Singlet fission (SF) materials hold the potential to increase the power conversion efficiency of solar cells by reducing the thermalization of high-energy excited states. The major hurdle in realizing this potential is the limited scope of SF-active materials with high fission efficiency, suitable energy levels, and sufficient chemical stability.

Adaptive aromaticity in the lowest singlet and triplet states is a rare property found among molecular systems. So far, only osmapentalene and osmapyridinium have been found to possess the adaptive aromaticity. Although it has been confirmed that the pattern of electron excitation is a key factor to achieve the adaptive aromaticity, further investigation of the metal center effect has not yet been made. Ruthenium, another Group 8 transition metal, can form metallacycles similar to the osmium counterparts.

According to Hückel’s and Baird’s rules, cyclic conjugated species are aromatic either in the ground state or in the excited state only. Thus, species with aromaticity in both states (denoted as adaptive aromaticity) are particularly rare. Here we carry out density functional theory calculations on a series of osmapyridine and osmapyridinium complexes (96 species) and find that two of them display adaptive aromaticity, which was verified by various aromaticity indices including HOMA, ELFπ, MCI, ACIDπ plots and the heat of hydrogenation.

Aromaticity is a fundamental chemical concept of ever-increasing diversity. According to Hückel’s and Baird’s rules, cyclic conjugated species with 4n+2 π-electrons are aromatic in the singlet electronic ground state (S0) and antiaromatic in the lowest triplet state (T1), and vice-versa. Thus, species with aromaticity in both states have not yet been reported.