Rational Design of Reversible Molecular Photoswitches Based on

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Rational Design of Reversible Molecular Photoswitches Based on Diarylethene Molecules Li Han, Xi Zuo, Heming Li, Yuan Li, Changfeng Fang, and Desheng Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10079 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

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The Journal of Physical Chemistry

Rational Design of Reversible Molecular Photoswitches Based on Diarylethene Molecules Li Han,† Xi Zuo,† Heming Li,† Yuan Li,¶ Changfeng Fang,*‡ and Desheng Liu*†§

†School

of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China

‡Advanced

Research Centre for Optics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China

§Department ¶School

of Physics, Jining University, Qufu 273155, China

of Information Science and Engineering, Shandong University, Qingdao 266237, China

Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

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ABSTRACT Reversible conductance photoswitching behaviors of single diarylethene molecule have garnered increasing interest in recent studies. It was revealed that the moleculeelectrode coupling strength plays a critical role in the realization of bidirectional conductance switching. Here, we report first-principles calculations of molecular devices based on photoswitching diarylethene molecules of two stable isomers, i.e., the ring-open form and the ring-closed form, respectively. A method of non-equilibrium Green's function combined with density functional theory is used to calculate the electronic transport properties and the switching mechanism of the devices. The results point to a large diversity in the electrical conductivity and the switching behavior depends essentially on the electronic structure of the molecule itself, regardless of the electrodes used in the devices, which is consistent with previous studies. Importantly, the on-off current ratio of the devices is predicted to be as large as 103, and negative differential resistance effect is observed in devices with graphene electrodes. These findings are helpful for the rational design of molecular photoswitches based on diarylethene and similar organic molecules.

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INTRODUCTION Miniaturization technologies based on advanced technologies hold the evolutionary trend of molecular electronic devices, which makes it possible to construct molecular devices with sophisticated experimental techniques. Being one of the fundamental components of integrated circuit, logic switches have received considerable attention and become one of the primary topics of research in the field of molecular electronics.1-2 Molecular switches can be classified into different categories in terms of distinct triggering conditions of their switching behaviors, such as redox reaction,3 electric field,4 light irradiation,5 and others. Among them, light-activated molecules, referred to as photoswitching molecules, have received increasing attention due to their simple triggering conditions, ultra-high density of data storage, and ultra-short response time. The switching processes of photoswitching molecules, such as azobenzene,5 diarylethene,6 spiropyran,7 fulgides,8 are spontaneously accompanied by the shift in absorption peaks and result in variation in wavelength of the triggering lights as a result of the different electronic structures of the molecules. In this regard, a representative molecule is diarylethenes characteristic of thermal stability and bistability, excellent fatigue resistance, ultra-short response time, and high quantum yield. As such, diarylethene-based molecular switches have been widely investigated in both experimental and theoretical studies.1, 9-11 To date, various photoswitching molecules have been synthesized and stable molecular junctions can be obtained by using different advanced technologies.12-16 However, the control of electrode-molecule interfaces still represents one of the most challenging issues in preparing reliable devices.17 For instance, using a mechanically controllable break-junction technique, Dùlic et al. investigated thiophene-based diarylethene molecule junctions self-assembled on gold electrodes and observed obvious switching behaviors. However, it turns out that the switching process was unidirectional and it can only change from the ring-closed form to the ring-open form.18 This is attributed to the strong coupling between the diarylethene molecule and the 3



