QIU YUNYAN (US)
STODDART JAMES (US)
WO2021237250A1 | 2021-11-25 | |||
WO2021253039A1 | 2021-12-16 | |||
WO2017075263A1 | 2017-05-04 |
US20210270771A1 | 2021-09-02 |
CLAIMS 1. A system for electron catalyzed molecular recognition, the system comprising: an electron source for providing an electron; a redox-active substrate capable of accepting the electron from the electron source; a catalytic intermediate formed noncovalently from the substrate and a second molecule, wherein the energy barrier for forming the catalytic intermediate is decreased by the redox-active substrate accepting the electron from the electron source; and, optionally, a final product. 2. The system of claim 1, wherein the electron source is an undivided electrochemical cell. 3. The system of claim 1, wherein the electron source is a divided electrochemical cell. 4. The system of claim 1, wherein the electron source is a chemical initiator. 5. The system of claim 4, wherein the chemical initiator is a homogeneous chemical initiator. 6. The system of claim 4, wherein the chemical initiator is a heterogeneous chemical initiator. 7. The system of any one of claims 1-6, wherein the catalytic intermediate is a bisradical host- guest complex. 8. The system of any one of claims 1-6 comprising the final product, wherein the final product is a trisradical host-guest complex. 9. The system of claim 8, wherein the catalytic intermediate is a bisradical host-guest complex. 10. The system of any one of claims 1-6, wherein the redox-active substrate comprises a marcrocyclic host ring R or a dumbbell-shaped guest D and the catalytic intermediate is a bisradical host-guest complex formed from the macrocyclic host ring and the dumbbell- shaped guest. 11. The system of claim 10, wherein the macrocyclic host ring comprises two bipyridinium (BIPY) units and/or wherein the dumbbell-shaped guest comprises a bipyridinium (BIPY) unit and a cationic terminus. 12. The system of any one of claims 10-11 comprising the final product, wherein the final product is a trisradical host-guest complex formed from the macrocyclic host ring and the dumbbell-shaped guest. 13. A method for electron catalyzed molecular recognition, the method comprising: providing, with an electron source, an electron to a redox-active substrate capable of accepting the electron from the electron source; and forming noncovalently a catalytic intermediate from the redox-active substrate and a second molecule, wherein the energy barrier for forming the catalytic intermediate is decreased by the redox-active substrate accepting the electron from the electron source; and, optionally, forming a final product. 14. The method of claim 13, wherein the electron source is an undivided electrochemical cell. 15. The method of claim 13, wherein the electron source is a divided electrochemical cell. 16. The method of claim 13, wherein the electron source is a chemical initiator. 17. The method of claim 16, wherein the chemical initiator is a homogeneous chemical initiator. 18. The method of claim 16, wherein the chemical initiator is a heterogeneous chemical initiator. 19. The method of any one of claims 13-18, wherein the catalytic intermediate is a bisradical host-guest complex. 20. The method of any one of claims 13-18 comprising forming the final product, wherein the final product is a trisradical host-guest complex. 21. The method of claim 20, wherein the catalytic intermediate is a bisradical host-guest complex. 22. The method of any one of claims 13-18, wherein the redox-active substrates comprises a marcrocyclic host ring R or a dumbbell-shaped guest D and the catalytic intermediate is a bisradical host-guest complex formed from the macrocyclic host ring and the dumbbell- shaped guest. 23. The method of claim 22, wherein the macrocyclic host ring comprises two bipyridinium (BIPY) units and/ or wherein the dumbbell-shaped guest comprises a bipyridinium (BIPY) unit and a cationic terminus. 24. The method of any one of claims 22-23 comprising forming the final product, wherein the final product is a trisradical host-guest complex formed from the macrocyclic host ring and the dumbbell-shaped guest. |
Scheme 2: Synthesis of Cat•6PF 6 using a radical-templated approach 3. Preparation of R 2(•+) and D +(•+) The starting materials of the molecular recognition, R 2(•+) and D +(•+) , were prepared by reducing the BIPY 2+ units in their parent compounds (R 4+ and D 3+ ) to BIPY •+ radical cations. Given the fact that the kinetics of the molecular recognition process is very sensitive to additional electrons, the BIPY units in this host–guest system should be entirely in the radical cationic state (BIPY •+ ), without the existence of either dicationic (BIPY 2+ ) or neutral (BIPY (0) ) states. It requires a complete single-electron reduction of BIPY 2+ units while avoiding over-reduction. Table 1: Comparison of different reduction methods used for producing BIPY •+ radical cations Different reduction methods of BIPY 2+ units were screened, and their outcomes were compared and summarized in Table 1. When using copper (Cu) as the reductant, the single- electron reduction is not complete, generating a mixture of BIPY •+ and BIPY 2+ . In contrast, the use of zinc (Zn) results 7,8 in the over-reduction, giving a mixture of BIPY •+ and BIPY (0) . Although a homogeneous, strong reductant, such as cobaltocene (CoCp 2 ), can induce a quantitative reduction of BIPY 2+ units, it is difficult in practice to control the amount of CoCp 2 to be exactly 1 molar equivalent. Finally, we selected the controlled potential electrolysis 9 (CPE) as an appropriate reduction method to produce BIPY •+ radical cations, because this approach is easy to operate, and its outcome is controllable by adjusting the working potential. The CPE experiments were performed inside a N 2 -filled glovebox using a custom-built H- cell electrolysis apparatus 8,10 . A three-electrode system was used, which included a reticular vitreous carbon (RVC) working electrode, an RVC counter electrode and an Ag/AgCl reference electrode. The two RVC electrodes were placed into individual half-cell chambers (100 mL) that were separated by an ionic exchange membrane (Fumasep FAPQ-375-PP from Fuel Cell Store) and held together by a clamp. The Ag/AgCl reference electrode was inserted inside the working cell. The whole apparatus was connected to a Gamry multipurpose instrument (Reference 600) interfaced to a PC. The experimental parameters were instructed using the software of Gamry Framework Version 6.30 under the chronocoulometry mode. The solid of R•4PF 6 (19.8 mg) or D•3PF 6 (18.3 mg) was dissolved in the MeCN solution of TBAPF 6 (0.05 M, 60 mL), and then transferred into the working cell. The counter cell was filled with a MeCN solution of (trimethylammonium methyl)ferrocene hexafluorophosphate, which served as the sacrificial electron donor. The working potentials for the electrolysis of R 4+ and D 3+ were set as –0.35 and –0.40 V, respectively, according to their CV and DPV data (Fig.10). The solutions were electrolysed under vigorous stirring (800 rpm) at room temperature for 40 min. The resulting stock solutions (0.3 mM) of R 2(•+) and D +(•+) were stored in the N 2 -filled glovebox for the use in subsequent experiments. It is noteworthy that the use of CPE method for electrolysis renders it practically difficult to achieve 100% conversion from BIPY 2+ units to BIPY •+ radical cations. Specifically, there was ca. 1 mol% residual BIPY 2+ in the solution after 40 min electrolysis. Therefore, 1 mol% CoCp 2 was added to both the electrochemically prepared solutions of R 2(•+) and D +(•+) in prior to the kinetic measurements of the molecular recognition process. 