Electrical Conductance and Diode-Like Behavior of Substituted

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Electrical Conductivity and Diode-Like Behavior of Substituted Azulene Ahmed M. El-Nahas, Aleksandar Staykov, and Kazunari Yoshizawa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10339 • Publication Date (Web): 13 Jan 2017 Downloaded from http://pubs.acs.org on January 16, 2017

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The Journal of Physical Chemistry

Electrical Conductivity and Diode-Like Behavior of Substituted Azulene Ahmed M. El-Nahas,*,1,2 Aleksandar Staykov,3 and Kazunari Yoshizawa,*,2,3

1

Chemistry Department, Faculty of Science, El-Menoufia University, Shebin El-Kom 32512, Egypt. 2

Institute for Materials Chemistry and Engineering, Kyushu University, Nishi-ku, Fukuoka, 8190395 Japan. 3

International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Nishi-ku, Fukuoka, 819-0395 Japan.

ABSTRACT: Nonequilibrium Green’s function approach combined with density functional theory has been used to study electrical conductivity of amino and cyano substituted azulene. The I-V curves show an increase in conductance when the amino groups exist at the 1,3 centers of the azulene rings and the cyano groups at the 4,8-postion. A modest current rectification ratio of ca. 2 was reported for substituted 2,6-azulene dithiolate while the value increases to 3-5 when bridging azulene through the 2,7-position based on the presence or absence of substituents and their positions. Diode-like behavior could be noticed in 1,3-azulene dithiolate when it is substituted with electron donors such as amino group at carbons 4 and 8 or when replacing the 2,4,6,8-CH groups by nitrogen atoms.

Keywords: Azulene, Electrical conductivity, Substituent effect, Nonequilibrium Green’s function, Current-Voltage curve, Molecular rectifiers.

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1. INTRODUCTION Molecular electronics represent an alternative to the traditional silicon-based electronic devices that have been highly miniaturized to nanometric range. Since the first prediction of single molecular rectifier by Aviram and Ratner1 in 1974 and its experimental verification by Metzger and his collaborators2 there is a great interest in the field of molecular electronics. Both theory and experiment have contributed to the progress of this area of research.3-21 Compared to alternant hydrocarbons, a limited number of experimental and theoretical electron transport investigations studied some nonalternant hydrocarbons such as azulene, see Chart 1.5-9 Azulene is an interesting system because of its unique electronic properties such as large permanent dipole moment.10-12 In azulene, the highest occupied molecular orbital (HOMO) has a large amplitude at positions 1, 3, 5, and 7 whereas the lowest unoccupied molecular orbital (LUMO) has the significant amplitude at positions 2, 4, 6, and 8. Therefore, substitution by electron donating and/or withdrawing groups at these positions can tune the optical and electrical properties of azulene.8

7

8

1

2

6

5

3

4

7

8

1 2

6

3 5

4

HOMO

LUMO

Chart 1

Current rectification was observed for azulene-like molecules through 2,6-wiring with a rectification ratio (RR) of ~2 though the parent 2,6-azulene dithiolate exhibits a very modest rectification ratio.8 Substitution by electron donor and withdrawing groups in these systems slightly affects the rectification ratio.8 Functionality of organic compounds can be fine-tuned to desired properties through substitution. Nature of electron demand of substituents influences electron transport positively or negatively.8,11-21 Therefore, our objective from this work is to study electron transport through substituted azulene when wired between semi-infinite gold electrodes from different positions along and across the molecule.


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2. METHODS OF CALCULATIONS Geometry optimizations of free and substituted 1,3-, 2,6-, and 2,7-azulene dithiols were performed with the hybrid B3LYP functional [22-24] and 6-31G(d,p) basis set25 using Gaussian 09 program.26 To study electron transport characteristics of these azulenes and their derivatives, each optimized structure was wired between two Au(1 1 1)-(4x4) electrodes through thiol sulfur atom in a threefold hollow site mode27 without thiol hydrogen atoms. A combination of nonequilibrium Green’s function (NEGF) formalism and exchange correlation DFT functional28 (LDA/PZ, the local density approximation with the Perdew–Zunger parameterization)

