Exceptionally Large Difference in Bond Length among Conformational Isomorphs of a Hexaphenylethane Derivative with a Dispiropyracene Skeleton^†

A large difference of 0.064(3) Å for a chemically equivalent Csp3−Csp3 bond was observed among crystallographically independent molecules with different conformations (conformational isomorphs) of a hexaphenylethane derivative, dispiro[(10-methylacridan)-9,1‘-pyracene-2‘,9‘ ‘-(10-methylacridan)] (1): triclinic, P1̄, a = 17.699(3) Å, b = 18.111(3) Å, c = 18.844(3) Å, α = 111.183(2)°, β = 93.828(1)°, γ = 102.130(2)°, V = 5438.5(14) Å3, T = −180 °C, Z = 8 (four independent molecules: A, A‘, B, and B‘). While B and B‘ adopt the skewed conformation to reduce steric repulsion in 1, in other positions of the crystal, molecules A and A‘ adopt the eclipsed conformation, which induces a much longer bond length [1.771(3) and 1.758(3) Å versus 1.712(2) and 1.707(2) Å].