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

Abstract

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.