Cation-Coupled Electron Transfer on

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Reversible Bond/Cation Coupled Electron Transfer on PhenylenediamineBased Rhodamine B and Its Application on Electrochromism Xiaojun Wang, Shuo Wang, Chang Gu, Weiran Zhang, Hongzhi Zheng, Jingjing Zhang, Geyu Lu, Yu-Mo Zhang, Minjie Li, and Sean Xiao-An Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 24, 2017

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ACS Applied Materials & Interfaces

Reversible Bond/Cation Coupled Electron Transfer on Phenylenediamine-Based Rhodamine B and Its Application on Electrochromism Xiaojun Wang1, 2, Shuo Wang2, Chang Gu2, Weiran Zhang1, 2, Hongzhi Zheng2, Jingjing Zhang2, Guyu Lu4, Yu-Mo Zhang2,* Minjie Li1, 2 and Sean Xiao-An Zhang1, 2, 3* 1

State Key Lab of Supramolecular Structure and Materials, Jilin University, Changchun,

130012, P. R. China. 2

College of Chemistry, Jilin University, Changchun, 130012, P. R. China.

3

Department of chemistry and pharmacy, Zhuhai College of Jilin University, Zhuhai, 519041, P.

R. China. 4

College of Electron Science and Engineering, Jilin University, Changchun 130012, P. R. China

Keywords: Electron transfer, bond coupled electron transfer, cation coupled electron transfer, molecular switch, electrochromic material

Abstract

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A bio-mimetic system on reversible bond coupled electron transfer (BCET) has been proposed and investigated in a switchable Rh-N molecule with redox active subunits. We discover that energy barrier of C-N bond breaking is reduced dramatically to less than 1/7 (from 40.4 kcal/mol to 5.5 kcal/mol), and 1/3 of the oxidation potential has been simultaneously lowered (from 0.67 V to 0.43 V) with the oxidation of Rh-N. The concept, cation coupled electron transfer (CCET), is highly recommended by analyzing existing proton coupled electron transfer (PCET) and metal coupled electron transfer (MCET) along with aforementioned BCET, which have same characteristic of transferring positive charges, such as proton, metal ion, and organic cation. Molecular switch can be controlled directly by electricity through BCET process. Solid electrochromic device was fabricated with extremely high color efficiency (720 cm2/C), great reversibility (no degradation for 600 cycles) and quick respond time (30 ms).

Introduction Electron transfer (ET) plays a critical role in a wide range of biological processes, including photosynthesis, energy conversion, respiration and various enzymatic oxidations/reductions.1-3 To avoid high energy barrier, electron transfer often couples with desired structural alteration and chemical transformation of positive ions, such as proton-coupled electron transfer (PCET) and metal ion-coupled electron transfer (MCET).4-8 Remarkable discoveries on characteristics of PCET and MCET indicate that structural alteration of molecule and electron density of its active sites are dramatically affected by migration of closely related proton and/or metal ion, which brings out a negative shift in oxidation process or a positive shift in reduction process visualized within cyclic voltammograms.9-11 Besides PCET and MCET, electron transfer coupled with covalent bond cleavage or formation, such as C-C12-16/C-O17/C-S18-20/S-S21/others22-25, also plays a critical role in photosensitization26-


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, polymer synthesis28, and other significant applications29-30. Even though these important

bond-state changes during electron transfer have attracted much attention, and several convincing theories have been proposed and reviewed.31 Less attention has been paid on reversible bond (cleavage and formation) coupling with electron transfer, which is an urgent need for further understanding why enzymes are fully reversible during their dynamic processes involving structure and function alteration coupling with electron transfer. Several challenges remain to be addressed for developing a system for the reversible bond coupled electron transfer (BCET). Firstly, a suitable system should be easily oxidized or reduced to initiate cleavage/formation of a specific reversible covalent bond. Secondly, the specific covalent bond should be relatively stable under ambient condition and could be affected readily by electron transfer. Thirdly, an effective visible indicator, which is strongly related with structural alteration, is also highly desired for monitoring the bond cleavage/formation (Scheme 1). Inspired by an excellent research and related reaction mechanism on phenylenediamine, which was proposed by Smith, D. K. and her coworkers32, we infer that cleavage/formation of the C-N bond on derivatives of phenylenediamine could be switched by electron transfer, if the energy of C-N bond is moderate. In addition, rhodamine B, which has excellent photo properties, might be a usable indicator for tracking the cleavage/formation states of its reversible C-N bond. Even though a number of molecular switches have been reported for the reversible bond, which is stimulated by photo33, acid34 and force35, however no examples can be found in literature on reversing the C-N bond of rhodamine B derivatives with electron transfer. Scheme 1. The schematic diagram for design of BCET molecule.


