Design, synthesis and single-molecular conductivity of isomeric naphthalene diimides

Abstract

<p indent="0mm">Two isomeric 4-methylthiophenylethynyl-substituted 1,4,5,8-naphthalene diimide (1,4,5,8-NDI) and 1,2,5,6-naphthalene diimide (1,2,5,6-NDI) derivatives <bold>1</bold> and <bold>2</bold> were designed and synthesized. The physicochemical properties of <bold>1</bold> and<bold> 2</bold> were studied by means of ultraviolet-visible (UV-vis) spectra, electrochemical measurements and density functional theory (DFT) calculations. The structural differences between <bold>1</bold> and <bold>2</bold> led to their different physicochemical properties and single-molecule device performance. Compound <bold>1</bold> based on 1,4,5,8-NDI that has lower highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels and narrower band gap is more conducive to electron transport relative to compound <bold>2</bold> with 1,2,5,6-NDI core. The single-molecular junctions of <bold>1</bold> and <bold>2</bold> were prepared by scanning tunneling microscope-break junctions (STM-BJ) technique, and their conductive behaviors were studied. When the potential of <sc>0.1 V</sc> was applied in the ionic liquid BMIPF₆, the conductance of single-molecular junctions of <bold>1</bold> and <bold>2</bold> was 10^−3.70<italic>G</italic>₀ (15.5 nS, 1<italic>G</italic>₀=77.5 μS) and 10^−3.90<italic>G</italic>₀ (9.8 nS), respectively, and the single-molecule device based on <bold>1</bold> has a better conductivity.