The First C₂‐Symmetric, Chiral Monomethine Dye—An Apparent Violation of the Helicity Rule for Optical Rotation

The P-helically twisted chromophore of the chiral monomethine dye (R,R)-1 (structure shown on the right) shows a negative Cotton effect for the long-wavelength cyanine band—in apparent violation of established helicity rules. Quantum mechanical calculations and a component analysis support the experimental findings, lending surprisingly additional support for those rules. X = C(CH3)C3H7, counterion: ClO.