4. Kinetic Studies on the Molecular Recognition Molecular recognition was conducted by combining the electrochemically prepared R 2(•+) and D +(•+) . The process was monitored by UV/Vis/NIR spectroscopy, because the [D⊂R] +3(•+) triradical complex—the product of this molecular recognition—displays a characteristic 11-13 NIR absorption band at 1080 nm. In a typical procedure, a screw cap cuvette was charged with 500 μL of R 2(•+) and 500 μL of D +(•+) (both are 0.3 mM in MeCN) under the N2 atmosphere. Different additives were added to the solution if needed. After quick mixing, the solution was transferred to the spectrometer to record the UV/Vis/NIR spectra with time. 4.1 Long-Time Kinetic Measurement Although the molecular recognition between R 2(•+) and D +(•+) is undetectable (Fig.2a) by UV/Vis/NIR spectroscopy within 70 min, we monitored the process in an extended time scale up to 10 h. This long-time measurement revealed a detectable but very slow increase (Fig.11) in the absorption band (λ max = 1080 nm) characteristic of the [D⊂R] +3(•+) trisradical complex. The low absorbance at 1080 nm indicated that the molecular recognition was still far from reaching equilibrium even after 10 h. Notably, during the process, the absorption (λ max = 600 nm) for uncomplexed BIPY •+ radical cations showed a clear decrease, possibly caused by the penetration of O 2 into the solution It has been reported 14-17 that the BIPY •+ radical cation can undergo a reversible disproportionation (Scheme 3) to form BIPY 2+ and BIPY (0) , although the equilibrium constant (K disp ) is as low as 10 −7 . Considering the mechanism of electron catalysis, we speculated that the trace amount of BIPY (0) —generated by the weak disproportionation of the BIPY •+ units in either R 2(•+) or D +(•+) —is responsible for the slow host–guest complexation between R 2(•+) and D +(•+) , as observed in the long-time kinetic measurement (Fig.11).
Scheme 3: The weak disproportionation (K disp ~ 10 −7 ) of the BIPY •+ radical cation can generate a trace amount of BIPY 2+ and BIPY (0) . In the host–guest system composed by R 2(•+) and D +(•+) , it could be the formation of a little amount of BIPY (0) unit that induces a slow but detectable molecular recognition. Furthermore, the equilibrium of disproportionation can be manipulated by redox chemistry, bringing about the changes in the concentration of BIPY (0) unit and thus enabling the regulation of the kinetics of the molecular recognition process. The equilibrium of disproportionation can be manipulated by redox chemistry. On the one hand, reduction—for example, by CoCp 2 —will increase the concentration of BIPY (0) unit, thereby accelerating (Fig.2) significantly the molecular recognition process. On the other hand, oxidation can decrease the concentration of BIPY (0) unit, which is expected to slow down the molecular recognition process. In an attempt to test this hypothesis, we introduced 20 mol% NOPF 6 (a chemical oxidant) into the equimolar mixture of R 2(•+) and D +(•+) , and recorded the evolution of UV/Vis/NIR spectra. The molecular recognition between R 2(•+) and D +(•+) under this condition was found to be slower (Fig.12) than that without NOPF 6 (Fig.11). Given the fact that the addition of 20 mol% NOPF 6 induces only a slight change in the concentrations of R 2(•+) and D +(•+) , the distinct kinetics of the molecular recognition is probably a result of the decreased amount of BIPY (0) unit. This result provides supporting evidence for the proposed effect of disproportionation on the molecular recognition process. 4.2 Estimation of the Binding Constant Host–guest complexation between R 2(•+) and D +(•+) is almost kinetically forbidden under ambient conditions, rendering it difficult to determine the binding constant by using traditional titration methods. Since the addition of a little amount (e.g., 4 mol%) of CoCp 2 can accelerate the molecular recognition process and has only a slight influence on the thermodynamic equilibrium, we can estimate the binding constant between R 2(•+) and D +(•+) using the experimental data under catalytic conditions. The reversible host–guest complexation can be written as the equation given in Figure 47. The binding constant K can be expressed as: When introducing 4 mol% CoCp 2 into the host–guest system composed by 150 μM R 2(•+) and D +(•+) , 6 μM of BIPY •+ radical cation was reduced to BIPY (0) unit. As a result, the initial concentrations of R 2(•+) (c 1 ) and D +(•+) (c 2 ) are 146 μM and 148 μM, respectively. According to the Lambert-Beer’s law: where A 1080,eq is the absorbance of the solution at 1080 nm after reaching equilibrium, and ε 1080 is the molar absorption coefficient of the [D⊂R] +3(•+) trisradical complex at 1080 nm, which can be estimated to be 1.12 × 10 4 M −1 cm −1 from the literature 13 . l is the path length and set as 0.4 cm throughout this research. Therefore, the equilibrium concentration (c eq ) of the [D⊂R] +3(•+) trisradical complex was calculated to be 56.1 μM. From the values of c 1 , c 2 and c eq , the binding constant between R 2(•+) and D +(•+) was calculated to be 6.8 × 10 3 M −1 , which is comparable to the values reported 13 in the literature. 