Preparation and Molecular Structures of 9,10-Dihydrophenanthrenes:  Substituent Effects on the Long Bond Length

9,10-Dihydrophenanthrene derivatives 1−3 with electron-donating and/or -accepting groups at their 9,10-positions were prepared, and their precise molecular structures were determined by X-ray analyses at 203 K. The long C9−C10 bond [1.646(4) Å] in the hexaarylethane-type compound 1 with four electron-donating groups is mainly caused by steric interaction. Push−pull type substitution does not induce the elongation of the central bond in the present system; the corresponding distance in 9,9-bis(4-dimethylaminophenyl)-10,10-dicyano derivative 2 [1.599(4) Å] is intermediate between those of 1 and the tetracyano compound 3 [1.587(2) Å].