and a naphthalene-1,8:4,5-bis(dicarboximide) radical anion (NDI─•). The occurrence of this type of previously unreported hetero radical-pairing interactions is attributed to enhancement effect of Coulombic attraction between these two radicals bearing opposite charges. The Coulombic-enhanced hetero radical pairing interactions are employed to drive host–guest recognition, as well as the reversible switching of a bistable rotaxane.
It has been known for several decades that aromatic compounds can undergo strong attractive interactions with each other by overlapping their π orbitals. These noncovalent interactions play an important role in the formation of a variety supramolecular architectures in both biological and artificial systems, including stabilizing DNA double helical strands through stacking of base pairs1, forming folded tertiary structures of proteins2, and host–guest recognition3,4, as well as π-electron aggregation for developing organic conductors and semiconductors5. In terms of a benzene ring, partial positive and negative charges are distributed within the σ-framework and above the benzene core, respectively. As a consequence, π-electron interactions between two benzene rings often occur in the form of edge-to-face or parallel displaced stacking interactions6 (Fig. 1a), in order to maximize π–σ attractions and minimize π–π repulsion. In this case, π-electron interactions are essentially not different from dipole–dipole interactions. When two aromatic moieties contain electron donating and withdrawing functional groups, their π-electron interactions could occur in a manner of face-centered stacking (Fig. 1b), owing to (i) efficient orbital overlap between the highest occupied molecular orbital (HOMO) of the former (the electron-rich one, e.g., tetrathiofuvalene (TTF) or dioxynaphthalene (DNP)) and lowest unoccupied molecular orbital (LUMO) of the latter (the electron-poor one, e.g., naphthalene-1,8:4,5-bis(dicarboximide) (NDI) or 4,4-bipyridinium dication (BPY2+)), and (ii) the electron-withdrawing group on the acceptor, which polarizes the electron density away and therefore suppresses π-electron repulsion. This type of π-electron interactions are often referred as donor–acceptor interactions7. π-electron stacking interactions are also observed between two identical aromatic radical compounds8 (Fig. 1c), which are often either cationic or anionic. This homoradical dimerization allows the unpaired radical electrons on the two identical singly occupied molecular orbitals (SOMOs) to undergo spin pairing, forming a diamagnetic radical dimer. This interaction is often extremely enthalpically favorable and can dominate the electrostatic repulsion between the two identical charges. This tendency of homo-dimerization is observed between a variety of aromatic radicals, including BPY+• radical cation9, NDI─• radical anion10, TTF+• radical cation11, and others12,13,14,15. These homoradical interactions have been harnessed to drive host–guest recognition16,17,18 and create molecular switches and machines19,20, as well as develop functional materials17,21,22 that have electromagnetic, optical, or magnetic properties.
Even though homoradical spin-pairing interactions between two identical aromatic radicals have been studies extensively, the examples that two different aromatic radicals could undergo hetero radical spin pairing to form supramolecular pimers are relative rare. The latter interactions are observed in a variety of radical-involved reaction intermediates. For example, it was reported that tetracyanoethylene radical anion (TCNE─•) can interact with a methyl vinyl ether radical cation (MVE+•), forming a radical pair as the reaction intermediate of a zwitterionic cycloaddition reaction23. However, hetero radical-pairing interactions are seldom employed to drive supramolecular recognition, especially in solution state. This might result from the fact that overlapping two different SOMOs is not as thermodynamically favorable as overlapping two identical SOMOs. For example, mixing TTF and tetracyanoquinodimethane (TCNQ) leads to electron transfer from the former to latter, producing a pair of TTF+• radical cation and TCNQ─• radical anion. In the context of supramolecular recognition, however, these TTF+• radical cations and TCNQ─• anions prefer to interact with themselves to form homoradical pairs, namely (TTF+•)2 or (TCNQ─•)2, instead of forming a heterodimer (TTF+•/TCNQ─•), as inferred from the single crystallographic analysis24. Recently, Stoddart et al.25 reported that, with the assistance of mechanically interlocked architectures, BPY+• radical cation can undergo radical spin–spin interaction with its more conjugated analog, namely 2,7-diazapyrenium radical cation (DAP+•). However, we are unaware of any other examples that two constitutionally distinct radicals could undergo radical spin–spin interactions to drive supramolecular recognition. We propose that this unlikely occurring hetero radical interaction might happen, if other supramolecular interactions such as Coulombic attractions are introduced and function cooperatively.
