Electric-Field-Induced Tautomerism in Metal-Free Benziporphyrins Enables Aromaticity-Controlled Conductance Switching.

Metal-free porphyrins can alternate between hydrogen-bonded tautomers, potentially enabling reversible molecular switching. However, controlled electric-field gating of porphyrin tautomerism has not been fully realized. We propose metal-free benziporphyrins (MFBPs), in which one pyrrole ring is replaced with a phenol group, as a new platform for tautomer-based molecular electronics. This approach allows for three distinct tautomers, each possessing a characteristic aromatic or antiaromatic electronic structure. Density functional theory and quantum transport calculations show that (i) experimentally realizable electric fields can induce selective tautomerization and (ii) each tautomer exhibits a characteristic conductance profile. The strong switching capability of MFBPs is demonstrated by on/off ratios exceeding 500 at low bias. Fused MFBPs further expand the functionality by providing multiple tautomeric states for multistate molecular registers and high-conductance wire-like architectures. These results establish MFBPs as versatile building blocks for electric-field-responsive molecular devices and open new research opportunities for molecular-scale logic and memory.