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A simple and desirable molecular system Rh-N (Figure 1a) is presented herein by interconnecting rhodamine B with phenylenediamine, which enables the C-N bond to be reversibly controlled by redox. Results indicate that strong absorption features of xanthene part provide distinct evidence for identifying the cleavage/formation states of the reversible C-N bond. And confused relationship on the reversible C-N bond alteration and electron transfer is prospectively clarified for the first time. A new kind of electrochromic device with excellent properties was build using bond coupled electron transfer mechanism. Results and discussion The bond cleavage/formation coupled electron transfer in Rh-N. To verify our assumptions, a potential candidate Rh-N was designed and synthesized firstly, which might have excellent acid responsive property from switchable rhodamine B and outstanding electricity-inducedisomerization property from redox-active p-phenylenediamine. In addition, two more reference molecules (Rh-H, Rh-O, Figure 1a) without p-phenylenediamine part were also prepared to further support current exploration. The reversible C-N bond (cleavage and formation) of the RhN molecule coupled with color change induced by acid-base is evaluated as shown in Figure S1. Cyclic voltammetry of the Rh-N along with two reference molecules (Rh-H, Rh-O) are firstly performed in Figure 1b. Although these molecules have similar structures, they exhibit remarkable difference. Both Rh-H and Rh-O show irreversible redox behavior with blunt and


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contiguous two oxidation peaks at 0.56 V and 0.70 V with similar peak area, which come from the oxidation of the N, N-diethylaniline.36 However, as for the oxidation of Rh-N, fully reversible two sharp and far apart peaks at 0.30 V and 0.85 V are observed. More interestingly, the two oxidation peaks of Rh-N exhibit different peak area with approximately 2:1 ratio. This indicates that the oxidation of Rh-N molecule may involve 3 electrons transfer processes, and two of them take place in the first oxidation peak. To make sure this interesting conjecture, the electron numbers of redox peaks is further measured by Randles-Sevcik equation37 (Figure S2 and S3). The electron numbers of the first and the second oxidation peak is respectively 1.69 and 1.02 based on the diffusion coefficient (D = 1.122×10-5 cm2s-1). These calculated electron numbers are based on the peak height. The electron number of the first oxidation peak is 1.69, lower than 2.0, which is because that the first oxidation peak is broader with a hidden peak behind in comparison with the second oxidation peak. This obviously results in peak height of the first oxidation peak less than 2.0. To further support our inference, the peak areas are also calculated to get the electron number. Obtained results are 0.040 95 mC and 0.020 23 mC respectively corresponding to the first and the second oxidation peak. That is, the observed peak area of the first oxidation peak is 2.02 times of the second peak, and this undoubtedly indicates that the first oxidation peak is indeed corresponding to a 2e- process.


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Figure 1. (a) The structure of Rh-N, Rh-H, Rh-O and AM-H. (b) The cyclic voltammograms of Rh-O, Rh-H and Rh-N in CH3CN (1.0 × 10-3 M) with 0.1 M TBAPF6, scan rate: 100 mV/s. (c) The electrochemical UV-Vis spectra before and after applied voltage for Rh-N (1.0 × 10-4 M) with 0.1 mol/L TBAPF6 in CH3CN comparing with the Rh-N (1.0 × 10-5 M) added 10 equivalent CF3COOH and 2 equivalent tris(p-bromophenyl) aminium hexachloroantimonate. (d) The cyclic voltammograms of Rh-N (1.0 × 10-3 M) with different equivalent HCl and cyclic voltammogram of AM-H (1.0 × 10-3 M) in 0.1 M TBAPF6/CH3CN. Based on the result that the first oxidation potential of Rh-O and Rh-H are both at 0.56 V, we can confirm confidently that electron-donor R-subunit within these molecules has negligible effect on their first oxidation on N, N-diethylaniline. It indicates that the first oxidation potential peak of Rh-N (0.30 V) is the oxidation at N, N-dimethylaniline group within displaced R subunit by pphenylenediamine. Normally, the second oxidation potential is higher than the first oxidation potential if the two electron transfer occurs in the same sites. This is because the ionized N-atom within the first oxidation intermediate will prefer attracting electron back instead of losing additional electron. This abnormal oxidation, with the two-electron transfer at only one oxidation peak, implies that some other process may affect the oxidation of Rh-N.


