Ortho­rhom­bic polymorph of (2E)-2-(2,3-dihydro-1H-inden-1-yl­idene)-2,3-di­hydro-1H-inden-1-one

The title compound, C(18)H(14)O, is polymorphic at 123 K. The ortho­rhom­bic form reported herein has two independent mol­ecules in the asymmetric unit, with mol­ecular volume 313.5 Å(3). The previously reported triclinic (P-1) form [Raston & Scott (2000 ▶). Green Chem., 2, 49–52] has mol­ecular volume 309.6 Å(3) at the same temperature. All three mol­ecules deviate significantly and systematically from the putative C(s) symmetry (δ(r.m.s.) = 0.0265, 0.0256, 0.0497 Å). Comparison of the two molecules in the orthorhombic polymorph shows that 16 of the 19 equivalent pairs of framework atoms have a mirror-image pattern of deviations (above/below plane), suggesting that the two are quasi-enanti­omorphs. The pattern of deviations in the triclinic form is nearly the same (13 of 19 atom pairs) as the ortho­rhom­bic form.