Cyclo[18]carbon: the smallest all-carbon electron acceptor

The recently synthesized C-18 molecule demonstrates strong electron acceptor properties similar to C-60. In this work, we study computationally the ground and excited state properties of C-18 and its complexes with several electron-donating molecules. We demonstrate that a high amount of the exact (HF) exchange in the DFT functional leads to a polyynic structure of C-18, in agreement with the experiment. We show that in complexes of C-18 with electron donors, the lowest excited states are charge separated states formed by electron transfer to C-18. This makes C-18 the smallest all-carbon electron acceptor reported so far. Because C-18 exhibits a larger internal reorganization energy compared to fullerene C-60, the electron transfer reactions with relatively high driving force will be shifted from the inverted to the normal Marcus regime when replacing C-60 by C-18.