Molecular Design for Electron-Driven Double-Proton Transfer: A New

Article Cite This: J. Phys. Chem. A 2018, 122, 9191−9198

pubs.acs.org/JPCA

Molecular Design for Electron-Driven Double-Proton Transfer: A New Scenario for Excited-State Proton-Coupled Electron Transfer Guanghua Ren,†,§ Qingchi Meng,†,§ Jinfeng Zhao,† and Tianshu Chu*,†,‡ †

Downloaded via UNIV OF WINNIPEG on December 23, 2018 at 19:58:16 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China ‡ Institute for Computational Sciences and Engineering, Laboratory of New Fiber Material and Modern Textile, the Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, PR China § University of Chinese Academy of Sciences, Beijing 100049, PR China S Supporting Information *

ABSTRACT: Proton-coupled electron transfer (PCET) reactions play important roles in solar energy conversion processes. Designing efficient artificial photosystems with PCET mechanisms is a promising solution for the growing demands of energy resources. Compared to ground states, inducing the PCET reactions directly from electronically excited states, named excited-state PCET (ES-PCET) reactions, is a more direct and efficient avenue to the formation of solar fuels. Here, based on benzimidazole phenolic derivatives, we have designed and studied some molecular structures that can undergo the electron-driven double-proton transfer (EDDPT) reactions within the ES-PCET framework. According to our DFT/TDDFT calculation results, the two protons transfer in a stepwise manner in the EDDPT process, and compared to the common way of electron-driven single-proton transfer (EDSPT) reactions, the proton transfer in the EDDPT process not only has a smaller energy barrier but also experiences a longer transferring distance, which has beneficial effects on producing solar fuels. The study of ES-PCET reactions under the mechanism of EDDPT may cast light on the regulation of proton transfer at defined distances and time scales, which is important in energy conversion processes.

1. INTRODUCTION With the depletion of fossil fuel, sustainable energy is being vigorously pursued for human’s growing energy demands.1 Finding efficient ways of converting and utilizing energy is also important for sustainable energy.2 As one of the techniques that promote sustainable energy, photosynthesis is a process that converts solar energy into chemical energy.3 Beginning with absorption of light, this process synthesizes organic fuel molecules via proton-coupled electron transfer (PCET) reactions.4−6 In natural photosynthesis, with the excitation of light, a chlorophylls P680 donates one electron to a pheophytin in its excited state. Then, a tyrosyl residue TyrZ participates a PCET reaction in its ground state via transferring its electron and proton to the oxidized P680 and abasic histidine, respectively.7 Artificial photosynthesis mimics the process of natural photosynthesis to turn carbon dioxide and water into clean fuels.8 One key factor of the mimicry is the building of PCET processes. For the PCET reactions, a great deal of researches have involved the so-called flash-quench techniques, which employ short laser pulses to excite highly reactive species and complete PCET reactions in electronic ground states.9,10 However, in this scheme, energy dissipation inevitably occurs when artificial molecules relax from excited © 2018 American Chemical Society

states to ground states. Therefore, excited-state PCET (ESPCET) reactions, where electrons and protons transfer directly in electronic excited states, were then proposed to increase energy conversion efficiencies.11 Photoexcitation affects both the properties of electron donating/withdrawing functional groups and the strength of hydrogen bonds (H-bonds) that consist of proton donors and acceptors.12−15 The well integration of electron donating/ withdrawing functional groups with H-bonds is of great importance to ES-PCET.16,17 Photoacid−base18 or photobase−acid19 complexes, which can be organic20 or transitional metal coordination compounds,21 are fine templates for these ES-PCET reactions. Within acid−base frameworks, ES-PCET can occur in both the unimolecular22 and bimolecular23,24 reaction modes. Because of the simpler configuration and higher efficiency, the way of unimolecular reactions has been widely applied into artificial photosynthetic systems.25 In the unimolecular ES-PCET reactions, the electron transfer characteristic is embodied by the intramolecular charge Received: September 22, 2018 Revised: November 5, 2018 Published: November 8, 2018 9191

