CHIRAL TRIMETHINIUM AND PENTAMETHINIUM CYANINE DYES WITH 1,2,3,4-TETRAHYDRO-6-METHYLQUINOLYL END-GROUPS - UV VIS AND CD SPECTROSCOPY AND STRUCTURE CORRELATIONS/

UV/Vis and CD spectra of the chiral trimethinium cyanine dyes T2 to T5 and the pentamethinium cyanine dyes P2 to P5 have been studied for po ssible correlations between chiroptical properties and molecular struc ture. With a non-planar conjugated K-system as a consequence of steric interaction between the two end groups and the polymethine chain and with the methyl substituents in the 2- or 3-ring positions acting as c hiral anchors, the inherent chirality of these dyes manifests itself i n the CD spectra which follow the same pattern throughout: monosignate CD absorptions in the long-wavelength cyanine band region and bisigna te in the aromatic-end-group region. A correlation between the sense o f twist of the polymethine chain and the observed and calculated sign of the cyanine band is established, which allows the assignment of the absolute configuration for the 2-, but not for the 3-methyl substitut ed dye derivatives. Neither in the trimethinium nor in the pentamethin ium dyes do the calculated energies, geometries or chiroptical propert ies indicate any interaction between the two end groups.