Structure of 1,5-bis(dimethylamino)-1,5-diphenylpentadienylium perchlorate and modelling of pentadienylium salts (cyanine dyes)

C 21 H 25 N 2 + .ClO 4 − (orange crystals), M r = 404.9, orthorhombic; P 2 1 2 1 2 1 , a = 7.810 (2), b = 15.007 (3), c = 18.326 (3) Å, V = 2147.8 (7) Å 3 . Graphite-monochromated Mo K α source at room temperature; λ = 0.71069 Å, μ = 0.22 mm −1 , Z = 4; F (000) = 856, D x = 1.25 g cm −3 , R = 0.057 for 1392 independent data. As for the unsubstituted pentamethinium dyes, the chain lies in an all- trans planar configuration. Molecular modelling calculations reproduce the X-ray structure with good accuracy, even in the case of more crowded pentadienylium cations, i.e. 1,5-bis(dimethylamino)-1,3,5-triphenylpentadienylium perchlorate.