DOI: 10.1021/acs.jpca.8b09264 J. Phys. Chem. A 2018, 122, 9191−9198

Article

The Journal of Physical Chemistry A

relatively limited compared with that by photochemical approaches. Therefore, a series of molecular structures have been designed for EDSPT and EDDPT under the mechanism of ES-PCET reactions. Figure 1a shows the schematics of

transfer (ICT) process and the proton transfer component is reflected through intramolecular proton transfer (IPT) process.26,27 After stimulating by light, the electron densities in photoacids or photobases are redistributed via ICT process, which is accompanied by the IPT reaction with the assistance of H-bonds. However, one limitation of this unimolecular ESPCET reactions is that the length of H-bonds constrains proton transfer distance, which dissatisfies the long-distance proton transfer demand in the production process of solar fuels.25 Here, we have adopted electron-driven double protontransfer (EDDPT) to lengthen the distance of proton transfer in ES-PCET reactions. 28−32 In our molecular design, derivatives of phenol with electron donating groups were chosen as electron and proton donors. Nitrogen bases, such as pyridine and amine, have acted as proton and electron acceptors. Benzimidazole plays the role of connectors for its ability to serve as the H-bond acceptor and donor simultaneously. On the basis of this design, our theoretical study showed that the two protons transfer in a stepwise manner in the EDDPT process. This not only expands the scope of proton transfer but also reduces the corresponding energy barriers for the subsequent proton transfer. Our designed molecular structures might provide insight into the development of artificial photosynthesis systems.

Figure 1. Design of electron-driven proton transfer molecules. (a) Schematics of design principles. I: electron donor. II: intermediate connector between donor and acceptor. III: electron acceptor. (b, c) Geometric structures of designed electron-driven single-proton transfer (b) and electron-driven double-proton transfer (c) molecules. R1 and R2 can be tert-buty1 or dimethylamino substituent groups. R3 is the pyridine or methanimine substituent group.

2. CALCULATION DETAILS The theoretical calculations were performed by dispersioncorrected33 DFT and TDDFT methods with the Gaussian 16 program suite.34 All the molecular geometries in the ground S0 state and the excited S1 state were fully optimized without any constraints at the level of M06-2X-D335/TZVP36,37 and TDM06-2X-D3/TZVP, respectively, because it is well-known that B3LYP is not a good choice for the charge transfer excited states as it suffers from the self-interaction error.38 Vibrational frequency calculations have been used to analyze the optimized structures to conform that these structures corresponded to the local minima with no imaginary frequency. On the basis of these optimized geometries, the Gibbs free energy, natural transition orbitals39 (NTOs), charge population analysis, and absorption and emission spectra were calculated by the M062X-D3 method with ma-def2-TZVP,40 a larger basis set than TZVP. The difference between using the ma-def2-TZVP and TZVP basis sets to describe the excited transitions is small in our case, which has been contrasted and shown in Table S4. The charge population analysis was dealt with Hirshfeld method via the Multiwfn suite.41 The ground-state potential energy surfaces (PESs) for the single- or double-proton transfer were obtained at the M06-2X-D3/TZVP level. And the excited-state PESs for the corresponding protons transfer were obtained at the same calculation level with the timedependent version. In all the calculations, the water solvent was selected and the solvent effects were included with the continuum solvation model based on solute electron density (SMD) method.42

design principles for EDSPT and EDDPT molecules. The general structure consists of three parts, named the donor of electrons and protons (I), the acceptor of electrons and protons (III), and the intermediate connector (II) between the donor and the acceptor. After absorbing a photon, the charge transfers from part I through part II to part III. Meanwhile, for adapting the changes in charge, protons can also be transferred if there have appropriate proton donors and acceptors in the three parts. In view of the design principles shown above, we have proposed and designed the molecules for EDSPT (Figure 1b) and EDDPT (Figure 1c) reactions. In our designed EDSPT molecules, a phenol derivative (pink color) works as a donor for electrons and protons. The benzimidazole (cyan color) serves as a connector. Besides, benzimidazole is also a proton donor due to the intramolecular H-bond between the N−H group and R3 substituent, shown in Figure 1b. For the purpose to serve as both the electron and the proton acceptor, R3 substituent can be selected from electron withdrawing groups that contain nitrogen atoms. Here, in this work, the pyridine or methanimine group has been chosen for serving as the electron and proton acceptor. In EDDPT molecular structures, the donors, acceptors and connectors are the same as EDSPT molecules. The only difference between the EDSPT and EDDPT molecules is the position of phenol hydroxyl, which means that, besides the intramolecular H-bond between the connector and acceptor (similarly as that in EDSPT molecule), there also exists another intramolecular H-bond between the donor and connector in EDDPT molecule (see Figure 1c). In this molecule, coupled with electrons transfer, protons can transfer from the phenol O−H to the N atom and from the N−H group to the R3 substituent in a stepwise or concerted manner. 3.2. Geometric Configurations and Charge Distributions. The ground-state optimized structures of all the molecules schemed in Figure 1 have been displayed in Figure 2. The molecular structures in sequence a and b are designed for the EDSPT and EDDPT reactions, respectively. Molecules

3. RESULTS AND DISCUSSION 3.1. Design Principles. According to the electrochemical experiments conducted by Moore et al., concerted oneelectron-two-proton transfer (E2PT) processes can be occurred in benzimidazole phenol derivatives.43 However, the application of E2PT by electrochemical methods was 9192