Herein, we demonstrate that a BPY+• radical cation and an NDI─• radical anion could undergo dimerization, when they are covalently connected. This unreported supramolecular interactions result from the marriage of the hetero radical spin–spin interactions and Coulombic attraction between these two aromatic radicals taking opposite charges. This supramolecular driving force is employed to establish the host–guest recognition between an NDI─• guest and a cyclobis(paraquat-p-phenylene) (CBPQT2(+•)) host, whose two BPY+• residues are able to undergo hetero radical pairing interactions simultaneously with the anionic guest. This interaction also allows us to develop a molecular switch in the form of a bistable rotaxane, which could be switched either chemically or electrochemically. This rotaxane is composed of a CBPQT4+ ring component that encircles a dumbbell component containing a DNP unit and an NDI unit. In oxidative conditions, the ring resides on DNP station, thanks to π-electron donor–acceptor interactions. Upon reduction of the CBPQT4+ ring and NDI unit to their dicationic diradical and anionic radical form, namely CBPQT2(+•) and NDI─•, respectively, the ring moves to encircle the latter binding station, on account of hetero radical-pairing interactions.
Hetero radical pairing in the presence of covalent bonds
We first investigated the possibility of BPY+• and NDI─• to undergo hetero-dimerization in the absence and presence of covalent connection. Compound 12+•2PF6─ and 2 (Fig. 2), which contain either a dicationic BPY2+ or an NDI functional group, respectively, were obtained. Cobaltocene was employed to reduce these two compounds to their radical forms in MeCN, namely 1+• and 2─• (PF6─ anion and cobaltocelium act as the counterions, respectively). The absorption spectra of 1+• and 2─• in MeCN were recorded (Fig. 3) by using ultraviolet/visible/near-infrared (UV/Vis/NIR) spectroscopy. As expected, 1+• and 2─• have characteristic absorption bands of BPY+• and NDI─•, respectively. The absorption spectrum of the 1:1 mixture of 1+• and 2─• is essentially the superimposition of their two individual spectra, indicating that 1+• and 2─• does not undergo significant supramolecular interactions (at least not at a concentration of 0.1 mM in MeCN). We then synthesized a compound 32+•2PF6─ (Fig. 2) bearing both a BPY2+ and an NDI functional group, which are covalently connected by a glycol chain. Upon reduction, the absorption spectrum of 3(+•)(─•) was recorded. To our delight, a broadband centered on 890 nm was observed (Fig. 3, pink trace) in the NIR region in the absorption spectrum, accompanied by a blueshifted characteristic absorption band centered on 571 nm, compared to the band centered on 606 nm in the case of 1+•. This result indicates that in the presence of covalent bond that helps to rule out entropy loss, BPY+• and NDI─• are able to undergo interactions.
The electron paramagnetic resonance (EPR) spectrum of 3(+•)(─•) was recorded (Fig. 4) to justify whether radical spin–spin pairing occurred. It was observed (Fig. 4b) that the EPR signal of 3(+•)(─•) was significantly weaker, compared to those of 1+• and 2─• with the same concentrations in their individual spectra (Supplementary Fig. 14a, b). This observation indicates that within 3(+•)(─•), the radical electron in BPY+• and NDI─• underwent spin paring and became diamagnetic to some extent. Addition of 12+•2PF6─ into the solution of 3(+•)(─•) produced (Supplementary Fig. 14c) a characteristic EPR spectrum of 1+• with high intensity. This observation results from electron transfer form 3(+•)(─•) to 12+•2PF6─, producing 3(+•) or 32+ and 1+•, respectively, because NDI─• is generally more reductive than BPY+•. We also performed EPR analysis of 3(+•)(─•) at variable temperatures ranging from ─30 °C to 75 °C in MeCN. At a higher temperature (e.g., 75 °C), the radical signal underwent a remarkable increase (Fig. 4a). This observation is not surprising because radical pairing between BPY+• and NDI─• in 3(+•)(─•) occurred at the expense of entropy loss. As a consequence, at a higher temperature, the BPY+•/NDI─• radical pair undergoes dissociation, increasing its EPR signal. At a lower temperature (─30 °C), when the entropic effect is suppressed, almost no EPR signal is detected (Fig. 4c).
