Retaining Huckel Aromaticity in the Triplet Excited State of Azobenzene

The implication of the potential concept of aromaticity in the relaxed lowest triplet state of azobenzene, an efficient molecular switch, using elementary aromaticity indices based on magnetic, electronic, and geometric criteria has been discussed. Azobenzene exhibits a major Huckel aromatic character retained in the diradical lowest relaxed triplet state (T-1) by virtue of a twisted geometry with partial delocalization of unpaired electrons in the perpendicular p-orbitals of two nitrogen atoms to the corresponding phenyl rings. The computational analysis has been expanded further to stilbene and N-diphenylmethanimine for an extensive understanding of the effect of closed-shell Huckel aromaticity in double-bond-linked phenyl rings. Our analysis concluded that stilbene has Huckel aromatic character in the relaxed T-1 state and N-diphenylmethanimine has a considerable Huckel aromaticity in the phenyl ring near the carbon atom while a paramount Baird aromaticity in the phenyl ring near the nitrogen atom of the C=N double bond. The results reveal the application of excited-state aromaticity as a general tool for the design of molecular switches.

Automated summary

The potential concept of aromaticity in the relaxed lowest triplet state of azobenzene, a molecular switch, has been discussed using different aromaticity indices. The study reveals the presence of Huckel aromatic character in the relaxed T-1 state of azobenzene, as well as stilbene and N-diphenylmethanimine, highlighting the significance of excited-state aromaticity in the design of molecular switches.