Article


Table 1. Primary Bond Lengths, Charge Distributions, and Relative Gibbs Free Energies in S0 and S1 States for a-1 and b-1 a-1 O1−H2b N3−H4 H4···N5 H2···N6 Q(D)c Q(C) Q(A) Gd

Figure 2. Molecular structures schemed in Figure 1. Structures in sequence a and b are for EDSPT and EDDPT molecules, respectively. Donor: 2,6-di-tert-butylphenol in x-1 and x-2, and 4-bis(dimethylamino)phenol in x-3 and x-4. Acceptor: pyridine in x-1 and x-3, and methanimine in x-2 and x-4. Benzimidazole as connector for all the structures. Numbered atoms are presented in a-1 and b-1. Same labels would be used for other structures but were omitted here. White: hydrogen. Red: oxygen. Blue: nitrogen. Gray: carbon. x: a or b.

a

b-1

S0

S1

0.962 1.011 2.224 − 0.068 −0.124 0.056 13.06

0.964 1.015 2.089 − 0.207 −0.161 −0.046 86.46

Δ

S0

S1

Δ

+0.002 +0.004 −0.135 − +0.139 −0.037 −0.102 −

0.992 1.012 2.164 1.705 −0.073 0.004 0.069 0

1.010 1.015 2.091 1.638 0.048 −0.037 −0.011 80.37

+0.018 +0.003 −0.073 −0.067 +0.121 −0.041 −0.080 −

a

The difference between S1 and S0. bBond values in Å. cCharge in au. Value of Gibbs free energy relative to the S0 state of b-1 in kcal/mol.

d

changed, which affects the process of ES-PCET. To investigate the ES-PCET mechanism of the designed molecules, we have compared the primary bond lengths and charge distributions in their S0 and S1 states. Table 1 summarized the results for a-1 and b-1. Bonds are represented by symbols of O1−H2, N3−H4, H4...N5, etc. The subscripts correspond to atom labels as shown in Figure 2. Symbols Q(D), Q(C), and Q(A) stand for the sum of atomic charges in donors, connectors and acceptors. Their quantitative values were obtained from Hirshfeld population analysis (HPA). For the a-1 molecule, after photoexcitation, the length of intramolecular H-bond, named H4...N5, are shortened from 2.224 to2.089 Å, indicating its strength has been enhanced.27 In addition, discovered by comparisons, the photoexcitation has little impacts on the lengths of bond O1−H2 and N3−H4 but has great influences on charge distributions. Compared with S0 states, Q(D) increased with 0.139 au, Q(C) decreased with 0.037 au, and Q(A) decreased with 0.102 au in S1 states. That is to say, electrons have transferred from donors through connectors to acceptors. The enhanced intramolecular H-bond and transferred electrons provide preconditions for the occurrence of the EDSPT process.17 For b-1, besides the similar results of the enhanced H4...N5 intramolecular H-bond and redistributed atomic charges with a-1, the length of the other intramolecular H-bond H2...N6 is also shortened after photoexcitation. In details, the length of the H-bond of H4···N5 decreased by 0.073 Å, and that of H2···N6 shortened from 1.705 to 1.638 Å, a shortening of 0.067 Å. Q(D) has an increment of 0.121 au, accompanied by a decrement of 0.041 au in Q(C) and 0.080 au in Q(A). These consequences are in support of the EDDPT mechanism in b-1 molecules. Moreover, in order to compare the stability of the isomers, the Gibbs free energy of a-1 and b1 was calculated in their S0 and S1 states. The results are presented in Table 1 and it shows that the S0 state geometry of b-1 is the most stable one, which has a 13.06 kcal/mol lower than the S0 state geometry of a-1. Similarly, for S1 state, the Gibbs free energy of b-1 is also lower than that of a-1, with a value of 6.09 kcal/mol. Thus, the more stable structure of b-1 suggests that the second H-bond H2···N6 further lows the energy of the molecular system. Bond lengths, HPA, and Gibbs free energies in S0 and S1 states for other molecular structures that listed in Figure 2 are summarized in Tables S1−S3 in the Supporting Information, which have undergone similar variations with a-1 and b-1 molecules. 3.3. Natural Transition Orbitals. To further confirm the electron transfer46 processes, orbital transitions from S0 to S1

