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Article Cite This: J. Phys. Chem. C 2018, 122, 6818−6825

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Theoretical Exploration of the Layered Sandwich Cobaltacarborane as a Multi-State NLO Molecular Switch Triggered by Redox Nana Ma,*,† Jinjin Gong,† Jie Zhang,† Shangning Han,‡ Mengxiao Song,† and Guisheng Zhang*,† †

Henan Key Laboratory of Organic Functional Molecule and Drug Innovation, Henan Key Laboratory of Boron Chemistry and Advanced Energy Materials, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, China ‡ Xinlian College of Henan Normal University, Xinxiang, Henan 453007, China S Supporting Information *

ABSTRACT: Layered sandwich metallacarboranes have various advantages including structural diversity, tailoring ability, rich electrochemical properties, and high thermal stability and thus could be potential multifunctional nonlinear optical (NLO) materials. In this study, second-order NLO properties and the multi-state switching effect of the layered cobaltacarborane, induced by electrochemical behavior, were explored using density functional theory (DFT). The calculated first hyperpolarizabilities (β) illustrated that the NLO responses are enhanced with the increasing of layers, which was mainly attributed to the C → B intraligand charge transfer (C2B3) (ILCT) and metal-centered d → d transition (MCT) as indicated by the time-dependent (TD)DFT results. Remarkable β contrasts were observed after undergoing two sequential one-electron redox processes. Therefore, the pronounced multi-state NLO switch can be achieved. In mixed-valence systems, the metal−metal interaction plays a crucial role in the NLO responses through intervalence charge transfer (IVCT) and the π → π* transition of the whole metal−bridge−metal system.

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INTRODUCTION Nonlinear optical (NLO) materials are of great importance for emerging optoelectronic and photonic applications such as optical data manipulations, storage, and transmission.1−6 As the molecular NLO field matures, designing multi-state NLO switching systems including several switchable units has been proposed, aiming at the improvement of the multiple storage capacity of optical memories.7−10 Excellent molecular multistate NLO switching should satisfy two details: (i) Complete reversibility and a high speed of switching are highly desirable for practical applications. That is, the stimuli effecting molecular-level NLO changes are significant. (ii) As potential molecular materials, molecular compounds should be air-stable, soluble in common solvents, and generally robust, combined with adjustable properties as needed by systematic modification.11 A class of organometallic complexes that fit these requirements to a remarkable degree are metallacarborane sandwich complexes, which form an extraordinarily diverse field starting with the original investigations of Hawthorne et al. in 1965.12 The sandwich metallacarborane compounds incorporate small planar RR′C2B3H54− (C2B3) or pyramidal carborane ligands RR′C2B4H62− (C2B4) or their substituted derivatives (Figure 1). These ligands adopt a six-electron binding mode to bond with metals; they are electronic analogues of C5H5− and C6H6. Because of the higher negative charge and the lower electronegativity of boron in comparison with carbon, they © 2018 American Chemical Society

Figure 1. Small carborane ligands RR′C2B4H62− (C2B4) and RR′C2B3H54− (C2B3).

coordinate to metals much more tightly and with greater versatility, compared with the corresponding hydrocarbon rings.13−19 Significantly, the sandwich metallacarboranes are remarkably versatile, accommodating a wide range of metals and organic substituents; they are soluble in most of organic solvents and typically resistant to air and moisture. They can be reversibly oxidized and reduced; in many cases, they are paramagnetic, exhibiting substantial electron delocalization of the unpaired electrons between metal centers.19 These advantages inspired researchers to design this class of compounds as molecular materials with switchable NLO Received: January 15, 2018 Revised: March 12, 2018 Published: March 12, 2018 6818

