One-dimensional staircase aggregates in crystals of 1,7-bis(dimethylamino)heptamethinium hexafluorophosphate, a polymethine dye

The title compound C 11 H 19 N 2 .PF 6 crystallises in the monoclinic space group C2/m (no. 12) with a = 1234.4 pm, b = 942.8 pm, c = 717.6 pm, β = 109.95°, Z = 2 and D x = 1.372 g cm -3 . R = 0.08 for 627 unique reflections with I > 2σ(I) at 298 K. The cation as well as the anion are disordered. A polarization of the polymethine system by the counterion is not observed, because the dye molecule exhibits a symmetrical polymethine structure. The chromophores are closely stacked in one-dimensional staircase aggregates which are separated by the counterions. Their parallel arrangement in the unit cell should lead to only one transition dipole moment.