in these two sequences are isomers, which have same functional groups but different configurations. Refer to the geometry of TyrZ, derivatives of phenol can function as donors of the electrons and protons.43 Here, due to their good abilities of donating electrons, tert-butyl and dimethylamino groups are selected as substituents, that is, tert-butyl substituent was presented in x-1 and x-2, and dimethylamino substituent was in x-3 and x-4, where x stands for series a or b molecules. In order to cater for the characteristics of accepting both electrons and protons, electron withdrawing groups containing nitrogen atoms, such as pyridine and methanimine, are chosen as acceptors of electrons and protons.44,45 Specially, pyridine is the acceptor for x-1 and x-3, and methanimine is the acceptor for x-2 and x-4. For connectors, on one hand, they should bridge donors and acceptors; On the other hand, constructing the EDS(D)PT architecture requires the connectors function as proton donors and proton acceptors simultaneously. Inspired by the structure of His190, here benzimidazole was selected as the connector.41 The number of intramolecular Hbonds determines which pathway the ES-PCET should take the EDSPT or the EDDPT. When phenolic hydroxyl is located far from benzimidazole as shown in sequence a, only one intramolecular H-bond can be formed between the N−H group in benzimidazole and the N atom of pyridine or methanimine, which makes the ES-PCET to follow the EDSPT pathway. However, when phenolic hydroxyl is located near benzimidazole as shown in sequence b, another intramolecular H-bond can be formed as phenolic hydroxyl plays the role of a proton donor and benzimidazole works as a proton acceptor. Thus, the EDDPT mechanism governs the ES-PCET processes of molecules in sequence b. All the molecular geometries have been fully optimized at their S0 and S1 states. The results of optimized coordinates were listed in the Supporting Information. As can be seen in a1 and b-1, some key atoms have been numbered for conveniently describing the major configurational changes after photoexcitation. Contrast results of S0 and S1 states for a1 and b-1 structures have been collected in Table 1. And others are summarized in Table S1 in the Supporting Information. After optical excitation, some molecular parameters such as bond lengths, bond angles and charge distributions are 9193


Article


stand for the regions where the electrons are excited from. Similarly, the orbitals in S1 states display the regions where electrons are transferred to. As can be seen from NTOs, electrons have been transferred from tert-butyl-substituted phenol (donor) to pyridine or methanimine (acceptor), which is consistent with results of HPA. NTOs for other designed molecules were presented in Figure S1 to S3. 3.4. Potential Energy Surfaces. For verifying the EDSPT and EDDPT mechanisms, potential energy surfaces (PESs) were calculated for the proton transfer in S0 and S1 states. Figure 4 shows the computational results for a-1 and b-1 molecules. All the PESs in Figure 4 were adiabatic, which means that the results in these situations were performed by the way of relaxation. Bond labels presented here are the same as those in Figure 2. The value of energy barrier for proton transfer was indicated by symbol Δ. As can be seen in parts a and b of Figure 4, energy barriers for single-proton transfer along N3−H4 bond in S0 and S1 states are 11.97 and 5.51 kcal/ mol, respectively. Besides, the process of the single-proton transfer in S1 state is exothermic but that in S0 state is endothermic, which indicates the proton transfer is likely to occur in S1 states but hardly achieved in S0 states. Considering that photoinduced charge transfer takes place in S1 state but is absent in S0 state, the transfer of protons was actually affected

states were studied by TDDFT theory. As multiple orbitals involved in the vertical transition from S0 to S1 states (see in Table S5), NTOs were adopted to convert many pairs of orbitals into one single pair, which describes the electronic configuration changes efficiently. NTOs for a-1 and b-1 molecules were shown in Figure 3. The orbitals in S0 states

Figure 3. Natural transition orbitals from S0 to S1 states for a-1 and b1 molecules.

Figure 4. Potential energy surfaces on EDSPT and EDDPT in S0 and S1 states for a-1 and b-1. Single-proton transfer along N3−H4 in S0 state (a) and S1 state (b) for a-1. Double-proton transfer along O1−H2 and N3−H4 in S0 state (c) and S1 state (d) for b-1. All situations in S0 and S1 states are adiabatic. Same labels as in Figure 2 for bond O1−H2 and N3−H4. 9194


Article


Figure 5. Calculated absorption (a) and fluorescence (b) spectra for a-1, b-1 and their corresponding proton-transfer products: a-1_pt, product after the proton transfer for a-1; b-1_pt1 and b-1_pt2, products after the first and second proton-transfer for b-1.

Figure 6. Energy levels and electronic configurations for S1 and T1 to T4 states. Parts a and b are standing for the situations in a-1 and b-1 molecules. H: highest occupied molecular orbital (HOMO). L: lowest occupied molecular orbital (LUMO). Red dotted line arrow: possible intersystem crossing tunnels. Black solid line arrow: vertical transition.

