Is Excited-State Aromaticity a Driving Force for Planarization of Dibenzannelated 8π-Electron Heterocycles?

Compounds with dibenzannelated heterocycles with eight pi-electrons are found in a range of applications. These molecules often adopt a bent structure in the ground state (S-0) but can become planar in the first excited states (S-1 and T-1) because of the cyclically conjugated 4n pi central ring, which fulfils the requirements for excited state aromaticity. We report on a quantum chemical investigation of the aromatic character in the S-1 and T-1 states of dibenzannelated seven- and six-membered heterocycles with one, two, or three heteroatoms in the 8 pi-electron ring. These states could have pi pi* or n pi* character. We find that compounds with one or two heteroatoms in the central ring have pi pi* states as their S-1 and T-1 states. They are to a significant degree influenced by excited state aromaticity, and their optimal structures are planar or nearly planar. Among the heteroatoms, nitrogen provides for the strongest excited state aromaticity whereas oxygen provides for the weakest, following the established trend of the S-0 state. Yet, dibenzannelated seven-membered-ring compounds with N=N bonds have non-aromatic n pi* states with strongly puckered structures as their S-1 and T-1 states.