Structure and properties of 9-(cycloheptatrienylidene)fluorene and its derivatives

One sesquifulvalene, 9-(cycloheptatrienylidene)fluorene, was found to be composed of a flat fluorenylidene part and a non-planar cycloheptatrienylidene moiety. 1-Substituted 9-(cycloheptatrienylidene)fluorene was not obtained because of a steric repulsion between the 1-substituent and the 7 ' -hydrogen of the seven-membered ring. Upon consideration of their C-13 NMR spectra, the electronic properties of the seven-membered ring are slightly influenced by introducing a substituent (halogen, methoxy group, etc.) on the fluorenylidene part. The dissolution of 9-(cycloheptatrienylidene)fluorene in trifluoroacetic acid-chloroform afforded the (9-fluorenyl)tropylium cation. The substituent attached to the fluorene part also slightly influences the tropylium moiety.