3.5. Absorption and Fluorescence Spectra. To better understand the properties of these designed molecules in excited states, the absorption and fluorescence spectra have been calculated, as shown in Figure 5 and Figures S7−S9. The details of these calculation results are also listed in Table S5. For the convenience of expressions, the proton transfer product of a-1 was marked as a-1_pt and the corresponding two products for b-1 were named as b-1_pt1 and b-1_pt2. The transition energies from S0 to higher singlets and triplets were also displayed in Figure 6. For a-1, the transition energy from S0 to S1 state was 4.18 eV (297 nm). And its fluorescence band centered at 378 nm. Meanwhile, a-1_pt fluoresced at 446 nm, which means the EDSPT process caused a red shift of 68 nm. This large fluorescence red shift indicates the charge of the excited state has been redistributed for adapting the configurational changes induced by the EDSPT. For b-1 molecules, the transition energy from S0 to S1 state was 4.10 eV (302 nm). And its fluorescence band centered at 371 nm. These characteristics are similar to a-1 molecules. This similarity indicates that the position of phenol hydroxyl in the donor part has a small impact on electronic state transitions before the corresponding protons transfer. However, compared with b-1, the fluorescence band of b-1_pt1 has a red shift of 74 nm,

by the charge transfer. For b-1, the PESs of double-proton transfer along bond O1−H2 and N3−H4 in S0 and S1 states are shown in Figure 4c and 4d, respectively. Here, the shapes of PESs reveal that the two protons transfer in a stepwise manner; that is, proton transfer along N3−H4 bond occurs only after the first proton completes its transfer along O1−H2 bond, reflecting a proton-relay process. Comparing with energy barriers of proton transfer along bond N3−H4 in a-1, their corresponding values in b-1 are much smaller in both the S0 and S1 states. These decrease means that proton transfer along bond O1−H2 will reduce the difficulty of proton transfer along bond N3−H4. Similar with a-1, for the S0 state of b-1, the energy level of products after proton transfer is also higher than that of reactants, which goes against the thermodynamic stability. In S1 state, energy barriers along O1−H2 and N3−H4 routes are 1.58 and 7.00 kcal/mol, respectively. Besides, products are more stable than reactants. These results support the inference that the EDDPT mechanism underlying the ESPCET processes in molecule b-1. PESs for other designed molecules are presented in Figures S4−S6. For the representative molecules, a-1 and b-1, molecules in series a undergo an EDSPT process, and in series b experience an EDDPT process. 9195


Article


mechanism. Spectral results reflect that the electron and proton transfer in the EDDPT process can affect each other. Triplet states can be involved in the excitations but with a limited rate of ISC. A significant feature in the EDDPT reaction is that the two protons transferred in a stepwise manner in the S1 state. Besides this, in addition to the traditional EDSPT, the EDDPT has a longer proton-transfer distance and a lower energy barrier, which may pave ways for producing solar fuels more efficiently. Our study provides a new strategy to control the proton transfer in excited states, which makes sense in the relevant fields of materials and energy.

centering at 445 nm. Furthermore, the b-1_pt2 emitted at 504 nm, which has another red shift of 59 nm relative to b-1_pt1. Through the PESs of proton transfer in excited states, shown in Figure 4, parts b and d, it has been verified that the intramolecular electron transfer indeed promotes the corresponding proton transfer. However, the large spectral shift in Figure 5 reveals a fact that the proton transfer can also exert an impact on the charge distribution in return to enhance the character of charge transfer. The other molecules tell the same stories as the a-1 and b-1, except the influences of the position of phenol hydroxyl in x-3 and x-4 molecules, seen from Figure S7−S9. That is, the absorption and fluorescence bands of b-3 and b-4 have obvious red shifts when compared to those of a-3 and a-4. This may be caused by the steric effect. When the position of phenol hydroxyl transforms from the site locating between the two dimethylamino functional groups to the site forming an intramolecular H-bond with N6 atom, the steric hindrance will be decreased significantly, which results in a more delocalized charge distribution in ground states. Therefore, in contrast to a-3 and a-4, the electrons in b-3 and b-4 are more easily to be excited by light. Through exploring the characteristics of these spectra, it can be concluded that different configurations can result in different electronic state transitions. Moreover, the excited intramolecular charge transfer can drive the proton transfer, and conversely, the proton transfer can also make the charge redistribute. Certainly, for the better use of solar energy, these molecules should be further modified, which will transfer the absorption bands to the visible region. 3.6. Role of Triplet States in Electronic Transition. For the purpose of examining whether triplet states can contribute to the electronic transition, the energy levels and electronic configurations of triplet states which located near the S1 state have been calculated at M06-2X-D3/ma-def2-TZVP level. Corresponding results have been shown for a-1 and b-1 molecules in Figure 6, parts a and b, respectively. According to the results, there are 4 triplet states locating near the S1 state, named from T1 to T4, thus, intersystem crossing (ISC) radiationless transition could happen among these triplet states and S1 state. However, based on El-Sayed’s rule,47 ISC tunnels can be existed if there involves a change in molecular orbital type. For example, a (π, π*) singlet state could transition to a (n, π*) triplet state, but not to a (π, π*) triplet state and vice versa. In other words, if standing in the perspective of space orbitals, the two states that proceeding ISC transformations should have same LOMOs but different HOMOs. Therefore, according to this rule, for both a-1 and b-1, only T1 and T2 states can be involved in the excited transitions. Besides, owing to these similar electronic configurations among S1, T1 and T2 states, the rate of ISC would be low. This speculation has been verified by the large configuration-interaction coefficients in the fluorescence calculations, shown in Table S5. The other molecules demonstrate similar results with a-1 and b-1, as seen from Figure S10−S12.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.8b09264. Optimized geometry parameters, HPA, Gibbs free energy, NTOs, PESs, absorption and emission spectra and electronic configurations of triplets for a-2, b-2, a-3, b-3, and a-4, b-4 molecules, and optimized Cartesian coordinates in S0 and S1 states for all the molecules (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*(T.C.) E-mail: [email protected]; [email protected]. ORCID