29

was

utilized, as implemented in the Atomistix ToolKit computer code (ATK 11.2.3) ,30 to compute the current. Single-zeta basis set (SZ) was used for gold and double-zeta (DZ) basis set for other elements. Core electrons were described by norm-conserving pseudopotentials.31 A mesh cutoff of 150 Ryd and an electronic temperature of 300 K were applied. The Brillouin zone was sampled with 1x1x100 k-point. It is documented that DZ and SZ basis sets without polarization are a source of inaccuracy.32 Therefore, for the investigated parent and some of the substituted molecular junctions, the current calculations have been conducted using single-zeta with polarization (SZP) for gold and double-zeta plus polarization (DZP) basis sets for other elements. Moreover, only for the unsubstitued molecular junctions the effect of utilizing DZP basis sets for all atoms was examined. Using the latter basis functions with polarization functions gives I-V characteristics similar to that of the former one but with higher current and does not significantly affect the rectification ratios as presented in the supporting information (Figure S1 and Table S1). For reducing the computational cost we studied the rest of azulenes’ junctions without polarization functions with a hope to keep, at least, the qualitative feature exhibited by the systems under consideration. Unless noted otherwise, all discussions will be considered at the cheapest level of theory that uses SZ and DZ basis sets without polarization functions. Figure 1 displays some of the azulene junctions. The Au-azulene-Au junction is composed of the left electrode, the central molecular region, and the right electrode. The central region includes three layers from each electrode (each layer contains 16 Au atoms, in total 96 Au atoms).



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Figure 1. Substituted azulene dithiolate molecular junctions.

The current (I) is calculated as a function of voltage from -2.0 to +2.0 V and the transmission spectra were calculated in the region -2 to +2 eV with respect to the Fermi energy. Molecular Projected Self-consistent-Hamiltonian (MPSH) analysis is carried out to assist in understanding electrical conductivity of substituted azulene. The MPSH states represent the eigenstates of the molecule in the presence of electrodes. The current pass through a molecular junction is calculated from transmission probability (T(E,V)) based on the Landauer-Buttiker formula:33,34

I(V) =

∞

∞ , 1

T(E,V) = Tr [ГL (E,V)GR(E,V) ГR (E,V)GA(E,V)]

(2)

where f is the Fermi function, µL/R are the electrochemical potential of the left/right electrodes and the difference between them is given by eV with the applied bias voltage V, i.e., µL = µ(0) – eV/2 and µR = µ(0) + eV/2. The µR/L(0)=EF is the Fermi energy. This µL - µR energy region is called bias window. ГL/R is the coupling function associated with the L/R electrodes and GR/A is the retarded and advanced Green’s functions of the system.

3. RESULTS AND DISCUSSION HOMO/LUMO from DFT calculations as transport channels Electron transport through a molecule bridged between two electrodes depends on the alignment of the HOMO and/or LUMO with the Fermi energy level of the electrodes. The energy difference between the Fermi level and these orbitals represents the tunneling barrier for 4 ACS Paragon Plus Environment

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electron or hole transport, the smaller the barrier the higher probability for electrical conductivity. Although the Kohn-Sham (KS) orbitals obtained from DFT calculations have no physical meaning, the shape and symmetry properties of the KS orbitals are very similar to those calculated by Hartree-Fock (HF) and extended Hückel (eH) methods.35-38 The energy order of HOMOs is in most cases are in agreement among various methods. However, the LUMOs are problematic. The underestimation of the HOMO–LUMO gap and the related close to the electrodes’ Fermi energy are the issues are not surprising as KS “orbitals” are mathematical objects rather than physical orbitals. Generally, the KS orbitals are a reasonable qualitative tool for interpretation of real molecular orbitals. Tables 1and 2 collected HOMO and LUMO energies for isolated unsubstituted azulenedithiols from Gaussian and ATK calculations using different functional and basis sets. An inspection of Table 1 reveals that the Perdew-Burke-Ernzerhof (PBE)39 generalized gradient approximation (GGA) and LDA functionals underestimate the HOMO-LUMO energy gap relative to B3LYP. The energy gap calculated with pseudopotential basis sets 0.3 eV higher than that one obtained from all-electron calculations using the LDA and PBE methods. Figure 2 displays the change of the HOMO and LUMO energies with bias voltages for the 1,3-azulene dithiolate molecular junction. It is evident that adding polarization function brings HOMO close to the Fermi level which will increase the current.