4.3 Estimation of the Rate Constant The rate constant of the molecular recognition between R 2(•+) and D +(•+) was estimated from the kinetic data measured by UV/Vis/NIR spectroscopy. We employed the kinetic model of a two- to-one reversible reaction to deduce the rate equation (Figure 48). The forward rate (r) of this process can be expressed as: wherein c 1 and c 2 are the initial concentrations of R 2(•+) and D +(•+) , respectively, x represents the concentration of the [D⊂R] +3(•+) trisradical complex at a certain moment, and k 1 and k 2 refer to the rate constants of the association and dissociation, respectively. As a result of the relationship between k 1 , k 2 and the equilibrium constant K: the equation (3) can be transformed to be: In order to simplify the equation, we defined two constants m and n as follows: By this way, the equation (5) was transformed to be: and then solved: wherein Q, a constant related to the data at t = 0, is out of our concern. Since the characteristic absorption for the [D⊂R] +3(•+) trisradical complex at 1080 nm follows the Lambert-Beer’s law: we finally obtained the relationship between the absorbance at 1080 nm (A 1080 ) and time (t) as follows: wherein a parameter y was defined to simplify the form, which can be calculated from the data of A 1080 together with a series of constants including ε 1080 , l, c 1 , c 2 and K. Fitting the data of A 1080 to the time (t), we determined (Fig.13) the rate constants (k 1 ) of the molecular recognition between R 2(•+) and D +(•+) under different conditions. The values of k 1 have been summarized in Table 2. Table 2 | The values of c 1 , c 2 and k 1 for the molecular recognition processes between R 2(•+) and D +(•+) under different conditions 4.4 Estimation of the Turnover Number Based on the kinetic data obtained by UV/Vis/NIR spectroscopy, we calculated the turnover number (TON) of the electron-catalysed molecular recognition between R 2(•+) and D +(•+) . During the kinetic measurements, O2 was found to penetrate slowly into the solution and oxidize the BIPY (0) units. As a result, the catalytic effect of electrons is gradually weakened and even quenched as time goes on. This happening, which is non-negligible for long-time molecular recognition processes catalysed by a small number of electrons, renders it difficult to determine accurately the TON value. Therefore, a lower limit of TON was estimated using the yield of the [D⊂R] +3(•+) trisradical complex after a moderate time (70 min), according to the following formula: wherein c(Com) is the concentration of the [D⊂R] +3(•+) trisradical complex at 70 min and can be determined from the absorbance of the solution at 1080 nm, while c(e − ), the molar concentration of injected electrons, is approximately equal to that of CoCp2. The estimated TON values under different conditions are summarized in Table 3. Table 3 | The estimated values of the turnover number (TON) for the electron-catalysed molecular recognition processes under different conditions In the case of 4 and 8 mol% CoCp 2 , the TON values are relatively small, because the yield of the [D⊂R] +3(•+) trisradical complex is limited by the thermodynamic equilibrium between substrates and complexes. In the case of 1 mol% CoCp 2 , the TON was estimated to be 13, i.e., each mole of electrons is able to catalyse the formation of at least 13 moles of the [D⊂R] +3(•+) trisradical complex. The modest value of TON, not only reflects the quenching effect by O 2 , but also results from the excessive stability of the catalytic intermediate, i.e., the [D⊂R] +2(•+) bisradical complex, which inhibits the completion of catalytic cycles. Further efforts need to be made in order to develop more efficient electron-catalysed molecular recognition systems involving less stable intermediates, so as to increase the TON value. 5. Electron Catalysis Evidenced by Cyclic Voltammetry In addition to UV/Vis/NIR spectroscopy, the electron-catalysed molecular recognition between R 2(•+) and D +(•+) has also been evidenced by cyclic voltammetry (CV). The CV scan of either R 4+ or D 3+ indicated two sequential, reversible reductions from BIPY 2+ units to BIPY •+ radical cations and further to BIPY (0) units. Specifically, the reduction peaks of R 4+ (Fig.14a) at −297 and −736 mV can be ascribed to the reduction from R 4+ to R 2(•+) and from R 2(•+) to R (0) , respectively. Likewise, the two-step reduction of D 3+ (Fig.14b) occurred at −306 and −717 mV successively. In contrast with the relatively simple redox behaviours of R 4+ or D 3+ alone, the CV scan of the mixture of R 4+ and D 3+ from +0.5 to –1.2 V displayed (Fig.14c) a complicated and irreversible trace. In particular, the negatively shifted reduction peak at –835 mV and the positively shifted oxidation peak at +30 mV, compared with R 4+ or D 3+ alone, are characteristic 12 of the formation of the [D⊂R] +3(•+) trisradical complex because the radical-pairing interactions involved in the complex can stabilize the BIPY •+ radical cations against (1) being reduced to BIPY (0) and (2) being oxidized to BIPY 2+ . The oxidation peak at +30 mV characteristic for the [D⊂R] +3(•+) trisradical complex, however, was found to disappear, when performing a short-range CV scan (Fig.14e) of the mixture of R 4+ and D 3+ from +0.5 to –0.6 V. Under this set of conditions, the BIPY 2+ units were only reduced to BIPY •+ radical cations before reoxidation, meaning that no generation of BIPY (0) units occurs during the process. The clear difference, therefore, between the CV traces with different scanning ranges indicates that the reduction from BIPY •+ radical cations to BIPY (0) units—i.e., the injection of additional electrons—is essential for promoting the molecular recognition between R 2(•+) and D +(•+) . When performing a middle-range CV scan (Fig. 14d) on a mixture of R 4+ and D 3+ from +0.5 to –0.75 V, we still observe the relevant peak at +30 mV, albeit with decreased intensity. This control experiment has excluded the possibility that the formation of the [D⊂R] +3(•+) trisradical complex originates from the reduction process at –835 mV. In order to reveal the influence of electron-catalysed molecular recognition on the redox properties of the host–guest system, we performed (Fig.15) CV measurements on the mixture of R 4+ and D 3+ at variable scan rates. The CV traces were found to be highly dependent on the scan rates, suggesting that a correlation exists between supramolecular association/dissociation and electron transfer events. By analysing the CV data at different scan rates and referring to the literature 11-13 , a whole redox cycle was divided into four stages and all the events occurring during the process were illustrated in Schemes 4–7 (Figures 49-52). Stage 1 (Scheme 4, Figure 49), scanning from +0.5 to –0.6 V, results in the reduction of BIPY 2+ units to BIPY •+ radical cations. In this stage, R 4+ and D 3+ are reduced to R 2(•+) and D +(•+) , respectively, at the same potential. Thus, only one reduction peak is observed regardless of the scan rate. Note that the complexation between R 2(•+) and D +(•+) is kinetically forbidden until more electrons are injected in Stage 2. Stage 2 (Scheme 5, Figure 50), scanning from –0.6 to –1.2 V, results in the reduction of BIPY •+ radical cations to BIPY (0) neutral units. When the scan rate is relatively slow (0.01–10 V/s), the mixture of R 2(•+) and D +(•+) accepts three electrons gradually. Frist of all, the injection of two electrons leads to the formation of the [D⊂R] +2(•+) bisradical complex. Further reduction of all the BIPY units to their neutral states occurs at a more negative potential, indicating the stabilizing effect of the radical-pairing interaction on the bisradical complex. Finally, the lack of interactions between R (0) and D + induces their dissociation. Therefore, two reduction peaks have been observed in this stage