DOI: 10.1021/acs.jpcc.8b00459 J. Phys. Chem. C 2018, 122, 6818−6825

Article

The Journal of Physical Chemistry C

higher negative charge, RR′C2B3H54− can stabilize a high metal oxidation state, and thus Co(IV) was located in the middle, but Co(III) was located at two ends of whole molecule. The response of a molecule to a homogeneous static electric field can be represented by the following Taylor expansion34

responses, because these superiorities are exactly what the NLO molecular switches should have.20−24 Previously, a theoretical study showed that the peanutshaped sandwich metallacarboranes have a significant secondorder NLO switching effect induced by redox,25 but this type of sandwich metallacarboranes can hardly be developed into NLO molecular switches because of their simple structures and simple electrochemical properties. Conversely, due to the structural diversity, tailoring ability, rich electrochemical properties, and high thermal stability, the multi-decker sandwich metallacarboranes are thus used to explore switchable NLO properties by density functional theory (DFT) in this work. Generally, many synthesized multi-decker sandwich metallacarboranes, from triple-decker to octa-decker in size, incorporate a variety of first- (Fe, Co, Ni), second- (Ru, Rh), and third-row (Ir, Pt) transition metals.19 With sophisticated synthetic methods13−19 and electrochemical studies,26−30 the family of cobaltacarboranes is selected to investigate the structure−property relationship of second-order NLO switching. It can be anticipated that these molecular compounds can exhibit switchable NLO properties and can be a potential multistate NLO switch. The computational models of triple-, tetra-, penta-, and hexa-decker cobalt(III/IV) sandwich metallacarboranes under the present investigation are shown in Figure 2.

E (F ) = E 0 −

∑ μi0 Fi − i

1 − 3

1 2

∑ αijFF i j ij

1 ∑ βijkFF i jFk − 4 ijk

∑ γijklFF i jFkFl + ··· (1)

ijkl

where E0 is the energy of the molecule in the absence of an electric field, μi is its permanent dipole moment, αij is the dipole polarizability, and βijk and γijkl are the first and second hyperpolarizabilities, respectively. To compute the polarizability and hyperpolarizability, one option is to take the derivates either analytically or numerically. In this work, the static first hyperpolarizabilities (βtot) were calculated by analytical third energy derivatives, which is more efficient and less expensive than numerical derivatives.35 The βtot value was calculated using the following equation βtot = (βx2 + βy2 + βz2)1/2

(2)

where βi = βiii +

1 3

∑ (βijj + βjij + βjji) i≠j

i, j = x, y, z (3)

To utilize the merits of range-separated and global hybrids, the CAM (Coulomb-attenuating model) was proposed as an applicable approach for the first hyperpolarizability.36,37 mPWPW91* is also an excellent hybrid DFT functional for the first hyperpolarizability of transition metal complexes.38,39 Therefore, the CAM-B3LYP and mPWPW91* functionals were employed to calculate the βtot values in this work. The 631+G(d) basis set is for the nonmetal elements and the transition metal Co adopted SDD basis set. The charge transfer (CT) character should be considered to gain more insights into the NLO responses. Time-dependent (TD)DFT is one of the most popular methods for the calculation of excitation energies in quantum chemistry due to its efficiency and accuracy. Therefore, calculations of the excitations were performed by the TDDFT method at the ωB97XD40/6-31+G(d)/SDD level. The solvent effect was considered for TDDFT calculations by the polarizable continuum model (PCM)41,42 in dichloromethane (CH2Cl2) solvent.

Figure 2. Computational models of decker sandwich cobaltacarboranes.

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COMPUTATIONAL DETAILS All calculations were performed by the Gaussian 09 program package.31 On the basis of the X-ray data,13−17 geometries were optimized without symmetry restriction by the functional B3LYP. Although other possible rotamer configurations were also optimized, they evolve to the final reported ones. The SDD32 basis set containing the Stuttgart Dresden effective core potentials was used for transition metal Co, whereas the nonmetal elements used the 6-31G(d) all-electron basis set. Frequency calculations at the same level were performed to confirm that the structures are the minima on the potential energy surfaces. The spin multiplicities of compounds were determined through the experimental ESR spectrum data.11,33 The spin density and inferred valence state of Co ions are shown in Figure S1, and the formal electronic configurations are d 6 d 6 (Co III Co III ), d 6 d 5 d 6 (Co III Co IV Co III ), d 6 d 5 d 5 d 6 (CoIIICoIVCoIVCoIII), and d6d5d5d5d6 (CoIIICoIVCoIVCoIVCoIII) for 1-Co2, 2-Co3, 3-Co4, and 4-Co5, respectively. Due to the