Table 1: HOMO/LUMO and Eg energies (eV) for azulenedithiols at method/6-31(d,p). FMO

Method B3LYP

HOMO LUMO

Eg

Azu-2,6-SH

3.24

-5.49 -1.94

-4.64 -2.61 -5.29 -3.28

Azu-2,7-SH -5.28 -2.04

Eg

Azu-1,3-SH

Eg

3.55

-5.60 -2.25

3.36

2.03

-4.84 -2.48

2.36

-5.01 -2.89

2.12

2.00

-5.49 -3.15

2.34

-5.65 -3.57

2.09

PBE HOMO LUMO LDA HOMO LUMO



Table 2: HOMO/LUMO and Eg energies (eV) for isolated 2,6-azulenedithiol at method/basis as implemented in ATK. Basis\DFT SZ HOMO LUMO

LDA HOMO/LUMO -1.03 1.03

HOMO LUMO

-1.01 1.01

HOMO LUMO

-1.20 1.20

HOMO LUMO

-1.12 1.12

2.05

HOMO LUMO

PBE HOMO/LUMO -1.01 1.01

2.03

2.02

HOMO LUMO

-1.010 1.01

2.01

2.40

HOMO LUMO

-1.22 1.22

2.44

2.24

HOMO LUMO

-1.14 1.14

2.27

Eg

Eg

SZP

DZ

DZP

1.0

0.5

HOMO/LUMO (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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HOMO/SZ LUMO/SZ HOMO/SZP LUMO/SZP HOMO/DZ LUMO/DZ HOMO/DZP LUMO/DZP

0.0

-0.5

-1.0

-1.5 -2

-1

0

1

2

Bias (V)

Figure 2. Change of MPSH HOMO and LUMO energies with bias voltages for the 1,3-azulene dithiolate molecular junction. 6 ACS Paragon Plus Environment

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Previously,9 we have computed electron transport through nine azulene molecular junctions and found high current when azulene is wired between two gold electrodes from four positions, namely1,3-, 1,5-, 2,6-, and 4,7-, and a diode-like behavior from position 2,7-. This work focuses on the electrical conductivity of substituted 1,3-, 2,6- and 2,7-azulene dithiolates. Figures 3-5 show the computed I-V properties of these molecular systems wired between two gold electrodes. More plots are presented in the Supporting Information (Figures S2 and S3). Substituents that stabilize (destabilizes) LUMO (HOMO) lead to a shift of these orbitals toward the Fermi level and therefore, to rising of the current and vice versa.11 Compared to the 2,6azulene dithiolate molecular junction, replacing H atoms at the 4,8 positions by cyano groups and at the 1,3 centers by amino groups enhances the current by ~23 µA at ±2 V. Exchange of the amino and cyano groups at these positions show current reduction almost by the same amount. The presence of electron donating groups at positions where HOMO has a maximum amplitude shifts its energy upward (destabilization) and brings it closer to the Fermi level.11 This leads to an improvement in the electrical conductivity. The HOMO-LUMO energy gap (Eg) in the free 2,6-azulene dithiol is smaller when the amino groups are located at the 1,3-position and larger when adding them to the 4,8- position (3.5, 2.4, and 3.8 eV, respectively), see the Supporting Information (Tables S2-4). However, the presence of electron accepting group at positions where LUMO has high orbital amplitude stabilizes it (Eg = 3 eV in 4,8-dicyano-2,6-azulene dithiol). Therefore, adding amino groups at centers 1,3 and cyano groups at positions 4,8 gives the smallest Eg of 1.8 eV with the highest current. The corresponding MPSH energy gap for the latter system is 1.2 eV at 2 V compared to 2.5 eV in the unsubstituted 2,6-azulene dithiolate (Table S5). The observed reduction in the energy gap when a molecule is sandwiched between two electrodes is attributed to the image effect.40



a) Disubstitution 60 2,6-azu 2,6-azu-1,3-CN2,6-azu-4,8-CN2,6-azu-1,3-NH22,6-azu-5,7-CN2,6-azu-5,7-NH22,6-azu-4,8-NH2-