for the slow CV scans. When the scan rate is very fast (10–50 V/s), however, the mixture of R 2(•+) and D +(•+) accepts three electrons rapidly and generates directly R (0) and D + , so that the formation of the bisradical complex cannot be detected. Therefore, only one reduction peak has been observed in this stage involving the fast CV scans. Stage 3 (Scheme 6, Figure 51), scanning from –1.2 to –0.4 V, results in the reoxidation of BIPY (0) neutral units to BIPY •+ radical cations. Three possible pathways are proposed for this stage: When the scan rate is slow (0.01–0.1 V/s), the mixture of R (0) and D + has the chance to lose two electrons first of all, generating the couple of R 2(•+) and D + or the couple of R •+ and D +(•+) . Both couples can be associated into the [D⊂R] +2(•+) bisradical complex. On account of the formation of this stable bisradical complex, the potential for the two-electron oxidation of the mixture of R (0) and D + is less positive than that required for the oxidation of R (0) or D + alone to its radical cation state. This two-electron oxidation peak is only observable using very slow scan rates because sufficient time is required to overcome the energy barrier for the assembly of the bisradical complex from its components. Subsequently, another peak at a more positive potential is observed and ascribed to oxidation of the [D⊂R] +2(•+) bisradical complex to the [D⊂R] +3(•+) trisradical complex. Thus, two reoxidation peaks have been observed in this stage for the slow CV scans, leading to the generation of the trisradical complex. When the scan rate is in the medium (0.1–10 V/s) range, we cannot observe a considerate amount of the [D⊂R] +2(•+) bisradical complex formed during the measurements. The mixture of R (0) and D + loses three electrons at a moderate speed, generating R 2(•+) and D +(•+) as well as a small amount of R •+ and D + . The latter two species will initiate the electron-catalysed molecular recognition between R 2(•+) and D +(•+) , affording the [D⊂R] +3(•+) trisradical complex. Thus, although only one reoxidation peak has been observed for the medium-rate CV scans, the trisradical complex is still generated in this stage. When the scan rate is fast (10–50 V/s), the mixture of R (0) and D + loses three electrons in an instantaneous manner. Hence, only one reoxidation peak has been observed in this stage for the fast CV scans, with neither the [D⊂R] +2(•+) bisradical complex nor the [D⊂R] +3(•+) trisradical complex being generated. Stage 4 (Scheme 7, Figure 52), scanning from –0.4 to +0.5 V, results in the reoxidation from BIPY •+ radical cations to BIPY 2+ units. Three possible pathways are proposed for this stage: When the scan rate is slow (0.01 V/s), the [D⊂R] +3(•+) trisradical complex generated in Stage 3 loses one electron first of all, forming a [D⊂R] 3+2(•+) bisradical complex composed of two BIPY •+ radical cations and one BIPY 2+ unit. When the CV scan is performed very slowly, this weakly bounded complex—as a result of increased Coulombic repulsion—has sufficient time to undergo dissociation and generate R 2+(•+) and D +(•+) . At the same potential, the resulting species lose two electrons to afford R 4+ and D 3+ . Therefore, only one reoxidation peak is observed in this stage for the slow CV scan. When the scan rate is in the medium (0.01–10 V/s) range, the [D⊂R] +3(•+) trisradical complex generated in Stage 3 loses one electron first of all, forming a [D⊂R] 3+2(•+) bisradical complex composed of two BIPY •+ radical cations and one BIPY 2+ unit. The oxidation of this complex is less favourable than that of the trisradical complex on account of the increased number of positive charges. Hence, when the CV scan is faster than the dissociation of the [D⊂R] 3+2(•+) bisradical complex, the complex will lose another electron at a more positive potential and produce a [D⊂R] 5+(•+) monoradical complex. Upon the formation of this transient complex, Coulombic repulsion is further increased, leading to (1) the dissociation of the complex to R 4+ and D +(•+) , or (2) the threading of the ring onto the collecting oligomethylene chain in the guest molecule, thus forming a [D < R] 5+(•+) rotaxane. Here the symbol “<” denotes that the ring encircles the dumbbell in a rotaxane. Either the mixture of R 4+ and D +(•+) or the [D < R] 5+(•+) rotaxane can be oxidized, at the same potential, to the mixture of R 4+ and D 3+ or the [D < R] 7+ rotaxane, respectively. Hence, the medium-rate CV scans in this stage give rise to two possible sets of products, both of which result from two reoxidation peaks. When the scan rate is fast (10–50 V/s), the products in Stage 3 are R 2(•+) and D +(•+) , both of which lose electrons independently at the same potential and restore R 4+ and D 3+ . Thus, only one reoxidation peak has been observed in this stage for the fast CV scans. After one cycle of a middle-rate (e.g., 100 mV s −1 ) CV scan on the mixture of R 4+ and D 3+ , some of the starting materials have been transformed to the [D < R] 7+ rotaxane. As a result, the second cycle of the CV scan displays (Fig.16) a new reduction peak at −63 mV, compared with the first cycle of scan. This positively shifted peak can be ascribed to a two-electron reduction (Fig. 16) of the [D < R] 7+ rotaxane to the [D⊂R] 3+2(•+) bisradical complex, considering that the intramolecular binding promotes the generation of two BIPY •+ radical cations in the rotaxane. 6. Mechanism of the Electron-Catalysed Molecular Recognition 6.1 Discussion on the Catalytic Pathways The electron-catalysed molecular recognition between R 2(•+) and D +(•+) is accomplished in two stages: (1) the formation of a key intermediate, the [D⊂R] +2(•+) bisradical complex, by injecting electrons into the starting materials, and (2) the transformation of this intermediate into the final product, i.e., the [D⊂R] +3(•+) trisradical complex. Since both R 2(•+) and D +(•+) contain the BIPY •+ radical cation and can accept the injected electron, there should be more than one possible pathway during electron catalysis. At the beginning, when an electron is injected into the system, two possible pathways (Scheme 8, Figure 53) exist, depending on whether R 2(•+) or D +(•+) accepts this electron. In Pathway 1, one of the BIPY •+ radical cations in R 2(•+) is reduced to BIPY (0) unit, and the resulting R •+ can bind rapidly with D +(•+) to afford the [D⊂R] +2(•+) bisradical complex. In Pathway 2, the BIPY •+ radical cation in D +(•+) is reduced to BIPY (0) unit, and the resulting D + can bind rapidly with R 2(•+) to afford the [D⊂R] +2(•+) bisradical complex. Given the fact that the reduction potential (Figs. 10 and 14) of D +(•+) is slightly more positive than that of R 2(•+) , the formation of the R 2(•+) / D + couple (i.e., Pathway 2) is relatively preferred. Both pathways decrease the number of positive charges in the system, and thereby lower the energy barrier associated with