Tianshu Chu: 0000-0002-3519-8737 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 11604333), the Science Challenging Program (JCKY2016212A501), and the Open Fund of the State Key Laboratory of Molecular Reaction Dynamics in DICP, CAS (SKLMRD-K201817).

■

REFERENCES

(1) Tester, J. W.; Drake, E. M.; Driscoll, M. J.; Golay, M. W.; Peters, W. A. Sustainable Energy: Choosing among Options; MIT Press: 2012. (2) Lior, N. Energy Resources and Use: The Present Situation and Possible Paths to the Future. Energy 2008, 33, 842−857. (3) Lambers, H.; Chapin, F. S.; Pons, T. L. Photosynthesis. Plant Physiological Ecology 2008, 11−99. (4) Meyer, T. J.; Huynh, M. H. V.; Thorp, H. H. The Possible Role of Proton-Coupled Electron Transfer (PCET) in Water Oxidation by Photosystem II. Angew. Chem., Int. Ed. 2007, 46, 5284−5304. (5) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Thermochemistry of Proton-Coupled Electron Transfer Reagents and Its Implications. Chem. Rev. 2010, 110, 6961−7001. (6) Weinberg, D. R.; Gagliardi, C. J.; Hull, J. F.; Murphy, C. F.; Kent, C. A.; Westlake, B. C.; Paul, A.; Ess, D. H.; McCafferty, D. G.; Meyer, T. J. Proton-Coupled Electron Transfer. Chem. Rev. 2012, 112, 4016−4093. (7) Carra, C.; Iordanova, N.; Hammes-Schiffer, S. Proton-Coupled Electron Transfer in a Model for Tyrosine Oxidation in Photosystem II. J. Am. Chem. Soc. 2003, 125, 10429−10436. (8) Kim, D.; Sakimoto, K. K.; Hong, D.; Yang, P. Artificial Photosynthesis for Sustainable Fuel and Chemical Production. Angew. Chem., Int. Ed. 2015, 54, 3259−3266. (9) Gagliardi, C. J.; Westlake, B. C.; Kent, C. A.; Paul, J. J.; Papanikolas, J. M.; Meyer, T. J. Integrating Proton Coupled Electron

4. CONCLUSIONS We have designed some new benzimidazole phenolic derivatives with proper electron/proton donating/withdrawing functional groups to investigate ES-PCET using DFT and TDDFT methods. Through comparison of the optimized geometries, charge distributions and PESs of proton transfer between S0 and S1 states, it can be concluded that the EDDPT reactions actually can be achieved under the ES-PCET 9196