50 40 30

Current (µA)

20 10 0 -10 -20 -30 -40 -50 -60 -2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Bias (V)

b) Tetrasubstitution

60

2,6-azu 2,6-azu-1,3-CN,4,8-NH22,6-azu-1,3-CN,5,7-NH22,6-azu-4,8-CN,1,3-NH22,6-azu-5,7-CN,1,3-NH22,6-azu-5,7-CN,4,8-NH22,6-azu-4,8-CN,5,7-NH2-

50 40 30 20

Current (µΑ)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

10 0 -10 -20 -30 -40 -50 -60 -2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Bias (V)

Figure 3. I-V characteristics of substituted 2,6-zulene dithiolates. 8 ACS Paragon Plus Environment

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50 40 1,3-azu1,3-azu-2,6-NH21,3-azu-2,6-CN-

30

Current (µA)

20 10 0 -10 -20 -30 -40 -50 -2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Bias (V)

Figure 4. I-V characteristics of substituted 1,3-azulene dithiolates.

2,7-azu2,7-azu-1,3-NH2,4,8-CN2,7-azu-1,3-NH22,7-azu-1,3-CN,4,8-NH22,7-azu-1,3-CN2,7-azu-4,8-NH22,7-azu-4,8-CN-

40 30 20 10 0

Current (µA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-10 -20 -30 -40 -50 -60 -70 -2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Bias (V)

Figure 5. I-V characteristics of substituted 2,7-azulene dithiolates. 9 ACS Paragon Plus Environment


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Inserting two cyano groups at the 1,3 carbons or other centers has the same effect as replacing 1,3-CH or other centers by nitrogen atoms as given in the Supporting Information (Figures S2 and S3). Similarly, 2,4,6,8-tetraaza-1,3-azulene dithiolate with an Eg of 1.7 eV (compared to 3.4 eV in the parent 1,3-azulene dithiolate) gives an increase in the current by 30 µA. However, 2,6-NH2 substitution enlarges the energy gap to 3.6 eV with current reduction of 15 µA. The cyano groups at the same positions enhance the current by 8 µA compared to the unsubstituted azulene dithiolate. The effect of substituents on the HOMO and LUMO energies and hence on the current has been discussed previously for different systems.8,11-13,

15-21

Replacing H atoms of the benzene ring by methoxy and cyano groups shifts the HOMO up and down and then to increase and decrease, respectively, electrical conductivity of the benzene molecular junction.11 For azulene-like molecules (with benzene and naphthalene rings sandwiched between the heptagon and pentagon rings), replacing H atoms of the azulene rings at positions 4,8- and 1,3- by F and NH2, respectively, increases the electron transport when the systems is wired between two gold electrodes from the 2,6-position.8 The transmission spectra are used to explain the I-V characteristics of the investigated azulenes. At zero bias, electrons can transfer through molecular junction via nonresonant tunneling. Applying a certain bias changes the position of the occupied and/or unoccupied molecular orbitals relative to the Fermi level, and therefore allow for electron transport through molecular junction using these orbital as dominant transport channel. Orbitals with energy close to the Fermi level will contribute to current and show peaks near the Fermi energy in the bias window of the transmission spectra. The transmission spectra (Figure 6) of the substituted azulene dithiolates at different bias voltages within the bias window will be analyzed. Compared to the parent azulene dithiolates, the three substituted azulene dithiolates indicate the appearance of more peaks with an enhanced intensity in the bias window at the same voltages range for cases accompanied by enhancing of the electrical conductivity with substitution at particular positions and a lack or shrinking of peaks for systems which exhibit weak current. Detailed transmission spectra of substituted azulene dithiolates are collected in the Supporting Information (Figures S4 and S5). The MPSH states of the 2,6- and 2,7-azulene dithiolates are presented in Figure 7. The MPSH orbitals that are delocalized over the whole molecular junction represent the predominant channels for electron transport in the investigated systems. The HOMO is delocalized over the 2,6-azulene and 4,8-diamino,1,3-dicyano dithiolate molecular 10 ACS Paragon Plus Environment

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junctions, while HOMO-1 and LUMO are the delocalized ones in the 1,3-amino-4,8-cycano-2,6azulene dithiolate. For 2,7-azulene dithiolate, the HOMO-1, HOMO, and LUMO states are extended over the whole molecular junction.