the binding between the ring and the dumbbell. See the DFT calculation results in Section 6.2. Since the electron is exchangeable, the R •+ / D +(•+) couple (coming from Pathway 1) and the R 2(•+) / D + couple (coming from Pathway 2) can co-exist in the system and undergo mutual transformation. Therefore, these two pathways are interconnected, leading jointly to the formation of the [D⊂R] +2(•+) bisradical complex, the key intermediate in the electron-catalysed molecular recognition process. When it comes to the transformation from the [D⊂R] +2(•+) bisradical complex to the [D⊂R] +3(•+) trisradical complex, there are also two possible pathways (Scheme 9, Figure 54). In Pathway 1, single-electron transfer (SET) occurs between the [D⊂R] +2(•+) bisradical complex and R 2(•+) , affording the [D⊂R] +3(•+) trisradical complex and R •+ . In Pathway 2, it is D +(•+) that participates in the SET with the [D⊂R] +3(•+) trisradical complex, generating the [D⊂R] +3(•+) trisradical complex and D + . Both pathways lead to the formation of the [D⊂R] +3(•+) trisradical complex—the final product of the molecular recognition—and meanwhile, the regeneration of either R •+ or D + to sustain the catalytic cycle. In summary, all the possible pathways during the molecular recognition process can be unified as electron catalysis, with the electron serving as the actual catalyst. 6.2 Quantum Mechanical Calculations of the Energy Barriers In order to elucidate the mechanism of the electron catalysis, we performed Quantum Mechanics (QM) calculations at the level of Density Functional Theory (DFT) to determine the energy barriers of the molecular recognition processes with or without injection of an electron. In these calculations, we optimized the geometries of molecules using Density Functional Theory at the M06-2X/6-31G* level 18 using Jaguar 19 v10.6 with the PBF Poisson-Boltzmann solvation model 20 based on acetonitrile (ɛ = 37.5 and R 0 = 2.18 Å). These single-point optimized structures were refined using the larger basis for M06-2X/6-311++G** to obtain the final energy. We included two Cl − anions to mimic the electrostatic effect of PF 6 − anions in the experimental system. The introduction of explicit counterions into the PBF implicit solvation model decreases the net charge of the whole system, reducing uncertainty in the calculated solvation energy based on the implicit solvation model. For the multi-radical systems in this study, we found the open-shell unrestricted wavefunction with low spin to be the ground state or almost degenerate with the high spin state. The energy of the true ground state of low spin wavefunction was estimated 21 by correcting the spin contamination to obtain the energy difference between the high-spin triplet and the low-spin singlet. Calculations were carried out on three host–guest systems: 1) R 2(•+) and D +(•+) , 2) R •+ and D +(•+) , and 3) R 2(•+) and D + . For each system, the potential energy surface was obtained by scanning the centre of the ring (defined as the average position of four methylene carbon atoms) through the atoms (numbered as position 1 to 16) in the dumbbell, as shown in Figs. 17–19. The position 0 corresponds to the infinite separation of two molecules (each surrounded by one Cl − anion). The energy of this state is defined as zero. In the potential energy surface obtained by DFT calculations, Coulombic repulsion between the ring and the dumbbell molecules contributes substantially to the energy barrier (ΔE ) during the threading process. As a result, the barrier heights differ substantially under different charge-number conditions. In the R 2(•+) –D +(•+) system (Fig.17), we determined ΔE = 15.0 kcal mol −1 from the energy difference between position 1 and 4. This large energy barrier arises from the Coulombic repulsion between R 2(•+) and D +(•+) . Specifically, both the positively charged BIPY •+ unit and PY + group in D +(•+) —working in concert on account of their close proximity—contribute to the Coulombic repulsion with the two positively charged BIPY •+ units in R 2(•+) . Therefore, the energy barrier can decrease substantially by reducing the charge number of either R 2(•+) or D +(•+) . In the R •+ –D +(•+) system (Fig. 18), we determined ΔE = 9.3 kcal mol −1 from the energy difference between position 3 and 7, which is 5.7 kcal mol −1 lower than that in the R 2(•+) –D +(•+) system. In the R 2(•+) –D + system (Fig. 19), we determined ΔE = 8.8 kcal mol −1 from the energy difference between position 3 and 5, which is 6.2 kcal mol −1 lower than that in the R 2(•+) –D +(•+) system. These results indicate that the injection of an electron into either R 2(•+) or D +(•+) can induce a remarkable decrease in the Coulombic repulsion, thereby decreasing the energy barrier for the molecular recognition process. Practically, upon the injection of an electron, there forms a mixture of the R •+ –D +(•+) couple and the R 2(•+) –D + couple, which are interconverted through single electron transfer between the ring and the dumbbell molecules. The energy barrier for the electron transfer is governed by the reorganization energy involving the conformational change of the dumbbell molecule, the movement of counterions, and the polarization field imposed by solvent molecules. If this energy barrier is negligible compared with the energy barrier for the complexation between R •+ and D +(•+) or between R 2(•+) and D + (which means that the extra electron can “freely” jump to the more favourable site with little barrier), we can combine the potential energy surfaces of R •+ –D +(•+) and R 2(•+) –D + systems to construct an overall potential energy surface for the molecular recognition associated with the injection of an electron. We found that for molecular recognition in the presence of an extra electron, the R •+ –D +(•+) couple is the energetically more favourable state or almost degenerate with the R 2(•+) –D + couple. By comparing the electronic energy of these two couples and taking the lower value as the electronic ground state at each position, we obtained the overall potential energy surface for the binding process with the injection of an electron, as plotted in Fig.20 together with the potential energy surface for the binding process without the injection of electron (i.e., the R 2(•+) –D +(•+) system). The comparison between these two energy profiles further confirms that the introduction of an electron can decrease the energy barrier of the molecular recognition. In order to estimate the change in Gibbs free energy (ΔG) of the molecular recognition with or without injection of an electron, the values of enthalpy change (ΔH) and entropy change (ΔS) need to be determined. Assuming that the volume change during this noncovalent binding event is small in solution, the enthalpy change approximately equals to the binding energy, i.e., ΔH ≈ ΔE. In order to estimate the entropy change, we computed the Hessian