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The in-situ UV−Vis spectroelectrochemical properties of Rh-N in CH3CN are studied to further understand the oxidation process of Rh-N. As shown in Figure 1c, when the potential holds at top of the first oxidation peak, two new absorption bands appear at 517 and 556 nm (red curve). The peak position is very similar with the spectrum of acid-form Rh-N containing two typical absorption peaks of fully conjugated xanthene subunit. The same absorption position and similar shape indicate clearly that the 2e- oxidized-form Rh-N, Rh-N-BO, has the same chromophore subunit with the acid-form Rh-N. This further indicates that the C-N bond is indeed broken at the first oxidation peak of Rh-N. Combined electrochemical UV-Vis spectra of Rh-H at the first oxidation potential appears a very faint broad peak at 561 nm with >150 nm absorption range (Figure S4, S5), which is a typical absorption of aniline radical cation.38, 39 This result implies that the radical cation on N, N-diethylaniline of the Rh-H is far less favorable to be converted to open-form rhodamine. It demonstrates that the strong and sharp absorption of oxidized Rh-N is not due to the oxidation of N, N-diethylaniline on the xanthene subunit. To understand the effect of C-N cleavage on reversible redox of Rh-N, the cyclic voltammetry with different equivalent HCl is performed. As shown in Figure 1d, with the concentration of HCl increasing, a new oxidation peak at 0.48 V (which is assigned to the oxidation of the acidform Rh-N) appears, and its height is increased gradually along with synchronous transition from good reversibility to irreversibility within its original first redox peak. The new peak is close to the oxidation peak of a well-known molecule AM-H (Figure 1a and Scheme S1) that contains irreversible one electron oxidation. These results suggest strongly that the oxidation mechanism of acid-form Rh-N should be a simple single electron oxidation. And compared with the first electron oxidation potential of the Rh-N containing C-N cleavage, the first electron oxidation of the acid-form Rh-N is much more difficult. Thus, the reversible 2e- oxidation of Rh-N is


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facilitated by C-N bond cleavage, and the cleavage of C-N bond is dependent on the electron transfer of Rh-N. Based on aforementioned experimental data, we propose that: 1) the cleavage of C-N bond is induced by the first electron oxidation of N, N-dimethylaniline; 2) the positive charge is dispersed simultaneously into the chromophore (xanthene part); 3) the pphenylenediamine group with radical state can be then further oxidized to cation form. This hypothesis could convincingly explain intricate relationship between the color emerging, the oxidation of N, N-dimethylaniline and 2 e- process in the first oxidation peak. And it will be further discussed in the computational part. As shown in the Figure 1d, the second oxidation peak has little change with increasing of HCl concentration, and there is no second oxidation peak in the cyclic voltammogram of AM-H. These demonstrate that the second oxidation peak is assigned to the ionized xanthene part (N, Ndiethylaniline subunit) of Rh-N-BO which is generated by C-N bond cleavage at the first oxidation peak as shown in Scheme S1. This result is also supported by the electrochemical UVVis spectra shown in Figure S6, S7, and S8. The cyclic voltammetry with different concentration of NaOH is also performed as shown in Figure S9. There is no change in cyclic voltammograms after adding NaOH, which demonstrates that the alkalinity has no influence on the electrochemical behaviour of Rh-N. The electrochemical UV-Vis spectra were also recorded as shown in Figure S10. Since there is no change of the absorption at 555 nm after stimulated with 0.3 V voltage, which indicates that the alkalinity has no effect on the color changing for the oxidative process of Rh-N. Therefore, the color changing during the oxidation of Rh-N is obviously not due to the acid induced color change of Rh-N.