Hetero radical pairing in host–guest recognition
CBPQT ring is a macrocylic host bearing two BPY units, which are separated by an approximate 6.8 Å distance on account of the two rigid phenylene bridges26. In its fully oxidized state, namely CBPQT4+, the electron-deficient cavity of the tetracationic host is capable of accommodating a variety of electron-rich guests, such as TTF and DNP, driven by π-electron donor–acceptor interactions. When the host is reduced to its dicationic diradical state, namely CBPQT2(+•), this host was reported17 to recognize guests containing BPY+• moieties, owing to homoradical-pairing interactions between the three BPY+• radicals in both the host and the guest. The distance (i.e., 6.8 Å) between two BPY units in CBPQT is twice of efficient π–π distance. This is of importance because host–guest recognition could occur in a manner that the guest is able to undergo π–π interactions simultaneously with each of the two BPY units in the host. Encouraged by the result that BPY+• and NDI─• undergo spin paring when they are covalently connected, we then investigated whether this hetero radical pairing interaction could drive the formation of a supramolecular host–guest complex, namely NDI─•⊂CBPQT2(+•).
We mixed the NDI derirative 2 and CBPQT4+•4PF6─ in MeCN in a 1:1 ratio and recorded (Fig. 5a) the absorption spectrum by using UV/Vis spectroscopy. No absorption band was observed in the visible light region, indicating that there are little or no binding affinity between 2 and CBPQT4+•4PF6─ in their fully oxidized states, given that both of these two compounds are electron deficient. Cobaltocene was added into the solution to reduce both the host and guest. Addition of no more than two equiv of cobaltocene resulted in the appearance of an absorption band (Fig. 5a) centered at 606 nm, which is the characteristic absorption band of BPY+• or the CBPQT2(+•) ring, indicating the reduction of the host. Upon addition of more cobaltocene, a broad absorption band centered on 937 nm started to appear (Fig. 5a), whose absorption coefficient reached its maximum after three equiv of cobaltocene was added. This absorption band in the NIR region results from the occurrence of hetero radical pairing interactions between the NDI─• in 2─• and one of the BPY+• radicals in the CBPQT2(+•) ring, by forming a pseudorotaxane 2─•⊂CBPQT2(+•). A dumbbell-shaped compound, namely 4, containing an NDI unit in the middle and two bulky stoppers at each end, was synthesized as a control compound. These two bulky groups are sufficiently large to prevent threading of the ring onto the dumbbell. After adding three equiv of cobaltocene into the 1:1 solution of 4 and CBPQT4+•4PF6─ in MeCN, no absorption band in NIR region was observed (Fig. 5b). This result acts as a control experiment, confirming that the binding event between CBPQT2(+•) and 2─• occurs in a manner that the ring threads onto the guest. The so-called alongside interactions between one BPY+• unit in CBPQT2(+•) ring and 2─• are not responsible for the occurrence of NIR absorption band, or at least not at this experimental concentration (i.e., 0.1 mM in MeCN). The association constant (Ka) of the 1:1 complex 2─•⊂CBPQT2(+•) was measured to be approximate 1.2 ± 0.6 × 105 M─1 in MeCN at 298 K, by tracking the absorbance at 937 nm in the UV/Vis/NIR absorption spectra of CBPQT2(+•) upon addition of different amount of 2─• (see Supplementary Fig. 20). A free energy of binding ΔG therefore is calculated to be approximate −7 kcal mol−1, by using the equation ΔG = ─RT ln Ka. The formation of 2─•⊂CBPQT2(+•) complex was convinced by using EPR spectroscopy (see Supplementary Figs. 15, 16). We also recorded (see Supplementary Fig. 21) the UV/Vis/NIR absorption spectra of the 1:1 complex 2─•⊂CBPQT2(+•) at variable temperatures between 283 and 303 K, by which ΔH and ΔS were determined to be ─7.6 and ─8 cal mol−1 K−1, respectively.