electrodes. As a result, the excited state of the ring-open molecule overlaps with the plasmon band of the gold electrode such that the excited state is quenched by the gold electrode.18-20 Subsequently, Kudernac et al. successfully realized the fabrication of reversible molecular junctions with phenylene-based diarylethene.21 The molecule and the metal electrodes were connected by sulfur atoms, which approach has been widely employed in the literature. However, the main drawback of this approach is that the system tends to undergo oxidative oligomerization.17, 22 For the purpose of obtaining stable and controllable contact, carbon-based materials, such as carbon nanotube and graphene, have been alternatively applied in molecular devices because of their robust structure and rich electronic properties.23-30 Graphene is a two-dimensional (2D) material with high electron mobility at room temperature, excellent thermal conductivity, superior mechanical properties, stable chemical properties, and fascinating electronic properties.31-33 As such, it represents one of the most promising candidates in replacement of traditional electrode materials. In 2016, reversible molecular junctions with unprecedented stability were reported.34 It was formed by diarylethene molecules sandwiched between graphene electrodes and can achieve reversible photoswitching in both directions by exposure to UV and visible lights. In this work, inspired by the aforementioned experiments,18, 21, 33-34 we aim to design reversible molecular photoswitches based on the diarylethene molecule synthesized by Fredrich et al.35 The reactivity of the photoswitching circle between the ring-open form (Figure 1a) and the ring-closed form (Figure 1b) can be triggered in both directions by using irradiation with visible light, and unprecedented stability and ultra-high fatigue resistance can be obtained. The switching core of the two isomers are shown in Figure 1c and Figure 1d. Starting with the diarylethene molecules, we first investigate the reaction mechanism for the photochemical and thermal properties of the molecules by means of state-of-the-art density functional theory (DFT) calculations.36 Then, we design a diarylethene-based molecular junctions with different electrodes and simulate the transport properties by using a method of the non-equilibrium Green's function 4


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combined with DFT. According to the calculations, we predict that the on-off ratio of the devices can be as large as 103 within a large bias range.

Figure 1. Schematic diagram of photochemical interconversion between the ring-open form (a) and the ring-closed form (b) of diarylethene molecules. The molecular structures of the switching core in the two isomers are shown in (c) and (d), respectively.

THEORETICAL METHODS AND COMPUTIONAL DETAILS As schematically illustrated in Figure 2, the molecular junctions are constructed with the ring-closed and the ring-open forms of diarylethene molecules. These molecules are embedded between two monolayer graphene nanoribbons (GNR) (Figure 2a and 2b) and two Au (111) electrodes (Figure 2c and 2d), respectively. All devices can be divided into three parts, i.e., the left electrode, the central region, and the right electrode. Specifically, we chose graphene nanoribbons with zig-zag edges saturated with H atoms. A vacuum padding of 15 Å was added along the x- and y-axes to prevent the interlayer interactions. We first optimized the geometries of the two isomers of diarylethene molecules in the gas phase. The solvent effect was considered by using the integral equation formulation of the polarizable continuum model (PCM). The calculations were performed with the Gaussian 16 suite of programs.37 It has been confirmed in our previous study that the range-separated and dispersion corrected ωB97X-D38 hybrid density functional shows good overall performance for both photochemical and thermal 5



transition steps.36 Thus, the ωB97X-D functional was used in combination with the double- SVP (split valence plus polarization) basis set in our calculations.36,39 However, for comparative purposes, these steps of diarylethene molecules were also explored by using a complementary global hybrid functional B3LYP40-42 and global hybrid meta functional M06-2X.43

Figure 2. Geometric structures of the diarylethene-based molecular junctions with different electrodes. (a) and (b) the closed and the open isomer of diarylethene molecules sandwiched between GNR electrodes. (c) and (d) the closed and open models with Au (111) electrodes. All devices are divided into three parts, i.e., left electrode, central region, and right electrode.

Calculations of geometric optimization of the devices and the subsequent electronic transport properties were performed by the combination of DFT and non-equilibrium Green’s function method (NEGF) integrated within the Atomistix toolkit (ATK) software package.44-46 This approach has been used successfully in explaining experimental results and predicting new physics in materials. Specifically, the Perdew– Zunger (PZ) parameterization of local density approximation (LDA) function were chosen to describe the exchange-correlation potential.34 For comparison purpose, calculations with generalized gradient approximation (GGA) functional were also 6


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performed for the systems with Au electrodes in order to assess the appropriateness of LDA functional used in our study, and the results can be found in Figure S1 of the Supporting Information. We expanded the valence electrons in single-zeta plus polarization base set (SZP) for the metal electrodes and double-zeta polarization base set (DZP) for other atoms.47 The Norm-conserving Troullier–Martins pseudo-potentials were used to describe the core electrons.48 The Brillouin zone was sampled with 1×1×100 points and 5×5×100 points within the Monkhorst–Pack K-point sampling scheme in the GNR and Au electrode system, respectively. A mesh cutoff of 150 Ry was adopted to balance the computing accuracy and efficiency. In terms of the electronic transport properties, the tunneling current through the molecular junctions was calculated from the Landauer–Büttiker formula:49 2𝑒

𝐼(𝑉) = ℎ ∫𝑇(𝐸,𝑉𝑏)[𝑓𝐿(𝐸 ― 𝜇𝐿) ― 𝑓𝑅(𝐸 ― 𝜇𝑅)]𝑑𝐸, where 𝑇(𝐸,𝑉𝑏) is the transmission function, 𝑓𝐿(𝑅)(𝐸 ― 𝜇𝐿(𝑅)) the Fermi function, and 𝜇𝐿(𝑅) the chemical potential of the left (right) electrode.