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Figure 2. 1H NMR spectrum of Rh-N (a), oxidized by 2.2 equivalent tris (p-bromophenyl) aminium hexachloroantimonate (b), then reduced by excessive N, N-Diisopropylethylamine (c) in CD2Cl2 recorded at 500 MHz at room temperature. To further confirm structural and electronic information, the 1H NMR spectrum of oxidative state of Rh-N need to be obtained by chemical oxidation method. One electron oxidant, tris (pbromophenyl) aminium hexachloroantimonate, was chosen as suitable oxidant due to its oxidbillity (0.7 V vs. Fc) and special electrochemical oxidative mechanism.40 To observe the oxidbillity of the oxidant, the CV of Rh-N, oxidant, and triphenylamine, which has the similar structure with the reduced oxidant, were measured as shown in Figure S11. It is noteworthy that the start voltage of the CV for this oxidant is higher than the redox peak, while for Rh-N and triphenylamine, the start voltage is lower than the redox peak of itself. This demonstrates that the oxidant is electron-deficient and has the oxidbillity. The redox peak of oxidant is between the


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first oxidation peak of Rh-N and the second oxidation peak. Which indicate that the oxidant has a suitable oxidbillity for oxidizing Rh-N. Therefore, the desired Rh-N-BO (the product of 2eoxidation at the first oxidation peak) could generates rather than tri-oxidized Rh-N (the product with the second oxidation peak). The absorption spectrum of chemical oxidation method was also measured (Figure 1c). The absorption spectrum within visible region of chemical-oxidized Rh-N using this oxidant is same with that of electro-oxidized Rh-N, while the strong absorption at 305 nm of reduced oxidant hindered the comparison the absorption in ultraviolet region (Figure S12). Therefore, diisopropylethylamine, without obvious absorption within ultraviolet and visible region (blue curve, Figure S12), was used as reductant to get the absorption of reduced oxidant (black curve, Figure S12). After the absorption of reduced oxidant was deducted, the absorption of chemical-oxidized Rh-N without the absorption of reduced oxidant was obtained as shown in Figure S13. All of the peaks within ultraviolet and visible region are identical with the electrochemical oxidized Rh-N. This confirms that the electro-oxidation and the chemical oxidation have the same product. The chemical oxidation process is shown in Scheme S2. Then, the 1H NMR spectrum of oxidative state of Rh-N is recorded. Compared with 1

H NMR spectrum of Rh-N (Figure 2a and Figure S14a), 1H NMR spectrum of Rh-N-BO (Figure

2b and Figure S14b) exhibits obviously downfield shifts overall, and the peak shift of this oxidation process is shown in Table S2 and Figure S15. 1H COSY was also used to confirm the structure of Rh-N-BO (Figure S16). H10 exhibits the largest shift about 1.12 ppm, which indicates that the p-phenylenediamine part is oxidized to cationic form owning strong electrondrawing properties. And this result is also proved by the downfield shift of 0.99 ppm for H8 and 0.65 ppm for H9. The H1, H2 and H3 of xanthene shift dramatically to δ = 6.84 ppm, 6.96 ppm and 7.14 ppm, respectively, which consistent with the change of Rh-N to an open-form Rh-N.


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H4, H5, H6 and H7 display a less downfield shift influenced by the electron-drawing cationic xanthene and cationic p-phenylenediamine part. These results help to confirm that the product of the first oxidation peak is surely Rh-N-BO, and the cleavage of C-N bond is coupled with electron transfer in the first oxidation peak. The reversibility of C-N bond cleavage/formation is studied by measuring the intensity of characteristic absorption at 556 nm applied with the cyclic voltage in situ (Figure S17). When the molecule Rh-N is oxidized at 0.30 V, the absorption at 556 nm increases simultaneously, which suggests that C-N bond is broken. Then, the absorption decreases with the oxidation product beginning to be reduced at 0.28 V. This decrease indicates that the conjugated state of xanthene begins to disappear, and the C-N bond is reformed. The closed cyclic curve of the absorption intensity reveals that the reduction regenerates the initial molecules. The cleavage/formation of C-N bond, via electron transfer, can be repeatedly mediated by electric field. The reversibility of the C-N bond cleavage/formation is also studied by 1H NMR spectroscopy. The addition of 5.0 equivalent N, N-diisopropylethylamine, as reductant to the Rh-N-BO, reduces surely it back to its neutral state as shown in Figure 2c and Figure S14c. Scheme 2. Calculated oxidation potentials (Ecalc vs. Fc), Gibbs free energy (∆G), and activation energy (Ea) for postulated oxidation pathways of Rh-N.


