Ultrastrong Regulation Effect of Electric Field on Cyclo[18]carbon: Phenomenon and Essence

Cyclo[18]carbon has a very unique geometry and electronic structure. Based on quantum chemistry calculations, we found that external electric field (EEF) has an ultrastrong regulation effect on various aspects of the cyclo[18]carbon. The main findings are as follows: (1) The EEF evidently affects geometric structure of the cyclo[18]carbon, and the ring is greatly elongated when EEF exceeds 0.02 a.u. (2) The EEF causes a huge polarization in the charge distribution of the system, and large EEF can even make some in-plane pi electrons nearly detached from the system (3) Presence of EEF significantly reduces HOMO-LUMO gap (4) The EEF not only greatly changes the position and intensity of the original absorption peak of the cyclo[18]carbon in the ultraviolet region, but large EEF also leads to a new absorption band in the visible light range and thus makes the cyclo[18]carbon show color. We made an in-depth explanation and discussion of the above phenomena from the aspects of energy, electronic structure, and atomic force. Furthermore, we demonstrated that it is feasible to equivalently apply strong EEF on the cyclo[18]carbon via a purely chemical and thus a more easily achieve way, namely introducing divalent alkaline earth metal cation. This work is not only of scientific interest in the interplay between the EEF and the cyclo[18]carbon, but may also open a novel path to explore different applications of this unusual molecule.