It is difficult to experimentally quantify the contribution of either Coulombic attraction or hetero radical spin–spin interactions in driving the folding of 3(+•)(─•) or the formation 2─•⊂CBPQT2(+•). This is because it is difficult to obtain a control system that can rule out one of these two supramolecular interactions. We observed that, upon addition of tetrabutylammonium hexafluorophosphate (TBA+•PF6─, 50 mM) into the solutions of 2─•⊂CBPQT2(+•) in MeCN, the broad absorption band centered on 937 nm disappeared (see Supplementary Fig. 18). This observation indicates that addition of TBA+•PF6─ results in dissociation of the NDI─•⊂CBPQT2(+•) complex. This phenomenon could be explained by the fact that Coulombic attraction between 2─• and CBPQT2(+•) was disrupted by TBA+•PF6─, which acts as a competitive ionic reagent to interact with the NDI─• anions or CBPQT2(+•) cations. We thus reasonably hypothesize that Coulombic attraction might play a more important role in their complexation between 2─• and CBPQT2(+•) than radical spin-spin interactions.
Density functional theory (DFT) calculation was performed, in order to better understand the nature of the binding between the two oppositely charged radicals. Theoretical calculation indicates that, center-aligned BPY+•/NDI─• radical pair represents the most thermodynamically optimized state, which has a binding energy (ΔH) of ─15.8 kcal mol−1, while the energy gap between the triplet and the singlet state of the BPY+•/NDI─• radical dimer is remarkably small, that is, 0.01 kcal mol─1 (see Supplementary Fig. 26). The implication is that when the stacking interactions between BPY+• and NDI─• occur in a center-aligned manner, the major driving force comes from Coulombic attractions. Radical spin–spin interactions do not occur in this case, because the two SOMOs of BPY+• and NDI─• radicals have mismatched symmetries (see Supplementary Fig. 28a, b). However, when BPY+• and NDI─• orientate in a parallel displaced manner, the triplet–singlet energy gap becomes larger (Supplementary Fig. 26), implying the occurrence of radical spin–spin interactions. It seems that the center-aligned stacking is more favored than the parallel displaced stacking in terms of binding energy (ΔH), because the former orientation maximizes the Coulombic attractions. However, the truth is that, in solution, Coulombic attractions between BPY+• and NDI─• are often suppressed by solvation or competitive Coulombic interactions from counter-ions, making the parallel displaced stacking more favored. In addition, in the case of 3(+•)(─•) when a glycol linker is present, the perfect center-aligned orientation of BPY+• and NDI─• might not be obtainable, also allowing the occurrence of radical spin–spin interactions in the parallel displaced manner.
We also performed DFT calculation to provide (see Fig. 6 and Supplementary Fig. 29) a quantum mechanical description of the NDI─•⊂CBPQT2(+•) complex. The NDI─• is partially encapsulated in the cavity of CBPQT2(+•), which reflects the fact that fully encapsulating the guest within the host cavity might result in large steric hindrance between the naphthalene protons in the NDI─• guest and the phenylene moieties in the CBPQT2(+•) host. This partial encapsulation results in a parallel displaced stacking between the NDI─• unit in the guest and each of the two BPY+• units in the macrocycle, which allows hetero radical spin–spin interactions to occur. The distance between NDI─• and each of the two BPY+• units of CBPQT2(+•) is around 3.3 Å, allowing efficient π–π interactions. By the examination of the frontier molecular orbitals of the radical complex, it has been confirmed that radical spin–spin interactions occurred between the corresponding SOMOs (Fig. 6) of the NDI─• and the two BPY+• units in the CBPQT2(+•) ring, that is, both the SOMO-1 and LUMO orbitals of the complex include significant molecular orbital overlap between NDI─• and CBPQT2(+•). Natural population analysis27 of the NDI─•⊂CBPQT2(+•) complex reveals that the charge on the NDI─• and CBPQT2(+•) fragments are ─0.96 and +1.96, respectively, indicating occurrence of charge transfer from NDI─• to CBPQT2(+•).
