Intrinsic flexing abilities of molecular muscles based on tetrakis(2,3-thienylene): a quantum mechanical study

The conformational and energetic properties of tetrakis(2,3-thienylene), which is the building block of a single-molecule electroactive mechanical actuator, and some substituted derivatives have been characterized using quantum mechanical calculations. More specifically, calculations have been used to investigate the actuation range and the ability to be oxidized of tetrakis(2,3-thienylene), its homodimer, and some derivatives containing charged substituents. The actuation range of tetrakis(2,3-thienylene) upon oxidation to the 1+ state was found to be about 6%, while oxidation to the 2+ state with triplet and singlet spin multiplicities produces dimensional changes of 5% and 11%, respectively. Interestingly, the singlet dicationic state was predicted to be significantly more stable than the triplet one. Similar changes in the molecular length were obtained for the dimer of tetrakis(2,3-thienylene) upon oxidation, in this case the actuation range of the dication being about 11% for the two spin multiplicities. However, it was found that the actuation range increases to about 14% and 19% when the 3+ and 4+ states are considered for the dimer. Finally, incorporation of substituents on the tetrakis(2,3-thienylene) unit led to stabilization of the charged states, even though no relevant improvement in the range of actuation was detected.