Adaptive σ‐Aromaticity in an Unsaturated Three‐Membered Ring

Abstract

Based on Hückel's and Baird's rules, species are aromatic either in the lowest singlet state (S 0) or the lowest triplet state (T 1) only. Thus, species with adaptive aromaticity (with aromaticity in both the S 0 and T 1 states) is particularly rare. On the other hand, σ-aromaticity in the T 1 state has been underdeveloped, let alone adaptive σ-aromaticity. Herein, via various aromaticity indices including NICS, ACID and EDDB, we demonstrate adaptive σ-aromaticity in an unsaturated three-membered ring, which is a traditional area [a

Adaptive Aromaticity in Metallasilapentalynes

“…In other words, species can display aromaticity either in the S 0 or T 1 state generally. On the contrary, adaptive aromaticity is a recently proposed concept, which means that species could be aromatic in two states, i.e., both the S 0 and T 1 states. − Besides, benzene with strong π-accepting substituent groups (CHO, COCH 3 , NO, and NO 2 ) was reported to display aromaticity in both S 0 and T 1 states . However, spin density is mainly localized at the substituent in these cases .…”

Computational predictions of adaptive aromaticity for the design of singlet fission materials

“…87 Later, tetraatomic boron species and compounds with σ-aromaticity in metallacycles were reported by experimentalists 88,89 and theoreticians. 72,74,90,91 Recently, our group used various aromaticity indices to prove that σ-aromaticity not only plays a key role in the ground-state of species containing a metallacyclopropene 74 or B4 unit 72 but also stabilizes the T 1 state. The origin of these compounds with adaptive σaromaticity is due to the excitation mode of π → π* from the S 0 to T 1 state, which has an ignorable effect on the σ-system and preserves the σ-aromaticity in the T 1 state.…”

“…The model complexes were taken from previously reported species with adaptive aromaticity. 59,69,71,72,74,77 We adopted the calculation method proposed by Zeng, Hoffmann, and Ananth (denoted as the ZHA method), 29 which was also used in Ottosson's work. 58 In the ZHA method, the vertical excitation energy E(S 1 ) v and E(T 2 ) v were based on the optimized geometries of the S 0 and T 1 states, respectively.…”

“…After that, adaptive aromaticity has also been found using theoretical calculations in osmapyridinium with phosphonium substituents, 68 metallapentalenes, and metallasilapentalynes with strong electron-donor ligands, 69,70 cyclo [10]carbon, 71 tetraatomic boron species, 72 pyrrole ring of the polycyclic dipyrrolonaphthyridinedione skeleton, 73 and the unsaturated three-membered rings. 74 In particular, the dipyrrolonaphthyridinedione has been reported by experimentalists and theoreticians as an excellent singlet fission chromophore. 73…”

Open questions on aromaticity in organometallics

“…1c), which is aromatic in both S 0 and T 1 states, and which we termed as adaptive aromaticity 20 . Later, the concept of adaptive aromaticity has been extended to osmapyridinium 21 , mono-substituted benzene, tetraatomic boron species, osmapentalene derivatives, cyclo [10] carbon, and the pyrrole ring in dipyrrolonaphthyridinedione 22 . As two-state aromaticity is particularly rare, discovery of such adaptive aromatics, caused by a novel excitation fashion, has been always challenging.…”