RESULTS AND DISCUSSION The open and close isomers of diarylethene More recently, new diarylethene molecules conjugated with biacetyl terminal groups have been synthesized successfully. It was demonstrated that two isomers of diarylethene could switch reversibly through photochemical reactions with visible light. To investigate the relative stability of the open and closed forms of isomers, we show in Table 1 the variations of key geometric parameter during the open–close transition, and the relative free energies and excitation energies of the two isomers obtained with different functionals in the gas phase and in acetonitrile solvent. For the excitation energies, the experimental data obtained in acetonitrile solvent are listed as well. By comparing the geometric parameters obtained from calculations of the three functionals in gas phase, it is noted that the results are more affected as a function of functionals in the case of open form than in the case of closed form. Since the long7



range-separated functionals (such as ωB97X-D) aim especially at predicting accurate excitation energies by including the description of weak interactions, it is seen that there are no substantial differences in the geometric parameters for these functionals. This can be further confirmed by the PCM results that the solvent only has a very little effect on both geometries and free energies. As for the excitation energies in the experiment, the ring-closure and ring-opening processes are alternating, under the irradiation of lights with different wavelengths. Specifically, the ring-closure process of the molecule can be powered by absorption with irr=405 nm (max=390 nm), and the reversible ringopening process was realized with irr>500 nm (max=608 nm). As can be seen from Table 1, all the methods consistently predict a blue shift for the open form isomer and a red shift for the closed form isomer in both vertical excitation energies and adiabatic excitation energies in solution. Most importantly, the ωB97X-D results of vertical excitation energies in acetonitrile solvent are in reasonable agreement with the result of experimental absorption maxima. Based on these results, the photoisomerization and thermal reaction paths in solution were modeled and the correponding stationary points were located. The results have been presented in Figure 3. As being revealed in experiment that the transition between the ring-open and ringclosed isomers was carried out through the triplet manifold, the S0, S1 and T1 stationary points of the reaction in Figure 3 are located by performing all of the requisite calculations (geometry optimizations and frequency and intrinsic reaction coordinate calculations) by using ωB97X-D functional in solution. First, it should be pointed out that the reactions are reversible in two directions, and the intersystem crossing singlet– triplet transitions could be anywhere between the vertically excited Franck–Condon point and the T1 minimum, which needs further dynamic simulations for a comprehensive understanding and is not the concern of the present study. Starting from the closed form minimum on the ground state S0, irradiation with visible light (irr>500 nm) triggers the photoisomerization to produce the triplet closed form minimum on the triplet state T1. According to the calculated results, further isomerization continues if 8


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the temperature is high enough as the close  open reaction is an energetically uphill process with the energy barrier about 33 kcal/mol at the ωB97X-D/SVP level of theory.

Table 1. Calculated Geometric Parameters and the Corresponding Energies with Different Functionals a Geometric parameters Isomer open

close

a

Energies b

Exp. max c

Method

Medium

C2–C7

C4–C5





G

VEE

AEE

ωB97X-D

Gas phase

3.4483

1.3504

3.53

19.48

0.0

2.73

2.66

M06-2X

Gas phase

3.3742

1.3512

3.98

19.29

0.0

2.59

2.53

B3LYP

Gas phase

3.6070

1.3597

5.95

23.98

0.0

2.44

2.35

ωB97X-D

acetonitrile

3.4713

1.3507

3.56

19.72

0.0

2.97

2.77

M06-2X

acetonitrile

3.3906

1.3517

3.97

19.29

0.0

2.74

2.57

B3LYP

acetonitrile

3.6499

1.3594

5.56

23.96

0.0

2.49

2.21

ωB97X-D

Gas phase

1.5404

1.4597

11.73

-1.42

11.2

2.43

1.93

M06-2X

Gas phase

1.5360

1.4597

11.12

-1.84

9.8

2.29

1.85

B3LYP

Gas phase

1.5457

1.4486

10.10

-1.99

12.2

1.60

1.45

ωB97X-D

acetonitrile

1.5417

1.4605

11.69

-1.76

10.8

2.37

1.68

M06-2X

acetonitrile

1.5371

1.4607

11.12

-2.14

9.5

2.23

1.60

B3LYP

acetonitrile

1.5471

1.4487

10.04

-2.35

12.0

1.48

1.13

--

d

3.18

--

d

2.04

Bond lengths in Å, dihedral angles in degrees, and excitation energies in eV. With reference to Figure

