Cyclohepta[]fluorene as a bistable molecule: first principles studies on its electronic structure and the effects of benzo-extension, substitution and solvation.

Cyclohepta[]fluorene 1 and its derivatives have received considerable attention due to possible technological applications as molecular devices. Despite efforts from both theory and experiment, the electronic structure of 1 has remained unclear. Herein, we report advanced first-principles calculations on 1 using a multireference and a coupled-cluster method. We confirm a bistability of 1 between a polar singlet state and a non-polar triplet state. We also study the effects of benzo-extension on the electronic structure and the influences of substitution and solvation on the ground state. Our results suggest that deliberate choice of substituents allows to toggle the multiplicity of the ground state. We also propose that due to its bistability, 1 represents an attractive building block for molecular devices.