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RESULTS AND DISCUSSION NLO Response Evolution with the Increasing of Layers. Frontier Molecular Orbital (FMO) Analysis. First, the interaction between the Co ions and C2B3 ligand was analyzed based on the FMO compositions shown in Figure 3. To have a visualized effect about the interaction, taking the triple-decker sandwich model 1-Co2 as a description, we provided the bonding character of this kind of compounds. As shown in Figure 4, the highest occupied orbitals (HOMO) are the antibonding combination of the dyz and σ orbitals of C2B3, as described by Grimes et al.,27 and HOMO−1 is the bonding combination of the dxy and π orbitals of C2B3, while the lowest unoccupied orbitals (LUMO) are the antibonding combination of the dyz and σ orbitals of C5H5 and σ orbitals of C2B3. When 6819


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disappears, and the bonding combination of the dyz and π orbitals of C2B3 appear (Figure 3). Moreover, the HOMO− LUMO energy gap is narrowing, and the low-lying HOMO− LUMO energy gap might enhance molecular second-order NLO responses. The calculated βtot values listed in Table 1 illustrate that the second-order NLO responses show an enhancement from the triple- to hexa-decker sandwich complexes. That is, the βtot values present in the order of 4-Co5 > 3-Co4 > 2-Co3 > 1Co2. Therefore, larger NLO responses of these sandwich complexes can be achieved by constructing carborane-bridged multi-decker structures. To clarify this interesting result, the components of βtot were also collected (Table 1). Based on these data, the βy component is the main contribution for βtot. The y direction is parallel with C2B3 and the C5H5 decker (Figure 2), so it can be inferred that the stacking of layers benefits a large second-order NLO response. Further charge transfer transitions were analyzed by TDDFT calculations. The TD-ωB97XD results of 2-Co3 show a satisfactory agreement between calculated and experiment absorptions (Table 2). The electron density difference maps

Figure 3. FMOs (isosurface value = 0.04) and energy level of the studied layered sandwich compounds.

Table 2. Calculated Absorption Spectra of Layered Sandwich Cobaltacarboranec

Figure 4. FMOs combined with metal and bridge ligand C2B3. a

Experimental value. bThe purple represents where the electrons are coming from, and the blue represents where the electrons are going. c From tripe- to hexa- by the ωB97XD/6-31+G(d)/SDD level.

two C2B3 ligands coordinate with Co ions, the HOMO of 2Co3 contains the bonding combination of the dxy and π orbitals of C2B3, and the HOMO energy level decreases gradually with the increasing of this bonding from 1-Co2 to 4-Co5. Meanwhile, the LUMO energy level reduces, because the antibonding combination of the dyz and π orbitals of C5H5

(EDDMs) of the ground state and the excited state corresponding to the primary absorption are also given. For 1-Co2, a high-energy absorption band (277 nm) appears to be

Table 1. β Values (a.u.) Calculated by CAM-B3LYP and mPWPW91* Functionals with 6-31+G(d)/SDD Basis Set βx

compound ref. compound 1-Co2 2-Co3 3-Co4 4-Co5

a

-0 0 −150 −2

βy -45 115 −443 −666

βz

βtot, CAM-B3LYP b

-0 0 68 0

43 (60−120) 45 115 423 666

c

βtot, mPWPW91* b

44 68 121 266 320

βtotd -333 623 1597 1480

a

[Fc(η6-BC5H5)CoCp]+. bCalculated value. cExperimental value43 (the calculated value agrees with the experimental value, illustrating that the functionals are suitable). dCalculated values of −Cl substituted compounds by the CAM-B3LYP functional. 6820