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Further verification of the reversible C-N bond via in-depth analysis with theoretical computation. Computational methods41 are employed to further analyze the oxidation pathway. From the previous experiment results, it can be confirmed so far that the first oxidation peak of Rh-N is 2e- process, and the first electron transfer is assigned as the oxidation of N, Ndimethylaniline, and Rh-N-S, the product of the first electron transfer, translates into Rh-N-BO, at the same time. However, it is not clear what the relationship between the C-N bond cleavage and the second electron transfer is. Based on the experiment results and conventional knowledge, we suspect that the further oxidation of Rh-N-S at the first peak may follow three possible pathways, as shown in Scheme 2. In Path A, the second electron loses from N, N-diethylaniline of xanthene group to generate Rh-N-A, and then C-N bond is broken to obtain the Rh-N-BO. However, the second oxidation potential (1.28 V, Table S3) is much higher than its initial first oxidation potential (Ecalc=0.43 V). And the activation energy of the ring open reaction (with the C-N bond cleavage) is 11.9 kcal/mol, which is much larger than other two pathways. Thus, Path A is not be the optimum reaction pathway. These results mean that the second electron should not be captured from N, N-diethylaniline of its xanthene subunit, but it is captured from N, N-


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dimethylaniline on its amide sub-structure. The difference between Path B and Path C is the sort order of electron transfer and ring-opening reaction (the C-N bond cleavage). In Path B, the activation energy of the ring-opening reaction is 5.5 kcal/mol, which is much lower than path A. And the reaction is favorable in thermodynamics, based on Gibbs free energy of the ring-opening reaction (-3.1 kcal/mol). More amazingly, the second oxidation potential (Ecalc=0.40 V) is even lower than the first oxidation potential (Ecalc=0.43 V), which indicates that this second oxidation proceeds immediately under the same e-field. This explains aforementioned results well on why only a single peak was observed with two electrons transfer. In contrast, the second oxidation potential of Rh-N (0.67 V) is higher about 240 mV than the first oxidation potential (Ecalc=0.43 V) in Path C, in which electron transfer occurs firstly, then ring opens. Therefore, Path C is not the optimum reaction pathway, even if the activation energy and Gibbs free energy of the ringopening reaction is favorable. That is to say, Path B is the best and actual reaction pathway. More interestingly, the energy barrier of C-N cleavage coupling with electron transfer is reduced by 34.9 kcal/mol, from 40.4 kcal/mol to 5.5 kcal/mol, compared the open reaction of Rh-N and Rh-N-S. Comparing the oxidation potential of Path B (0.40 V) and C (0.67 V), we can deduce easily that more energy will be needed in the process of the second electron transfer if the cleavage of C-N bond is not occurred before. Meanwhile, the cleavage of C-N bond will be more difficult without the first electron transfer, such as the ring-opening reaction of neutral molecule Rh-N. Thus, the second electron transfer of Rh-N is promoted by the cleavage of C-N bond which is induced by the first electron transfer. The 2e- transfer and the C-N cleavage occurs synchronously based on the very low activation energy of the ring-opening reaction which is far below 10 kcal/mol. Thus


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the electron transfer and C-N cleavage are coupled with each other, which is called as bond coupled electron transfer.

Figure 3. Calculated energy profiles for the Path B, and the Geometric structure of the transition state (TS). The energies given are relative to the energy of Rh-N-S. Analysis of the calculated potential energy surface in different pathway provides a qualitative view of the ring-opening reaction (Figure 3, S19, S20 and S21). As shown in Figure S19, the ring-opening reaction of Rh-N has only one transition state (TS), in which the bond length of CN is 3.48 Å. The length change of its C-N bond and rotation of the dihedral angle of Ph1 and Ph2 is obviously main obstacle in process of its ring-opening reaction. And the activation energy of ring opening reaction is 40.4 kcal/mol. When the single electron is lost and Rh-N-S is generated, the length of C-N increases and the dihedral angle of the amine plane (Ph2) and the benzene plane (Ph1) decreases to -4.5°. In addition, the pathway of ring-opening reaction of Rh-N-S is changed through two stepwise transition states. Its first transition state is obtained by increasing the length of C-N bond, as the structure of TSB-1 (Figure 3). Turning the dihedral angle of the amine plane (Ph2) and the benzene plane (Ph1) from -4.5° to 37.0° is the next step to obtain the second transition state TSB-2. Meanwhile the activation energy of ring-opening reaction is decreased to 5.5 kcal/mol. Thus, the first electron transfer is the key to change the pathway of CN cleavage and to decrease the activation energy. Compared with Rh-N-S, the electron density of


