Electrochemical Gating of Charge Transport in Single Macrocycle Molecular Switches

Molecular switches, i.e., molecular scale devices that can be turned on and off, are of long standing interest.  In the quest for miniaturization of electronic devices, the development of electrical switches of molecular dimensions has been identified as an important target of practical importance. While much progress has been made in designing functional molecular scale electrical switches, i.e., molecular cores whose intrinsic conductivity can be externally modulated, the question of contact resistance has not been sufficiently addressed.

Our novel strategy for the regulation of charge transport through single molecule junctions combines external stimuli using a variation of electrode potential in an electrochemical environment, internal modulation of molecular structures and optimization of anchoring groups.  Our synthetic collaborators have designed redox-active benzodifuran (BDF) compounds as functional electronic units which we have used to fabricate metal-molecule-metal (m-M-m) junction devices by scanning tunneling microscopy (STM) and mechanically controllable break junctions (MCBJ).[1]  The conductance of BDF terminated thiol, the standard anchoring group for molecular scale electronic components, can be tuned by changing the electrode potentials in an ionic liquid electrolyte to show a clear single molecule field effect transistor (FET) behavior.

We hypothesized that we could optimize the response by appropriate choice of molecular anchoring group.[2]  Guided by our earlier investigations, a BDF molecule tailored with carbodithioate (-CS₂-) anchoring groups was synthesized.  Our studies show that replacement of thiol by carbodithioate not only increases the junction conductance, but also substantially improves the FET response by enhancing the on/off ratio from 2.5 to 8.

[1] Regulating a Benzodifuran Single Molecule Redox Switch via Electrochemical Gating and Optimization of Molecule/Electrode Coupling, Zhihai Li, Hui Li, Songjie Chen, Toni Froehlich, Christian Schönenberger, Michel Calame, Silvio Decurtins, Shi-Xia Liu and Eric Borguet, Journal of the American Chemical Society 136, 25 8867-8870 (2014).

[2] Optimizing Single-Molecule Conductivity of Conjugated Organic Oligomers with Conjugated Carbodithioate Linkers, Yangjun Xing, Tae-Hong Park, Ravindra Venkatramani, Shahar Keinan, David  N. Beratan, Michael J. Therien, and Eric Borguet, Journal of the American Chemical Society 132 (23), 7946-7956 (2010).