matrix (the second derivative of energy) of simplified model systems in which the ring molecule remained unchanged while the long alkyl chain and the 2,6-diisopropylphenyl terminal group in the dumbbell molecule were replaced with a propyl unit. The calculation of these model systems was performed in gas phase with no counterion. In contrast to species in the gas phase, species in the solution phase cannot translate or rotate freely, leading to liberational modes. To account for the solvent confinement, we scale the corresponding entropic contributions due to translation and rotation down by a factor 22 of 0.5. The calculated values of ΔH, ΔS and ΔG for the three host–guest systems are summarized in Table 4. Table 4: The calculated thermodynamic parameters ΔH, TΔS, and ΔG at 298 K for the molecular recognition in the three host–guest systems Table 5: The charge population on the ring molecule at different positions in the three host– guest systems. These are electrostatic potential (ESP) derived charges using the M06-2X/6- 311++G** level of DFT. 6.3 Spectroscopic Detection of the Key Intermediate [D⊂R] +2(•+) bisradical complex, the key intermediate of the electron-catalysed molecular recognition process, was detected (Fig.21) by UV/Vis/NIR spectroscopy. Although the addition of 4 mol% CoCp 2 gives rise to the catalytic formation of [D⊂R] +3(•+) trisradical complex—as indicated by the absorption band at 1080 nm—increasing the amount of CoCp 2 from 4 to 100 mol% brings about the decrease of the absorption band at 1080 nm, together with the appearance of a new, red-shifted absorption band at 1700 nm. We ascribe this absorption band to the [D⊂R] +2(•+) bisradical complex. The SET between the [D⊂R] +2(•+) bisradical complex and R 2(•+) or D + —a process leading to the formation of the [D⊂R] +3(•+) trisradical complex—was probed by UV/Vis/NIR spectroscopy. When introducing R 2(•+) into the solution of [D⊂R] +2(•+) bisradical complex, we observed (Fig. 24a) the decay of the absorption (λmax = 1700 nm) for the bisradical complex, accompanied with the increase of the absorption (λ max = 1080 nm) for the trisradical complex. Notably, more than 1 eq of R 2(•+) was required (Fig.24b) to render this evolution being complete. This result is reasonable because the reduction potential of the [D⊂R] +3(•+) / [D⊂R] +2(•+) redox couple is close to (instead of much lower than) that of the R 3(•+) / R 2(•+) couple. Considering the fact that the electron is injected in a catalytic amount (e.g., 4 mol%) during the molecular recognition process, there is always excess of R 2(•+) in relation to the low concentration of [D⊂R] +2(•+) bisradical complex, guaranteeing the sufficiency of the SET process and the sustainability of the catalytic cycle. In a similar manner, we have also demonstrated (Fig.25) experimentally the SET between the [D⊂R] +2(•+) bisradical complex and D +(•+) , a process that affords the [D⊂R] +3(•+) trisradical complex. These results provide supportive evidence for the two possible SET pathways towards the final product of the molecular recognition, as illustrated in Scheme 9 (Figure 54). 6.4 Investigation of the [2]Catenane Model Compound In order to unravel the superstructure and investigate the properties of the bisradical complex, we synthesized a [2]catenane (Cat•6PF 6 ) as a model compound for detailed studies. Although the bisradical complex was originally detected during the molecular recognition process, the combination of R 2(•+) , D +(•+) and CoCp 2 constitutes a complicated supramolecular system (Fig. 26), involving seven kinds of BIPY-based chemical species and seven reversible processes. In contrast, the [2]catenane—as a result of the mechanical bond 23,24 between two component rings— is a well-defined molecular system (Fig. 26), including no more than two kinds of BIPY-based species and only one process. The simplicity of this molecular system allows us to obtain entirely a bisradical-state [2]catenane, namely Cat 2(•+) , by controlling the amount of CoCp 2 , which is helpful for the investigation of its properties. In addition, Cat 2(•+) has proven to be crystallizable, enabling the direct observation of its structure. The trisradical tricationic state of the [2]catenane, Cat 3(•+) , was prepared by stirring the MeCN solution of Cat•6PF 6 (150 μM) with excess of activated Zn dust for 30 min. Thereafter, the solid was filtered out to afford a purple solution. This solution of Cat 3(•+) underwent titration with 1.0 eq CoCp 2 , and the evolution of the UV/Vis/NIR spectra was recorded. During the titration, the decrease of the absorption band at 1080 nm was observed (Fig.27a), accompanied by increase of the absorption at 1640 nm, with isobestic points at 506 nm and 1219 nm. This result indicates the quantitative transformation from Cat 3(•+) to Cat 2(•+) . The narrow band gap (less than 0.62 eV, deduced 25 from λ onset ≥ 2000 nm) of Cat 2(•+) is probably because of the combination of radical- pairing and donor–acceptor interactions. Furthermore, when excess of R 2(•+) was introduced into the solution of Cat 2(•+) , as expected, the absorption band at 1640 nm was found (Fig.27b) to decay, along with the recovery of the absorption band at 1080 nm. This spectral change, indicating that Cat 2(•+) was oxidized back to Cat 3(•+) , confirms the ability of the bisradical state to undergo SET with R 2(•+) . In order to grow the single crystal of Cat 2(•+) , 4.0 eq CoCp 2 was added into the MeCN solution of Cat•6PF 6 (1.0 mM). Thereafter, slow vapor diffusion of Et 2 O into the MeCN solution was allowed to occur in the N2-filled glovebox over one week, affording black crystals suitable for X-ray crystallographic analysis. Methods. A suitable crystal was selected and mounted on a MITIGEN holder with Paratone oil on a XtaLAB Synergy R, DW system, HyPix diffractometer. The crystal was kept at 100.01(10) K during data collection. Using Olex2 (Ref 26 ), the structure was solved with the ShelXT structure solution program 27 using Intrinsic Phasing and refined with the XL refinement package 28 using Least Squares minimisation. Crystal data for C 76 H 84 CoF 18 N 12 P 3 (M =1659.39). Tetragonal, space group P4 � c2 (no. 116), a = 16.09100(10) Å, c = 30.6313(4) Å, V = 7931.06(14) Å 3 , Z = 4, T = 100.01(10) K, μ(CuK α ) = 3.075 mm −1 , D calc = 1.390 g cm −3 , 54393 reflections measured (5.492 ≤ 2Θ ≤ 157.412), 8323 unique (R int = 0.0342, R sigma = 0.0215) which were used in all calculations. The final R 1 was 0.0851 (I > 2σ(I)) and wR 2 was 0.2472 (all data). Refinement Details. Distance restraints were imposed on the carbon chain and disordered PF 6 − anions. The enhanced rigid-bond restraint (SHELX keyword RIGU) was applied 29 on the carbon chain and disordered PF 6 − anions. Restraints on similar amplitudes separated by less than 1.7 Å. were also imposed on the disordered carbon chain. The solid-state (super)structures are shown in Figs.28 and 29.