Article

The Journal of Physical Chemistry A Transfer (PCET) and Excited States. Coord. Chem. Rev. 2010, 254, 2459−2471. (10) Wenger, O. S. Proton-Coupled Electron Transfer with Photoexcited Metal Complexes. Acc. Chem. Res. 2013, 46, 1517− 1526. (11) Wenger, O. S. Proton-Coupled Electron Transfer with Photoexcited Ruthenium (II), Rhenium (I), and Iridium (III) Complexes. Coord. Chem. Rev. 2015, 282-283, 150−158. (12) Zhao, G. J.; Liu, J. Y.; Zhou, L. C.; Han, K. L. Site-Selective Photoinduced Electron Transfer from Alcoholic Solvents to the Chromophore Facilitated by Hydrogen Bonding: A New Fluorescence Quenching Mechanism. J. Phys. Chem. B 2007, 111, 8940− 8945. (13) Zhao, G. J.; Han, K. L. Site-Specific Solvation of the Photoexcited Protochlorophyllide a in Methanol: Formation of the Hydrogen-Bonded Intermediate State Induced by Hydrogen-Bond Strengthening. Biophys. J. 2008, 94, 38−46. (14) Zhao, G. J.; Han, K. L. Hydrogen Bonding in the Electronic Excited State. Acc. Chem. Res. 2012, 45, 404−413. (15) Yu, F.; Li, P.; Li, G.; Zhao, G.; Chu, T.; Han, K. A near-Ir Reversible Fluorescent Probe Modulated by Selenium for Monitoring Peroxynitrite and Imaging in Living Cells. J. Am. Chem. Soc. 2011, 133, 11030−11033. (16) Lennox, J. C.; Kurtz, D. A.; Huang, T.; Dempsey, J. L. ExcitedState Proton-Coupled Electron Transfer: Different Avenues for Promoting Proton/Electron Movement with Solar Photons. ACS Energy Lett. 2017, 2, 1246−1256. (17) Chai, S.; Zhao, G. J.; Song, P.; Yang, S. Q.; Liu, J. Y.; Han, K. L. Reconsideration of the Excited-State Double Proton Transfer (ESDPT) in 2-Aminopyridine/Acid Systems: Role of the Intermolecular Hydrogen Bonding in Excited States. Phys. Chem. Chem. Phys. 2009, 11, 4385−4390. (18) Ren, G. H.; Meng, Q. C.; Zhao, J. F.; Chu, T. S. Zwitterions of the Excited 4-([2, 2′-Bipyridine]-4-Yl) Phenol Photoacid Molecules: Formation and Fluorescence. J. Mol. Liq. 2018, 264, 48−53. (19) Xie, Y.; Ilic, S.; Skaro, S.; Maslak, V.; Glusac, K. D. ExcitedState Hydroxide Ion Release from a Series of Acridinol Photobases. J. Phys. Chem. A 2017, 121, 448−457. (20) Verma, S.; Aute, S.; Das, A.; Ghosh, H. N. Proton-Coupled Electron Transfer in a Hydrogen-Bonded Charge-Transfer Complex. J. Phys. Chem. B 2016, 120, 10780−10785. (21) Chen, J. S.; Zhao, G. J.; Cook, T. R.; Han, K. L.; Stang, P. J. Photophysical Properties of Self-Assembled Multinuclear Platinum Metallacycles with Different Conformational Geometries. J. Am. Chem. Soc. 2013, 135, 6694−6702. (22) Demchenko, A. P.; Tang, K. C.; Chou, P. T. Excited-State Proton Coupled Charge Transfer Modulated by Molecular Structure and Media Polarization. Chem. Soc. Rev. 2013, 42, 1379−1408. (23) Young, E. R.; Rosenthal, J.; Hodgkiss, J. M.; Nocera, D. G. Comparative Pcet Study of a Donor-Acceptor Pair Linked by Ionized and Nonionized Asymmetric Hydrogen-Bonded Interfaces. J. Am. Chem. Soc. 2009, 131, 7678−7684. (24) Chen, J. S.; Zhou, P. W.; Yang, S. Q.; Fu, A. P.; Chu, T. S. Sensing Mechanism for a Fluoride Chemosensor: Invalidity of Excited-State Proton Transfer Mechanism. Phys. Chem. Chem. Phys. 2013, 15, 16183−16189. (25) Mora, S. J.; Odella, E.; Moore, G. F.; Gust, D.; Moore, T. A.; Moore, A. L. Proton-Coupled Electron Transfer in Artificial Photosynthetic Systems. Acc. Chem. Res. 2018, 51, 445−453. (26) Luber, S.; Adamczyk, K.; Nibbering, E. T.; Batista, V. S. Photoinduced Proton Coupled Electron Transfer in 2-(2′-Hydroxyphenyl)-Benzothiazole. J. Phys. Chem. A 2013, 117, 5269−5279. (27) Chu, T. S.; Liu, B. T. Establishing New Mechanisms with Triplet and Singlet Excited-State Hydrogen Bonding Roles in Photoinduced Liquid Dynamics. Int. Rev. Phys. Chem. 2016, 35, 187−208. (28) Bonin, J.; Costentin, C.; Robert, M.; Savéant, J. M.; Tard, C. Hydrogen-Bond Relays in Concerted Proton-Electron Transfers. Acc. Chem. Res. 2012, 45, 372−381.