Hetero radical pairing in a bistable rotaxane
A bistable rotaxane 54+•4PF6─ was designed and synthesized (see Fig. 7 and the details in SI). 54+•4PF6─ contains a CBPQT4+ ring, which encircles a dumbbell bearing a π-electron-rich DNP recognition site and a π-electron-deficient NDI unit. An NDI derivative 6 bearing an azide functional group and a DNP derivative 7 containing an alkyne functional group were obtained in relatively high yields in six and four steps, respectively. The bistable rotaxane 54+•4PF6─ and its dumbbell counterpart 8 were obtained in 62% and 90% yields, respectively, by means of copper(I)-catalyzed azide–alkyne cycloadditions28,29 between 6 and 7 in Me2CO in the presence or absence of CBPQT4+•4PF6─. The structure of rotaxane 54+•4PF6─ was fully characterized by nuclear magnetic resonance (NMR) spectroscopy (see Supplementary Figs. 23, 24) and mass spectrometry (see Supplementary Fig. 25). Both 1D and 2D 1H NMR spectra of 54+•4PF6─ in CD3CN revealed that, in the fully oxidized state, the CBPQT4+ ring encircles the DNP unit in the rotaxane.
The switching behavior of 54+•4PF6─ by using redox stimuli was monitored (Fig. 8) by using UV/Vis spectroscopy. The absorption 527 nm with a relatively low coefficient was observed (Fig. 8, black trace) before addition of reductant. This characteristic absorption band is on account of the donor–acceptor interactions between the DNP unit in the dumbbell and one of the two BPY2+ unit in the ring, indicating that the ring encircles the DNP station. Upon addition of zinc dust into the solution, another characteristic absorption band centered on 577 nm, accompanied by the appearance of a broadband centered on 937 nm, was observed. This observation is consistent with the appearance of the NIR absorption band in the 2─•⊂CBPQT2(+•) complex, indicating that after reduction, the CBPQT2(+•) ring shuttled and encircled the NDI─• station in 52(+•)(─•). After exposing the solution to air, the absorption bands centered at 571 and 937 nm were no longer observed, indicating that after oxidation of the ring and the NDI─• unit in the dumbbell, the CBPQT4+ ring moved back and encircling the DNP unit.
The redox-stimulated switching behavior of the rotaxane 54+•4PF6─ in MeCN can also be justified by comparing the cyclic voltammetry (CV) data of 54+•4PF6─ (Fig. 9, red trace), with those of the CBPQT4+•4PF6─ (Fig. 9, blue trace) and the dumbbell 8 (Fig. 9, black trace). The CV of CBPQT4+•4PF6─ reveals two consecutive reversible redox processes. The reduction peaks observed at ─0.21 and ─0.65 V can be assigned to two two-electron reductions, namely CBPQT4+/CBPQT2(+•) and CBPQT2(+•)/CBPQT0. The CV of the dumbbell 8 also reveals two consecutive reversible redox processes. The reduction peaks observed at ─0.50 and ─0.89 V can be assigned to two one-electron reductions of the NDI functional group, namely NDI/NDI─• and NDI─•/NDI2─, respectively. The CV of the rotaxane 54+•4PF6─ reveals six reversible redox processes. The first two reduction peaks observed at ─0.21 and ─0.31 V can be assigned to the first two one-electron reduction of the ring, namely CBPQT4+/CBPQT(+•)(2+) and CBPQT(+•)(2+)/CBPQT2(+•). The nonequivalence of the first two reduction potentials of the ring in the rotaxane results from the fact that, in the fully oxidized state of 54+•4PF6─, one of the two BPY2+ units in the ring is more strongly engaged in the π-electron donor–acceptor interactions than the other. The former unit is therefore more difficult to reduce. The third reduction peak of 54+•4PF6─ observed at ─0.40 V can be assigned to the one-electron reduction of the NDI unit, namely NDI/NDI─•. Compared to the first reduction potential (─0.50 V) of the dumbbell 8, this reduction peak is shifted to a substantially more positive value by 0.10 V, indicating that the hetero radical-pairing interactions act as the driving force, which makes NDI easier to be reduced. This observation supports our assumption that, upon reduction of the rotaxane 54+•4PF6─, the CBPQT2(+•) ring shuttled and encircled the NDI─• unit as the secondary binding station. The fourth reduction peak (─0.65 V) observed for the rotaxane can be assigned to the one-electron reduction of one of the two BPY+• units in the CBPQT2(+•) ring (CBPQT2(+•)/CBPQT(+•)), which is not strongly engaged in hetero radical-paring interactions with the NDI─• radical anion of the dumbbell component of 54+•4PF6─. The consequence