Molecular Design to Enhance the Thermal Stability of a Photo
Switchable Molecular Junction Based on Dimethyldihydropyrene
and Cyclophanediene Isomerization

Photoswitchable molecular devices based on dimethyldihydropyrene (DHP) and cyclophanediene (CPD) isomers in gold junctions have been systematically studied by using first-principles calculations. The reaction pathways for the forth- and back-isomerization between DHP and CPD have been explored. It is found that along the ground state, the calculated barrier for the back-isomerization from CPD to DHP is as high as 23.2 kcal/mol. The forth- and back-isomerization on excited state was found to be much easier compared to that on the ground state. Our calculations have shown that the same conclusions about the reaction pathways can be drawn for the DHP/CPD derivatives that were experimentally studied. It is revealed that the thermal stability of the molecular switch can be significantly enhanced when certain substitutions are employed. A desirable substitution that gives a larger ON/OFF ratio and higher thermal stability is proposed for these isomeric systems. We have also found that the electrode distance has a huge impact on the electron transport properties, as well as the switching performance, of these junctions, which nicely explains some puzzling experimental observations.