Current Rectification Introducing asymmetry on azulene dithiolate by replacing H atoms or the CH groups of the molecular backbone by groups or heteroatoms, respectively, increases or decreases its diodelike behavior compared to the unsubstituted one. Plots of current rectification ratios (RR = I+/I-) for different azulene dithiolates are presented in Figure 8. No current rectification was recorded for unsubstituted 1,3-azulene dithiolate and modest rectification ratio (RR) of 1.2 at 1.4 V was obtained from the 2,6-position. However, the 2,7-junction exhibits diode-like behavior with an RR value of 4 at 2 V. This latter RR is increased to 5 by 1,3-CN-4,8-NH2 substitution. On the other hand, the current rectification ratio is reduced to 1.6 upon exchanging the position of amino and cyano groups. Double substitution of amino and cyano groups at positions 1,3- and 4,8respectively, in 2,7-azulene dithiolate increases the current by ca. 25 µA in both directions where the Eg is 1.7 eV in this molecular junction compared to 3.2 eV in the unsubstituted one and 3.9 eV in 1,3-CN-48-NH2,2,7-azulene dithiolate. It has been reported that current rectification could be explained in terms of asymmetric shift of frontier molecular orbitals (FMO) under positive and negative bias voltages.15 For 1,3azulene dithiolate, the conductance is dominated by the delocalized HOMO, HOMO-1 and LUMO+1 states. These orbitals are affected equally under positive and negative voltages, see Table S6. Similarly, in the 2,6-azulene dithiolate junction the energies of HOMO-1 and LUMO, which are responsible for electron transport through this connection, undergo almost equal change upon reversing bias. However, the situation is different for the junction that show a significant diode-like behavior such as 2,7-azulene dithiolate; the energies of HOMO-1 to LUMO+2 exhibit asymmetric shift under negative and positive bias voltages. The negative bias shifts the LUMOs close to the Fermi level while the positive bias shifts them away leading to high and low current, respectively, as observed in the I-V curve of this connection, see Figure 3. We have also calculated the current rectification through the 2,7-naphthalene dithiolate molecular junction. An RR of 6 was reduced to 2 upon substitution with two cyano groups at the 1,4-positions and two amino groups at the 5,8-positions. Moreover, the current produced from 11 ACS Paragon Plus Environment


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the 2,7-azulene dithiolate is much higher than from the 2,7-naphthalene dithiolate in both directions (-41↔10 vs. -11.2↔2 µA, respectively, at -/+2 V). Adding -NH2 at positions 4,8 or -CN at the 1,3 centers almost switches off the diode-like behavior of the 2,7-azulene dithiolate molecular junction over the entire range of the voltages used in this work while adding these substituents at the same positions significantly reduces the current in both directions but keeps the asymmetric feature of the I-V curve with a rectification ratio of 5 compared to 4 in the unsubstituted 2,7-azulene dithiolate at 2 V. However, reversing the position of the amino and cyano groups enhances the current asymmetrically in both directions with a reduction in RR to 1.7 at 2 V. When using -NH2 at positions 1,3- or -CN at 4,8-, the RR values become 3.3 and 3.5 at 0.8 and 1.5 V, respectively. A modest RR of 2.1 at 1.4 V could be obtained from the 2,6-connection when it has 5,7-CN and 4,8-NH2 groups. Diode-like behavior (RR=2.2, at 1.2 V) can be also obtained when wiring azulene through the 1,3connection by replacing CH of 2, 6, 4, and 8 positions by nitrogen atoms. Other substitution patterns give RR between 1.2 1nd 1.8. Current rectification in parent and substituted 2,7- and 2,6-azulene dithiolate junctions is higher than that recorded for substituted and unsubstituted azulene-like systems where Zhou et al.15 reported a maximum RR value of 2.4 compared to 3-5 in our systems. The authors described their compounds as superior to substituted oligo(phenyleneethylene) both from the strength of current and the rectification power. Wiring azulene through the 1,3-connection with replacing CH of 2, 6, 4, and 8 positions by nitrogen atoms gives a rectification ratio of 2.2 at 1.2 V. For the investigated molecular junctions and at different basis sets, the diode-like behavior of the investigated systems still exits at voltages between 1 and 1.5 V if there is a worry about damaging the whole junction at higher voltage of 2 V. For example, 2,7-azulene dithiolate shows rectification ratios of 3.5, 3.2, and 3.4 at 1.5V using SZ, SZP, DZP, respectively.


