1,3-Diradicals Embedded in Curved Paraphenylene Units: Singlet versus Triplet State and In-Plane Aromaticity

Abstract

Curved π-conjugated molecules and open-shell structures have attracted much attention from the perspective of fundamental chemistry, as well as materials science. In this study, the chemistry of 1,3-diradicals (<b>DR</b>s) embedded in curved cycloparaphenylene (<b>CPP</b>s) structures, <b>DR</b>-(<i>n</i>+3)<b>CPP</b>s (<i>n</i> = 0-5), was investigated to understand the effects of the curvature and system size on the spin-spin interactions and singlet versus triplet state, as well as their unique characteristics such as in-plane aromaticity. A triplet ground state was predicted for the larger 1,3-diradicals, such as the seven- and eight-paraphenylene-unit-containing diradicals <b>DR</b>-<b>7CPP</b> (<i>n</i> = 4) and <b>DR</b>-<b>8CPP</b> (<i>n</i> = 5), by quantum chemical calculations. The smaller-sized diradicals <b>DR</b>-(<i>n</i>+3)<b>CPP</b>s (<i>n</i> = 0-3) were found to possess singlet ground states. Thus, the ground-state spin multiplicity is controlled by the size of the paraphenylene cycle. The size effect on the ground-state spin multiplicity was confirmed by the experimental generation of <b>DR</b>-<b>6CPP</b> in the photochemical denitrogenation of its azo-containing precursor (<b>AZ</b>-<b>6CPP</b>). Intriguingly, a unique type of in-plane aromaticity emerged in the smaller-sized singlet states such as S-<b>DR</b>-<b>4CPP</b> (<i>n</i> = 1), as proven by nucleus-independent chemical shift calculations (NICS) and an analysis of the anisotropy of the induced current density (ACID), which demonstrate that homoconjugation between the 1,3-diradical moiety arises because of the curved and distorted bonding system.