Regulating enol-keto tautomerism at the single-molecule level with a confined optical field.

The keto-enol tautomerism, involving a reversible isomerization of the molecule, plays a critical role in organic synthesis, biological activity, and molecular-scale charge transport. It is therefore essential to manipulate the process of keto-enol tautomerism. Unlike typical ketones, β-diketones exist dominantly in the enol form and it is a great challenge to realize enol-keto tautomerism due to the formation of intramolecular hydrogen bonds in the enol form. Here, via in situ monitoring of the conductance evolution of thousands of single-molecule junctions, we demonstrated that the enol → keto transformation can be significantly promoted by confined ultraviolet (UV) irradiation at an extremely low intensity (1‰ of sunshine) employing antenna electrodes. Our study reveals that the conductance of the enol form is an order of magnitude larger than that of the keto form although both have similar molecular lengths and identical anchoring groups, and the enol form shows a current rectification behaviour which is completely absent in the keto form. Supported by UV-vis measurements, wavelength-dependent conductance measurements, and theoretical calculations, the mechanism for the enol → keto transformation promoted by the gap-electrode-confined optical field was elucidated, offering a new strategy to regulate the tautomerism processes at the single molecule level, and implying a potential multi-functional application of β-diketones in the fabrication of rectifiers and synchronous switches.