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The Journal of Physical Chemistry C dominated by excitations of a metal-centered d → d transition (MCT), a C → B intraligand charge transfer (ILCT) within C2B3, and the ligand to metal charge transfer (LMCT) transition from the π molecular orbital of C2B3 to the d orbital of the Co ion. Therefore, the βy value arises mainly from the ILCT and MCT, while the βx component is almost zero due to the two opposite LMCT transitions. With respect to 1-Co2, intervalence charge transfer (IVCT)44 along the x-axis was observed in the mixed-valence system 2-Co3 containing CoIII and CoIV, but the CoIII−CoIV−CoIII electronic configuration leads to two IVCTs in opposite directions, and thus, the βx component is almost zero. The βy value also results from the ILCT and MCT. For 3-Co4, there is no obvious IVCT, but there are MCT and MLCT from the metal to B3 component of the C2B3 ligand, which contribute to the βx and βy components. For 4-Co5, IVCTs in opposite directions, induced by the CoIII−CoIV−CoIV−CoIV−CoIII electronic configuration, result in the little βx component, but the MCT of CoIII and π → π* of the C2B3 ligand contribute to the larger βy value. In summary, the second-order NLO responses characteristic of this type of cobaltacarborane mainly arise from ILCT, π → π* of the C2B3 ligand, and MCT. The involved charge transfer transitions for the NLO responses are collected in Figure 5. Moreover, the

and some of these compounds have been reported.47 As expected, the βtot value of systems by including the −Cl substituent increases by 4 times at least; in particular, when the number of layers is six, the βtot value could reach to 46 092 au (Table 1). Multi-State Redox Switching of NLO Responses. For a pronounced switching effect, the molecule must be stable in two (or more) states that exhibit a large contrast of NLO responses. Almost all of the studies to date on the switching of NLO properties in molecular materials have involved the modulation of β responses. In addition, complete reversibility and a high speed of switching are also highly desirable for practical applications on NLO molecular materials.48 Because of the rich electrochemical properties,26 these layered sandwich cobaltacarboranes can be explored to design the NLO switches according to some investigations.49−52 Therefore, the oxidation−reduction properties of these molecules and the corresponding second-order NLO properties are of interest in attempting to achieve the multi-state NLO switching effect by investigating the nature of the metal−metal interaction and the degree to which delocalization may occur. Geometric and Electronic Structures of Redox States. With the injection or loss of electrons, the geometry changes evidently on the basis of the related bond distances (Table S1− 4). These changes of bond distances ultimately result in a conspicuous feature of the structure, bending of the cobalt chain. We used the bond angles as a measurement of distortion induced by redox for 2-Co3, 3-Co4, and 4-Co5. For 3-Co4, the bending can be viewed as the deviation of the Co(1)−Co(2)− Co(3)−Co(4) array from linearity [Co(l)−Co(2)−Co(3) A1, Co(2)−Co(3)−Co(4) A2] and the deviation of the Co(1)− Co(2)−Co(3)−Co(4)−Co(5) array from linearity [Co(l)− Co(2)−Co(3) A1, Co(2)−Co(3)−Co(4) A2, Co(3)−Co(4)− Co(5) A3] for 4-Co5. These optimized angles are presented in Figure 6. The results indicate that the bending of structure is larger than that of the neutral form in oxidation processes but is smaller in reduction processes, and the metal atoms are almost in a straight line. These parameters, especially for the equal bond length and bending in a system, suggest that the redox states almost have delocalized electronic structures, except for the oxidation states of 1-Co2 and the oxidation and reduction states of 3-Co4. Moreover, the changes of the electronic properties are related to the HOMO and affect the LUMO, thus resulting in the change of redox properties. According to the related FMOs and spin density (Figure S2) combined with the related optimized parameters of redox states, probable electronic configurations of cobalt ions for each redox state are given in Figure 7. Remarkable Multi-State NLO Switching Effect. The switching effect of NLO stimulated by a redox reaction is what we considered. The calculated βtot values by mPWPW91* and

Figure 5. Involved charge transfer transitions for NLO responses.