1, dihedral angles are defined as follows:  =C3–C4–C5–C6,  =S1–C4–C5–S8. b G are relative free energies, VEE are vertical excitation energies, and AEE are adiabatic excitation energies. Experimental absorption maximum (in eV) from ref. 35. d Experimental data not available.

Then, followed with another intersystem crossing triplet-singlet transition, the reaction returns the system to the other S0 minimum, thus completing the ring-opening and allowing for the ring-closure process to begin. The ring-closure reaction, by contrast, is analogous to the ring-opening process in that there are also photoisomerization and intersystem crossing transitions. However, there are also differences for these two processes. First, the initially triggered excitation that produces the singlet–triplet transition is estimated to need an energy (max=417 nm) higher than that for the ringopening reaction (max=523 nm), which is in reasonable agreement with the experimental values (390 nm and 608 nm, respectively). Moreover, the energy barrier for the energetically downhill open  close reaction is about 6 kcal/mol at the ωB97X9


c


D/SVP level of theory. Compared with the energy barrier of 33 kcal/mol for the ringopening process, it can be predicted that the ring-closure process should be easier to accomplish, which is also in accordance with the experimental results of quantum yield of 0.028 for the ring-opening and 30 for the ring-closure, respectively. 35 Similar to the case of ωB97X-D, functionals M06-2X and B3LYP yield the same trend for the process, as shown in Figure S2 of the Supporting Information.

Figure 3. Energy profiles for the reactants involved in the open-close transition process, calculated by using B97X-D functional and SVP basis set in acetonitrile solvent. Excitations are in electronic energies and thermal reactions are in free energies.

I-V characteristics and transmission spectra Based on the open and closed form minima in the ground state, we built molecular junction models with different electrodes to simulate the structures of actual devices (shown in Figure 2). To elucidate the transport properties of the molecular junctions, the I-V curves and transmission spectra of GNR-GNR and Au-Au systems within bias range [0.0V, 1.0V] were obtained from the self-consistent calculations. As shown in Figure 4, the current for the closed configuration is always larger than the open configuration for both GNR-GNR and Au-Au systems. In order to visualize the different transport behaviors, the transmission spectra variation against the external bias voltage of GNR-GNR system are plotted in Figure 5a and Figure 5b and the biasdependent transmission spectra in Au-Au system are shown in Figure 5c and Figure 5d. 10


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In contrast to the case of multiple transmission peaks located in the bias window for the closed configuration (Figure 5a and 5c), the transmission spectra are negligible in the case of the open configuration (Figure 5b and 5d). According to the Landauer-Büttiker formula, the current is obtained from the integral of the transmission spectra within the bias window. Hence, there is a big difference in the currents of the two isomers and it shows an excellent switching effect. Besides, we calculated the conductance of molecular conjunctions at zero-bias, as shown in Table 2. It can be seen that the electronic conductivity of the closed configuration is quite distinct from the open case. When diarylethene molecule is switched from the closed form to the open form, the molecular junction is converted from the on-state (high conductivity) into the off-state (low conductivity) simultaneously. To quantitatively evaluate the switch effects, we define the on-off ratio by R(V) = I(𝑐𝑙𝑜𝑠𝑒𝑑 ― form)/I(𝑜𝑝𝑒𝑛 ― form) to describe the switching effect, where I(𝑐𝑙𝑜𝑠𝑒𝑑 ― form) and I(𝑜𝑝𝑒𝑛 ― form) represent the current through the closed form and the open form, respectively. The maximum on-off ratio can reach 1045 in the GNR-GNR system and 1582 in the Au-Au system. These results indicate that excellent switching effect can be obtained in diarylethene-based molecular junctions by transformation between the open and closed form in light-driven process.

