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 (DRs) embedded in curved cycloparaphenylene (CPPs) structures, DR-(n+3)CPPs (n = 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 1,3diradicals, such as the seven-and eight-paraphenylene-unitcontaining diradicals DR-7CPP (n = 4) and DR-8CPP (n = 5), by quantum chemical calculations. The smaller-sized diradicals DR-(n+3)CPPs (n = 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 DR-6CPP in the photochemical denitrogenation of its azo-containing precursor (AZ-6CPP). Intriguingly, a unique type of in-plane aromaticity emerged in the smaller-sized singlet states such as S-DR-4CPP (n = 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.

SOMO–HOMO Conversion in Triplet Cyclopentane-1,3-diyl Diradicals

“…The slightly higher HOMO energy in DR3F1 than that in DR4F1 is explained by the curved structure of the anthryl unit in DR3F1 . A similar curve effect was reported for cycloparaphenylene compounds. − …”

SOMO–HOMO Conversion in Triplet Carbenes

“…Thus, the HOMO energy in 4C was higher than that in DPC by ∼2 eV. A similar effect, i.e., increased HOMO energy due to curved π-electron systems, has been observed for cycloparaphenylene (CPP) derivatives …”

“…According to the natural bond orbital (NBO) analysis, the s character increased from 18.6% ( 11C C2v ) to 26.3% ( 4C C2v ) (Table S1). As evident from the curve effect of π-conjugated systems, , the quinoidal contribution increased with decreasing ring size. Thus, the HOMO energy increased despite the decrease in the number of benzene rings.…”

New Insights into Bond Homolysis Process and Discovery of Novel Bonding System (C–π–C) by Generating Long-lived Singlet Diradicals

“…Under high pressure conditions with viscous solvents, which are known to increase in viscosity with increasing pressure, the logarithm of the lifetime of 11 was proportional to the induced pressure (Figure 5b). We further aimed to determine whether the longevity of π-single-bonded chemical species can be achieved by kinetic stabilization through a novel molecular design� In this context, inspired by recent synthetic studies on cyclic paraphenylene, [33][34][35][36] we designed a molecule with a diradical skeleton in the macrocyclic ring (Scheme 3) 37 . Specifically, the diradical skeleton has a planar structure and a larger macrocyclic ring than the corresponding ring-closing structure; therefore, we expected it to be less molecularly distorted and thus more kinetically stabilized in the reaction yielding the ring-closed product.…”