$\pi \text{–}\pi$ stacking, a fundamental noncovalent interaction between aromatic molecules, is essential for enabling charge transport across molecular fragments—a process vital to both chemistry and biology.1^,2 Owing to the widespread importance of $\pi \text{–}\pi$ interactions in diverse fields, there is considerable interest in precisely modulating the interaction at the single-molecule scale. Studies have shown that $\pi \text{–}\pi$ interaction can be influenced by adjusting molecular concentration,3 applying mechanical forces,4 or designing target molecules with enhanced attraction.5 However, achieving precise control of intermolecular $\pi \text{–}\pi$ interaction at the single-molecule level without altering molecular conformation or concentration remains a significant challenge.

Reporting in Physical Review Letters, Prof. Dong Xiang and co-workers developed a nondestructive approach to realize the precise control of $\pi \text{–}\pi$ stacking at a single-molecule scale by laser illumination.6 Through conductance measurements, they demonstrated a significant enhancement of intermolecular $\pi \text{–}\pi$ coupling induced by an appropriately polarized laser acting as an optical tweezer within a metallic nanocavity.

The scanning tunneling microscopy break junction technique was employed to investigate the effect of light illumination on the $\pi \text{–}\pi$ interaction between 2,6-naphthalediamine (2,6-NDA) molecules by monitoring their conductance traces. Two distinct conductance peaks were clearly observed, corresponding to the molecular monomer junctions [the high-conductance (HC)] and molecular dimer junctions [the low-conductance (LC)]. Under light illumination, the HC peak shifted to a higher conductance value, attributed to photon-assisted transport.7 Simultaneously, the LC peak exhibited a significant increase in intensity and a pronounced rightward shift, indicating both an enhanced yield of molecular dimer junctions and stronger coupling between $\pi \text{–}\pi$ stacked molecules.

Further investigation revealed that the LC peak intensity increase was strongly dependent on the polarization of the incident light, with p- and s-polarized light yielding distinct effects. Changes in light intensity primarily influenced the LC peak intensity without affecting other parameters. In addition, the flicker noise power spectral density analysis provided strong evidence that the LC peak corresponds to $\pi \text{–}\pi$ stacked dimers.

Mechanistic studies revealed that the enhancement of $\pi \text{–}\pi$ stacking arises from light-induced plasmonic gradient forces. Control experiments using $\beta$-naphthylamine, 5,6,7,8-tetrahydronaphthalen-2-amine, and 2,6-naphthyridine provided clear evidence of this phenomenon: the intensity of the LC peak of the $\pi \text{–}\pi$ stacked dimer significantly increased, and its position shifted to higher values under light illumination. Furthermore, computational results based on finite element analysis and the quasi-quantum approach support the hypothesis that the observed effects of light on dimer formation originate from optical gradient forces, which are significantly amplified within plasmonic nanogaps.

Overall, this work demonstrates that intermolecular $\pi \text{–}\pi$ interactions can be regulated by light illumination, driven by optical plasmonic trapping forces. Revealing the nanogapped electrodes serves as optical tweezers, enabling precise manipulation of the relative positions of extremely small objects, even at the single-molecule scale in solution. This innovative approach is likely to captivate researchers across chemistry, physics, and materials science disciplines.

Liwei Wang is a lecturer at Shanghai Normal University, Shanghai, China. She received her BS from East China University of Science and Technology in 2010 and her PhD from Beijing University of Chemical Technology in 2016. Then she worked as a postdoctoral fellow at Washington State University in the United States. Her research includes organic optoelectronic materials and photo-responsive polymer materials. She was selected as Highly Cited Researcher in Cross-Field by Clarivate Analytics in 2024.

Shengxiong Xiao is a professor at Shanghai Normal University, Shanghai, China. He received his BS from Wuhan University in 1999 and his MS under the supervision of Prof. Yuliang Li from Institute of Chemistry, Chinese Academy of Sciences, in 2002. He obtained his PhD with Prof. Colin Nuckolls at Columbia University in 2007. Then he worked as a postdoctoral fellow with Prof. Julius Rebek at the Scripps Research Institute. His research focuses on the design and synthesis of organic functional materials and fabrication of molecular-scale devices for energy transport and conversion. Prof. Xiao was listed among the World’s Top 2% Scientists in 2024 by Stanford/Elsevier.