Polyethylated aromatic rings: Conformation and rotational barriers of 1,2,3,4,5,6,7,8-octaethylanthracene, 1,2,3,4,6,7,8-heptaethylfluorene, and 1,2,3,4,5,6,7,8-octaethylfluorene

1,2,3,4,5,6,7,8-Octaethylanthracene (5), 1,2,3,4,6,7,8-heptaethylfluorene ( 7) and 1,2,3,4,5,6,7,8-octaethylfluorene (8) were synthesized by Friedel-Cr afts ethylations of 9,10-dihydroanthracene and fluorene. MM3 calculations i ndicate that the two ethylated six-membered rings of 5 and 7 are conformati onally independent. According to the calculations, two low-energy conformer s of each compound are possible with the ethyl groups attached to the exter nal aryl rings arranged in an alternated "up-down" orientation. MM3 calcula tions indicate that in the lowest energy conformation the central fluorene core of 8 adopts a twisted conformation to avoid repulsive steric interacti ons between the ethyls at the bay region. Two fully alternated up-down conf ormations are possible for 8, differing in the orientation ("in" or "out") of the ethyls in the bay region. MM3 calculations predict that the lowest e nergy conformer is the fully alternated "out" form of C-2 symmetry. The rot ational barriers of 5, 7, and 8 are in the 8.7-11.3 kcal mol(-1) range, the largest barrier corresponding to the more crowded octaethylfluorene 8. Ant hracene 5 adopts in the crystal a conformation of approximate C-2h symmetry with pairs of peri groups on the same edge of the molecule oriented syn. T he conformations adopted in the crystal by 7 and 8 do not correspond to the calculated lowest energy form. In the conformation of 7 in the crystal the ethyl groups on the trisubstituted ring adopt the unusual all syn arrangem ent. Octaethylfluorene 8 adopts a conformation with a twisted central fluor ene core but with a syn arrangement of a pair of vicinal ethyl groups.