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amide in Rh-N-B molecule, as the product of the ring-opening reaction, increases significantly due to that the positive charge is dispersed into the xanthene group, as shown in Figure S18. And the distribution of the electron density of the amide in Rh-N-B is similar to that of Rh-N. Thus, the oxidation potential of Rh-N-B is very close to that of Rh-N. Thus, by analysis of the potential energy surface, we can undoubtedly conclude that the coupling of the electron transfer and C-N bond cleavage is the key to the oxidation process of Rh-N.

Figure 4. (a) The cyclic voltammograms of Rh-N (1.0 × 10-3 M) with TBAPF6 (0.1 M) in acetonitrile at different scan speed at 25°C. (b) The cyclic voltammograms of Rh-N (1.0 × 10-3 M) with TBAPF6 (0.1 M) in acetonitrile at different temperature, the scan speed is 5000 mV/s. Normalized by the height of the second redox peak. To further verify and support the result of related theoretical computation, the cyclic voltammograms of Rh-N at different scan speed and temperature were measured. With the improvement of the scan speed as shown in Figure 4a, a new oxidation peak at 0.58 V emerges, and meanwhile the first oxidation peak decreases accordingly. The rate constant of the first electron oxidation was measured by using cyclic voltammetry at different scan rate according to the known method reported by Laviron as shown in Figure S22.42 With the temperature increasing, the rate constant is increased from 0.25 (at 10 °C) to 0.46 (at 40 °C). Because the rate of electron transfer is well known relatively quick, the electron losing from both Rh-N and Rh-N-B to generate Rh-N-S and Rh-N-BO just needs very short time. Therefore, in the oxidative process of Rh-N, the transform from Rh-N-S to Rh-N-B should be the slowest process to


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overcome the activation energy of the ring-opening reaction (5.5 kcal/mol). With improvement of the scan speed, there should be no enough time for Rh-N-S to be transformed to Rh-N-B, thus there will be more remaining Rh-N-S on the surface of the electrode, and once the voltage scan up to 0.58 V, a new observed oxidation peak appear immediately. Since this new oxidation peak should not come from the oxidation of Rh-N-B, because the open-ring reaction of Rh-N-S is independent of the voltage. Therefore, the new oxidation peak is obviously due to the direct oxidation of remaining Rh-N-S through path C, in which the remaining Rh-N-S is further oxidized to Rh-N-C at high scan speed at 0.58 V. The difference between value of the oxidation voltage of Rh-N-S through path C and the oxidation voltage of Rh-N is measured as 0.22 V, which is very similar with the calculated value for 0.24 V. The same phenomenon can be found also in the high-scan-speed CV measurement at different temperature (Figure 4b). With the temperature increasing, the first oxidation peak rises, and meanwhile the new oxidation peak decreases accordingly. This is because that, when the temperature is relatively low, the open-ring reaction of Rh-N-S is slower. There will be more remaining Rh-N-S to be oxidized through path C at the new oxidation peak. However, when the temperature is improved, the open-ring reaction of Rh-N-S becomes fast, and remaining Rh-N-S will be much less than before. This will surely lead to the new oxidation peak decrease. The CV measurement results at different scan speed and temperature consist with the theoretical computation. Another important phenomenon is that, even if the temperature is decreased to 10 °C and the scan speed is fast as 5000 mV/s, the peak area of original first oxidation peak is still much larger than the new oxidation peak as shown in Figure 4b. This indicates that, once the Rh-N-S is transferred to Rh-N-B, the Rh-N-B will be oxidized immediately. Many analytical methods were


