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 C(9)-C(10) bond [1.646(4) A] 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) A] is intermediate between those of 1 and the tetracyano compound 3 [1. 587(2) A].