Structure and dynamics of helically twisted cyanine dyes Original Research Article

Optimized geometries of the helically twisted C2-symmetric monomethine dyes 1 and 2 and of the trimethine dyes 3 and 4 have been obtained at the RHF/6-31G** and (for 1 and 2) the B3LYP/6-31G** level. The non-planar deformation of the chromophore is reproduced quantitatively by the calculations. Chiral substitution of the dye, as in 2 and 4, renders oppositely twisted conformations diastereomeric. The calculated energy difference between the diastereomers is in satisfactory agreement with the experimental data where available and is found to be hyperconjugative in nature. The inversion of 1 involves a planar transition state. The relaxation from there is followed by ab initio molecular dynamics.