Table 2. Values of zero-bias conductance and the on-off ratio of different systems Conductance(G0) c form o form The on-off ratio

GNR-GNR system 1.58×10-5 8.87×10-8 1045

Au-Au system 3.72×10-3 3.64×10-6 1582

Moreover, negative differential resistance (NDR) phenomena can be observed in the closed form of the GNR-GNR system within the bias ranges of [0.40V, 0.50V] and [0.90V, 1.00V], respectively. Taking the former case as an example, the transmission spectra of 0.46V, 0.48V and 0.50V (corresponding to the voltage of peaks and valley in the I-V curves) have been plotted in Figure 6(a) in order to describe the NDR effect. 11



As shown in Figure 6(a), there is a narrow transmission peak near the Fermi level, which comes from LUMO that becomes more localized as the bias increases. Thus, the reason why the current decreases with the bias voltage increases is just because the integral of transmission function in the energy range of [0.40V, 0.50V] gets smaller. To understand the underlying mechanisms, we further calculated the projected density of states (PDOS) of the scattering region and the local density of states (LDOS) of electrons around the energy of 0.25eV, as shown in Figure 6(b) and Figure 6(c). Figure 6(b) shows that the PDOS peak of the molecule overlaps with the peak of the right GNR at first, and then moves toward the Fermi level and separate gradually with the peak of the right GNR. Meanwhile, the electronic states of the scattering region are localized in the central molecule and shift to the edge of the right GNR electrode. The molecule, as a bridge between the two electrodes, provides a crucial channel for electronic transport. The transfer of electronic states from the molecule to the edge will inevitably reduce the electronic conductivity of the whole system and, finally, result in the NDR effect. In the case of bias range of [0.90V, 1.00V], the origin of the NDR behavior is similar to that of the foregoing analysis, and more details have been shown in Figure S3 of the Supporting Information. Interestingly, the NDR effect is not observed in the Au-Au electrode system. The distinct transport behavior between the two systems is as a result of the different electronic properties of the electrodes. As compared with gold, which is a commonly used material of electrodes with stable physical chemical properties and conductivity, graphene has more complex electronic structures. As external bias voltage increases, the mismatch of energy levels between graphene electrodes and molecule leads to the change of conductivity of the devices, and finally gives rise to the NDR effect in the GNR-GNR system.

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Figure 4. I-V curves of different molecular junctions. As illustrated in the inset, the red (blue) and black (green) lines represent the closed and the open configurations in the GNR (Au) electrode system, respectively.

Figure 5. Bias-dependent transmission spectra of the (a) closed and (b) open configuration with GNR electrodes; (c) and (d) the same case with Au electrodes. The white solid lines indicate the bias window.

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Figure 6. (a) The transmission spectra within [0.46V, 0.50V]. (b) The corresponding projected density of states (PDOS) for the central scattering region of the closed configuration in the GNRGNR system. The projection subspace of the scattering region is divided into three part, i.e., the left GNR, the molecule, and the right GNR. (c) The local density of states (LDOS) for the central scattering region around the energy of 0.25 eV. An isovalue of 0.005 is chosen for all plots.

View of molecular levels To further understand the switching effect, the zero-bias transmission spectra of molecular junctions based on the GNR-GNR system were calculated, and the results have been plotted in Figure 7a. It can be seen that the transmission spectra are completely different in the two configurations. In the case of molecular junctions with GNR electrodes, all the transmission peaks are within the energy range between 0.00 eV and 1.00 eV. There are three sharp transmission peaks for the closed configuration while there is only one transmission peak for the open configuration, which stays far away from the Fermi level and is lower in energy than the peaks for the closed configuration. In the case of molecular junctions with Au electrodes (Figure 7b), the situation is similar except for the location and magnitude of the peaks. The transmission 14


