Switchable Rhodamines for Molecular Electronics

Rhodamines are widely used in bioimaging as fluorogenic sensors that reversibly switch between homoconjugated lactone (dark) and conjugated zwitterionic (bright) states in response to external stimuli. Here we transpose the lactone-zwitterion equilibrium (<i>K</i>_L-Z) concept from fluorescence microscopy to molecular electronics to create the first rhodamine-based single-molecule conductance switch. Installing thioanisole end groups onto rhodamines enables their study in scanning tunneling microscope break-junction (STM-BJ) measurements. We use optical absorbance, STM-BJ, and density functional theory studies to show that trifluoroacetic acid (TFA) triggers switching between insulating and conducting states in rhodamine molecular junctions with an on/off conductance ratio of 46, which is among the highest reported switching factors for chemically responsive single-molecule junctions. Control studies with a permanently conjugated methyl ester derivative supports that the switching mechanism describes spirolactone interconversion between closed and open states. We demonstrate reversible acid/base switching over three cycles and also show we can drive conductance switching with lithium ion or sonication as external stimuli. Given the known sensitivity of rhodamine electronic structure to minor synthetic modifications, this work establishes rhodamines as an exciting yet untapped platform for the design of functional molecular electronics.