first hyperpolarizability is related to the energy and oscillator strength (f) of permitted UV−vis excitations according to the two-level model.45,46 It is worth to note that all the calculated βtot values vary according to λmax, namely, the highest λmax corresponds to the highest βtot value. The C → B charge transfer inspired us to view the C2B3 ligand as part of the donor−acceptor (D−A) model, and this observation has interesting implications for the design of D−A metal complex architectures. For example, the electronwithdrawing substituent −Cl replaces the hydrogen of the middle boron in the each C2B3 ring, leading to new structures,

Figure 6. Structure parameters (bond angles) induced by redox processes (blue circles represent cobalt ion and black thick lines represent ligands). 6821


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the triple-decker compound 1-Co2, the first one-electron oxidation can significantly enhance the βtot value by ∼12 times and decrease it by ∼6 times due to further one-electron oxidation. One-electron reduction also induces the enhancement of the βtot value by ∼8 times, and the βtot value of red2 drops by ∼3 times because of a second one-electron reduction. Therefore, the βtot contrasts among these redox states indicate that the multi-state NLO switching was achieved as a five-state “off−on−off−on−off” style. For the tetra-decker compound 2Co3, compared to the neutral form, the βtot value of ox1 changes slightly, and then, that of ox2 increases by ∼6 times; similarly, the first one-electron reduction only enhances the βtot value by ∼1 time, but the βtot value of red2 increases further and is ∼43 times as large as that of red1. The changes of the NLO responses caused by the redox reaction could be designed as a three-state “on−off−on” switching. For the penta-decker compound 3-Co4, the oxidation processes increase the NLO response by ∼15 times for ox1, and the βtot value of ox2 decreases by ∼3 times due to further one-electron oxidation; first, one-electron reduction enhances the NLO response by ∼2 times, and then, the second one-electron reduction reduces the NLO response by ∼1 times. Depending on the magnitude of βtot value and the βtot contrast, a five-state switching “off−on− off−on−off” can be formed. For the hexa-decker compound 4Co5, the βtot value of the ox1 state changes slightly compared with that of neutral form, and the ox2 state increases by ∼2 times with respect to the ox1 state. First, one-electron reduction also enhances the NLO response by ∼4 times, while the second one-electron reduction drops the βtot value by ∼39 times compared with the red1 state. The results indicate that 4-Co5 has a four-state switching effect “on−off−on−off”. For a concise visual effect, a histogram about the βtot contrast is shown as Figure 8. The absorption wavelength, oscillator strength, kinds of transitions assigned from EDDMs, and directions of transitions are shown in Table S2 to explain the main reasons resulting in the βtot contrast. Some absorptions have a high oscillator strength due to containing various transitions, while the directions of these transitions is different. In general, larger β responses are associated with systems with smaller excitation energies. The β components result from the corresponding directional transitions. Specifically, for 1-Co2, in the first oxidation state ox1, where the metal−metal interaction is weaker (Class II according to the Robin and Day classification53), the valences are localized in the mixed-valence state and described as CoIIICoIV. The IVCT along with the xaxis causes a large βx value. Then, further electron loss is localized on the CoIII ion and the state ox2, with the electronic configuration of the CoIVCoIV forms. In reduction processes, the extra electron of red1 is delocalized between two metals, which is also the case for the red2 state. There are strong M−M interactions in the two reduction states belonging to Robin− Day Class III.53 As concluded above, the βy component is contributed from C → B of the C2B3 ILCT, MCT, and partial MLCT transitions. Aoso, based on the EDDMs, π → π* transition from one orbital delocalized over the whole metal− bridge−metal system to a higher-energy orbital, which is likewise delocalized, is also the major contribution to βy; although, the absorption is weak. Also, all the oxidation and reduction states of 2-Co3 and 4-Co5 under investigation have a good indication of the delocalized nature due to strong M−M interaction, and the specific electronic configurations are shown in Figure 7. Unlike the above-mentioned systems, for 3-Co4,

Figure 7. Electronic configurations of sandwich cobaltacarboranes 1Co2, 2-Co3, 3-Co4, and 4-Co5 (from top to bottom) and their redox states.