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peaks for the closed configuration junction with Au electrodes spread in a large range between -0.50 eV and 1.00 eV. Moreover, the peak for the open configuration is suppressed to a great extent. It is well known that the eigenvalues of the molecular orbitals that can be impacted by the interaction between molecule and electrodes match well with the position of the transmission peaks, and the delocalization degree of these orbitals determines the height of the transmission peaks. Comparing with the energy levels of the isolated molecule, which can be obtained through the diagonalization of the Hamiltonian matrix, the energy levels of the molecular junctions are shifted and broadened. In order to reveal the origin of these transmission peaks, a technique named as molecule projected self-consistent Hamiltonian (MPSH) that has been used extensively in theoretical analyses were employed in our study.50 In this technique, the self-consistent Hamiltonian of the whole system is projected onto the central molecule, and the eigenvalues of the molecules are recalculated to involve the impact of the electrodes. However, choosing only the molecule part as the projection subspace cannot fully describe the interaction between the molecule and the electrodes due to the molecule-electrode coupling. Therefore, we calculated the MPSH eigenvalues containing the anchoring atoms (C atoms for GNR electrodes and S atoms for Au electrodes, respectively) and plotted the spatial distribution of the frontier molecular orbitals, as listed in Table 3 and shown in Figure 8. It shows clearly that, in the closed form, the perturbed highest occupied molecular orbital (HOMO) is delocalized in the molecular part but localized at the two ends, making the orbital a non-conducting channel. Besides, the lowest occupied molecular orbital (LUMO) and LUMO+1 are all delocalized over the whole scattering region, and the energy level of LUMO (0.22 eV) is closer to the Fermi level than that of the LUMO+1 (0.54 eV), which provide the dominant transmission channels and generate the significant transmission peaks (0.22 eV) near the Fermi level. For the electronic states in the open form, however, the HOMO and HOMO-1 are all localized on one side of the molecule and the only

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transmission peak (~ 0.40 eV) comes from the combination of delocalized LUMO+1 and LUMO.

Figure 7. Transmission spectra at zero bias in the GNR-GNR and Au-Au systems. The Fermi level is set to zero.

Moreover, for the purpose of comparison, the transmission spectra of Au electrodes system were calculated by the same methodology in order to evaluate the effect derived from different electrodes. The electronic structures behave similarly with that in the GNR system in several aspects. For the open form, although the HOMO and HOMO-1 are all delocalized states, their energy levels (-1.32 eV and -1.39 eV) are far away from the Fermi level such that they do not contribute to electronic transport. The orbital distributions of LUMO (0.48 eV) and LUMO+1 (0.53 eV) show that there is a strong interaction between the molecule and the electrode at one end but weak interaction at the other, leading to diminished tunneling effect and a small wide transmission peak (~ 0.50 eV). As a result, the transport properties of the Au-Au system is in accordance with that of the GNR-GNR system, which indicates that the switching effect originates from the intrinsic difference in electronic structures of the different isomers. When a molecule bridges the left and right electrodes together, the molecule provides possible transport channels for electron tunneling through the molecular junction, and the spatial distribution of the molecular orbitals near the Fermi level determines the different electron transport capability and finally results in distinct conductivity. This trend can 16


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be also seen from the results in Table 3 where we compare the energy levels of the isolated molecules with that of the molecular junctions. Table 3. (a)

Energy levels and MPSH eigenvalues of molecules in different systems. Systems

Molecule

Configurations

c form

o form

c form

o form

LUMO+1(eV)

0.73

0.83

0.54

0.41

LUMO(eV)

0.38

0.79

0.22

0.36

HOMO(eV)

-0.35

-0.71

-0.45

-1.19

HOMO-1(eV)

-0.83

-0.72

-1.19

-1.21

0.73

1.50

0.67

1.55

HOMO-LUMO gap (b)

GNR-GNR system

Systems

Molecule

Configurations

Au-Au system

c form

o form

c form

o form

LUMO+1(eV)

0.71

0.88

0.61

0.53

LUMO(eV)

0.37

0.84

0.23

0.48

HOMO(eV)

-0.36

-0.84

-0.49

-1.32

HOMO-1(eV)

-1.09

-0.84

-1.24

-1.39

0.73

1.68

0.72

1.80

HOMO-LUMO gap

To visualize the modification of electrodes, we calculated the energy levels of isolated molecules and HOMO-LUMO gap in all systems, and the results have been shown in Table 3a and 3b. Comparing the two isomers in different systems, the HOMOLUMO energy gap of the open configuration is largely more influenced by the modulation of the electrodes than that of the closed configuration. For instance, in the Au-Au system (Table 3b), the perturbed HOMO-LUMO gap of the closed form moves only slightly while the energy gap increases as large as 0.12 eV in the open form. These results show that the HOMO-LUMO gap can be modulated by changing the electrodes from graphene nanoribbon to Au.

