Optical and charge transport characteristics of photoswitching plasmonic molecular systems

Probing the optical and charge transport characteristics in molecular junctions not only provides fundamental understanding of light–matter interactions and quantum transport at the atomic and molecular scale, but also holds great promise for the development of molecular-scale optical and electronic devices. Herein, an overview of recent progress in fabricating and characterizing photoswitching molecular systems using both the current measured from single molecule circuits as well as the light signals monitored in photodetectors is presented. We review four groups of azobenzene, diarylethene, dihydroazulene, spiropyran photoswitching molecules that have been used to construct photoswitching molecular devices by scanning tunneling microscope-based or mechanically controlled break-junction techniques, focusing on the impact of light-induced reactions on the charge transport processes at the single molecule level. We also discuss key optical properties of photoswitching systems, uncovered by a range of optical methods including transient absorption and ultrafast spectroscopies, that are critically related to structural symmetry or nonlinear optical effects.