Benz[c] indeno[2,1-a] fluorene: a 2,3-naphthoquinodimethane incorporated into an indenofluorene frame

Synthesis of 13,14-dimesitylbenz[c] indeno[2,1-a] fluorene (9b), a 2,3-naphthoquinodimethane derivative isolated for the first time, was achieved. The X-ray crystallographic structure and spectroscopic and electrochemical properties of 9b indicate substantial singlet biradical character which is higher than that of 11,12-dimesitylindeno[2,1-a] fluorene (8b), in accord with the quantum chemical study. Cycloadditions with oxygen and dichlorodicyano-p-benzoquinone took place at the inner naphthalene carbons rather than the most reactive exocyclic carbons because of the effective steric protection by the mesityl groups at the latter positions. [4 + 2] Cyclodimerization also took place at the inner naphthalene position as a 4 pi component and at the outer naphthalene ring as a 2 pi component.