Table 6: Comparison between the two possible resonance isomers of the bisradical binding state Cat 2(•+) , which involves the through-space delocalization of two electrons over three BIPY units, can be presented as the hybridization of two possible resonance isomers. These two isomers exhibit different structures and properties, as summarized in Table 6. According to 1) the narrow bandgap estimated from the NIR absorption, 2) the diamagnetic nature indicated by the silent EPR signal, and 3) the uneven distances between adjacent BIPY units observed in the X-ray crystallographic data, we determined the asymmetric structure as the major isomer of Cat 2(•+) , wherein the BIPY (0) neutral unit is located on one side rather than in the middle. The structural feature of Cat 2(•+) can be generalized to its non-interlocked counterpart, i.e., a noncovalent bisradical complex. 7. Molecular Recognition Promoted by Various Chemical Initiators The mechanism of electron catalysis prompted us to screen a variety of chemical initiators (Fig.30) for promoting the molecular recognition between R 2(•+) and D +(•+) . Firstly, a small amount of either D + and R (0) —the reduction product of R 2(•+) or D +(•+) , respectively—can be used directly to initiate the molecular recognition process. Secondly, the bisradical-state [2]catenane Cat 2(•+) exhibits a similar effect. Thirdly, some other reductants without BIPY structural units—such as bis(cyclopentadienyl) cobalt(II) (cobaltocene, CoCp 2 ), bis(pentamethylcyclopentadienyl) cobalt(II), Co(Cp*) 2 for short, and tetrakis(dimethylamino)ethylene (TDAE)—have also shown their potency to trigger the molecular recognition process. Finally, active metals, including magnesium (Mg), aluminium (Al), iron (Fe) and zinc (Zn), can serve as heterogenous initiators, although copper (Cu) has no such effect because of the insufficient reducing power. The related experimental data are shown in Figs.32–38. In summary, the reducing power (Fig.31) required for triggering this molecular recognition process is around −700 mV. It means that any reagents— with 1) the appropriate reduction potential that is lower than, or comparable to, this value and 2) the ability to transfer an electron at a reasonable rate—are eligible chemical initiators. 8. Molecular Recognition Initiated or Controlled by Electricity The mechanism of electron catalysis renders it possible to initiate or control the molecular recognition between R 2(•+) and D +(•+) using electricity. For this purpose, the electrolysis of the mixture of R 2(•+) and D +(•+) was performed in a divided or undivided cell. These two kinds of electrochemical set-ups 9 have shown distinct effects on the molecular recognition process. Specifically, the divided-cell approach, which enables the permanent injection of catalytic amounts of electrons, is analogous to the introduction of chemical initiators. In contrast, during the continuous electrolysis in an undivided cell, electrons are simultaneously injected into and withdrawn from the solution, allowing temporal control of the molecular recognition process. 8.1 Electrochemically Initiated Molecular Recognition in a Divided Cell Electrochemically initiated molecular recognition between R 2(•+) and D +(•+) was performed in a N 2 -filled glovebox using a divided cell. A three-electrode system was employed, which included a reticular vitreous carbon (RVC) working electrode, an RVC counter electrode and an Ag/AgCl reference electrode. The working cell (Fig. 39a, left) was filled with a MeCN solution (50 mL) containing R 2(•+) (150 μM), D +(•+) (150 μM) and TBAPF 6 (0.05 M), while the counter cell (Fig. 39a, right) was filled with a MeCN solution (50 mL) of excess of (trimethylammonium methyl)ferrocene hexafluoro-phosphate, which serves as the sacrificial electron donor. The two cells were separated by an ionic exchange membrane and held together by a clamp. The whole apparatus was connected to a Gamry multipurpose instrument (Reference 600) interfaced to a PC. The experimental parameters were instructed using the software of Gamry Framework Version 6.30 under the chronocoulometry mode. In order to inject a certain number (catalytic amount) of electrons into the host–guest system, the solutions were electrolysed under stirring at a constant potential of −0.65 V for only a few seconds before switching off the electricity. This approach of using divided-cell electrolysis has displayed a similar catalytic effect to that involving the introduction of chemical initiators. The injection of 100 mC (14 mol%) of electrons allows (Fig.39b) the molecular recognition between R 2(•+) and D +(•+) to be completed instantaneously (within 10 s). In contrast, by injecting 25 mC (3.5 mol%) of electrons and then switching off the electricity, the molecular recognition proceeds (Fig.40) at a relatively slow rate, allowing the process to be monitored by UV/Vis/NIR spectroscopy. 8.2 Electrochemically Controlled Molecular Recognition in an Undivided Cell Electrochemically controlled molecular recognition between R 2(•+) and D +(•+) was conducted in a N 2 -filled glovebox using an IKA ® Electrasyn 2.0 Device. The set-up was an undivided cell in which both the cathode and anode are reticular vitreous carbon electrodes. In a typical procedure, a MeCN solution (9 mL) containing R 2(•+) (150 μM), D +(•+) (150 μM) and TBAPF 6 (0.05 M) underwent electrolysis for 3 min at a constant current (e.g., 1.0 mA) and a constant stirring rate (e.g., 300 rpm). Subsequently, the electricity was switched off and the solution was allowed to stand for 3 min. The overall process consisted of three on/off cycles. Molecular recognition during the intermittent electrolysis was monitored by sampling the solution every 3 min and recording its UV/Vis/NIR spectrum. In order to reveal the influence of current intensity and stirring rate on the kinetics of molecular recognition, two arrays of electrolysis experiments were carried out. In the first array, the stirring rate was held at 300 rpm, while the current was set to 0.5, 1.0 or 2.0 mA. In the second array, the current was held at 1.0 mA, while the stirring rate was set to 200, 300 or 400 rpm. Four possible pathways involving the molecular recognition between R 2(•+) and D +(•+) during the undivided-cell electrolysis are illustrated in Figs. 41–44. All the pathways follow a similar mechanism, while the difference lies in which component in the system is reduced / oxidized at the interface of the cathode / anode, respectively, in the initiating step. In a combined view of the above four possible pathways, the electrochemically controlled molecular recognition between R 2(•+) and D +(•+) occurs through an “initiation–propagation– termination” process, which can be summarized as below. Note that the electron at the cathode and the anode is labelled as e c − and e a − , respectively. Initiation: Propagation: Termination: The overall process: These equations cover the possible processes at the beginning stage of electrolysis. When the electrochemically controlled molecular recognition has proceeded for a period of time, the [D⊂R] +3(•+) trisradical