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

The recently synthesized C18 molecule demonstrates strong electron acceptor properties similar to C60. In this work, we study computationally the ground and excited state properties of C18 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 C18, in agreement with the experiment. We show that in complexes of C18 with electron donors, the lowest excited states are charge separated states formed by electron transfer to C18. This makes C18 the smallest all-carbon electron acceptor reported so far. Because C18 exhibits a larger internal reorganization energy compared to fullerene C60, the electron transfer reactions with relatively high driving force will be shifted from the inverted to the normal Marcus regime when replacing C60 by C18.