2, Chalmers University, Gothenburg, , Sweden
Molecular photoswitches have been the focus of intense research for decades as these systems are potentially useful as optoelectronic switches for controllable optical filters, data recording, signal transduction in sensing systems etc. Despite the variety of uses of these systems, many photochromic molecules have proven to be difficult to switch when placed on a metal surface, limiting their utility for integration with electronic systems. This work demonstrates that isomerization from the norbornadiene (NB)-state to the quadricyclane (QC)-state can be electrically induced on metal surfaces. We utilize the STM-Break Junction (STM-BJ) technique to make contact to and investigate the switching properties of these photoswitching molecules synthesized by Moth-Poulsen et al. Specifically, at room temperature under high vacuum, at bias=0.05V, the high conductance NB-state is detected, meanwhile, at bias=0.2V, the low conductance QC-state is observed. Alternatively, when the STM-BJ experiments are performed in a mesitylene solvent environment, the switching does not occur until reaching a bias of ~1.2V. This change in the switching voltage suggests that the solvent serves as a cold bath and that a local heating effect may be responsible for the observed switching behavior. A series of STM-BJ tests under different ambient temperatures ranging from 78K to 300K shows the correlation between the switching bias and the ambient temperatures. The bias-temperature dependence fits well with a model of local heating effect due to electron-phonon interaction by Di Ventra et al. which yields a local effective temperature of 325K at the molecular junction when switching occurs. By modeling the behavior with an Arrhenius-type relation we are able to examine the effects of surface binding on the switching energy barrier. Compared with our previous work of switching molecules from NB-state to QC-state through radiating laser light of 405nm, we demonstrate that electrically controlled excitation from NB to QC could also be achieved. This work brings deeper insight of chemical reaction at a single molecule level on the metal surface as well as provide a new approach to electrically control and engineer photoswitching molecule behavior from a surface science perspective.