Reversible Broad-Bias Photoswitching of DNA Molecular Junctions via Azobenzene-Induced π-Stack Disruption

Reversible photoswitches that operate directly inside the DNA π-stack itself have remained elusive. Here, we introduce one or more azobenzene moieties into a DNA duplex while leaving the opposite position baseless. Reversible trans-cis photoisomerization strongly modulates longitudinal π-conjugation: the planar trans isomer partially preserves π-overlap across the defect, whereas the nonplanar cis isomer disrupts it dramatically. Both isomers substantially lower junction conductance compared to pristine DNA, but the cis-rich state induces a far more severe reduction. Conductive-probe atomic force microscopy reveals robustly reversible photoswitching: the cis-rich state lowers current by up to 2 orders of magnitude relative to the trans-rich state. On/off ratios increase superlinearly with the number of azobenzene units (up to ∼ 150 for three units) and remain >50 even at bias voltages of 1.5 V─exceptionally stable for a molecular photoswitch. Ultraviolet photoelectron spectroscopy confirms that the HOMO level of the DNA π-system is virtually unaffected both by azobenzene incorporation and by trans-cis isomerization. The observed switching therefore originates from a stronger suppressed rate of charge transfer across the cis defect, fully consistent with enhanced backscattering and possible destructive quantum interference due to its higher asymmetry location.