The stabilization of 4π-electron systems remains a fundamental challenge in chemistry, stemming from their intrinsic antiaromaticity and thermodynamic instability as dictated by Hückel’s rule. A recent experimental study has demonstrated that rational molecular design using N-heterocyclic carbenes (NHCs) can surmount these limitations by inducing aromaticity. However, the issue of T1 aromaticity in such systems remains unresolved. Elucidating the substituent-modulated behavior of T1 (anti)aromaticity is crucial for deciphering the physicochemical properties of these systems.

In accordance with the constraints by Hückel’s and Baird’s rules, species generally exhibit aromaticity in one state (the lowest singlet state S0 or the lowest triplet state T1). Consequently, species with adaptive aromaticity (being aromatic in both the S0 and T1 states) are particularly rare. In this study, density functional theory (DFT) was employed to investigate adaptive aromaticity in 14e–, 16e– and 18e– metallabenzenes.

Adaptive aromaticity has attracted substantial attention due to its unconventional two-state aromaticity in both the lowest singlet state (S0) and the lowest triplet state (T1) and its application to the singlet fission. Here we carry out a comprehensive investigation on a series of pyrrole derivatives by density functional theory (DFT) calculations and machine learning.

The concept of adaptive aromaticity, which denotes two-state aromaticity in both the lowest singlet and triplet states, stands in marked contrast to the traditional one-state aromaticity governed by the Hückel and Baird rules. Nonetheless, organic compounds exhibiting adaptive aromaticity remain particularly rare.

Complexes with aromaticity in both the lowest singlet state (S0) and the lowest triplet state (T1) (denoted as adaptive aromaticity) are rare because according to Hückel’s and Baird’s rules, a species could be aromatic in either the S0 or T1 state in most cases. Thus, it is particularly challenging to design species with adaptive aromaticity. Previous reports on adaptive aromaticity were mainly focused on 16e metallapentalenes.

Species generally exhibit one-state aromaticity either in the lowest singlet state (S0) or the lowest triplet state (T1) according to the Hückel's and Baird's rules. Hence, it is rare for species exhibit two-state aromaticity in both the S0 and T1 states (termed as adaptive aromaticity), let alone adaptive σ aromaticity. Here, we report adaptive σ aromaticity in unsaturated rhenacyclopropene rings via density functional theory (DFT) calculations.

In general, compounds exhibit one-state aromaticity in either the ground or excited state according to the Hückel’s and Baird’s rules. Thus, species with two-state aromaticity in the lowest singlet and triplet states (termed as adaptive aromaticity) are rare. Understanding the underlying mechanism for achieving adaptive aromaticity is important to enrich this rare family. Here we carry out density functional theory (DFT) calculations to probe the origin of adaptive aromaticity in metallapyridiniums.

Discovery of species with adaptive aromaticity, being aromatic in both the lowest-lying singlet and triplet states (S0 and T1), is a significant challenge because cyclic conjugated complexes are commonly aromatic in one state only according to Hückel’s and Baird’s rules. On the other hand, the carbone ligands containing two lone pair electrons at the carbon(0) atom have attracted considerable attention from both theoretical and experimental chemists recently.

Singlet fission has attracted extensive attention from experimentalists and theoreticians due to its ability to improve photovoltaic conversion efficiency. Still, designing singlet fission materials remains challenging. In this work, we explored the relationship between adaptive aromaticity and singlet fission potentials by computationally screening the adaptive aromatic species reported by our group.

According to Hückel’s and Baird’s rules, cyclic species are generally aromatic only either in the lowest singlet state (S0) or in the lowest-lying triplet ππ* excited state (T1). Thus, species with aromaticity both in S0 and T1 states (termed as adaptive aromaticity) are particularly rare. Herein, we carry out density functional theory (DFT) calculations to examine the aromaticity of 16e metallapentalenes containing heteroatoms (N, O).