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1,3-CN-4,8-NH2-2,6azulene dithiolate 2V

1V

0V

1,3- NH2-4,8-CN-2,6azulene dithiolate

1,3- diaza-2,6-azulene dithiolate

Figure 6. Transmission spectra of substituted azulene dithiolates at different bias voltages.


-1 V

-2 V


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

HOMO-1

HOMO

LUMO

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LUMO+1

4,8- NH2-1,3-CN2,6-SH-Azu

2,6-SH- Azu

4,8-CN-1,3-NH2-2,6SH-Azu

2,7-SH-Azu

Figure 7. Zero-bias MPSH states of substituted azulene dithiolates.


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a) 2,6-azulene dithiolate

2,6-azu 2,6-azu-1,3-CN,4,8-NH22,6-azu-1,3-CN,5,7-NH22,6-azu-4,8-CN,1,3-NH22,6-azu-5,7-CN,1,3-NH22,6-azu-5,7-CN,4,8-NH22,6-azu-4,8-CN,5,7-NH2-

2.4 2.2

Rectification Ratio

2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.0

0.5

1.0

1.5

2.0

Bias (V)

b) 2,7-azulene dithiolate

6

2,7-azu2,7-azu-1,3-NH2,4,8-CN2,7-azu-1,3-NH22,7-azu-1,3-CN,4,8-NH22,7-azu-1,3-CN2,7-azu-4,8-NH22,7-azu-4,8-CN-

5

Rectification Ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

3

2

1 0.0

0.5

1.0

1.5

2.0

Bias (V)

Figure 8. Rectification ratios of substituted azulene dithiolates. 15 ACS Paragon Plus Environment


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4. CONCLUSIONS Electrical conductivity of substituted azulenes sandwiched between two gold electrodes from three different orientations has been modeled a combination of non-equilibrium Green’s function approach and density functional theory (NEGF-DFT). The results draw the following conclusions: 1. The current obtained from the 1,3-, 2,6- and 2,7- connections can be enhanced or reduced by substitution with electron donating groups at centers of higher or lower orbital amplitudes, respectively, through tuning of HOMO and/or LUMO energies. 2. A good diode-like feature with rectification ratio (RR) of 3-5 is produced from 2,7azulene dithiolate and its derivatives such as the one with 1,3-CN-4,8-NH2 substitution of RR = 5 at 2 V, whereas a modest RR of 2.1 at 1.4 V could be obtained from the 2,6-connection when it has 5,7-CN and 4,8-NH2 substituents. 3. Wiring azulene through the 1,3-connection with replacing CH of 2, 6, 4, and 8 positions by nitrogen atoms gives a rectification ratio of 2.2 at 1.2 V.

AUTHOR INFORMATION Corresponding Author Ahmed M. El-Nahas. Phone +2-1064607974 E-Mail: [email protected] Kazunari Yoshizawa. Phone +81-92-802-2529 E-Mail: [email protected]

ACKNOWLEDGMENTS K.Y. thanks Grant-in-Aid for Scientific Research (Nos. 24109014 and 15K13710) from the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education of Education, Culture, Sports, Science, and Technology of Japan (MEXT) and the MEXT Projects of “Integrated Research on Chemical Synthesis” and “Elements Strategy Initiative to Form Core Research Center”. A.M.E.-N. thanks Kyushu University for nice hospitality during his stay as visiting professor. A.S. would like to thank World Premier International Research Center 16 ACS Paragon Plus Environment

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Initiative (WPI), Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT), Japan.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DO: I-V curves of azulene dithiolates, detailed transmission spectra for azuelene dithiolates from DFT-NEGF calculations, HOMO/LUMO energies, and detailed reference 26.

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Graphical Abstract NH2

CN CN

S

NH2

S

S

S

CN

NH2

NH2

Weak current

CN

Strong current


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Electrical Conductance and Diode-Like Behavior of Substituted

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