CAM-B3LYP are listed in Table 3, and the data indicate that the notable NLO switch effect can be achieved by redox Table 3. β Values (a.u.) Calculated at CAM-B3LYP and mPWPW91* Functionals with 6-31+G(d)/SDD Basis Set

a

compound

βx

βy

βz

βtota

βtotb

ox2 ox1 1-Co2 red1 red2 ox2 ox1 2-Co3 red1 red2 ox2 ox1 3-Co4 red1 red2 ox2 ox1 4-Co5 red1 red2

0 519 0 0 0 0 0 0 0 −6 −2485 6759 −150 −537 66 −8 0 −2 2 3

81 −70 45 323 −123 −722 −81 115 209 −8902 −10 −1325 −443 −461 −479 1580 −691 666 −2719 73

0 0 0 −202 −51 0 0 0 0 3 489 −181 68 −383 −196 0 0 0 0 0

81 523 45 368 133 722 81 115 209 8902 2533 6890 473 805 521 1580 691 666 2719 73

93 1651 68 521 39 723 103 121 271 1574 3529 2553 266 1799 663 3983 1047 320 2438 700

CAM-B3LYP results. bmPWPW91* results.

behavior; although, there is a difference between mPWPW91* and CAM-B3LYP results, but their change trends of βtot values are consistent basically except for those of ox1 and ox2 for 3Co4. Here, we use mPWPW91* results as an example to discuss the βtot contrast with the injection or loss of electron, because the βtot value order of ox1 and ox2 for 3-Co4 seems to be reasonable based on the TDDFT results (see below). For 6822


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Figure 8. βtot contrasted for the multi-state NLO switches of studied systems.

the odd electron of the mixed-valence state ox2 is localized on one metal as well as red1. Also, among all the mixed-valence states, the ox1 state of 1-Co2 and the two oxidation states and two reduction states of 3-Co4 have an asymmetric electronic distribution of Co ions. The orbitals involved in the major transitions are delocalized between the C and B atoms of C2B3 rather than among B−B−B or C−C, as they are in the other systems. Therefore, their direction and distance of CT related to NLO responses would be different from other states. As shown in Table S2, the x- and z-direction CTs also benefit the larger βtot values, combined with the IVCT along the x-axis and ILCT and LMCT along the y-axis. Finally, for 3-Co4, as mentioned above, the βtot values of ox1 and ox2 calculated by the mPWPW91* functional, present in the order of ox1 < ox2 but ox1 > ox2 for the CAM-B3LYP results. As the two-level model expressed, the higher the λmax, the higher the βtot value. Therefore, ox1 has larger βtot value due to the lower-energy absorption band (371 nm) rather than ox2, whose absorption band is about 310 nm.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (N.M.) *E-mail: [email protected] (G.Z.) ORCID

Nana Ma: 0000-0003-3225-9554 Jie Zhang: 0000-0002-7693-6388 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (grant no. 21603062), the Research Starting Funded Project of Henan Normal University (qd14124), and the Youth Scientific Funds of Henan Normal University (2015QK11). The calculations are supported by the High Performance Computing (HPC) Centre of Henan Normal University.

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CONCLUSION In this work, we employed DFT to investigate the NLO properties of layered cobaltacarboranes and proposed that this type of sandwich complex may serve as multi-state NLO molecular switches by redox reaction. The TDDFT calculations have insight on the NLO responses of these compounds: (i) the β component along the y-axis arises mainly from the ILCT, C → B charge transfer of intraligand (C2B3), and MCT; (ii) in some mixed-valence systems, the βx component is attributed to IVCT due to the metal−metal interaction, and the π → π* transition of the whole metal−bridge−metal, which mainly contributes to the βx value. In addition, the intrinsic stability, tailoring ability, and versatility of these compounds allow further wide-ranging studies on NLO switching that focus on families of systematically related sandwich metallacarboranes complexes, such as linked-sandwich, network-sandwich systems, and so on.

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Details on the spin density and inferred valence state of Co, HOMO of ox1, LUMO of red1, spin density distributions, some structural parameters, the calculated absorption data of redox states (PDF)

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b00459. 6823


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