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tried to further detect the possible Rh-N-B, unfortunately however all of that are failed so far. This phenomenon strongly supports to the result of theoretical computation that the oxidative voltage of Rh-N-B, as the product of C-N bond cleavage, is smaller than the oxidative voltage of Rh-N. This clearly demonstrates that the second electron transfer is accelerated significantly by the C-N bond cleavage. There are two possibilities for this oxidation process. The first is that, once the Rh-N-B is generated, it will be oxidized immediately. The other possibility is that both steps of the open-ring reaction of Rh-N-S and the further oxidation of Rh-N-B are closely coupled together, which means that the open-ring reaction and oxidation are taking place in a single concerted step, and this results in no observable Rh-N-B in such oxidative process of RhN. The application on electrochromism of bond coupled electron transfer. Electrochromic materials have received great attention in recent years.43-44 They are interesting materials that their absorption and/or reflection could be modulated by the voltage or electric field.45 Related electrochromic device can be used at rearview mirrors, smart windows, electronic papers, etc.46 The most popular electrochromic materials, researched so far, are conducted polymers47-49, small molecules (viologen derevetives)50-52, and metal oxides (WO3)5356

. Existing electrochromic materials however do not satisfy many anticipated practical

applications, and they are usually still suffering from low reversibility, expensive processing, slow respond speed, and low color efficiency, etc. Thus exploration of new electrochromic materials are urgently needed.57-59 Organic molecule switches is a class of color changeable dyes with good respond properties stimulated by the external stimuli. Using these organic dyes as electrochromic material may solve some highly difficult existing problems in electrochromic field.60 Their high molar absorption coefficient would contribute to the color efficiency; their


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attractive various colors of the dyes would enrich color choice of the electrochromic materials; and the good reversibility of related molecular switchs would possibly improve the durability of related electrochromic devices.

Figure 5. (a) The UV-Vis spectra of ON and OFF states for solid device. (ON: +1.5 V 10 s) (b) The color efficiency of the solid device. (c) The respond time at +1.5 V for solid device. (d) The reversibility (1.1 V 3 s, -0.7 V 10 s, 600 cycles) of the solid electrochromic device. The b, c and d was recorded at 561 nm. The good reversibility of bond coupled electron transfer has been demonstrated, and the color changing is accompanied by the oxidation/reduction of Rh-N. Therefore, the Rh-N molecule would have good electrochromic properties. The solid device was fabricated using sandwich structure with two layers ITO glass. And P-benzoquinone was used as ion storage layer. The mechanism of solid electrochromic device was shown in Figure S23. The UV-Vis spectra of solid device and picture was shown in Figure 5a, there is no absorption at visible region at OFF state. After stimulated by positive voltage at 1.1 V, the device turn to ON state with obvious absorption at 561 nm. The absorption ranged from 400 nm to 450 nm of ON-state device is come from the radical anion of p-benzoquinone, which exhibits absorption at 422 nm and 448 nm as


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shown in Figure S24. Because the absorption of benzoquinone radical anion is much lower than the absorption of oxidized Rh-N, the color-changing of benzoquinone has negligible influence on the color display of this electrochromic device. The color efficiency was measured and calculated as shown in Figure 5b, the color efficiency could reach as high as 720 cm2/C. And meanwhile the color efficiency of traditional small molecule and metal oxide electrochromic device is commonly lower than 100 cm2/C.50,51,54,56 This illustrates that our current electrochromic device fabricated by using Rh-N has extremely high color efficiency. There are two reasons causing the high color efficiency. The first is because the molecular switching is directly controlled by the electron transfer through BCET. Every two electron transfer induced one molecular switch to be turn ON, which has high molecular switching efficiency. The second is due to that rhodamine derivatives usually have high molar absorption coefficient. The respond time is also measured (Figure 5c). When stimulated by a 1.1 V positive voltage for 30 ms, there is still absorption changing response of the device, which demonstrated that the electrochromic devices have very sensitive respond properties. Finally, the reversibility of the electrochromic device was recorded with no degradation for 600 cycles (Figure 5d). The good reversibility shows a good application prospect for this electrochromic material using BCET mechanism. Scheme 3. Bonding alteration process of Rh-N via bond coupled electron transfer.