is that this reduction peak does not undergo remarkable shift, compared to the second reduction peak (─0.65 V) of the free CBPQT4+. In contrast, the fifth reduction peak of the rotaxane 54+•4PF6─, which can be attributed to one-electron reduction (CBPQT(+•)/CBPQT), is shifted to a substantially more negative value (─0.73 V). This negative shift can be attributed to the stabilizing influence of the hetero radical-pairing interaction between the NDI─• in the dumbbell component of the rotaxane and the BPY+• in the CBPQT(+•) ring, which renders the reduction of BPY+• more difficult. This negative shift also further convinces the encirclement of the CBPQT2(+•) or CBPQT(+•) around the NDI─• unit of the dumbbell component. The sixth reduction peak observed ─0.89 V can be attributed to one-electron reduction (NDI─•/NDI2─). This peak does not undergo remarkable shift compared to the second reduction peak of the NDI─• unit in the dumbbell compound 8, an observation indicating that the hetero radical pairing was jeopardized after the ring is fully reduced to its neutral state, namely CBPQT, which no longer encircles the NDI─• anion.
π–π radical spin–spin pairing interactions between two identical π-electron radical compounds have been studied extensively in the field of supramolecular chemistry. However, the π–π interactions between two different π-electron radicals are seldom employed in driving supramolecular recognition. The lack of hetero radical spin–spin interactions might be explained by the often-occurring mismatch of the SOMOs of two different radical moieties. Here, we demonstrate that with the assistance of a covalently connected system, a bipyridinium radical cation and a naphthalene diimide radical anion can undergo spin–spin interactions. This previously unreported supramolecular recogntion, takes advantage of the marriage of hetero radical spin–spin pairing interactions and Coulombic attraction. Theoretical calculation demonstrates that the latter interaction might play a more predominant role, especially in the case when these two radical moieties orientate in a center-aligned manner. We employed this noncovalent interaction to drive the formation of a pseudorotaxane, which is composed of a cyclophane host containing two bipyridinium radical cations encircling a naphthalene diimide radical anionic guest. This recognition motif can also be harnessed to drive the intramolecular mechanical movement of a bistable rotaxane that can function as a redox-stimulated molecular switch. The discovery of this nocovalent interaction opens up opportunities for rationally designing supramolecular or mechanically interlocked architectures with more diverse functions.
The syntheses of model compounds 2, 32+•2PF6─, and 4, as well as azide and alkyne derivatives, namely 6 and 7, were described in Supplementary Figs. 1–12. The dumbbell 8 and the rotaxane 54+•4PF6− were synthesized by using a copper(I)-catalyzed alkyne-azide cyclization.
NMR spectra were recorded at ambient temperature using Bruker AVANCE III 400/500 spectrometers, with working frequencies of 400/500 and 100/125 MHz for 1H and 13C, respectively. Chemical shifts are reported in ppm relative to the residual internal non-deuterated solvent signals (CD3CN: δ = 1.94 ppm; CDCl3: δ = 7.26 ppm). High-resolution mass spectra were recorded on an Fourier transform ion cyclotron resonance mass spectrometry. EPR spectra were taken on a cw-EPR spectrometer (Bruker A300). CV experiments was carried out at room temperature in argon-purged solutions in MeCN with a Gamry Multipurpose instrument (Reference 600) interfaced to a PC. UV/Vis/NIR absorption spectra were taken on a Cary Series UV-Vis-NIR spectrophotometer.
The authors declare that all other data supporting the findings of this study are available from the article and its Supplementary Information files or available from the authors upon reasonable request.
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This work was supported by the Chinese ‘‘Thousand Youth Talents Plan’’, the National Natural Science Foundation of China (No. 21772173 to H.L. and No. 51771174 to F.C.), and the Natural Science Foundation of Zhejiang Province of China (No. LR18B020001). We are grateful to Prof. Yizhen Jin (Department of Chemistry, Zhejiang University) for the UV/Vis/NIR instruments.
The authors declare no competing interests.
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