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Figure 8. Spatial distribution of the frontier molecular orbitals in the GNR-GNR system and AuAu system. The isovalue is set to 0.03 for all plots.

To evaluate the impact of terminal methyl groups, which have been removed in the calculations presented above, we also calculated the I-V curves in the case of diarylethene molecules with terminal methyl groups (see Figure S4 and S5 of the Supporting Information). The obtained on-off ratio is still more than 103, indicating excellent switching behavior.

CONCLUSIONS In summary, we have constructed molecular junction models from the ring-open and ring-closed isomers of diarylethene molecules connected with GNR and gold electrodes. The electronic structures and transport properties of the devices were investigated by using NEGF combined with DFT methods. The transition processes between the two isomers activated with visible lights were simulated, and the results are in reasonable agreement with experimental data. Our theoretical results demonstrate that the closed and open isomers do possess large differences in conductivity, and the switching effect is related with the distinction in their electronic configurations. The on-off ratio in both 18


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systems can reach as large as 103 in a wide bias voltage range. It also indicates that the switch based on the light sensing diarylethene molecule is incredibly stable. Furthermore, NDR effect is found in the molecular junctions with graphene nanoribbon electrode. These findings provide an alternative strategy of designing diarylethenebased molecular photoswitches with high on-off ratio.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website: http://pubs.acs.org. Supporting Information Available: (Details on the LDA and GGA transport calculations of the Au-Au system; details on energy profiles for the reactants involved in the open-close transition process calculated by ωB97X-D, M06-2X, and B3LYP functionals with SVP basis set in acetonitrile solvent; details on analysis of NDR effect within [0.90V, 1.00V] in GNR-GNR system; details on configurations and I-V curves of molecular junctions bridged with terminal methyl groups.)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

ORCID Changfeng Fang: 0000-0001-5869-7996

Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 11574118, 21473102, 61874068), China Postdoctoral Science Foundation (No. 2018M632660), Key Research and Development Program of Shandong Province (No. 2018GGX102008), and the Fundamental Research Funds of Shandong University.

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TOC Graphic

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TOC Graphic 80x77mm (300 x 300 DPI)



Figure 1.Schematic diagram of photochemical interconversion between the ring-open form (a) and the ringclosed form (b) of diarylethene molecules. The molecular structures of the switching core in the two isomers are shown in (c) and (d), respectively. 80x46mm (300 x 300 DPI)


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Figure 2. Geometric structures of the diarylethene-based molecular junctions with different electrodes. (a) and (b) the closed and the open isomer of diarylethene molecules sandwiched between GNR electrodes. (c) and (d) the closed and open models with Au (111) electrodes. All devices are divided into three parts, i.e., left electrode, central region, and right electrode. 80x91mm (300 x 300 DPI)



Figure 3. Energy profiles for the reactants involved in the open-close transition process, calculated by using ωB97X-D functional and SVP basis set in acetonitrile solvent. Excitations are in electronic energies and thermal reactions are in free energies. 80x61mm (600 x 600 DPI)


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Figure 4. I-V curves of different molecular junctions. As illustrated in the inset, the red (blue) and black (green) lines represent the closed and the open configurations in the GNR (Au) electrode system, respectively. 80x61mm (600 x 600 DPI)



Figure 5. Bias-dependent transmission spectra of the (a) closed and (b) open configuration with GNR electrodes; (c) and (d) the same case with Au electrodes. The white solid lines indicate the bias window. 170x140mm (300 x 300 DPI)


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Figure 6. (a) The transmission spectra within [0.46V, 0.50V]. (b) The corresponding projected density of states (PDOS) for the central scattering region of the closed configuration in the GNR-GNR system. The projection subspace of the scattering region is divided into three part, i.e., the left GNR, the molecule, and the right GNR. (c) The local density of states (LDOS) for the central scattering region around the energy of 0.25 eV. An isovalue of 0.005 is chosen for all plots. 70x52mm (300 x 300 DPI)





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Figure 7. Transmission spectra at zero bias in the GNR-GNR and Au-Au systems. The Fermi level is set to zero. 80x55mm (300 x 300 DPI)



Figure 8. Spatial distribution of the frontier molecular orbitals in the GNR-GNR system and Au-Au system. The isovalue is set to 0.03 for all plots. 160x74mm (300 x 300 DPI)


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Rational Design of Reversible Molecular Photoswitches Based on

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