(29) Megiatto, J. D., Jr.; Méndez-Hernández, D. D.; Tejeda-Ferrari, M. E.; Teillout, A. L.; Llansola-Portolés, M. J.; Kodis, G.; Poluektov, O. G.; Rajh, T.; Mujica, V.; Groy, T. L.; et al. A Bioinspired Redox Relay That Mimics Radical Interactions of the Tyr-His Pairs of Photosystem II. Nat. Chem. 2014, 6, 423−428. (30) Liu, X. Y.; Zhao, J. F.; Zheng, Y. J. Insight into the Excited-State Double Proton Transfer Mechanisms of Doxorubicin in Acetonitrile Solvent. RSC Adv. 2017, 7, 51318−51323. (31) Yu, X. F.; Yamazaki, S.; Taketsugu, T. Solvent Effects on the Excited-State Double Proton Transfer Mechanism in the 7-Azaindole Dimer: A TDDFT Study with the Polarizable Continuum Model. Phys. Chem. Chem. Phys. 2017, 19, 23289−23301. (32) Sikorska, E.; Khmelinskii, I.; Hoffmann, M.; Machado, I. F.; Ferreira, L. F.; Dobek, K.; Karolczak, J.; Krawczyk, A.; Insińska-Rak, M.; Sikorski, M. Ground-and Excited-State Double Proton Transfer in Lumichrome/Acetic Acid System: Theoretical and Experimental Approach. J. Phys. Chem. A 2005, 109, 11707−11714. (33) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104−154122. (34) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; et al., Gaussian 16 Rev. B.01; Gaussian: Wallingford, CT, 2016. (35) Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Functionals. Theor. Chem. Acc. 2008, 120, 215−241. (36) Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (37) Fan, G. H.; Han, K. L.; He, G. Z. Time-Dependent Density Functional-Based Tight-Bind Method Efficiently Implemented with Openmp Parallel and Gpu Acceleration. Chin. J. Chem. Phys. 2013, 26, 635−645. (38) Dreuw, A.; Head-Gordon, M. Failure of Time-Dependent Density Functional Theory for Long-Range Charge-Transfer Excited States: The Zincbacteriochlorin−Bacteriochlorin and Bacteriochlorophyll−Spheroidene Complexes. J. Am. Chem. Soc. 2004, 126, 4007− 4016. (39) Martin, R. L. Natural Transition Orbitals. J. Chem. Phys. 2003, 118, 4775−4777. (40) Papajak, E.; Zheng, J.; Xu, X.; Leverentz, H. R.; Truhlar, D. G. Perspectives on Basis Sets Beautiful: Seasonal Plantings of Diffuse Basis Functions. J. Chem. Theory Comput. 2011, 7, 3027−3034. (41) Lu, T.; Chen, F. Multiwfn: A Multifunctional Wavefunction Analyzer. J. Comput. Chem. 2012, 33, 580−592. (42) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. Universal Solvation Model Based on Solute Electron Density and on a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions. J. Phys. Chem. B 2009, 113, 6378−6396. (43) Huynh, M. T.; Mora, S. J.; Villalba, M.; Tejeda-Ferrari, M. E.; Liddell, P. A.; Cherry, B. R.; Teillout, A. L.; Machan, C. W.; Kubiak, C. P.; Gust, D.; et al. Concerted One-Electron Two-Proton Transfer Processes in Models Inspired by the Tyr-His Couple of Photosystem II. ACS Cent. Sci. 2017, 3, 372−380. (44) Díaz-Gavilán, M.; Rodríguez-Serrano, F.; Gómez-Vidal, J. A.; Marchal, J. A.; Aránega, A.; Gallo, M. Á .; Espinosa, A.; Campos, J. M. Synthesis of Tetrahydrobenzoxazepine Acetals with Electron-Withdrawing Groups on the Nitrogen Atom. Novel Scaffolds Endowed with Anticancer Activity against Breast Cancer Cells. Tetrahedron 2004, 60, 11547−11557. (45) Yu, F. B.; Li, P.; Wang, B. S.; Han, K. L. Reversible nearInfrared Fluorescent Probe Introducing Tellurium to Mimetic 9197


Article

The Journal of Physical Chemistry A Glutathione Peroxidase for Monitoring the Redox Cycles between Peroxynitrite and Glutathione in Vivo. J. Am. Chem. Soc. 2013, 135, 7674−7680. (46) Deng, W. Q.; Sun, L.; Huang, J. D.; Chai, S.; Wen, S. H.; Han, K. L. Quantitative Prediction of Charge Mobilities of Π-Stacked Systems by First-Principles Simulation. Nat. Protoc. 2015, 10, 632− 642. (47) El-Sayed, M. Spin-Orbit Coupling and the Radiationless Processes in Nitrogen Heterocyclics. J. Chem. Phys. 1963, 38, 2834− 2838.

9198


Molecular Design for Electron-Driven Double-Proton Transfer: A New

Recommend Documents