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Discussion and reflection. Reaction mechanism, via a mechanistic and thermodynamic framework, on bond coupled electron transfer has been outlined in Scheme 3. The vertical coordinate represents the C-N bond cleavage/formation, and horizontal axis stands for the electron transfer. Anticipated BCET reaction on forming Rh-N-BO might proceed mainly via one of three conjectured pathways. Two of the pathways are step-wise reactions, with either carrying cleavage/formation of the C-N bond or conducting electron transfer (ET) as the first step, denoted as pathway I and II respectively. In pathway I, a presumed intermediate Rh-N-C is generated initially with 2e- oxidation (Ecalc=+0.67 V versus Fc+/Fc), and then its activated C-N bond can be broken by overcoming at least 6.3 kcal/mol activation energy. In pathway II, the expected intermediate Rh-N-O might be generated initially, which needs to overcome a tough activation barrier (Ea= 40.4 kcal/mol), and then is further oxidized to final Rh-N-BO at 0.40 V versus Fc+/Fc. The third and most convincing pathway is conjectured via an amazing process of BCET reaction, represented by the diagonal arrow in Scheme 3. This distinctive pathway will proceed much easily with both a low oxidation potential (Ecalc=+0.43 V versus Fc+/Fc) and a low overall energy barrier (Ea=5.5 kcal/mol). And this is presumably much favorable route for energy-efficient catalysis. In this BCET system, an organic cation takes away the positive charge from the oxidative part right after its C-N bond cleavage. This amazing process have similar characteristic with PCET and MCET transferring proton and metal ion along with electron transfer. These delightful similarities motivate us to highly recommend a unified concept, cation coupled electron transfer (CCET)61-63, for simplifying those existing concepts on similar couplings (such as, PCET, MCET and BCET). It is well known that, in versatile biological process, very low activation energy is required to effectively achieve multistep complex reaction, while involving ion (such as, proton,


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metal ion, electron, and small organic ion etc.) transfer, bond cleavage/formation, structural alteration and configuration inversion etc. It is also well known that ion introducing or eliminating will greatly alter electron density on active side of the molecule or molecular aggregate under enzyme catalysis. The CCET recommended herein might concisely explain well on how some biological processes proceed and why enzymes can amazingly promote these processes via efficiently reducing activation energy. Conclusions We systematically investigated a fully reversible bond coupled electron transfer in Rh-N molecule. This specific molecule is designed by ingenious interconnecting color switchable rhodamine with a redox active phenylenediamine. Based on experimental and computational methods, we have demonstrated that the cleavage of C-N bond accelerate the second electron transfer simultaneously by eliminating its initial positive charge, right after the first electron is removed. And similarly right after its first electron is recaptured under a lower voltage, recombining its C-N bond can also facilitate the second electron reduction spontaneously. BCET provided a pathway with low activation energy for redox reaction to avoid both high redox potential and high energy barrier. Thus, the coupling relationship of the reversible C-N bond and electron transfer is clearly confirmed from the redox process of Rh-N. This meaningful understanding will not only accelerate and stimulate the development of bond coupled electron transfer, but also help to understand, further and more deeply, those elusive enzymatic oxidation / reduction and other biological processes. Besides, a unified concept of CCET is recommended herein to expediently simplify existing concepts on describing various couplings, such as PCET, MCET, BCET and etc., based on the facts of all involving both cation transformation and electron transfer. This simplified concept and unconventional model will undoubtedly inspire


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new molecular design and synthesis for glamorous biomimetic chemical catalysis. A new kind of electrochromic device was fabricated based on BCET mechanism, with extremely high color efficiency (720 cm2/C), the good reversibility (no degradation among 600 cycles) and quick respond time (30 ms). ASSOCIATED CONTENT Supporting Information. Synthesis and characterization of Rh-N, Rh-O and Rh-H, crystal data of Rh-N, Randles-Sevcik equation calculation, cyclic voltammetry, Spectroelectrochemical spectra of Rh-N and Rh-H, calculated oxidation potentials for Rh-N, molecular orbital for Rh-N, Rh-N-B, and Rh-N-S, calculated potential energy surface for Rh-N, and calculated structures by DFT. Corresponding Author * [email protected] / * [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This study was supported by the National Science Foundation of China (Grant No. 51373068, 51303063) and the program of Chang Jiang Scholars and Innovative Research Team in the University (IRT101713018) for financial support. REFERENCES


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Cation-Coupled Electron Transfer on

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