Applicability of Baird’s Rule for In-Plane Aromaticity

Abstract: The concept of aromaticity is essential for chemists to understand the nature of many chemical substances. “In-plane” aromaticity is a unique type of aromaticity found in some π-conjugated systems consisting of radially oriented p-orbitals. Although in-plane aromaticity has been theoretically predicted about 40 years ago, its intrinsic properties are much less well understood than those of conventional Hückel aromatic systems with perpendicularly oriented p-orbitals because of lack of available examples of in-plane aromatic molecules. Here we demonstrate, from both theoretical and experimental aspects, that in-plane aromaticity of cycloparaphenylenes is reversed upon excitation to the lowest triplet state. In-plane non-aromatic neutral forms with a 4N-electron system and in-plane aromatic dications with a 4N+2-electron system turn out to be aromatic and anti-aromatic, respectively, indicating that Baird’s rule holds for the in-plane aromatic system. Armchair carbon nanotube fragments based on the cycloparaphenylene were also found to follow Baird’s rule from a magnetic viewpoint.<br /

Radical Ions of Cycloparaphenylenes: Size Dependence Contrary to the Neutral Molecules

Cycloparaphenylenes (CPPs) have attracted wide attention because of their interesting properties owing to distorted and strained aromatic systems and radially oriented p orbitals. For application of CPPs, information on their charged states (radical cation and radical anion) is essential. Here, we measured absorption spectra of the radical cations and the radical anions of CPPs with various ring sizes over a wide spectral region by means of radiation chemical methods. The peak position of the near-IR bands for both the radical cation and the radical anion shifted to lower energies with an increase in the ring size. This trend is contrary to what is observed for transitions between the HOMO and LUMO of the neutral CPP. The observed spectra of the CPP radical ions were reasonably assigned based on time-dependent density functional theory. These results indicate that the next HOMO and the next LUMO levels are important in the electronic transitions of radical ions

Radical Ions of Cyclopyrenylene: Comparison of Spectral Properties with Cycloparaphenylene

Hoop-shaped π-conjugated molecules have attracted much attention. In this study, the radical ions of [4]­cyclo-2,7-pyrenylene ([4]­CPY), a cyclic tetramer of pyrene, and [4]­cyclo-4,5,9,10-tetrahydro-2,7-pyrenylene ([4]­CHPY) were investigated using radiation chemical methods, namely, γ-ray radiolysis and pulse radiolysis. The absorption spectra of the radical ions of [4]­CPY and [4]­CHPY showed clear peaks in the near-IR and UV–vis regions similar to those of [8]­cycloparaphenylene ([8]­CPP). Theoretical calculations using time-dependent density functional theory provided reasonable assignments of the observed absorption bands. It was indicated that the C4–C5 and C9–C10 ethylene bonds of [4]­CHPY do not contribute to the electronic transitions, resulting in absorption spectra similar to those of [8]­CPP. On the other hand, it was confirmed that the allowed electronic transitions of the radical ions of [4]­CPY are different from those of the radical ions of [4]­CHPY and [8]­CPP