Modeling of a Molecular Device: The Quaterthiophene-Substituted Azobenzene Molecule

The [2,2';5',2'',5'',2''] quaterthiophene-substituted azobenzene molecule exhibits a length change of >30% upon photoisomerization and therefore holds promise for application as a molecular actuator. A computational investigation has been undertaken for the ground state of this system to correlate its structural features to energy. Full torsional-space conformational searching was performed first to generate an ensemble of chemically possible conformations, which was followed by geometry optimizations at the semiempirical AM1 level and single point calculations at higher DFT level for all conformations. Eleven lowest energy structures were selected, each from the 11 categories of conformations in terms of the configurations of its four constitutional functional groups, and full geometry optimizations were then performed for these 11 structures with DFT method. The results were analyzed to correlate the structural features to energy. Structures with the trans-azobenzene configuration were found to be more extended and have lower energy than those with the cis-azobenzene configuration, in agreement with the structural features reported experimentally. The DFT optimizations provide a precise energy order of each category of conformations. This computational scheme holds promise as a tool for molecular device design.