complex is formed and accumulated. At this time, besides the starting materials R 2(•+) and D +(•+) , the complex can also accept / lose an electron at the cathode / anode, rendering the pathways more complicated. Table 7: The calculated conversions of electrochemically controlled molecular recognition under different conditions Table 8: The estimated values of Faradic efficiencies of electrochemically controlled molecular recognition under different conditions The conversions and Faradic efficiencies of electrochemically controlled molecular recognition are estimated and summarized in Tables 7 and 8. As expected, the conversion can be improved with the increased current intensity and is lowered with the increased stirring rate. In the case of 1.0 mA, 200 rpm, the conversion after 9 min electrolysis is 31.3%, a value close to the conversion at the thermodynamic equilibrium (37.4%). In order to understand the values of Faradic efficiency, we probed the electrolysis process in the undivided cell. When a BIPY •+ radical cation, whether in R 2(•+) or D +(•+) , picks up an electron from the cathode to form a BIPY (0) unit, there can be three possible subsequent pathways: In Pathway 1, the species bearing the BIPY (0) unit moves quickly to the anode and returns an electron, or encounters the BIPY 2+ unit generated from the anodic oxidation. As a result, BIPY •+ is restored before molecular recognition has happened. The Faradic efficiency is 0. In Pathway 2, a [D⊂R] +2(•+) bisradical complex is generated, which deposits an electron at the anode or undergo single electron transfer with a BIPY 2+ unit to generate the final product, i.e., the [D⊂R] +3(•+) trisradical complex. As a result, the transfer of one electron from cathode to anode contributes to one round of molecular recognition. This process is a stoichiometric one, and the Faradic efficiency is 100%. In Pathway 3, a [D⊂R] +2(•+) bisradical complex is generated, which transfers an electron to BIPY •+ , thereby affording a [D⊂R] +3(•+) trisradical complex and a BIPY (0) unit. Subsequently, this newly formed BIPY (0) can propagate molecular recognition in a “chain reaction” manner. As a result, one electron can induce more than one round of molecular recognition. This process is justified as catalysis, and the Faradic efficiency is larger than 100%. In a practical setting, the electrolysis experiments result in a combination of these three pathways. The distributions of them rely on the mass transport of chemical species. Since the solution is stirred during the process, convection should be the major form of mass transport and play an important role in regulating the lifetime of catalytic intermediates. Pathway 1 is the major one when convection is very fast, so that the lifetime of BIPY (0) is not long enough to induce molecular recognition before returning to BIPY •+ . In contrast, Pathway 3 is dominant when convection is very slow, so that BIPY (0) has sufficient time to sustain many cycles of molecular recognition. Pathway 2 is somewhere in between. All the values of Faradic efficiencies (Table 8) are lower than 100%, an observation which indicates that Pathway 1 plays a dominant role under the conditions that we have employed. The Faradic efficiency could be improved by lowering the stirring rate. 9. Theoretical Analyses of the Molecular Recognition Processes 9.1 Chemically Initiated Molecular Recognition From the theoretical perspective, when the electron is introduced chemically by a reductant (Red), it will trigger the molecular recognition. In order to simplify the expression, we only consider the case in which R 2(•+) accepts an electron from the reductant, and so the process can be described as: Initiation: Propagation: The overall process: The overall process is a catalysis by the electron, that is to say, molecular recognition proceeds faster than it would without the reductant, but its equilibrium constant remains the same. An arbitrarily small (catalytic) amount of reductant can in principle facilitate an arbitrarily large number of conversions from R 2(•+) and D +(•+) to the [D⊂R] +3(•+) trisradical complex, i.e., bring the molecular recognition to its equilibrium which, based on the free energy differences, is essentially to completion. 9.2 Electrochemically Controlled Molecular Recognition The electrochemically controlled molecular recognition contains two possible processes, depending on the convection rates of intermediates during the electrolysis in an undivided cell. If the convection is slow, the number of catalytic cycles prior to termination will be large and the system can be considered to operate in a catalytic regime. This process is identical with the chemically initiated molecular recognition in terms of the theoretical consideration described above. If the convection is fast, the number of catalytic cycles will be small and the system can be considered to operate in a stoichiometric regime, where the transfer of one electron from cathode to anode promotes the formation of only one supramolecular complex. In order to describe the stoichiometric process, let us only consider the case in which R 2(•+) accepts an electron from the cathode. During the event, R 2(•+) picks up an electron at the cathode (e c − ) and binds with D +(•+) to form a [D⊂R] +2(•+) bisradical complex as the intermediate, which deposits the electron at the anode (e a − ) to afford the final product, i.e., the [D⊂R] +3(•+) trisradical complex: The overall process is the combination of these three steps: where F is Faraday constant, V is the applied voltage, R is the universal gas constant and T is temperature. According to this equation, the assembly of R 2(•+) and D +(•+) into the [D⊂R] +3(•+) trisradical complex is coupled to the electron transfer from cathode to anode, and so the steady-state position of this supramolecular system is influenced by the voltage. Therefore, we must conclude that the effect of electrons supplied electrochemically in this stochiometric process is not catalysis, even though the rate of molecular recognition is much larger than that in the absence of an electron supply. Furthermore, according to trajectory thermodynamics 33 , we should also consider another p air of forward and microscopic reverse processes: Combining the process I and II, the steady-state concentration ratio among R 2(•+) , D +(•+) and the [D⊂R] +3(•+) trisradical complex is determined by a kinetically weighted average of two voltage-constrained equilibrium constants: In this equation, a and b are weighting coefficients of the process I and II, respectively, and their ratio is defined as q, whose value in general depends on the applied voltage. ^^^^ is the kinetic asymmetry 34 parameter that is bounded between e FV/RT and e −FV/RT . Therefore, the stoichiometric process during the electrolysis has two features. Firstly, there is a one-to-one correspondence between the number of (i) electrons accepted by BIPY •+ units at the cathode and transferred subsequently to the anode and (ii) complexes formed during the process. 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