Theoretical Study on Electrical Properties of Molecular Junctions of Viologen Derivatives

Zhuan-Yun Cai, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China;
Jia Liu, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China;
Si-Yuan Guan, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China;
De-Yin Wu, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China;Follow
Zhong-Qun Tian, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China;

Corresponding Author

De-Yin Wu(dywu@xmu.edu.cn)

Abstract

In this paper, the electrical properties of molecular junctions formed N,N′-bis(4-thioalkyl)-4,4′-bipyridinium (viologen) moiety between two gold (Au) electrodes have been investigated by combining density functional theory and non-equilibrium Green’s functional approach. To modulate the viologen molecule to be a cation with one and two positive charges (V⁺ and V²⁺), we introduce one and two trifluoroacetic acid ions (TFA^-) around the molecule, respectively. The valence states of V⁺ and V²⁺ are confirmed by checking Mulliken and NBO charges. Then the relationship between molecular conductance and electronic structures of the neutral state V, the radical state V⁺ and dication V²⁺ are analyzed in detail. The results in analyzing transmission spectra of the three states reveal that the conductance values of V and V⁺ are two orders of magnitude larger than that of V²⁺. This suggests that the redox states of viologen molecules can be used to realize the function of molecular switches. Our calculated results also show that increasing the torsion angle between two pyridine rings of the S-4V4-S molecule will decrease the conductance. By comparing different ions of TFA、PF₆ and BF₄, the calculated results show that the molecular junction conductance decreases about 3 times when the torsion angle increases by about 6°. It indicates that increasing the torsion angle of the dication V²⁺ can improve significantly switching ratio of viologen derivatives molecules. At the same time, the calculated results show that increasing the number of methylene groups in alky chains (HS-nVn-SH, n = 2 ~ 7), the conductance values of molecular junctions decrease exponentially, and the attenuation factor of each methylene is about 1 close to alkanedithiol molecules in literatures experimentally and theoretically. This also shows that as the alkyl chain length increases, the DFT-NEGF theoretical method can better predict the zero-bias conductance of the viologen derivative molecule.

Graphical Abstract

Keywords

viologen, redox active center, molecule conductance, density functional theory, non-equilibrium green’s function

DOI Link

https://doi.org/10.13208/j.electrochem.200621

Publication Date

2021-02-28

Online Available Date

2020-07-16

Revised Date

2020-07-15

Received Date

2020-06-21

Recommended Citation

Zhuan-Yun Cai, Jia Liu, Si-Yuan Guan, De-Yin Wu, Zhong-Qun Tian. Theoretical Study on Electrical Properties of Molecular Junctions of Viologen Derivatives[J]. Journal of Electrochemistry, 2021 , 27(1): 92-99.
DOI: In this paper, the electrical properties of molecular junctions formed N,N′-bis(4-thioalkyl)-4,4′-bipyridinium (viologen) moiety between two gold (Au) electrodes have been investigated by combining density functional theory and non-equilibrium Green’s functional approach. To modulate the viologen molecule to be a cation with one and two positive charges (V⁺ and V²⁺), we introduce one and two trifluoroacetic acid ions (TFA^-) around the molecule, respectively. The valence states of V⁺ and V²⁺ are confirmed by checking Mulliken and NBO charges. Then the relationship between molecular conductance and electronic structures of the neutral state V, the radical state V⁺ and dication V²⁺ are analyzed in detail. The results in analyzing transmission spectra of the three states reveal that the conductance values of V and V⁺ are two orders of magnitude larger than that of V²⁺. This suggests that the redox states of viologen molecules can be used to realize the function of molecular switches. Our calculated results also show that increasing the torsion angle between two pyridine rings of the S-4V4-S molecule will decrease the conductance. By comparing different ions of TFA、PF₆ and BF₄, the calculated results show that the molecular junction conductance decreases about 3 times when the torsion angle increases by about 6°. It indicates that increasing the torsion angle of the dication V²⁺ can improve significantly switching ratio of viologen derivatives molecules. At the same time, the calculated results show that increasing the number of methylene groups in alky chains (HS-nVn-SH, n = 2 ~ 7), the conductance values of molecular junctions decrease exponentially, and the attenuation factor of each methylene is about 1 close to alkanedithiol molecules in literatures experimentally and theoretically. This also shows that as the alkyl chain length increases, the DFT-NEGF theoretical method can better predict the zero-bias conductance of the viologen derivative molecule.