An extremely long-lived localised singlet diradical with π-single bonding character is found in a macrocyclic structure that retards the radical–radical coupling reaction by the “stretch and solvent-dynamic effects”.

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.

Localized singlet diradicals have attracted much attention, not only in the field of bond-homolysis chemistry, but also in nonlinear optical materials. In this study, an extremely long lived localized singlet diradical was obtained by using a new molecular design strategy in which it is kinetically stabilized by means of a macrocycle that increases the molecular strain of the corresponding σ-bonded compound. Notably, the lifetime of this diradical (14 μs) is two orders of magnitude longer than that of a standard singlet diradical without a macrocyclic structure (≈0.2 μs) at 293 K. The species is persistent below a temperature of 100 K. In addition to the kinetic stabilization of the singlet diradical, the spontaneous oxidation of its corresponding ring-closed compound at 298 K produced oxygenated products under atmospheric conditions. Apparently, the "stretch effect" induced by the macrocyclic structure plays a crucial role in extending the lifetime of localized singlet diradicals and increasing the reactivity of their corresponding σ-bonded compounds.

Singly occupied molecular orbital−highest occupied molecular orbital (SOMO−HOMO) conversion (inversion), SHC, is a phenomenon in which the SOMO is lower in energy than the doubly occupied molecular orbitals (DOMO, HOMO). A non-Aufbau electronic structure leads to unique properties such as a switch in bond dissociation energy and the generation of high-spin species on one-electron oxidation. In addition, the pronounced photostability of these species has been reported recently for application in organic light-emitting devices. In this review article, we summarise the chemistry of SOMO−HOMO converted (inverted) species reported to date.

In this study, the SOMO−HOMO conversion has been shown for the first time in triplet carbenes embedded in cycloparaphenylene units. The high-lying HOMO originating from the curved π-conjugated system and the low-lying SOMO−1 originating due to the small carbene angle are the key to endowing this interesting electronic configuration. Furthermore, simple planar triplet carbenes such as fluorenylidene were found to possess SOMO− HOMO energy-converted electronic configurations.

According to the Aufbau principle, singly occupied molecular orbitals (SOMOs) are energetically higher lying than a highest doubly occupied molecular orbital (HOMO) in the electronically ground state of radicals. However, in the last decade, SOMO–HOMO-converted species have been reported in a limited group of radicals, such as distonic anion radicals and nitroxides. In this study, SOMO–HOMO conversion was observed in triplet 2,2-difluorocyclopentane-1,3-diyl diradicals DR3F1, DR4F1, and 2-fluorocyclopentante-1,3-diyl diradical DR3HF1, which contain the anthracyl unit at the remote position. The high HOMO energy in the anthracyl moiety and the low-lying SOMO–1 due to the fluoro-substituent effect are the key to the SOMO–HOMO conversion phenomenon. Furthermore, the cation radical DR3F1 + generated through the one-electron oxidation of DR3F1 was found to be a SOMO–HOMO-converted monoradical.

The evolution of prenucleation clusters in the prenucleation stage of colloidal semiconductor quantum dots (QDs) has remained unexplored. With CdTe as a model system, we show that substances form and isomerize prior to the nucleation and growth of QDs. Called precursor compounds (PCs), the prenucleation clusters are relatively optically transparent and can transform to absorbing magic‐size clusters (MSCs). When a prenucleation‐stage sample at 25, 45, or 80 °C is dispersed in a mixture of cyclohexane (CH) and octylamine (OTA) at room temperature, either MSC‐371, MSC‐417, or MSC‐448 evolves with absorption peaking at 371, 417, or 448 nm, respectively. We propose that PC‐371 forms at 25 °C, and isomerizes to PC‐417 at 45 °C and to PC‐448 at 80 °C. The PCs and MSCs are quasi isomers. Relatively large and small amounts of OTA favor PC‐371 and PC‐448 in dispersion, respectively. The present findings suggest the existence of PC‐to‐PC isomerization in the QD prenucleation stage.

The evolution of prenucleation clusters in the prenucleation stage of colloidal semiconductor quantum dots (QDs) has remained unexplored. With CdTe as a model system, we show that substances form and isomerize prior to the nucleation and growth of QDs. Called precursor compounds (PCs), the prenucleation clusters are relatively optically transparent and can transform to absorbing magic‐size clusters (MSCs). When a prenucleation‐stage sample at 25, 45, or 80 °C is dispersed in a mixture of cyclohexane (CH) and octylamine (OTA) at room temperature, either MSC‐371, MSC‐417, or MSC‐448 evolves with absorption peaking at 371, 417, or 448 nm, respectively. We propose that PC‐371 forms at 25 °C, and isomerizes to PC‐417 at 45 °C and to PC‐448 at 80 °C. The PCs and MSCs are quasi isomers. Relatively large and small amounts of OTA favor PC‐371 and PC‐448 in dispersion, respectively. The present findings suggest the existence of PC‐to‐PC isomerization in the QD prenucleation stage.